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Versions: (draft-schaad-cose-msg) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

COSE Working Group                                             J. Schaad
Internet-Draft                                            August Cellars
Intended status: Informational                            March 21, 2016
Expires: September 22, 2016


                      CBOR Encoded Message Syntax
                         draft-ietf-cose-msg-11

Abstract

   Concise Binary Object Representation (CBOR) is data format designed
   for small code size and small message size.  There is a need for the
   ability to have the basic security services defined for this data
   format.  This document specifies processing for signatures, message
   authentication codes, and encryption using CBOR.  This document also
   specifies a representation for cryptographic keys using CBOR.

Contributing to this document

   The source for this draft is being maintained in GitHub.  Suggested
   changes should be submitted as pull requests at <https://github.com/
   cose-wg/cose-spec>.  Instructions are on that page as well.
   Editorial changes can be managed in GitHub, but any substantial
   issues need to be discussed on the COSE mailing list.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 22, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Design changes from JOSE  . . . . . . . . . . . . . . . .   5
     1.2.  Requirements Terminology  . . . . . . . . . . . . . . . .   6
     1.3.  CBOR Grammar  . . . . . . . . . . . . . . . . . . . . . .   6
     1.4.  CBOR Related Terminology  . . . . . . . . . . . . . . . .   7
     1.5.  Document Terminology  . . . . . . . . . . . . . . . . . .   8
   2.  Basic COSE Structure  . . . . . . . . . . . . . . . . . . . .   8
   3.  Header Parameters . . . . . . . . . . . . . . . . . . . . . .  10
     3.1.  Common COSE Headers Parameters  . . . . . . . . . . . . .  12
   4.  Signing Objects . . . . . . . . . . . . . . . . . . . . . . .  16
     4.1.  Signing with One or More Signers  . . . . . . . . . . . .  16
     4.2.  Signing with One Signer . . . . . . . . . . . . . . . . .  18
     4.3.  Externally Supplied Data  . . . . . . . . . . . . . . . .  19
     4.4.  Signing and Verification Process  . . . . . . . . . . . .  20
     4.5.  Computing Counter Signatures  . . . . . . . . . . . . . .  21
   5.  Encryption Objects  . . . . . . . . . . . . . . . . . . . . .  22
     5.1.  Enveloped COSE Structure  . . . . . . . . . . . . . . . .  22
       5.1.1.  Recipient Algorithm Classes . . . . . . . . . . . . .  24
     5.2.  Encrypted COSE structure  . . . . . . . . . . . . . . . .  24
     5.3.  Encryption Algorithm for AEAD algorithms  . . . . . . . .  25
     5.4.  Encryption algorithm for AE algorithms  . . . . . . . . .  27
   6.  MAC Objects . . . . . . . . . . . . . . . . . . . . . . . . .  28
     6.1.  MAC Message with Recipients . . . . . . . . . . . . . . .  28
     6.2.  MAC Messages with Implicit Key  . . . . . . . . . . . . .  30
     6.3.  How to compute and verify a MAC . . . . . . . . . . . . .  30
   7.  Key Structure . . . . . . . . . . . . . . . . . . . . . . . .  32
     7.1.  COSE Key Common Parameters  . . . . . . . . . . . . . . .  32
   8.  Signature Algorithms  . . . . . . . . . . . . . . . . . . . .  35
     8.1.  ECDSA . . . . . . . . . . . . . . . . . . . . . . . . . .  36
       8.1.1.  Security Considerations . . . . . . . . . . . . . . .  38
   9.  Message Authentication (MAC) Algorithms . . . . . . . . . . .  39
     9.1.  Hash-based Message Authentication Codes (HMAC)  . . . . .  39
       9.1.1.  Security Considerations . . . . . . . . . . . . . . .  41
     9.2.  AES Message Authentication Code (AES-CBC-MAC) . . . . . .  41
       9.2.1.  Security Considerations . . . . . . . . . . . . . . .  42
   10. Content Encryption Algorithms . . . . . . . . . . . . . . . .  42



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     10.1.  AES GCM  . . . . . . . . . . . . . . . . . . . . . . . .  43
       10.1.1.  Security Considerations  . . . . . . . . . . . . . .  44
     10.2.  AES CCM  . . . . . . . . . . . . . . . . . . . . . . . .  44
       10.2.1.  Security Considerations  . . . . . . . . . . . . . .  47
     10.3.  ChaCha20 and Poly1305  . . . . . . . . . . . . . . . . .  47
       10.3.1.  Security Considerations  . . . . . . . . . . . . . .  48
   11. Key Derivation Functions (KDF)  . . . . . . . . . . . . . . .  48
     11.1.  HMAC-based Extract-and-Expand Key Derivation Function
            (HKDF) . . . . . . . . . . . . . . . . . . . . . . . . .  49
     11.2.  Context Information Structure  . . . . . . . . . . . . .  51
   12. Recipient Algorithm Classes . . . . . . . . . . . . . . . . .  54
     12.1.  Direct Encryption  . . . . . . . . . . . . . . . . . . .  55
       12.1.1.  Direct Key . . . . . . . . . . . . . . . . . . . . .  55
       12.1.2.  Direct Key with KDF  . . . . . . . . . . . . . . . .  56
     12.2.  Key Wrapping . . . . . . . . . . . . . . . . . . . . . .  57
       12.2.1.  AES Key Wrapping . . . . . . . . . . . . . . . . . .  58
     12.3.  Key Encryption . . . . . . . . . . . . . . . . . . . . .  59
     12.4.  Direct Key Agreement . . . . . . . . . . . . . . . . . .  59
       12.4.1.  ECDH . . . . . . . . . . . . . . . . . . . . . . . .  60
     12.5.  Key Agreement with KDF . . . . . . . . . . . . . . . . .  64
       12.5.1.  ECDH . . . . . . . . . . . . . . . . . . . . . . . .  64
   13. Keys  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  65
     13.1.  Elliptic Curve Keys  . . . . . . . . . . . . . . . . . .  65
       13.1.1.  Double Coordinate Curves . . . . . . . . . . . . . .  66
     13.2.  Symmetric Keys . . . . . . . . . . . . . . . . . . . . .  67
   14. CBOR Encoder Restrictions . . . . . . . . . . . . . . . . . .  68
   15. Application Profiling Considerations  . . . . . . . . . . . .  68
   16. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  69
     16.1.  CBOR Tag assignment  . . . . . . . . . . . . . . . . . .  69
     16.2.  COSE Header Parameter Registry . . . . . . . . . . . . .  70
     16.3.  COSE Header Algorithm Label Table  . . . . . . . . . . .  70
     16.4.  COSE Algorithm Registry  . . . . . . . . . . . . . . . .  71
     16.5.  COSE Key Common Parameter Registry . . . . . . . . . . .  72
     16.6.  COSE Key Type Parameter Registry . . . . . . . . . . . .  73
     16.7.  COSE Elliptic Curve Registry . . . . . . . . . . . . . .  73
     16.8.  Media Type Registrations . . . . . . . . . . . . . . . .  74
       16.8.1.  COSE Security Message  . . . . . . . . . . . . . . .  74
       16.8.2.  COSE Key media type  . . . . . . . . . . . . . . . .  75
     16.9.  CoAP Content Format Registrations  . . . . . . . . . . .  77
     16.10. Expert Review Instructions . . . . . . . . . . . . . . .  78
   17. Security Considerations . . . . . . . . . . . . . . . . . . .  79
   18. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  80
   19. References  . . . . . . . . . . . . . . . . . . . . . . . . .  80
     19.1.  Normative References . . . . . . . . . . . . . . . . . .  80
     19.2.  Informative References . . . . . . . . . . . . . . . . .  81
   Appendix A.  Making Mandatory Algorithm Header Optional . . . . .  84
     A.1.  Algorithm Identification  . . . . . . . . . . . . . . . .  84
     A.2.  Counter Signature Without Headers . . . . . . . . . . . .  87



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   Appendix B.  Three Levels of Recipient Information  . . . . . . .  88
   Appendix C.  Examples . . . . . . . . . . . . . . . . . . . . . .  89
     C.1.  Examples of Signed Message  . . . . . . . . . . . . . . .  90
       C.1.1.  Single Signature  . . . . . . . . . . . . . . . . . .  90
       C.1.2.  Multiple Signers  . . . . . . . . . . . . . . . . . .  91
       C.1.3.  Counter Signature . . . . . . . . . . . . . . . . . .  92
       C.1.4.  Signature w/ Operation Time and Criticality . . . . .  93
     C.2.  Single Signer Examples  . . . . . . . . . . . . . . . . .  94
       C.2.1.  Single ECDSA signature  . . . . . . . . . . . . . . .  94
     C.3.  Examples of Enveloped Messages  . . . . . . . . . . . . .  95
       C.3.1.  Direct ECDH . . . . . . . . . . . . . . . . . . . . .  95
       C.3.2.  Direct plus Key Derivation  . . . . . . . . . . . . .  96
       C.3.3.  Counter Signature on Encrypted Content  . . . . . . .  97
       C.3.4.  Encrypted Content with External Data  . . . . . . . .  99
     C.4.  Examples of Encrypted Messages  . . . . . . . . . . . . .  99
       C.4.1.  Simple Encrypted Message  . . . . . . . . . . . . . .  99
       C.4.2.  Encrypted Message w/ a Partial IV . . . . . . . . . . 100
     C.5.  Examples of MAC messages  . . . . . . . . . . . . . . . . 100
       C.5.1.  Shared Secret Direct MAC  . . . . . . . . . . . . . . 100
       C.5.2.  ECDH Direct MAC . . . . . . . . . . . . . . . . . . . 101
       C.5.3.  Wrapped MAC . . . . . . . . . . . . . . . . . . . . . 102
       C.5.4.  Multi-recipient MAC message . . . . . . . . . . . . . 103
     C.6.  Examples of MAC0 messages . . . . . . . . . . . . . . . . 104
       C.6.1.  Shared Secret Direct MAC  . . . . . . . . . . . . . . 104
     C.7.  COSE Keys . . . . . . . . . . . . . . . . . . . . . . . . 105
       C.7.1.  Public Keys . . . . . . . . . . . . . . . . . . . . . 105
       C.7.2.  Private Keys  . . . . . . . . . . . . . . . . . . . . 106
   Appendix D.  Document Updates . . . . . . . . . . . . . . . . . . 108
     D.1.  Version -09 to -10  . . . . . . . . . . . . . . . . . . . 108
     D.2.  Version -08 to -09  . . . . . . . . . . . . . . . . . . . 109
     D.3.  Version -07 to -08  . . . . . . . . . . . . . . . . . . . 109
     D.4.  Version -06 to -07  . . . . . . . . . . . . . . . . . . . 109
     D.5.  Version -05 to -06  . . . . . . . . . . . . . . . . . . . 109
     D.6.  Version -04 to -05  . . . . . . . . . . . . . . . . . . . 109
     D.7.  Version -03 to -04  . . . . . . . . . . . . . . . . . . . 110
     D.8.  Version -02 to -03  . . . . . . . . . . . . . . . . . . . 110
     D.9.  Version -02 to -03  . . . . . . . . . . . . . . . . . . . 110
     D.10. Version -01 to -2 . . . . . . . . . . . . . . . . . . . . 110
     D.11. Version -00 to -01  . . . . . . . . . . . . . . . . . . . 111
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 111

1.  Introduction

   There has been an increased focus on the small, constrained devices
   that make up the Internet of Things (IOT).  One of the standards that
   has come out of this process is the Concise Binary Object
   Representation (CBOR).  CBOR extended the data model of the
   JavaScript Object Notation (JSON) by allowing for binary data among



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   other changes.  CBOR is being adopted by several of the IETF working
   groups dealing with the IOT world as their encoding of data
   structures.  CBOR was designed specifically to be both small in terms
   of messages transport and implementation size as well having a schema
   free decoder.  A need exists to provide message security services for
   IOT and using CBOR as the message encoding format makes sense.

   The JOSE working group produced a set of documents
   [RFC7515][RFC7516][RFC7517][RFC7518] using JSON [RFC7159] that
   specified how to process encryption, signatures and message
   authentication (MAC) operations, and how to encode keys using JSON.
   This document does the same work for use with the CBOR [RFC7049]
   document format.  While there is a strong attempt to keep the flavor
   of the original JOSE documents, two considerations are taken into
   account:

   o  CBOR has capabilities that are not present in JSON and should be
      used.  One example of this is the fact that CBOR has a method of
      encoding binary directly without first converting it into a base64
      encoded string.

   o  COSE is not a direct copy of the JOSE specification.  In the
      process of creating COSE, decisions that were made for JOSE were
      re-examined.  In many cases different results were decided on as
      the criteria were not always the same as for JOSE.

1.1.  Design changes from JOSE

   o  Define a top level message structure so that encrypted, signed and
      MACed messages can easily identified and still have a consistent
      view.

   o  Signed messages separate the concept of protected and unprotected
      parameters that are for the content and the signature.

   o  MAC messages are separated from signed messages.

   o  MAC messages have the ability to use the same set of recipient
      algorithms as enveloped messages do to obtain the MAC
      authentication key.

   o  Use binary encodings for binary data rather than base64url
      encodings.

   o  Combine the authentication tag for encryption algorithms with the
      ciphertext.





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   o  The set of cryptographic algorithms has been expanded in some
      directions, and trimmed in others.

1.2.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

   When the words appear in lower case, their natural language meaning
   is used.

1.3.  CBOR Grammar

   There currently is no standard CBOR grammar available for use by
   specifications.  We therefore describe the CBOR structures in prose.

   The document was developed by first working on the grammar and then
   developing the prose to go with it.  An artifact of this is that the
   prose was written using the primitive type strings defined by CDDL.
   In this specification, the following primitive types are used:

      any - non-specific value that permits all CBOR values to be placed
      here.

      bool - a boolean value (true: major type 7, value 21; false: major
      type 7, value 20).

      bstr - byte string (major type 2).

      int - an unsigned integer or a negative integer.

      nil - a null value (major type 7, value 22).

      nint - a negative integer (major type 1).

      tstr - a UTF-8 text string (major type 3).

      uint - an unsigned integer (major type 0).

   There is a version of a CBOR grammar in the CBOR Data Definition
   Language (CDDL) [I-D.greevenbosch-appsawg-cbor-cddl].  Since CDDL has
   not be published as an RFC, this grammar may not work with the final
   version of CDDL when it is published.  For those people who prefer
   using a formal language to describe the syntax of the CBOR, an
   informational version of the CBOR grammar is interweaved into the




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   text as well.  The CDDL grammar is informational, the prose
   description is normative.

   The collected CDDL can be extracted from the XML version of this
   document via the following XPath expression below.  (Depending on the
   XPath evaluator one is using, it may be necessary to deal with &gt;
   as an entity.)

   //artwork[@type='CDDL']/text()

   CDDL expects the initial non-terminal symbol to be the first symbol
   in the file.  For this reason the first fragment of CDDL is presented
   here.

   start = COSE_Messages / COSE_Key / COSE_KeySet / Internal_Types

   ; This is define to make the tool quieter
   Internal_Types = Sig_structure / Enc_structure / MAC_structure /
           COSE_KDF_Context

   The non-terminal Internal_Types is defined for dealing with the
   automated validation tools used during the writing of this document.
   It references those non-terminals that are used for security
   computations, but are not emitted for transport.

1.4.  CBOR Related Terminology

   In JSON, maps are called objects and only have one kind of map key: a
   string.  In COSE, we use strings, negative integers and unsigned
   integers as map keys.  The integers are used for compactness of
   encoding and easy comparison.  Since the work "key" is mainly used in
   its other meaning, as a cryptographic key, we use the term "label"
   for this usage as a map key.

   The presence of a label in a map which is not a string or an integer
   is an error.  Applications can either fail processing or process
   messages with incorrect labels, however they MUST NOT create messages
   with incorrect labels.

   A CDDL grammar fragment is defined that defines the non-terminals
   'label' as in the previous paragraph and 'values' which permits any
   value to be used.

   label = int / tstr
   values = any






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1.5.  Document Terminology

   In this document we use the following terminology: [CREF1]

   Byte is a synonym for octet.

   Constrained Application Protocol (CoAP) is a specialized web transfer
   protocol for use in constrained systems.  It is defined in [RFC7252].

   Key management is used as a term to describe how a key at level n is
   obtained from level n+1 in encrypted and MACed messages.  The term is
   also used to discuss key life cycle management, this document does
   not discuss key life cycle operations.

2.  Basic COSE Structure

   The COSE Message structure is designed so that there can be a large
   amount of common code when parsing and processing the different
   security messages.  All of the message structures are built on the
   CBOR array type.  The first three elements of the array contain the
   same information.

   1.  The set of protected header parameters wrapped in a bstr.

   2.  The set of unprotected header parameters as a map.

   3.  The content of the message.  The content is either the plain text
       or the cipher text as appropriate.  (The content may be detached,
       but the location is still used.)

   Elements after this point are dependent on the specific message type.

   Identification of which type of message has been presented is done by
   the following method:

   1.  The specific message type is known from the context.  This may be
       defined by a marker in the containing structure or by
       restrictions specified by the application protocol.

   2.  The message type is identified by a CBOR tag.  This document
       defines a CBOR tag for each of the message structures.  These
       tags can be found in Table 1.

   3.  When a COSE object is carried in a media type of application/
       cose, the optional parameter 'cose-type' can be used to identify
       the embedded object.  The parameter is OPTIONAL if the tagged
       version of the structure is used.  The parameter is REQUIRED if
       the untagged version of the structure is used.  The value to use



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       with the parameter for each of the structures can be found in
       Table 1.

   4.  When a COSE object is carried as a CoAP payload, the CoAP content
       type parameter can be used to identify the message content.  The
       CoAP content types can be found in Table 23.  The CBOR Tag for
       the message structure is not required as each security message is
       uniquely identified.

   +---------+----------------+-----------------+----------------------+
   | Tag     | cose-type      | Data Item       | Semantics            |
   | Value   |                |                 |                      |
   +---------+----------------+-----------------+----------------------+
   | TBD1    | cose-sign      | COSE_Sign       | COSE Signed Data     |
   |         |                |                 | Object               |
   |         |                |                 |                      |
   | TBD7    | cose-sign1     | COSE_Sign1      | COSE Single Signer   |
   |         |                |                 | Data Object          |
   |         |                |                 |                      |
   | TBD2    | cose-enveloped | COSE_Enveloped  | COSE Enveloped Data  |
   |         |                |                 | Object               |
   |         |                |                 |                      |
   | TBD3    | cose-encrypted | COSE_Encrypted  | COSE Encrypted Data  |
   |         |                |                 | Object               |
   |         |                |                 |                      |
   | TBD4    | cose-mac       | COSE_Mac        | COSE Mac-ed Data     |
   |         |                |                 | Object               |
   |         |                |                 |                      |
   | TBD6    | cose-mac0      | COSE_Mac0       | COSE Mac w/o         |
   |         |                |                 | Recipients Object    |
   |         |                |                 |                      |
   | TBD5    | N/A            | COSE_Key,       | COSE Key or COSE Key |
   |         |                | COSE_KeySet     | Set Object           |
   +---------+----------------+-----------------+----------------------+

                    Table 1: COSE Object Identification

   The following CDDL fragment identifies all of the top level messages
   defined in this document.  Separate non-terminals are defined for the
   tagged and the untagged versions of the messages for the convenience
   of applications.










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   COSE_Messages = COSE_Untagged_Message / COSE_Tagged_Message

   COSE_Untagged_Message = COSE_Sign / COSE_Sign1 /
       COSE_Enveloped / COSE_Encrypted /
       COSE_Mac / COSE_Mac0

   COSE_Tagged_Message = COSE_Sign_Tagged / COSE_Sign1_Tagged /
       COSE_Enveloped_Tagged / COSE_Encrypted_Tagged /
       COSE_Mac_Tagged / COSE_Mac0_Tagged

3.  Header Parameters

   The structure of COSE has been designed to have two buckets of
   information that are not considered to be part of the payload itself,
   but are used for holding information about content, algorithms, keys,
   or evaluation hints for the processing of the layer.  These two
   buckets are available for use in all of the structures except for
   keys.  While these buckets can be present, they may not all be usable
   in all instances.  For example, while the protected bucket is defined
   as part of the recipient structure, some of the algorithms used for
   recipient structures do not provide for authenticated data.  If this
   is the case, the protected bucket should be left empty.

   Both buckets are implemented as CBOR maps.  The map key is a 'label'
   (Section 1.4).  The value portion is dependent on the definition for
   the label.  Both maps use the same set of label/value pairs.  The
   integer and string values for labels has been divided into several
   sections with a standard range, a private range, and a range that is
   dependent on the algorithm selected.  The defined labels can be found
   in the 'COSE Header Parameters' IANA registry (Section 16.2).

   Two buckets are provided for each layer:

   protected:  Contains parameters about the current layer that are to
      be cryptographically protected.  This bucket MUST be empty if it
      is not going to be included in a cryptographic computation.  This
      bucket is encoded in the message as a binary object.  This value
      is obtained by CBOR encoding the protected map and wrapping it in
      a bstr object.  Senders SHOULD encode an empty protected map as a
      zero length binary object (it is both shorter and the version used
      in the authentication structures).  Recipients MUST accept both a
      zero length binary value and a zero length map encoded in the
      binary value.  The wrapping allows for the encoding of the
      protected map to be transported with a greater chance that it will
      not be altered in transit.  (Badly behaved intermediates could
      decode and re-encode, but this will result in a failure to verify
      unless the re-encoded byte string is identical to the decoded byte




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      string.)  This finesses the problem of all parties needing to be
      able to do a common canonical encoding.

   unprotected:  Contains parameters about the current layer that are
      not cryptographically protected.

   Only parameters that deal with the current layer are to be placed at
   that layer.  As an example of this, the parameter 'content type'
   describes the content of the message being carried in the message.
   As such, this parameter is placed only in the content layer and is
   not placed in the recipient or signature layers.  In principle, one
   should be able to process any given layer without reference to any
   other layer.  (With the exception of the COSE_Sign structure, the
   only data that needs to cross layers is the cryptographic key.)

   The buckets are present in all of the security objects defined in
   this document.  The fields in order are the 'protected' bucket (as a
   CBOR 'bstr' type) and then the 'unprotected' bucket (as a CBOR 'map'
   type).  The presence of both buckets is required.  The parameters
   that go into the buckets come from the IANA "COSE Header Parameters"
   (Section 16.2).  Some common parameters are defined in the next
   section, but a number of parameters are defined throughout this
   document.

   Labels in each of the maps MUST be unique.  When processing messages,
   if a label appears multiple times the message MUST be rejected as
   malformed.  Applications SHOULD perform the same checks that the
   labels appearing in the protected and unprotected headers are unique
   as well.  If the message is not rejected as malformed, attributes
   MUST be obtained from the protected bucket before they are obtained
   from the unprotected bucket.

   The following CDDL fragment represents the two header buckets.  A
   group Headers is defined in CDDL which represents the two buckets in
   which attributes are placed.  This group is used to provide these two
   fields consistently in all locations.  A type is also defined which
   represents the map of header values.  It uses forward references to a
   group definition of headers for generic and algorithms.













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   Headers = (
       protected : bstr,                  ; Contains a header_map
       unprotected : header_map
   )

   header_map = {
       Generic_Headers,
       ; Algorithm_Headers,
       * label => values
   }


3.1.  Common COSE Headers Parameters

   This section defines a set of common header parameters.  A summary of
   these parameters can be found in Table 2.  This table should be
   consulted to determine the value of label as well as the type of the
   value.

   The set of header parameters defined in this section are:

   alg  This parameter is used to indicate the algorithm used for the
      security processing.  This parameter MUST be present at each level
      of a signed, encrypted or authenticated message except the
      COSE_Sign structure.  When the algorithm supports authenticating
      associated data, this parameter MUST be in the protected header
      bucket.  The value is taken from the 'COSE Algorithm Registry'
      (see Section 16.4).

   crit  The parameter is used to indicate which protected header labels
      an application that is processing a message is required to
      understand.  Parameters defined in this document do not need to be
      included as they should be understood by all implementations.
      When present, this parameter MUST be placed in the protected
      header bucket.  The array MUST have at least one value in it.
      Not all labels need to be included in the 'crit' parameter.  The
      rules for deciding which header labels are placed in the array
      are:

      *  Integer labels in the range of 0 to 8 SHOULD be omitted.

      *  Integer labels in the range -1 to -255 can be omitted as they
         are algorithm dependent.  If an application can correctly
         process an algorithm, it can be assumed that it will correctly
         process all of the common parameters associated with that
         algorithm.  (The algorithm range is -1 to -65536, the higher
         end is for more optional algorithm specific items.)




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      *  Labels for parameters required for an application MAY be
         omitted.  Applications should have a statement if the label can
         or cannot be omitted.

      The header parameter values indicated by 'crit' can be processed
      by either the security library code or by an application using a
      security library, the only requirement is that the parameter is
      processed.  If the 'crit' value list includes a value for which
      the parameter is not in the protected bucket, this is a fatal
      error in processing the message.

   content type  This parameter is used to indicate the content type of
      the data in the payload or ciphertext fields.  Integers are from
      the 'CoAP Content-Formats' IANA registry table.  Strings are from
      the IANA 'Media Types' registry.  Applications SHOULD provide this
      parameter if the content structure is potentially ambiguous.

   kid  This parameter identifies one piece of data that can be used as
      input to find the needed cryptographic key.  The value of this
      parameter can be matched against the 'kid' member in a COSE_Key
      structure.  Other methods of key distribution can define an
      equivalent field to be matched.  Applications MUST NOT assume that
      'kid' values are unique.  There may be more than one key with the
      same 'kid' value, it may be required that all of the keys need to
      be checked to find the correct one.  The internal structure of
      'kid' values is not defined and cannot be relied on by
      applications.  Key identifier values are hints about which key to
      use.  They are not directly a security critical field.  For this
      reason, they can be placed in the unprotected headers bucket.

   Initialization Vector  This parameter holds the Initialization Vector
      (IV) value.  For some symmetric encryption algorithms this may be
      referred to as a nonce.  As the IV is authenticated by encryption
      process, it can be placed in the unprotected header bucket.

   Partial Initialization Vector  This parameter holds a part of the IV
      value.  When using the COSE_Encrypted structure, frequently a
      portion of the IV is part of the context associated with the key
      value.  This field is used to carry a value that causes the IV to
      be changed for each message.  As the IV is authenticated by the
      encryption process, this value can be placed in the unprotected
      header bucket.  The 'Initialization Vector' and 'Partial
      Initialization Vector' parameters MUST NOT be present in the same
      security layer.
      The message IV is generated by the following steps:

      1.  Left pad the partial IV with zeros to the length of IV.




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      2.  XOR the padded partial IV with the context IV.

   counter signature  This parameter holds a counter signature value.
      Counter signatures provide a method of having a second party sign
      some data.  The counter signature can occur as an unprotected
      attribute in any of the following structures: COSE_Sign,
      COSE_Sign1, COSE_Signature, COSE_Enveloped, COSE_recipient,
      COSE_Encrypted, COSE_Mac and COSE_Mac0.  These structures all have
      the same beginning elements so that a consistent calculation of
      the counter signature can be computed.  Details on computing
      counter signatures are found in Section 4.5.

   operation time  This parameter provides the time the content
      cryptographic operation is performed.  For signatures and
      recipient structures, this would be the time that the signature or
      recipient key object was created.  For content structures, this
      would be the time that the content structure was created.  The
      unsigned integer value is the number of seconds, excluding leap
      seconds, after midnight UTC, January 1, 1970.  The field is
      primarily intended to be to be used for counter signatures,
      however it can additionally be used for replay detection as well.






























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   +-----------+-------+----------------+-------------+----------------+
   | name      | label | value type     | value       | description    |
   |           |       |                | registry    |                |
   +-----------+-------+----------------+-------------+----------------+
   | alg       | 1     | int / tstr     | COSE        | Cryptographic  |
   |           |       |                | Algorithm   | algorithm to   |
   |           |       |                | Registry    | use            |
   |           |       |                |             |                |
   | crit      | 2     | [+ label]      | COSE Header | Critical       |
   |           |       |                | Label       | headers to be  |
   |           |       |                | Registry    | understood     |
   |           |       |                |             |                |
   | content   | 3     | tstr / int     | CoAP        | Content type   |
   | type      |       |                | Content-    | of the payload |
   |           |       |                | Formats or  |                |
   |           |       |                | Media Types |                |
   |           |       |                | registry    |                |
   |           |       |                |             |                |
   | kid       | 4     | bstr           |             | key identifier |
   |           |       |                |             |                |
   | IV        | 5     | bstr           |             | Full           |
   |           |       |                |             | Initialization |
   |           |       |                |             | Vector         |
   |           |       |                |             |                |
   | Partial   | 6     | bstr           |             | Partial        |
   | IV        |       |                |             | Initialization |
   |           |       |                |             | Vector         |
   |           |       |                |             |                |
   | counter   | 7     | COSE_Signature |             | CBOR encoded   |
   | signature |       | / [+           |             | signature      |
   |           |       | COSE_Signature |             | structure      |
   |           |       | ]              |             |                |
   |           |       |                |             |                |
   | operation | 8     | uint           |             | Time the COSE  |
   | time      |       |                |             | structure was  |
   |           |       |                |             | created        |
   +-----------+-------+----------------+-------------+----------------+

                     Table 2: Common Header Parameters

   The CDDL fragment that represents the set of headers defined in this
   section is given below.  Each of the headers is tagged as optional
   because they do not need to be in every map, headers required in
   specific maps are discussed above.







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   Generic_Headers = (
       ? 1 => int / tstr,  ; algorithm identifier
       ? 2 => [+label],    ; criticality
       ? 3 => tstr / int,  ; content type
       ? 4 => bstr,        ; key identifier
       ? 5 => bstr,        ; IV
       ? 6 => bstr,        ; Partial IV
       ? 7 => COSE_Signature / [+COSE_Signature], ; Counter signature
       ? 8 => uint         ; Operation time
   )

4.  Signing Objects

   COSE supports two different signature structures.  COSE_Sign allows
   for one or more signers to be applied to a single content.
   COSE_Sign1 is restricted to a single signer.  The structures cannot
   be converted between each other, the signature computation includes a
   parameter identifying which structure is being used.

4.1.  Signing with One or More Signers

   The signature structure allows for one or more signatures to be
   applied to a message payload.  There are provisions for parameters
   about the content and parameters about the signature to be carried
   along with the signature itself.  These parameters may be
   authenticated by the signature, or just present.  An example of a
   parameter about the content is the content type.  Examples of
   parameters about the signature would be the algorithm and key used to
   create the signature, when the signature was created, and counter
   signatures.

   When more than one signature is present, the successful validation of
   one signature associated with a given signer is usually treated as a
   successful signature by that signer.  However, there are some
   application environments where other rules are needed.  An
   application that employs a rule other than one valid signature for
   each signer must specify those rules.  Also, where simple matching of
   the signer identifier is not sufficient to determine whether the
   signatures were generated by the same signer, the application
   specification must describe how to determine which signatures were
   generated by the same signer.  Support of different communities of
   recipients is the primary reason that signers choose to include more
   than one signature.  For example, the COSE_Sign structure might
   include signatures generated with the RSA signature algorithm and
   with the Elliptic Curve Digital Signature Algorithm (ECDSA) signature
   algorithm.  This allows recipients to verify the signature associated
   with one algorithm or the other.  (The original source of this text




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   is [RFC5652].)  More detailed information on multiple signature
   evaluation can be found in [RFC5752].

   The signature structure can be encoded either with or without a tag
   depending on the context it will be used in.  The signature structure
   is identified by the CBOR tag TBD1.  The CDDL fragment that
   represents this is.

   COSE_Sign_Tagged = #6.991(COSE_Sign) ; Replace 991 with TBD1

   A COSE Signed Message is divided into two parts.  The CBOR object
   that carries the body and information about the body is called the
   COSE_Sign structure.  The CBOR object that carries the signature and
   information about the signature is called the COSE_Signature
   structure.  Examples of COSE Signed Messages can be found in
   Appendix C.1.

   The COSE_Sign structure is a CBOR array.  The fields of the array in
   order are:

   protected  as described in Section 3.

   unprotected  as described in Section 3.

   payload  contains the serialized content to be signed.  If the
      payload is not present in the message, the application is required
      to supply the payload separately.  The payload is wrapped in a
      bstr to ensure that it is transported without changes.  If the
      payload is transported separately (i.e. detached content), then a
      nil CBOR object is placed in this location and it is the
      responsibility of the application to ensure that it will be
      transported without changes.

      Note: When a signature with message recovery algorithm is used
      (Section 8), the maximum number of bytes that can be recovered is
      the length of the payload.  The size of the payload is reduced by
      the number of bytes that will be recovered.  If all of the bytes
      of the payload are consumed, then the payload is encoded as a zero
      length binary string rather than as being absent.

   signatures  is an array of signatures.  Each signature is represented
      as a COSE_Signature structure.

   The CDDL fragment which represents the above text for COSE_Sign
   follows.






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   COSE_Sign = [
       Headers,
       payload : bstr / nil,
       signatures : [+ COSE_Signature]
   ]

   The COSE_Signature structure is a CBOR array.  The fields of the
   array in order are:

   protected  as described in Section 3.

   unprotected  as described in Section 3.

   signature  contains the computed signature value.  The type of the
      field is a bstr.

   The CDDL fragment which represents the above text for COSE_Signature
   follows.

   COSE_Signature =  [
       Headers,
       signature : bstr
   ]

4.2.  Signing with One Signer

   The signature structure can be encoded either with or without a tag
   depending on the context it will be used in.  The signature structure
   is identified by the CBOR tag TBD7.  The CDDL fragment that
   represents this is:

   COSE_Sign1_Tagged = #6.997(COSE_Sign1) ; Replace 997 with TBD7

   The CBOR object that carries the body, the signature and the
   information about the body and signature is called the COSE_Sign1
   structure.  Examples of COSE Single signature messages can be found
   in Appendix C.2.

   The COSE_Sign1 structure is a CBOR array.  The fields of the array in
   order are:

   protected  as described in Section 3.

   unprotected  as described in Section 3.

   payload  as described in Section 4.1.





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   signature  contains the computed signature value.  The type of the
      field is a bstr.

   The CDDL fragment which represents the above text for COSE_Sign1
   follows.

   COSE_Sign1 = [
       Headers,
       payload : bstr / nil,
       signature : bstr
   ]

4.3.  Externally Supplied Data

   One of the features that we supply in the COSE document is the
   ability for applications to provide additional data to be
   authenticated as part of the security, but that is not carried as
   part of the COSE object.  The primary reason for supporting this can
   be seen by looking at the CoAP message structure [RFC7252] where the
   facility exists for options to be carried before the payload.  An
   example of data that can be placed in this location would be CoAP
   options for transaction ids and nonces to check for replay
   protection.  If the data is in the options section, then it is
   available for routers to help in performing the replay detection and
   prevention.  However, it may also be desired to protect these values
   so that if they are be modified in transit it can be detected.  This
   is the purpose of the externally supplied data field.

   This document describes the process for using a byte array of
   externally supplied authenticated data, however the method of
   constructing the byte array is a function of the application.
   Applications that use this feature need to define how the externally
   supplied authenticated data is to be constructed.  Such a
   construction needs to take into account the following issues:

   o  If multiple items are included, care needs to be taken that data
      cannot bleed between the items.  This is usually addressed by
      making fields fixed width and/or encoding the length of the field.
      Using options from CoAP [RFC7252] as an example, these fields use
      a TLV structure so they can be concatenated without any problems.

   o  If multiple items are included, a defined order for the items
      needs to be defined.  Using options from CoAP as an example, an
      application could state that the fields are to be ordered by the
      option number.






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4.4.  Signing and Verification Process

   In order to create a signature, a consistent byte stream is needed.
   This algorithm takes in the body information (COSE_Sign), the signer
   information (COSE_Signature), and the application data (External).  A
   CBOR array is used to construct the byte stream.  The fields of the
   array in order are:

   1.  A text string identifying the context of the signature.  The
       context string is:

       "Signature"  for signatures using the COSE_Signature structure.

       "Signature1"  for signatures using the COSE_Sign1 structure.

       "CounterSignature"  for signatures used as counter signature
          attributes.

   2.  The protected attributes from the body structure encoded in a
       bstr type.  If there are no protected attributes, a bstr of
       length zero is used.

   3.  The protected attributes from the signer structure encoded in a
       bstr type.  If there are no protected attributes, a bstr of
       length zero is used.  This field is omitted for the COSE_Sign1
       signature structure.

   4.  The protected attributes from the application encoded in a bstr
       type.  If this field is not supplied, it defaults to a zero
       length binary string.  (See Section 4.3 for application guidance
       on constructing this field.)

   5.  The payload to be signed encoded in a bstr type.  The payload is
       placed here independent of how it is transported.

   The CDDL fragment which describes the above text is.

   Sig_structure = [
       context: "Signature" / "Signature1" / "CounterSignature",
       body_protected: bstr,
       ? sign_protected: bstr,
       external_aad: bstr,
       payload: bstr
   ]

   How to compute a signature:





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   1.  Create a Sig_structure and populate it with the appropriate
       fields.

   2.  Create the value ToBeSigned by encoding the Sig_structure to a
       byte string using the encoding described in Section 14.

   3.  Call the signature creation algorithm passing in K (the key to
       sign with), alg (the algorithm to sign with) and ToBeSigned (the
       value to sign).

   4.  Place the resulting signature value in the 'signature' field of
       the array.

   How to verify a signature:

   1.  Create a Sig_structure object and populate it with the
       appropriate fields.

   2.  Create the value ToBeSigned by encoding the Sig_structure to a
       byte string using the encoding described in Section 14..

   3.  Call the signature verification algorithm passing in K (the key
       to verify with), alg (the algorithm used sign with), ToBeSigned
       (the value to sign), and sig (the signature to be verified).

   In addition to performing the signature verification, one must also
   perform the appropriate checks to ensure that the key is correctly
   paired with the signing identity and that the appropriate
   authorization is done.

4.5.  Computing Counter Signatures

   Counter signatures provide a method of having a different signature
   occur on some piece of content.  This is normally used to provide a
   signature on a signature allowing for a proof that a signature
   existed at a given time (i.e. a Timestamp).  In this document we
   allow for counter signatures to exist in a greater number of
   environments.  As an example, it is possible to place a counter
   signature in the unprotected attributes of a COSE_Enveloped object.
   This would allow for an intermediary to either verify that the
   encrypted byte stream has not been modified, without being able to
   decrypt it.  Or for the intermediary to assert that an encrypted byte
   stream either existed at a given time or passed through it in terms
   of routing (i.e. a proxy signature).

   An example of a counter signature on a signature can be found in
   Appendix C.1.3.  An example of a counter signature in an encryption
   object can be found in Appendix C.3.3.



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   The creation and validation of counter signatures over the different
   items relies on the fact that the structure of the objects have the
   same structure.  The elements are a set of protected attributes, a
   set of unprotected attributes and a body in that order.  This means
   that the Sig_structure can be used in a uniform manner to get the
   byte stream for processing a signature.  If the counter signature is
   going to be computed over a COSE_Enveloped structure, the
   body_protected and payload items can be mapped into the Sig_structure
   in the same manner as from the COSE_Sign structure.

   It should be noted that only a signature algorithm with appendix (see
   Section 8) can be used for counter signatures.  This is because the
   body should be able to be processed without having to evaluate the
   counter signature, and this is not possible for signature schemes
   with message recovery.

5.  Encryption Objects

   COSE supports two different encryption structures.  COSE_Encrypted is
   used when a recipient structure is not needed because the key to be
   used is known implicitly.  COSE_Enveloped is used the rest of time
   time.  This includes cases where there are multiple recipients, a
   recipient algorithm other than direct is to be used, or the key to be
   used is not known.

5.1.  Enveloped COSE Structure

   The enveloped structure allows for one or more recipients of a
   message.  There are provisions for parameters about the content and
   parameters about the recipient information to be carried in the
   message.  The protected parameters associated with the content are
   authenticated by the content encryption algorithm.  The protected
   parameters associated with the recipient are authenticated by the
   recipient algorithm (when the algorithm supports it).  Examples of
   parameters about the content are the type of the content, and the
   content encryption algorithm.  Examples of parameters about the
   recipient are the recipient's key identifier, the recipient
   encryption algorithm.

   The same techniques and structures are used for encrypting both the
   plain text and the keys used to protect the text.  This is different
   from the approach used by both [RFC5652] and [RFC7516] where
   different structures are used for the content layer and for the
   recipient layer.  Two structures are defined: COSE_Enveloped to hold
   the encrypted content, and COSE_recipient to hold the encrypted keys
   for recipients.  Examples of encrypted messages can be found in
   Appendix C.3.




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   The COSE Enveloped structure can be encoded either with or without a
   tag depending on the context it will be used in.  The COSE Enveloped
   structure is identified by the CBOR tag TBD2.  The CDDL fragment that
   represents this is.

  COSE_Enveloped_Tagged = #6.992(COSE_Enveloped) ; Replace 992 with TBD2

   The COSE_Enveloped structure is a CBOR array.  The fields of the
   array in order are:

   protected  as described in Section 3.

   unprotected  as described in Section 3.

   ciphertext  contains the cipher text encoded as a bstr.  If the
      ciphertext is to be transported independently of the control
      information about the encryption process (i.e. detached content)
      then the field is encoded as a null object.

   recipients  contains an array of recipient information structures.
      The type for the recipient information structure is a
      COSE_recipient.

   The CDDL fragment that corresponds to the above text is:

   COSE_Enveloped = [
       Headers,
       ciphertext: bstr / nil,
       recipients: [+COSE_recipient]
   ]

   The COSE_recipient structure is a CBOR array.  The fields of the
   array in order are:

   protected  as described in Section 3.

   unprotected  as described in Section 3.

   ciphertext  contains the encrypted key encoded as a bstr.  If there
      is not an encrypted key, then this field is encoded as a nil
      value.

   recipients  contains an array of recipient information structures.
      The type for the recipient information structure is a
      COSE_recipient.  (And example of this can be found in Appendix B.)
      If there are no recipient information structures, this element is
      absent.




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   The CDDL fragment that corresponds to the above text for
   COSE_recipient is:

   COSE_recipient = [
       Headers,
       ciphertext: bstr / nil,
       ? recipients: [+COSE_recipient]
   ]

5.1.1.  Recipient Algorithm Classes

   A typical encrypted message consists of an encrypted content and an
   encrypted CEK for one or more recipients.  The CEK is encrypted for
   each recipient, using a key specific to that recipient.  The details
   of this encryption depends on which class the recipient algorithm
   falls into.  Specific details on each of the classes can be found in
   Section 12.  A short summary of the five recipient algorithm classes
   is:

   direct:  The CEK is the same as the identified previously distributed
      symmetric key or derived from a previously distributed secret.  No
      CEK is transported in the message.

   symmetric key-encryption keys:  The CEK is encrypted using a
      previously distributed symmetric KEK.

   key agreement:  The recipient's public key and a sender's private key
      are used to generate a pairwise secret, a KDF is applied to derive
      a key, and then the CEK is either the derived key or encrypted by
      the derived key.

   key transport:  The CEK is encrypted with the recipient's public key.
      No key transport algorithms are defined in this document.

   passwords:  The CEK is encrypted in a KEK that is derived from a
      password.  No password algorithms are defined in this document.

5.2.  Encrypted COSE structure

   The encrypted structure does not have the ability to specify
   recipients of the message.  The structure assumes that the recipient
   of the object will already know the identity of the key to be used in
   order to decrypt the message.  If a key needs to be identified to the
   recipient, the enveloped structure ought to be used.

   The structure defined to hold an encrypted message is COSE_Encrypted.
   Examples of encrypted messages can be found in Appendix C.3.




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   The COSE_Encrypted structure can be encoded either with or without a
   tag depending on the context it will be used in.  The COSE_Encrypted
   structure is identified by the CBOR tag TBD3.  The CDDL fragment that
   represents this is.

  COSE_Encrypted_Tagged = #6.993(COSE_Encrypted) ; Replace 993 with TBD3

   The COSE_Enveloped structure is a CBOR array.  The fields of the
   array in order are:

   protected  as described in Section 3.

   unprotected  as described in Section 3.

   ciphertext  as described in Section 5.1.

   The CDDL fragment for COSE_Encrypted that corresponds to the above
   text is:

   COSE_Encrypted = [
       Headers,
       ciphertext: bstr / nil,
   ]

5.3.  Encryption Algorithm for AEAD algorithms

   The encryption algorithm for AEAD algorithms is fairly simple.  The
   first step is to create a consistent byte stream for the
   authenticated data structure.  For this purpose we use a CBOR array,
   the fields of the array in order are:

   1.  A text string identifying the context of the authenticated data
       structure.  The context string is:

       "Encrypted"  for the content encryption of an encrypted data
          structure.

       "Enveloped"  for the first level of an enveloped data structure
          (i.e. for content encryption).

       "Env_Recipient"  for a recipient encoding to be placed in an
          enveloped data structure.

       "Mac_Recipient"  for a recipient encoding to be placed in a MAC
          message structure.

       "Rec_Recipient"  for a recipient encoding to be placed in a
          recipient structure.



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   2.  The protected attributes from the body structure encoded in a
       bstr type.  If there are no protected attributes, a bstr of
       length zero is used.

   3.  The protected attributes from the application encoded in a bstr
       type.  If this field is not supplied, it defaults to a zero
       length bstr.  (See Section 4.3 for application guidance on
       constructing this field.)

   The CDDL fragment which describes the above text is:

   Enc_structure = [
       context : "Enveloped" / "Encrypted" / "Env_Recipient" /
           "Mac_Recipient" / "Rec_Recipient",
       protected: bstr,
       external_aad: bstr
   ]

   How to encrypt a message:

   1.  Create a Enc_structure and populate it with the appropriate
       fields.

   2.  Encode the Enc_structure to a byte stream (AAD) using the
       encoding described in Section 14.

   3.  Determine the encryption key.  This step is dependent on the
       class of recipient algorithm being used.  For:

       No Recipients:  The key to be used is determined by the algorithm
          and key at the current level.

       Direct and Direct Key Agreement:  The key is determined by the
          key and algorithm in the recipient structure.  The encryption
          algorithm and size of the key to be used are inputs into the
          KDF used for the recipient.  (For direct, the KDF can be
          thought of as the identity operation.)

       Other:  The key is randomly generated.

   4.  Call the encryption algorithm with K (the encryption key to use),
       P (the plain text) and AAD.  Place the returned cipher text into
       the 'ciphertext' field of the structure.

   5.  For recipients of the message, recursively perform the encryption
       algorithm for that recipient using the encryption key as the
       plain text.




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   How to decrypt a message:

   1.  Create a Enc_structure and populate it with the appropriate
       fields.

   2.  Encode the Enc_structure to a byte stream (AAD) using the
       encoding described in Section 14.

   3.  Determine the decryption key.  This step is dependent on the
       class of recipient algorithm being used.  For:

       No Recipients:  The key to be used is determined by the algorithm
          and key at the current level.

       Direct and Direct Key Agreement:  The key is determined by the
          key and algorithm in the recipient structure.  The encryption
          algorithm and size of the key to be used are inputs into the
          KDF used for the recipient.  (For direct, the KDF can be
          thought of as the identity operation.)

       Other:  The key is determined by decoding and decrypting the
          recipient structure.

   4.  Call the decryption algorithm with K (the decryption key to use),
       C (the cipher text) and AAD.

5.4.  Encryption algorithm for AE algorithms

   How to encrypt a message:

   1.  Verify that the 'protected' field is absent.

   2.  Verify that there was no external additional authenticated data
       supplied for this operation.

   3.  Determine the encryption key.  This step is dependent on the
       class of recipient algorithm being used.  For:

       No Recipients:  The key to be used is determined by the algorithm
          and key at the current level.

       Direct and Direct Key Agreement:  The key is determined by the
          key and algorithm in the recipient structure.  The encryption
          algorithm and size of the key to be used are inputs into the
          KDF used for the recipient.  (For direct, the KDF can be
          thought of as the identity operation.)

       Other:  The key is randomly generated.



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   4.  Call the encryption algorithm with K (the encryption key to use)
       and the P (the plain text).  Place the returned cipher text into
       the 'ciphertext' field of the structure.

   5.  For recipients of the message, recursively perform the encryption
       algorithm for that recipient using the encryption key as the
       plain text.

   How to decrypt a message:

   1.  Verify that the 'protected' field is absent.

   2.  Verify that there was no external additional authenticated data
       supplied for this operation.

   3.  Determine the decryption key.  This step is dependent on the
       class of recipient algorithm being used.  For:

       No Recipients:  The key to be used is determined by the algorithm
          and key at the current level.

       Direct and Direct Key Agreement:  The key is determined by the
          key and algorithm in the recipient structure.  The encryption
          algorithm and size of the key to be used are inputs into the
          KDF used for the recipient.  (For direct, the KDF can be
          thought of as the identity operation.)

       Other:  The key is determined by decoding and decrypting the
          recipient structure.

   4.  Call the decryption algorithm with K (the decryption key to use),
       C (the cipher text) and AAD.

6.  MAC Objects

   COSE supports two different MAC structures.  COSE_MAC0 is used when a
   recipient structure is not needed because the key to be used is
   implicitly known.  COSE_MAC is used for all other cases.  These
   include a requirement for multiple recipients, the key being unknown,
   a recipient algorithm of other than direct.

6.1.  MAC Message with Recipients

   In this section we describe the structure and methods to be used when
   doing MAC authentication in COSE.  This document allows for the use
   of all of the same classes of recipient algorithms as are allowed for
   encryption.




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   When using MAC operations, there are two modes in which it can be
   used.  The first is just a check that the content has not been
   changed since the MAC was computed.  Any class of recipient algorithm
   can be used for this purpose.  The second mode is to both check that
   the content has not been changed since the MAC was computed, and to
   use the recipient algorithm to verify who sent it.  The classes of
   recipient algorithms that support this are those that use a pre-
   shared secret or do static-static key agreement (without the key wrap
   step).  In both of these cases, the entity that created and sent the
   message MAC can be validated.  (This knowledge of sender assumes that
   there are only two parties involved and you did not send the message
   yourself.)

   The MAC message uses two structures, the COSE_Mac structure defined
   in this section for carrying the body and the COSE_recipient
   structure (Section 5.1) to hold the key used for the MAC computation.
   Examples of MAC messages can be found in Appendix C.5.

   The MAC structure can be encoded either with or without a tag
   depending on the context it will be used in.  The MAC structure is
   identified by the CBOR tag TBD4.  The CDDL fragment that represents
   this is:

   COSE_Mac_Tagged = #6.994(COSE_Mac)         ; Replace 994 with TBD4

   The COSE_Mac structure is a CBOR array.  The fields of the array in
   order are:

   protected  as described in Section 3.

   unprotected  as described in Section 3.

   payload  contains the serialized content to be MACed.  If the payload
      is not present in the message, the application is required to
      supply the payload separately.  The payload is wrapped in a bstr
      to ensure that it is transported without changes.  If the payload
      is transported separately (i.e. detached content), then a null
      CBOR object is placed in this location and it is the
      responsibility of the application to ensure that it will be
      transported without changes.

   tag  contains the MAC value.

   recipients  as described in Section 5.1.

   The CDDL fragment which represents the above text for COSE_Mac
   follows.




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   COSE_Mac = [
      Headers,
      payload: bstr / nil,
      tag: bstr,
      recipients: [+COSE_recipient]
   ]

6.2.  MAC Messages with Implicit Key

   In this section we describe the structure and methods to be used when
   doing MAC authentication for those cases where the recipient is
   implicitly known.

   The MAC message uses the COSE_Mac0 structure defined in this section
   for carrying the body.  Examples of MAC messages with an implicit key
   can be found in Appendix C.6.

   The MAC structure can be encoded either with or without a tag
   depending on the context it will be used in.  The MAC structure is
   identified by the CBOR tag TBD6.  The CDDL fragment that represents
   this is:

   COSE_Mac0_Tagged = #6.996(COSE_Mac0)    ; Replace 996 with TBD6

   The COSE_Mac0 structure is a CBOR array.  The fields of the array in
   order are:

   protected  as described in Section 3.

   unprotected  as described in Section 3.

   payload  as described in Section 6.1.

   tag  contains the MAC value.

   The CDDL fragment which corresponds to the above text is:

   COSE_Mac0 = [
      Headers,
      payload: bstr / nil,
      tag: bstr,
   ]

6.3.  How to compute and verify a MAC

   In order to get a consistent encoding of the data to be
   authenticated, the MAC_structure is used to have a canonical form.




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   The MAC_structure is a CBOR array.  The fields of the MAC_structure
   in order are:

   1.  A text string that identifies the structure that is being
       encoded.  This string is "MAC" for the COSE_Mac structure.  This
       string is "MAC0" for the COSE_Mac0 structure.

   2.  The protected attributes from the COSE_MAC structure.  If there
       are no protected attributes, a zero length bstr is used.

   3.  If the application has supplied external authenticated data,
       encode it as a binary value and place in the MAC_structure.  If
       there is no external authenticated data, then use a zero length
       'bstr'.  (See Section 4.3 for application guidance on
       constructing this field.)

   4.  The payload to be MAC-ed encoded in a bstr type.  The payload is
       placed here independent of how it is transported.

   The CDDL fragment that corresponds to the above text is:

   MAC_structure = [
        context: "MAC" / "MAC0",
        protected: bstr,
        external_aad: bstr,
        payload: bstr
   ]

   The steps to compute a MAC are:

   1.  Create a MAC_structure and populate it with the appropriate
       fields.

   2.  Encode the MAC_structure to a byte stream using the encoding
       described in Section 14.

   3.  Call the MAC creation algorithm passing in K (the key to use),
       alg (the algorithm to MAC with) and ToBeMaced (the value to
       compute the MAC on).

   4.  Place the resulting MAC in the 'tag' field of the COSE_Mac
       structure.

   5.  Encrypt and encode the MAC key for each recipient of the message.

   How to verify a MAC are:





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   1.  Create a MAC_structure object and populate it with the
       appropriate fields.

   2.  Encode the MAC_structure to a byte stream using the encoding
       described in Section 14.

   3.  Obtain the cryptographic key from one of the recipients of the
       message.

   4.  Call the MAC creation algorithm passing in K (the key to use),
       alg (the algorithm to MAC with) and ToBeMaced (the value to
       compute the MAC on).

   5.  Compare the MAC value to the 'tag' field of the COSE_Mac
       structure.

7.  Key Structure

   A COSE Key structure is built on a CBOR map object.  The set of
   common parameters that can appear in a COSE Key can be found in the
   IANA registry 'COSE Key Common Parameter Registry' (Section 16.5).
   Additional parameters defined for specific key types can be found in
   the IANA registry 'COSE Key Type Parameters' (Section 16.6).

   A COSE Key Set uses a CBOR array object as its underlying type.  The
   values of the array elements are COSE Keys.  A Key Set MUST have at
   least one element in the array.

   The element "kty" is a required element in a COSE_Key map.

   The CDDL grammar describing COSE_Key and COSE_KeySet is:

   COSE_Key = {
       key_kty => tstr / int,
       ? key_ops => [+ (tstr / int) ],
       ? key_alg => tstr / int,
       ? key_kid => bstr,
       * label => values
   }

   COSE_KeySet = [+COSE_Key]

7.1.  COSE Key Common Parameters

   This document defines a set of common parameters for a COSE Key
   object.  Table 3 provides a summary of the parameters defined in this
   section.  There are also parameters that are defined for specific key
   types.  Key type specific parameters can be found in Section 13.



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   +---------+-------+----------------+-----------+--------------------+
   | name    | label | CBOR type      | registry  | description        |
   +---------+-------+----------------+-----------+--------------------+
   | kty     | 1     | tstr / int     | COSE      | Identification of  |
   |         |       |                | General   | the key type       |
   |         |       |                | Values    |                    |
   |         |       |                |           |                    |
   | key_ops | 4     | [+ (tstr/int)] |           | Restrict set of    |
   |         |       |                |           | permissible        |
   |         |       |                |           | operations         |
   |         |       |                |           |                    |
   | alg     | 3     | tstr / int     | COSE      | Key usage          |
   |         |       |                | Algorithm | restriction to     |
   |         |       |                | Values    | this algorithm     |
   |         |       |                |           |                    |
   | kid     | 2     | bstr           |           | Key Identification |
   |         |       |                |           | value - match to   |
   |         |       |                |           | kid in message     |
   |         |       |                |           |                    |
   | Base IV | 5     | bstr           |           | Base IV to be xor- |
   |         |       |                |           | ed with Partial    |
   |         |       |                |           | IVs                |
   +---------+-------+----------------+-----------+--------------------+

                          Table 3: Key Map Labels

   kty:  This parameter is used to identify the family of keys for this
      structure, and thus the set of key type specific parameters to be
      found.  The set of values defined in this document can be found in
      Table 19.  This parameter MUST be present in a key object.
      Implementations MUST verify that the key type is appropriate for
      the algorithm being processed.  The key type MUST be included as
      part of the trust decision process.

   alg:  This parameter is used to restrict the algorithms that are to
      be used with this key.  If this parameter is present in the key
      structure, the application MUST verify that this algorithm matches
      the algorithm for which the key is being used.  If the algorithms
      do not match, then this key object MUST NOT be used to perform the
      cryptographic operation.  Note that the same key can be in a
      different key structure with a different or no algorithm
      specified, however this is considered to be a poor security
      practice.

   kid:  This parameter is used to give an identifier for a key.  The
      identifier is not structured and can be anything from a user
      provided string to a value computed on the public portion of the
      key.  This field is intended for matching against a 'kid'



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      parameter in a message in order to filter down the set of keys
      that need to be checked.

   key_ops:  This parameter is defined to restrict the set of operations
      that a key is to be used for.  The value of the field is an array
      of values from Table 4.

   Base IV:  This parameter is defined to carry the base portion of an
      IV.  It is designed to be used with the partial IV header
      parameter defined in Section 3.1.  This field provides the ability
      to associate a partial IV with a key that is then modified on a
      per message basis with the parital IV.

      Care needs to be taken that this is only used as part of a key
      distribution algorithm that will ensure that it will be given only
      to parties that will use it correctly.  This is due to the fact
      that many of the content encryption algorithms defined for COSE
      require that IVs be unique for every message.  Use of this field
      will easily allow for this rule to be broken if not used
      carefully.  This field MUST be ignored unless an application
      specifically calls for its use.






























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   +---------+-------+-------------------------------------------------+
   | name    | value | description                                     |
   +---------+-------+-------------------------------------------------+
   | sign    | 1     | The key is used to create signatures.  Requires |
   |         |       | private key fields.                             |
   |         |       |                                                 |
   | verify  | 2     | The key is used for verification of signatures. |
   |         |       |                                                 |
   | encrypt | 3     | The key is used for key transport encryption.   |
   |         |       |                                                 |
   | decrypt | 4     | The key is used for key transport decryption.   |
   |         |       | Requires private key fields.                    |
   |         |       |                                                 |
   | wrap    | 5     | The key is used for key wrapping.               |
   | key     |       |                                                 |
   |         |       |                                                 |
   | unwrap  | 6     | The key is used for key unwrapping.  Requires   |
   | key     |       | private key fields.                             |
   |         |       |                                                 |
   | derive  | 7     | The key is used for deriving keys.  Requires    |
   | key     |       | private key fields.                             |
   |         |       |                                                 |
   | derive  | 8     | The key is used for deriving bits.  Requires    |
   | bits    |       | private key fields.                             |
   +---------+-------+-------------------------------------------------+

                       Table 4: Key Operation Values

   The following provides a CDDL fragment which duplicates the
   assignment labels from Table 3.

   ;key_labels
   key_kty=1
   key_kid=2
   key_alg=3
   key_ops=4

8.  Signature Algorithms

   There are two signature algorithm schemes.  The first is signature
   with appendix.  In this scheme, the message content is processed and
   a signature is produced, the signature is called the appendix.  This
   is the scheme used by algorithms such as ECDSA and RSASSA-PSS.  (In
   fact the SSA in RSASSA-PSS stands for Signature Scheme with
   Appendix.)

   The signature functions for this scheme are:




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   signature = Sign(message content, key)

   valid = Verification(message content, key, signature)

   The second scheme is signature with message recovery.  (An example of
   such an algorithm is [PVSig].)  In this scheme, the message content
   is processed, but part of it is included in the signature.  Moving
   bytes of the message content into the signature allows for smaller
   signatures, the signature size is still potentially large, but the
   message content has shrunk.  This has implications for systems
   implementing these algorithms and for applications that use them.
   The first is that the message content is not fully available until
   after a signature has been validated.  Until that point the part of
   the message contained inside of the signature is unrecoverable.  The
   second is that the security analysis of the strength of the signature
   is very much based on the structure of the message content.  Messages
   which are highly predictable require additional randomness to be
   supplied as part of the signature process.  In the worst case, it
   becomes the same as doing a signature with appendix.  Finally, in the
   event that multiple signatures are applied to a message, all of the
   signature algorithms are going to be required to consume the same
   number of bytes of message content.  This means that mixing of the
   different schemes in a single message is not supported, and if a
   recovery signature scheme is used then the same amount of content
   needs to be consumed by all of the signatures.

   The signature functions for this scheme are:

   signature, message sent = Sign(message content, key)

   valid, message content = Verification(message sent, key, signature)

   At this time, only signatures with appendixes are defined for use
   with COSE, however considerable interest has been expressed in using
   a signature with message recovery algorithm due to the effective size
   reduction that is possible.  Implementations will need to keep this
   in mind for later possible integration.

8.1.  ECDSA

   ECDSA [DSS] defines a signature algorithm using ECC.

   The ECDSA signature algorithm is parameterized with a hash function
   (h).  In the event that the length of the hash function output is
   greater than the group of the key, the left-most bytes of the hash
   output are used.

   The algorithms defined in this document can be found in Table 5.



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              +-------+-------+---------+------------------+
              | name  | value | hash    | description      |
              +-------+-------+---------+------------------+
              | ES256 | -7    | SHA-256 | ECDSA w/ SHA-256 |
              |       |       |         |                  |
              | ES384 | -35   | SHA-384 | ECDSA w/ SHA-384 |
              |       |       |         |                  |
              | ES512 | -36   | SHA-512 | ECDSA w/ SHA-512 |
              +-------+-------+---------+------------------+

                      Table 5: ECDSA Algorithm Values

   This document defines ECDSA to work only with the curves P-256, P-384
   and P-521.  This document requires that the curves be encoded using
   the 'EC2' key type.  Implementations need to check that the key type
   and curve are correct when creating and verifying a signature.  Other
   documents can defined it to work with other curves and points in the
   future.

   In order to promote interoperability, it is suggested that SHA-256 be
   used only with curve P-256, SHA-384 be used only with curve P-384 and
   SHA-512 be used with curve P-521.  This is aligned with the
   recommendation in Section 4 of [RFC5480].

   The signature algorithm results in a pair of integers (R, S).  These
   integers will the same length as length of the key used for the
   signature process.  The signature is encoded by converting the
   integers into byte strings of the same length as the key size.  The
   length is rounded up to the nearest byte and is left padded with zero
   bits to get to the correct length.  The two integers are then
   concatenated together to form a byte string that is the resulting
   signature.

   Using the function defined in [RFC3447] the signature is:
   Signature = I2OSP(R, n) | I2OSP(S, n)
   where n = ceiling(key_length / 8)

   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present and it MUST be 'EC2'.

   o  If the 'alg' field present, it MUST match the ECDSA signature
      algorithm being used.

   o  If the 'key_ops' field is present, it MUST include 'sign' when
      creating an ECDSA signature.




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   o  If the 'key_ops' field is present, it MUST include 'verify' when
      verifying an ECDSA signature.

8.1.1.  Security Considerations

   The security strength of the signature is no greater than the minimum
   of the security strength associated with the bit length of the key
   and the security strength of the hash function.

   System which have poor random number generation can leak their keys
   by signing two different messages with the same value 'k' (the per-
   message random value).  [RFC6979] provides a method to deal with this
   problem by making 'k' be deterministic based on the message content
   rather than randomly generated.  Applications that specify ECDSA
   should evaluate the ability to get good random number generation and
   require this when it is not possible.

   Note: Use of this technique a good idea even when good random number
   generation exists.  Doing so both reduces the possibility of having
   the same value of 'k' in two signature operations and allows for
   reproducible signature values which helps testing.

   There are two substitution attacks that can theoretically be mounted
   against the ECDSA signature algorithm.

   o  Changing the curve used to validate the signature: If one changes
      the curve used to validate the signature, then potentially one
      could have a two messages with the same signature each computed
      under a different curve.  The only requirement on the new curve is
      that its order be the same as the old one and it be acceptable to
      the client.  An example would be to change from using the curve
      secp256r1 (aka P-256) to using secp256k1.  (Both are 256 bit
      curves.)  We current do not have any way to deal with this version
      of the attack except to restrict the overall set of curves that
      can be used.

   o  Change the hash function used to validate the signature: If one
      has either two different hash functions of the same length, or one
      can truncate a hash function down, then one could potentially find
      collisions between the hash functions rather than within a single
      hash function.  (For example, truncating SHA-512 to 256 bits might
      collide with a SHA-256 bit hash value.)  This attack can be
      mitigated by including the signature algorithm identifier in the
      data to be signed.







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9.  Message Authentication (MAC) Algorithms

   Message Authentication Codes (MACs) provide data authentication and
   integrity protection.  They provide either no or very limited data
   origination.  (One cannot, for example, be used to prove the identity
   of the sender to a third party.)

   MACs use the same scheme as signature with appendix algorithms.  The
   message content is processed and an authentication code is produced.
   The authentication code is frequently called a tag.

   The MAC functions are:

   tag = MAC_Create(message content, key)

   valid = MAC_Verify(message content, key, tag)

   MAC algorithms can be based on either a block cipher algorithm (i.e.
   AES-MAC) or a hash algorithm (i.e.  HMAC).  This document defines a
   MAC algorithm for each of these two constructions.

9.1.  Hash-based Message Authentication Codes (HMAC)

   The Hash-base Message Authentication Code algorithm (HMAC)
   [RFC2104][RFC4231] was designed to deal with length extension
   attacks.  The algorithm was also designed to allow for new hash
   algorithms to be directly plugged in without changes to the hash
   function.  The HMAC design process has been vindicated as, while the
   security of hash algorithms such as MD5 has decreased over time, the
   security of HMAC combined with MD5 has not yet been shown to be
   compromised [RFC6151].

   The HMAC algorithm is parameterized by an inner and outer padding, a
   hash function (h) and an authentication tag value length.  For this
   specification, the inner and outer padding are fixed to the values
   set in [RFC2104].  The length of the authentication tag corresponds
   to the difficulty of producing a forgery.  For use in constrained
   environments, we define a set of HMAC algorithms that are truncated.
   There are currently no known issues with truncation, however the
   security strength of the message tag is correspondingly reduced in
   strength.  When truncating, the left-most tag length bits are kept
   and transmitted.

   The algorithm defined in this document can be found in Table 6.







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   +-----------+-------+---------+----------+--------------------------+
   | name      | value | Hash    | Tag      | description              |
   |           |       |         | Length   |                          |
   +-----------+-------+---------+----------+--------------------------+
   | HMAC      | 4     | SHA-256 | 64       | HMAC w/ SHA-256          |
   | 256/64    |       |         |          | truncated to 64 bits     |
   |           |       |         |          |                          |
   | HMAC      | 5     | SHA-256 | 256      | HMAC w/ SHA-256          |
   | 256/256   |       |         |          |                          |
   |           |       |         |          |                          |
   | HMAC      | 6     | SHA-384 | 384      | HMAC w/ SHA-384          |
   | 384/384   |       |         |          |                          |
   |           |       |         |          |                          |
   | HMAC      | 7     | SHA-512 | 512      | HMAC w/ SHA-512          |
   | 512/512   |       |         |          |                          |
   +-----------+-------+---------+----------+--------------------------+

                      Table 6: HMAC Algorithm Values

   Some recipient algorithms carry the key while others derive a key
   from secret data.  For those algorithms that carry the key (i.e.
   AES-KeyWrap), the size of the HMAC key SHOULD be the same size as the
   underlying hash function.  For those algorithms that derive the key
   (i.e.  ECDH), the derived key MUST be the same size as the underlying
   hash function.

   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present and it MUST be 'Symmetric'.

   o  If the 'alg' field present, it MUST match the HMAC algorithm being
      used.

   o  If the 'key_ops' field is present, it MUST include 'sign' when
      creating an HMAC authentication tag.

   o  If the 'key_ops' field is present, it MUST include 'verify' when
      verifying an HMAC authentication tag.

   Implementations creating and validating MAC values MUST validate that
   the key type, key length, and algorithm are correct and appropriate
   for the entities involved.








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9.1.1.  Security Considerations

   HMAC has proved to be resistant to attack even when used with
   weakening hash algorithms.  The current best method appears to be a
   brute force attack on the key.  This means that key size is going to
   be directly related to the security of an HMAC operation.

9.2.  AES Message Authentication Code (AES-CBC-MAC)

   AES-CBC-MAC is defined in [MAC].  (Note this is not the same
   algorithm as AES-CMAC [RFC4493]).

   AES-CBC-MAC is parameterized by the key length, the authentication
   tag length and the IV used.  For all of these algorithms, the IV is
   fixed to all zeros.  We provide an array of algorithms for various
   key lengths and tag lengths.  The algorithms defined in this document
   are found in Table 7.

   +-------------+-------+----------+----------+-----------------------+
   | name        | value | key      | tag      | description           |
   |             |       | length   | length   |                       |
   +-------------+-------+----------+----------+-----------------------+
   | AES-MAC     | 14    | 128      | 64       | AES-MAC 128 bit key,  |
   | 128/64      |       |          |          | 64-bit tag            |
   |             |       |          |          |                       |
   | AES-MAC     | 15    | 256      | 64       | AES-MAC 256 bit key,  |
   | 256/64      |       |          |          | 64-bit tag            |
   |             |       |          |          |                       |
   | AES-MAC     | 25    | 128      | 128      | AES-MAC 128 bit key,  |
   | 128/128     |       |          |          | 128-bit tag           |
   |             |       |          |          |                       |
   | AES-MAC     | 26    | 256      | 128      | AES-MAC 256 bit key,  |
   | 256/128     |       |          |          | 128-bit tag           |
   +-------------+-------+----------+----------+-----------------------+

                     Table 7: AES-MAC Algorithm Values

   Keys may be obtained either from a key structure or from a recipient
   structure.  Implementations creating and validating MAC values MUST
   validate that the key type, key length and algorithm are correct and
   appropriate for the entities involved.

   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present and it MUST be 'Symmetric'.





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   o  If the 'alg' field present, it MUST match the AES-MAC algorithm
      being used.

   o  If the 'key_ops' field is present, it MUST include 'sign' when
      creating an AES-MAC authentication tag.

   o  If the 'key_ops' field is present, it MUST include 'verify' when
      verifying an AES-MAC authentication tag.

9.2.1.  Security Considerations

   A number of attacks exist against CBC-MAC that need to be considered.

   o  A single key must only be used for messages of a fixed and known
      length.  If this is not the case, an attacker will be able to
      generate a message with a valid tag given two message, tag pairs.
      This can be addressed by using different keys for different length
      messages.  The current structure mitigates this problem as a
      specific encoding structure which includes lengths is build and
      signed.  (CMAC mode also addresses this issue.)

   o  If the same key is used for both encryption and authentication
      operations, using CBC modes an attacker can produce messages with
      a valid authentication code.

   o  If the IV can be modified, then messages can be forged.  This is
      addressed by fixing the IV to all zeros.

10.  Content Encryption Algorithms

   Content Encryption Algorithms provide data confidentiality for
   potentially large blocks of data using a symmetric key.  They provide
   integrity on the data that was encrypted, however they provide either
   no or very limited data origination.  (One cannot, for example, be
   used to prove the identity of the sender to a third party.)  The
   ability to provide data origination is linked to how the symmetric
   key is obtained.

   COSE restricts the set of legal content encryption algorithms to
   those that support authentication both of the content and additional
   data.  The encryption process will generate some type of
   authentication value, but that value may be either explicit or
   implicit in terms of the algorithm definition.  For simplicity sake,
   the authentication code will normally be defined as being appended to
   the cipher text stream.  The encryption functions are:






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   ciphertext = Encrypt(message content, key, additional data)

   valid, message content = Decrypt(cipher text, key, additional data)

   Most AEAD algorithms are logically defined as returning the message
   content only if the decryption is valid.  Many but not all
   implementations will follow this convention.  The message content
   MUST NOT be used if the decryption does not validate.

10.1.  AES GCM

   The GCM mode is a generic authenticated encryption block cipher mode
   defined in [AES-GCM].  The GCM mode is combined with the AES block
   encryption algorithm to define an AEAD cipher.

   The GCM mode is parameterized with by the size of the authentication
   tag and the size of the nonce.  This document fixes the size of the
   nonce at 96-bits.  The size of the authentication tag is limited to a
   small set of values.  For this document however, the size of the
   authentication tag is fixed at 128 bits.

   The set of algorithms defined in this document are in Table 8.

      +---------+-------+------------------------------------------+
      | name    | value | description                              |
      +---------+-------+------------------------------------------+
      | A128GCM | 1     | AES-GCM mode w/ 128-bit key, 128-bit tag |
      |         |       |                                          |
      | A192GCM | 2     | AES-GCM mode w/ 192-bit key, 128-bit tag |
      |         |       |                                          |
      | A256GCM | 3     | AES-GCM mode w/ 256-bit key, 128-bit tag |
      +---------+-------+------------------------------------------+

                   Table 8: Algorithm Value for AES-GCM

   Keys may be obtained either from a key structure or from a recipient
   structure.  Implementations encrypting and decrypting MUST validate
   that the key type, key length and algorithm are correct and
   appropriate for the entities involved.

   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present and it MUST be 'Symmetric'.

   o  If the 'alg' field present, it MUST match the AES-GCM algorithm
      being used.




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   o  If the 'key_ops' field is present, it MUST include 'encrypt' or
      'key wrap' when encrypting.

   o  If the 'key_ops' field is present, it MUST include 'decrypt' or
      'key unwrap' when decrypting.

10.1.1.  Security Considerations

   When using AES-GCM, the following restrictions MUST be enforced:

   o  The key and nonce pair MUST be unique for every message encrypted.

   o  The total amount of data encrypted for a single key MUST NOT
      exceed 2^39 - 256 bits.  An explicit check is required only in
      environments where it is expected that it might be exceeded.

   Consideration was given to supporting smaller tag values, the
   constrained community would desire tag sizes in the 64-bit range.
   Doing show drastically changes both the maximum messages size
   (generally not an issue) and the number of times that a key can be
   used.  Given that CCM is the usual mode for constrained environments
   restricted modes are not supported.

10.2.  AES CCM

   Counter with CBC-MAC (CCM) is a generic authentication encryption
   block cipher mode defined in [RFC3610].  The CCM mode is combined
   with the AES block encryption algorithm to define a commonly used
   content encryption algorithm used in constrained devices.

   The CCM mode has two parameter choices.  The first choice is M, the
   size of the authentication field.  The choice of the value for M
   involves a trade-off between message expansion and the probably that
   an attacker can undetectably modify a message.  The second choice is
   L, the size of the length field.  This value requires a trade-off
   between the maximum message size and the size of the Nonce.

   It is unfortunate that the specification for CCM specified L and M as
   a count of bytes rather than a count of bits.  This leads to possible
   misunderstandings where AES-CCM-8 is frequently used to refer to a
   version of CCM mode where the size of the authentication is 64 bits
   and not 8 bits.  These values have traditionally been specified as
   bit counts rather than byte counts.  This document will follow the
   tradition of using bit counts so that it is easier to compare the
   different algorithms presented in this document.

   We define a matrix of algorithms in this document over the values of
   L and M.  Constrained devices are usually operating in situations



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   where they use short messages and want to avoid doing recipient
   specific cryptographic operations.  This favors smaller values of
   both L and M.  Less constrained devices do will want to be able to
   user larger messages and are more willing to generate new keys for
   every operation.  This favors larger values of L and M.

   The following values are used for L:

   16 bits (2)  limits messages to 2^16 bytes (64 KiB) in length.  This
      sufficiently long for messages in the constrained world.  The
      nonce length is 13 bytes allowing for 2^(13*8) possible values of
      the nonce without repeating.

   64 bits (8)  limits messages to 2^64 bytes in length.  The nonce
      length is 7 bytes allowing for 2^56 possible values of the nonce
      without repeating.

   The following values are used for M:

   64 bits (8)  produces a 64-bit authentication tag.  This implies that
      there is a 1 in 2^64 chance that a modified message will
      authenticate.

   128 bits (16)  produces a 128-bit authentication tag.  This implies
      that there is a 1 in 2^128 chance that a modified message will
      authenticate.

























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   +--------------------+-------+----+-----+-----+---------------------+
   | name               | value | L  | M   | k   | description         |
   +--------------------+-------+----+-----+-----+---------------------+
   | AES-CCM-16-64-128  | 10    | 16 | 64  | 128 | AES-CCM mode        |
   |                    |       |    |     |     | 128-bit key, 64-bit |
   |                    |       |    |     |     | tag, 13-byte nonce  |
   |                    |       |    |     |     |                     |
   | AES-CCM-16-64-256  | 11    | 16 | 64  | 256 | AES-CCM mode        |
   |                    |       |    |     |     | 256-bit key, 64-bit |
   |                    |       |    |     |     | tag, 13-byte nonce  |
   |                    |       |    |     |     |                     |
   | AES-CCM-64-64-128  | 12    | 64 | 64  | 128 | AES-CCM mode        |
   |                    |       |    |     |     | 128-bit key, 64-bit |
   |                    |       |    |     |     | tag, 7-byte nonce   |
   |                    |       |    |     |     |                     |
   | AES-CCM-64-64-256  | 13    | 64 | 64  | 256 | AES-CCM mode        |
   |                    |       |    |     |     | 256-bit key, 64-bit |
   |                    |       |    |     |     | tag, 7-byte nonce   |
   |                    |       |    |     |     |                     |
   | AES-CCM-16-128-128 | 30    | 16 | 128 | 128 | AES-CCM mode        |
   |                    |       |    |     |     | 128-bit key,        |
   |                    |       |    |     |     | 128-bit tag,        |
   |                    |       |    |     |     | 13-byte nonce       |
   |                    |       |    |     |     |                     |
   | AES-CCM-16-128-256 | 31    | 16 | 128 | 256 | AES-CCM mode        |
   |                    |       |    |     |     | 256-bit key,        |
   |                    |       |    |     |     | 128-bit tag,        |
   |                    |       |    |     |     | 13-byte nonce       |
   |                    |       |    |     |     |                     |
   | AES-CCM-64-128-128 | 32    | 64 | 128 | 128 | AES-CCM mode        |
   |                    |       |    |     |     | 128-bit key,        |
   |                    |       |    |     |     | 128-bit tag, 7-byte |
   |                    |       |    |     |     | nonce               |
   |                    |       |    |     |     |                     |
   | AES-CCM-64-128-256 | 33    | 64 | 128 | 256 | AES-CCM mode        |
   |                    |       |    |     |     | 256-bit key,        |
   |                    |       |    |     |     | 128-bit tag, 7-byte |
   |                    |       |    |     |     | nonce               |
   +--------------------+-------+----+-----+-----+---------------------+

                   Table 9: Algorithm Values for AES-CCM

   Keys may be obtained either from a key structure or from a recipient
   structure.  Implementations encrypting and decrypting MUST validate
   that the key type, key length and algorithm are correct and
   appropriate for the entities involved.





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   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present and it MUST be 'Symmetric'.

   o  If the 'alg' field present, it MUST match the AES-CCM algorithm
      being used.

   o  If the 'key_ops' field is present, it MUST include 'encrypt' or
      'key wrap' when encrypting.

   o  If the 'key_ops' field is present, it MUST include 'decrypt' or
      'key unwrap' when decrypting.

10.2.1.  Security Considerations

   When using AES-CCM, the following restrictions MUST be enforced:

   o  The key and nonce pair MUST be unique for every message encrypted.

   o  The total number of times the AES block cipher is used MUST NOT
      exceed 2^61 operations.  This limitation is the sum of times the
      block cipher is used in computing the MAC value and in performing
      stream encryption operations.  An explicit check is required only
      in environments where it is expected that it might be exceeded.

   [RFC3610] additionally calls out one other consideration of note.  It
   is possible to do a pre-computation attack against the algorithm in
   cases where the portions encryption content is highly predictable.
   This reduces the security of the key size by half.  Ways to deal with
   this attack include adding a random portion to the nonce value and/or
   increasing the key size used.  Using a portion of the nonce for a
   random value will decrease the number of messages that a single key
   can be used for.  Increasing the key size may require more resources
   in the constrained device.  See sections 5 and 10 of [RFC3610] for
   more information.

10.3.  ChaCha20 and Poly1305

   ChaCha20 and Poly1305 combined together is a new AEAD mode that is
   defined in [RFC7539].  This is a new algorithm defined to be a cipher
   that is not AES and thus would not suffer from any future weaknesses
   found in AES.  These cryptographic functions are designed to be fast
   in software-only implementations.

   The ChaCha20/Poly1305 AEAD construction defined in [RFC7539] has no
   parameterization.  It takes a 256-bit key and a 96-bit nonce as well
   as the plain text and additional data as inputs and produces the



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   cipher text as an option.  We define one algorithm identifier for
   this algorithm in Table 10.

   +-------------------+-------+---------------------------------------+
   | name              | value | description                           |
   +-------------------+-------+---------------------------------------+
   | ChaCha20/Poly1305 | 24    | ChaCha20/Poly1305 w/ 256-bit key,     |
   |                   |       | 128-bit tag                           |
   +-------------------+-------+---------------------------------------+

                   Table 10: Algorithm Value for AES-GCM

   Keys may be obtained either from a key structure or from a recipient
   structure.  Implementations encrypting and decrypting MUST validate
   that the key type, key length and algorithm are correct and
   appropriate for the entities involved.

   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present and it MUST be 'Symmetric'.

   o  If the 'alg' field present, it MUST match the ChaCha algorithm
      being used.

   o  If the 'key_ops' field is present, it MUST include 'encrypt' or
      'key wrap' when encrypting.

   o  If the 'key_ops' field is present, it MUST include 'decrypt' or
      'key unwrap' when decrypting.

10.3.1.  Security Considerations

   The pair of key, nonce MUST be unique for every invocation of the
   algorithm.  Nonce counters are considered to be an acceptable way of
   ensuring that they are unique.

11.  Key Derivation Functions (KDF)

   Key Derivation Functions (KDFs) are used to take some secret value
   and generate a different one.  The secret value comes in three
   flavors:

   o  Secrets that are uniformly random: This is the type of secret
      which is created by a good random number generator.

   o  Secrets that are not uniformly random: This is type of secret
      which is created by operations like key agreement.



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   o  Secrets that are not random: This is the type of secret that
      people generate for things like passwords.

   General KDF functions work well with the first type of secret, can do
   reasonable well with the second type of secret and generally do
   poorly with the last type of secret.  None of the KDF functions in
   this section are designed to deal with the type of secrets that are
   used for passwords.  Functions like PBSE2 [RFC2898] need to be used
   for that type of secret.

   The same KDF function can be setup to deal with the first two types
   of secrets different.  The KDF function defined in Section 11.1 is
   such a function.  This is reflected in the set of algorithms defined
   for HKDF.

   When using KDF functions, one component that is included is context
   information.  Context information is used to allow for different
   keying information to be derived from the same secret.  The use of
   context based keying material is considered to be a good security
   practice.  This document defines a single context structure and a
   single KDF function.

11.1.  HMAC-based Extract-and-Expand Key Derivation Function (HKDF)

   The HKDF key derivation algorithm is defined in [RFC5869].

   The HKDF algorithm takes these inputs:

      secret - a shared value that is secret.  Secrets may be either
      previously shared or derived from operations like a DH key
      agreement.

      salt - an optional value that is used to change the generation
      process.  The salt value can be either public or private.  If the
      salt is public and carried in the message, then the 'salt'
      algorithm header parameter defined in Table 12 is used.  While
      [RFC5869] suggests that the length of the salt be the same as the
      length of the underlying hash value, any amount of salt will
      improve the security as different key values will be generated.
      This parameter is protected by being included in the key
      computation and does not need to be separately authenticated.  The
      salt value does not need to be unique for every message sent.

      length - the number of bytes of output that need to be generated.

      context information - Information that describes the context in
      which the resulting value will be used.  Making this information
      specific to the context that the material is going to be used



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      ensures that the resulting material will always be tied to the
      context.  The context structure used is encoded into the algorithm
      identifier.

      PRF - The underlying pseudo-random function to be used in the HKDF
      algorithm.  The PRF is encoded into the HKDF algorithm selection.
      u

   HKDF is defined to use HMAC as the underlying PRF.  However, it is
   possible to use other functions in the same construct to provide a
   different KDF function that is more appropriate in the constrained
   world.  Specifically, one can use AES-CBC-MAC as the PRF for the
   expand step, but not for the extract step.  When using a good random
   shared secret of the correct length, the extract step can be skipped.
   For the AES algorithm versions, the extract step is always skipped.

   The extract step cannot be skipped if the secret is not uniformly
   random, for example if it is the result of an ECDH key agreement
   step.  (This implies that the AES HKDF version cannot be used with
   ECDH.)  If the extract step is skipped, the 'salt' value is not used
   as part of the HKDF functionality.

   The algorithms defined in this document are found in Table 11.

   +---------------+-----------------+---------------------------------+
   | name          | PRF             | context                         |
   +---------------+-----------------+---------------------------------+
   | HKDF SHA-256  | HMAC with       | HKDF using HMAC SHA-256 as the  |
   |               | SHA-256         | PRF                             |
   |               |                 |                                 |
   | HKDF SHA-512  | HMAC with       | HKDF using HMAC SHA-512 as the  |
   |               | SHA-512         | PRF                             |
   |               |                 |                                 |
   | HKDF AES-     | AES-CBC-MAC-128 | HKDF using AES-MAC as the PRF   |
   | MAC-128       |                 | w/ 128-bit key                  |
   |               |                 |                                 |
   | HKDF AES-     | AES-CBC-MAC-256 | HKDF using AES-MAC as the PRF   |
   | MAC-256       |                 | w/ 256-bit key                  |
   +---------------+-----------------+---------------------------------+

                         Table 11: HKDF algorithms










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                   +------+-------+------+-------------+
                   | name | label | type | description |
                   +------+-------+------+-------------+
                   | salt | -20   | bstr | Random salt |
                   +------+-------+------+-------------+

                    Table 12: HKDF Algorithm Parameters

11.2.  Context Information Structure

   The context information structure is used to ensure that the derived
   keying material is "bound" to the context of the transaction.  The
   context information structure used here is based on that defined in
   [SP800-56A].  By using CBOR for the encoding of the context
   information structure, we automatically get the same type and length
   separation of fields that is obtained by the use of ASN.1.  This
   means that there is no need to encode the lengths for the base
   elements as it is done by the encoding used in JOSE (Section 4.6.2 of
   [RFC7518]).

   The context information structure refers to PartyU and PartyV as the
   two parties which are doing the key derivation.  Unless the
   application protocol defines differently, we assign PartyU to the
   entity that is creating the message and PartyV to the entity that is
   receiving the message.  By doing this association, different keys
   will be derived for each direction as the context information is
   different in each direction.

   The context structure is built from information that is known to both
   entities.  This information can be obtained from a variety of
   sources:

   o  Fields can be define by the application.  This is commonly used to
      assign fixed names to parties, but can be used for other items
      such as nonces.

   o  Fields can be defined by usage of the output.  Examples of this
      are the algorithm and key size that are being generated.

   o  Fields can be defined by parameters from the message.  We define a
      set of parameters in Table 13 which can be used to carry the
      values associated with the context structure.  Examples of this
      are identities and nonce values.  These parameters are designed to
      be placed in the unprotected bucket of the recipient structure.
      (They do not need to be in the protected bucket since they already
      are included in the cryptographic computation by virtue of being
      included in the context structure.)




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   +---------------+-------+-----------+-------------------------------+
   | name          | label | type      | description                   |
   +---------------+-------+-----------+-------------------------------+
   | PartyU        | -21   | bstr      | Party U identity Information  |
   | identity      |       |           |                               |
   |               |       |           |                               |
   | PartyU nonce  | -22   | bstr /    | Party U provided nonce        |
   |               |       | int       |                               |
   |               |       |           |                               |
   | PartyU other  | -23   | bstr      | Party U other provided        |
   |               |       |           | information                   |
   |               |       |           |                               |
   | PartyV        | -24   | bstr      | Party V identity Information  |
   | identity      |       |           |                               |
   |               |       |           |                               |
   | PartyV nonce  | -25   | bstr /    | Party V provided nonce        |
   |               |       | int       |                               |
   |               |       |           |                               |
   | PartyV other  | -26   | bstr      | Party V other provided        |
   |               |       |           | information                   |
   +---------------+-------+-----------+-------------------------------+

                  Table 13: Context Algorithm Parameters

   We define a CBOR object to hold the context information.  This object
   is referred to as CBOR_KDF_Context.  The object is based on a CBOR
   array type.  The fields in the array are:

   AlgorithmID  This field indicates the algorithm for which the key
      material will be used.  This field is required to be present.  The
      field exists in the context information so that if the same
      environment is used for different algorithms, then completely
      different keys will be generated each of those algorithms.  (This
      practice means if algorithm A is broken and thus can is easier to
      find, the key derived for algorithm B will not be the same as the
      key for algorithm B.)

   PartyUInfo  This field holds information about party U.  The
      PartyUInfo is encoded as a CBOR array.  The elements of PartyUInfo
      are encoded in the order presented, however if the element does
      not exist no element is placed in the array.  The elements of the
      PartyUInfo array are:

      identity  This contains the identity information for party U.  The
         identities can be assigned in one of two manners.  Firstly, a
         protocol can assign identities based on roles.  For example,
         the roles of "client" and "server" may be assigned to different
         entities in the protocol.  Each entity would then use the



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         correct label for the data they send or receive.  The second
         way is for a protocol to assign identities is to use a name
         based on a naming system (i.e.  DNS, X.509 names).
         We define an algorithm parameter 'PartyU identity' that can be
         used to carry identity information in the message.  However,
         identity information is often known as part of the protocol and
         can thus be inferred rather than made explicit.  If identity
         information is carried in the message, applications SHOULD have
         a way of validating the supplied identity information.  The
         identity information does not need to be specified and can be
         left as absent.

      nonce  This contains a nonce value.  The nonce can either be
         implicit from the protocol or carried as a value in the
         unprotected headers.
         We define an algorithm parameter 'PartyU nonce' that can be
         used to carry this value in the message However, the nonce
         value could be determined by the application and the value
         determined from elsewhere.
         This item is optional and can be absent.

      other  This contains other information that is defined by the
         protocol.
         This item is optional and can be absent.

   PartyVInfo  This field holds information about party V.  The
      PartyVInfo is encoded as a CBOR array.  For store and forward
      environments, the party V information may be minimal or even
      absent.  The elements of PartyVInfo are encoded in the order
      presented, however if the element does not exist no element is
      placed in the array.  The elements of the PartyVInfo array are:

      identity  See description of PartyUInfo identity.

      nonce  See description of PartyUInfo nonce.

      other  See description of PartyUInfo other.

   SuppPubInfo  This field contains public information that is mutually
      known to both parties.

      keyDataLength  This is set to the number of bits of the desired
         output value.  (This practice means if algorithm A can use two
         different key lengths, the key derived for longer key size will
         not contain the key for shorter key size as a prefix.)






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      protected  This field contains the protected parameter field.  If
         there are no elements in the protected field, then use a zero
         length bstr.

      other  The field other is for free form data defined by the
         application.  An example is that an application could defined
         two different strings to be placed here to generate different
         keys for a data stream vs a control stream.  This field is
         optional and will only be present if the application defines a
         structure for this information.  Applications that define this
         SHOULD use CBOR to encode the data so that types and lengths
         are correctly include.

   SuppPrivInfo  This field contains private information that is
      mutually known information.  An example of this information would
      be a pre-existing shared secret.  (This could for example, be used
      in combination with an ECDH key agreement to provide a secondary
      proof of identity.)  The field is optional and will only be
      present if the application defines a structure for this
      information.  Applications that define this SHOULD use CBOR to
      encode the data so that types and lengths are correctly included.

   The following CDDL fragment corresponds to the text above.

   PartyInfo = (
       ? nonce : bstr / int,
       ? identity : bstr,
       ? other : bstr,
   )

   COSE_KDF_Context = [
       AlgorithmID : int / tstr,
       PartyUInfo : [ PartyInfo ],
       PartyVInfo : [ PartyInfo ],
       SuppPubInfo : [
           keyDataLength : uint,
           protected : bstr,
           ? other : bstr
       ],
       ? SuppPrivInfo : bstr
   ]

12.  Recipient Algorithm Classes

   Recipient algorithms can be defined into a number of different
   classes.  COSE has the ability to support many classes of recipient
   algorithms.  In this section, a number of classes are listed and then
   a set of algorithms are specified for each of the classes.  The names



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   of the recipient algorithm classes used here are the same as are
   defined in [RFC7516].  Other specifications use different terms for
   the recipient algorithm classes or do not support some of the
   recipient algorithm classes.

12.1.  Direct Encryption

   The direct encryption class algorithms share a secret between the
   sender and the recipient that is used either directly or after
   manipulation as the content key.  When direct encryption mode is
   used, it MUST be the only mode used on the message.

   The COSE_Enveloped structure for the recipient is organized as
   follows:

   o  The 'protected' field MUST be a zero length item unless it is used
      in the computation of the content key.

   o  The 'alg' parameter MUST be present.

   o  A parameter identifying the shared secret SHOULD be present.

   o  The 'ciphertext' field MUST be a zero length item.

   o  The 'recipients' field MUST be absent.

12.1.1.  Direct Key

   This recipient algorithm is the simplest, the identified key is
   directly used as the key for the next layer down in the message.
   There are no algorithm parameters defined for this algorithm.  The
   algorithm identifier value is assigned in Table 14.

   When this algorithm is used, the protected field MUST be zero length.
   The key type MUST be 'Symmetric'.

                  +--------+-------+-------------------+
                  | name   | value | description       |
                  +--------+-------+-------------------+
                  | direct | -6    | Direct use of CEK |
                  +--------+-------+-------------------+

                           Table 14: Direct Key








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12.1.1.1.  Security Considerations

   This recipient algorithm has several potential problems that need to
   be considered:

   o  These keys need to have some method to be regularly updated over
      time.  All of the content encryption algorithms specified in this
      document have limits on how many times a key can be used without
      significant loss of security.

   o  These keys need to be dedicated to a single algorithm.  There have
      been a number of attacks developed over time when a single key is
      used for multiple different algorithms.  One example of this is
      the use of a single key both for CBC encryption mode and CBC-MAC
      authentication mode.

   o  Breaking one message means all messages are broken.  If an
      adversary succeeds in determining the key for a single message,
      then the key for all messages is also determined.

12.1.2.  Direct Key with KDF

   These recipient algorithms take a common shared secret between the
   two parties and applies the HKDF function (Section 11.1) using the
   context structure defined in Section 11.2 to transform the shared
   secret into the necessary key.  The 'protected' field can be of non-
   zero length.  Either the 'salt' parameter of HKDF or the partyU
   'nonce' parameter of the context structure MUST be present.  The
   salt/nonce parameter can be generated either randomly or
   deterministically.  The requirement is that it be a unique value for
   the key/IV pair in question.

   If the salt/nonce value is generated randomly, then it is suggested
   that the length of the random value be the same length as the hash
   function underlying HKDF.  While there is no way to guarantee that it
   will be unique, there is a high probability that it will be unique.
   If the salt/nonce value is generated deterministically, it can be
   guaranteed to be unique and thus there is no length requirement.

   A new IV must be used if the same key is used in more than one
   message.  The IV can be modified in a predictable manner, a random
   manner or an unpredictable manner.  One unpredictable manner that can
   be used is to use the HKDF function to generate the IV.  If HKDF is
   used for generating the IV, the algorithm identifier is set to "IV-
   GENERATION".

   When these algorithms are used, the key type MUST be 'symmetric'.




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   The set of algorithms defined in this document can be found in
   Table 15.

   +---------------------+-------+-------------+-----------------------+
   | name                | value | KDF         | description           |
   +---------------------+-------+-------------+-----------------------+
   | direct+HKDF-SHA-256 | -10   | HKDF        | Shared secret w/ HKDF |
   |                     |       | SHA-256     | and SHA-256           |
   |                     |       |             |                       |
   | direct+HKDF-SHA-512 | -11   | HKDF        | Shared secret w/ HKDF |
   |                     |       | SHA-512     | and SHA-512           |
   |                     |       |             |                       |
   | direct+HKDF-AES-128 | -12   | HKDF AES-   | Shared secret w/ AES- |
   |                     |       | MAC-128     | MAC 128-bit key       |
   |                     |       |             |                       |
   | direct+HKDF-AES-256 | -13   | HKDF AES-   | Shared secret w/ AES- |
   |                     |       | MAC-256     | MAC 256-bit key       |
   +---------------------+-------+-------------+-----------------------+

                           Table 15: Direct Key

   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present and it MUST be 'Symmetric'.

   o  If the 'alg' field present, it MUST match the KDF algorithm being
      used.

   o  If the 'key_ops' field is present, it MUST include 'deriveKey or
      'deriveBits'.

12.1.2.1.  Security Considerations

   The shared secret needs to have some method to be regularly updated
   over time.  The shared secret forms the basis of trust.  Although not
   used directly, it should still be subject to scheduled rotation.

12.2.  Key Wrapping

   In key wrapping mode, the CEK is randomly generated and that key is
   then encrypted by a shared secret between the sender and the
   recipient.  All of the currently defined key wrapping algorithms for
   COSE are AE algorithms.  Key wrapping mode is considered to be
   superior to direct encryption if the system has any capability for
   doing random key generation.  This is because the shared key is used
   to wrap random data rather than data has some degree of organization
   and may in fact be repeating the same content.  The use of Key



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   Wrapping loses the weak data origination that is provided by the
   direct encryption algorithms.

   The COSE_Enveloped structure for the recipient is organized as
   follows:

   o  The 'protected' field MUST be absent if the key wrap algorithm is
      an AE algorithm.

   o  The 'recipients' field is normally absent, but can be used.
      Applications MUST deal with a recipient field present, not being
      able to decrypt that recipient is an acceptable way of dealing
      with it.  Failing to process the message is not an acceptable way
      of dealing with it.

   o  The plain text to be encrypted is the key from next layer down
      (usually the content layer).

   o  At a minimum, the 'unprotected' field MUST contain the 'alg'
      parameter and SHOULD contain a parameter identifying the shared
      secret.

12.2.1.  AES Key Wrapping

   The AES Key Wrapping algorithm is defined in [RFC3394].  This
   algorithm uses an AES key to wrap a value that is a multiple of 64
   bits.  As such, it can be used to wrap a key for any of the content
   encryption algorithms defined in this document.  The algorithm
   requires a single fixed parameter, the initial value.  This is fixed
   to the value specified in Section 2.2.3.1 of [RFC3394].  There are no
   public parameters that vary on a per invocation basis.  The protected
   header field MUST be empty.

   Keys may be obtained either from a key structure or from a recipient
   structure.  If the key obtained from a key structure, the key type
   MUST be 'Symmetric'.  Implementations encrypting and decrypting MUST
   validate that the key type, key length and algorithm are correct and
   appropriate for the entities involved.

   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present and it MUST be 'Symmetric'.

   o  If the 'alg' field present, it MUST match the AES Key Wrap
      algorithm being used.





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   o  If the 'key_ops' field is present, it MUST include 'encrypt' or
      'key wrap' when encrypting.

   o  If the 'key_ops' field is present, it MUST include 'decrypt' or
      'key unwrap' when decrypting.

        +--------+-------+----------+-----------------------------+
        | name   | value | key size | description                 |
        +--------+-------+----------+-----------------------------+
        | A128KW | -3    | 128      | AES Key Wrap w/ 128-bit key |
        |        |       |          |                             |
        | A192KW | -4    | 192      | AES Key Wrap w/ 192-bit key |
        |        |       |          |                             |
        | A256KW | -5    | 256      | AES Key Wrap w/ 256-bit key |
        +--------+-------+----------+-----------------------------+

                  Table 16: AES Key Wrap Algorithm Values

12.2.1.1.  Security Considerations for AES-KW

   The shared secret need to have some method to be regularly updated
   over time.  The shared secret is the basis of trust.

12.3.  Key Encryption

   Key Encryption mode is also called key transport mode in some
   standards.  Key Encryption mode differs from Key Wrap mode in that it
   uses an asymmetric encryption algorithm rather than a symmetric
   encryption algorithm to protect the key.  This document does not
   define any Key Encryption mode algorithms.

   When using a key encryption algorithm, the COSE_Enveloped structure
   for the recipient is organized as follows:

   o  The 'protected' field MUST be absent.

   o  The plain text to be encrypted is the key from next layer down
      (usually the content layer).

   o  At a minimum, the 'unprotected' field MUST contain the 'alg'
      parameter and SHOULD contain a parameter identifying the
      asymmetric key.

12.4.  Direct Key Agreement

   The 'direct key agreement' class of recipient algorithms uses a key
   agreement method to create a shared secret.  A KDF is then applied to
   the shared secret to derive a key to be used in protecting the data.



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   This key is normally used as a CEK or MAC key, but could be used for
   other purposes if more than two layers are in use (see Appendix B).

   The most commonly used key agreement algorithm is Diffie-Hellman, but
   other variants exist.  Since COSE is designed for a store and forward
   environment rather than an on-line environment, many of the DH
   variants cannot be used as the receiver of the message cannot provide
   any dynamic key material.  One side-effect of this is that perfect
   forward secrecy (see [RFC4949]) is not achievable.  A static key will
   always be used for the receiver of the COSE message.

   Two variants of DH that are supported are:

      Ephemeral-Static DH: where the sender of the message creates a one
      time DH key and uses a static key for the recipient.  The use of
      the ephemeral sender key means that no additional random input is
      needed as this is randomly generated for each message.

      Static-Static DH: where a static key is used for both the sender
      and the recipient.  The use of static keys allows for recipient to
      get a weak version of data origination for the message.  When
      static-static key agreement is used, then some piece of unique
      data for the KDF is required to ensure that a different key is
      created for each message.

   When direct key agreement mode is used, there MUST be only one
   recipient in the message.  This method creates the key directly and
   that makes it difficult to mix with additional recipients.  If
   multiple recipients are needed, then the version with key wrap needs
   to be used.

   The COSE_Enveloped structure for the recipient is organized as
   follows:

   o  At a minimum, headers MUST contain the 'alg' parameter and SHOULD
      contain a parameter identifying the recipient's asymmetric key.

   o  The headers SHOULD identify the senders key for the static-static
      versions and MUST contain the senders ephemeral key for the
      ephemeral-static versions.

12.4.1.  ECDH

   The mathematics for Elliptic Curve Diffie-Hellman can be found in
   [RFC6090].

   ECDH is parameterized by the following:




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   o  Curve Type/Curve: The curve selected controls not only the size of
      the shared secret, but the mathematics for computing the shared
      secret.  The curve selected also controls how a point in the curve
      is represented and what happens for the identity points on the
      curve.  In this specification, we allow for a number of different
      curves to be used.  A set of curves are defined in Table 20.
      Since the only the math is changed by changing the curve, the
      curve is not fixed for any of the algorithm identifiers we define.
      Instead, it is defined by the points used.

   o  Ephemeral-static or static-static: The key agreement process may
      be done using either a static or an ephemeral key for the sender's
      side.  When using ephemeral keys, the sender MUST generate a new
      ephemeral key for every key agreement operation.  The ephemeral
      key is placed in the 'ephemeral key' parameter and MUST be present
      for all algorithm identifiers that use ephemeral keys.  When using
      static keys, the sender MUST either generate a new random value or
      otherwise create a unique value to be placed in either in the KDF
      parameters or the context structure.  For the KDF functions used,
      this means either in the 'salt' parameter for HKDF (Table 12) or
      in the 'PartyU nonce' parameter for the context structure
      (Table 13) MUST be present.  (Both may be present if desired.)
      The value in the parameter MUST be unique for the pair of keys
      being used.  It is acceptable to use a global counter that is
      incremented for every static-static operation and use the
      resulting value.  When using static keys, the static key should be
      identified to the recipient.  The static key can be identified
      either by providing the key ('static key') or by providing a key
      identifier for the static key ('static key id').  Both of these
      parameters are defined in Table 18

   o  Key derivation algorithm: The result of an ECDH key agreement
      process does not provide a uniformly random secret.  As such, it
      needs to be run through a KDF in order to produce a usable key.
      Processing the secret through a KDF also allows for the
      introduction of both context material, how the key is going to be
      used, and one time material in the event of a static-static key
      agreement.

   o  Key Wrap algorithm: No key wrap algorithm is used.  This is
      represented in Table 17 as 'none'.  The key size for the context
      structure is the content layer encryption algorithm size.

   The set of direct ECDH algorithms defined in this document are found
   in Table 17.

   +-----------+-------+---------+------------+--------+---------------+
   | name      | value | KDF     | Ephemeral- | Key    | description   |



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   |           |       |         | Static     | Wrap   |               |
   +-----------+-------+---------+------------+--------+---------------+
   | ECDH-ES + | -25   | HKDF -  | yes        | none   | ECDH ES w/    |
   | HKDF-256  |       | SHA-256 |            |        | HKDF -        |
   |           |       |         |            |        | generate key  |
   |           |       |         |            |        | directly      |
   |           |       |         |            |        |               |
   | ECDH-ES + | -26   | HKDF -  | yes        | none   | ECDH ES w/    |
   | HKDF-512  |       | SHA-512 |            |        | HKDF -        |
   |           |       |         |            |        | generate key  |
   |           |       |         |            |        | directly      |
   |           |       |         |            |        |               |
   | ECDH-SS + | -27   | HKDF -  | no         | none   | ECDH SS w/    |
   | HKDF-256  |       | SHA-256 |            |        | HKDF -        |
   |           |       |         |            |        | generate key  |
   |           |       |         |            |        | directly      |
   |           |       |         |            |        |               |
   | ECDH-SS + | -28   | HKDF -  | no         | none   | ECDH SS w/    |
   | HKDF-512  |       | SHA-512 |            |        | HKDF -        |
   |           |       |         |            |        | generate key  |
   |           |       |         |            |        | directly      |
   |           |       |         |            |        |               |
   | ECDH-ES + | -29   | HKDF -  | yes        | A128KW | ECDH ES w/    |
   | A128KW    |       | SHA-256 |            |        | Concat KDF    |
   |           |       |         |            |        | and AES Key   |
   |           |       |         |            |        | wrap w/ 128   |
   |           |       |         |            |        | bit key       |
   |           |       |         |            |        |               |
   | ECDH-ES + | -30   | HKDF -  | yes        | A192KW | ECDH ES w/    |
   | A192KW    |       | SHA-256 |            |        | Concat KDF    |
   |           |       |         |            |        | and AES Key   |
   |           |       |         |            |        | wrap w/ 192   |
   |           |       |         |            |        | bit key       |
   |           |       |         |            |        |               |
   | ECDH-ES + | -31   | HKDF -  | yes        | A256KW | ECDH ES w/    |
   | A256KW    |       | SHA-256 |            |        | Concat KDF    |
   |           |       |         |            |        | and AES Key   |
   |           |       |         |            |        | wrap w/ 256   |
   |           |       |         |            |        | bit key       |
   |           |       |         |            |        |               |
   | ECDH-SS + | -32   | HKDF -  | no         | A128KW | ECDH SS w/    |
   | A128KW    |       | SHA-256 |            |        | Concat KDF    |
   |           |       |         |            |        | and AES Key   |
   |           |       |         |            |        | wrap w/ 128   |
   |           |       |         |            |        | bit key       |
   |           |       |         |            |        |               |
   | ECDH-SS + | -33   | HKDF -  | no         | A192KW | ECDH SS w/    |
   | A192KW    |       | SHA-256 |            |        | Concat KDF    |



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   |           |       |         |            |        | and AES Key   |
   |           |       |         |            |        | wrap w/ 192   |
   |           |       |         |            |        | bit key       |
   |           |       |         |            |        |               |
   | ECDH-SS + | -34   | HKDF -  | no         | A256KW | ECDH SS w/    |
   | A256KW    |       | SHA-256 |            |        | Concat KDF    |
   |           |       |         |            |        | and AES Key   |
   |           |       |         |            |        | wrap w/ 256   |
   |           |       |         |            |        | bit key       |
   +-----------+-------+---------+------------+--------+---------------+

                      Table 17: ECDH Algorithm Values

   +-----------+-------+----------+-----------+------------------------+
   | name      | label | type     | algorithm | description            |
   +-----------+-------+----------+-----------+------------------------+
   | ephemeral | -1    | COSE_Key | ECDH-ES   | Ephemeral Public key   |
   | key       |       |          |           | for the sender         |
   |           |       |          |           |                        |
   | static    | -2    | COSE_Key | ECDH-ES   | Static Public key for  |
   | key       |       |          |           | the sender             |
   |           |       |          |           |                        |
   | static    | -3    | bstr     | ECDH-SS   | Static Public key      |
   | key id    |       |          |           | identifier for the     |
   |           |       |          |           | sender                 |
   +-----------+-------+----------+-----------+------------------------+

                    Table 18: ECDH Algorithm Parameters

   This document defines these algorithms to be used with the curves
   P-256, P-384, P-521.  Implementations MUST verify that the key type
   and curve are correct.  Different curves are restricted to different
   key types.  Implementations MUST verify that the curve and algorithm
   are appropriate for the entities involved.

   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present and it MUST be 'EC2'.

   o  If the 'alg' field present, it MUST match the Key Agreement
      algorithm being used.

   o  If the 'key_ops' field is present, it MUST include 'derive key' or
      'derive bits' for the private key.

   o  If the 'key_ops' field is present, it MUST be empty for the public
      key.



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12.5.  Key Agreement with KDF

   Key Agreement with Key Wrapping uses a randomly generated CEK.  The
   CEK is then encrypted using a Key Wrapping algorithm and a key
   derived from the shared secret computed by the key agreement
   algorithm.

   The COSE_Enveloped structure for the recipient is organized as
   follows:

   o  The 'protected' field is fed into the KDF context structure.

   o  The plain text to be encrypted is the key from next layer down
      (usually the content layer).

   o  The 'alg' parameter MUST be present in the layer.

   o  A parameter identifying the recipient's key SHOULD be present.  A
      parameter identifying the sender's key SHOULD be present.

12.5.1.  ECDH

   These algorithms are defined in Table 17.

   ECDH with Key Agreement is parameterized by the same parameters as
   for ECDH Section 12.4.1 with the following modifications:

   o  Key Wrap Algorithm: Any of the key wrap algorithms defined in
      Section 12.2.1 are supported.  The size of the key used for the
      key wrap algorithm is fed into the KDF function.  The set of
      identifiers are found in Table 17.

   When using a COSE key for this algorithm, the following checks are
   made:

   o  The 'kty' field MUST be present and it MUST be 'EC2'.

   o  If the 'alg' field present, it MUST match the Key Agreement
      algorithm being used.

   o  If the 'key_ops' field is present, it MUST include 'derive key' or
      'derive bits' for the private key.

   o  If the 'key_ops' field is present, it MUST be empty for the public
      key.






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13.  Keys

   The COSE_Key object defines a way to hold a single key object.  It is
   still required that the members of individual key types be defined.
   This section of the document is where we define an initial set of
   members for specific key types.

   For each of the key types, we define both public and private members.
   The public members are what is transmitted to others for their usage.
   We define private members mainly for the purpose of archival of keys
   by individuals.  However, there are some circumstances in which
   private keys may be distributed to entities in a protocol.  Examples
   include: entities that have poor random number generation,
   centralized key creation for multi-cast type operations, and
   protocols in which a shared secret is used as a bearer token for
   authorization purposes.

   Key types are identified by the 'kty' member of the COSE_Key object.
   In this document, we define four values for the member:

    +-----------+-------+--------------------------------------------+
    | name      | value | description                                |
    +-----------+-------+--------------------------------------------+
    | EC2       | 2     | Elliptic Curve Keys w/ X,Y Coordinate pair |
    |           |       |                                            |
    | Symmetric | 4     | Symmetric Keys                             |
    |           |       |                                            |
    | Reserved  | 0     | This value is reserved                     |
    +-----------+-------+--------------------------------------------+

                         Table 19: Key Type Values

13.1.  Elliptic Curve Keys

   Two different key structures could be defined for Elliptic Curve
   keys.  One version uses both an x and a y coordinate, potentially
   with point compression.  This is the traditional EC point
   representation that is used in [RFC5480].  The other version uses
   only the x coordinate as the y coordinate is either to be recomputed
   or not needed for the key agreement operation.  Currently no
   algorithms are defined using this key structure.










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     +-------+----------+-------+------------------------------------+
     | name  | key type | value | description                        |
     +-------+----------+-------+------------------------------------+
     | P-256 | EC2      | 1     | NIST P-256 also known as secp256r1 |
     |       |          |       |                                    |
     | P-384 | EC2      | 2     | NIST P-384 also known as secp384r1 |
     |       |          |       |                                    |
     | P-521 | EC2      | 3     | NIST P-521 also known as secp521r1 |
     +-------+----------+-------+------------------------------------+

                            Table 20: EC Curves

13.1.1.  Double Coordinate Curves

   The traditional way of sending EC curves has been to send either both
   the x and y coordinates, or the x coordinate and a sign bit for the y
   coordinate.  The latter encoding has not been recommended in the IETF
   due to potential IPR issues.  However, for operations in constrained
   environments, the ability to shrink a message by not sending the y
   coordinate is potentially useful.

   For EC keys with both coordinates, the 'kty' member is set to 2
   (EC2).  The key parameters defined in this section are summarized in
   Table 21.  The members that are defined for this key type are:

   crv  contains an identifier of the curve to be used with the key.
      The curves defined in this document for this key type can be found
      in Table 20.  Other curves may be registered in the future and
      private curves can be used as well.

   x  contains the x coordinate for the EC point.  The integer is
      converted to an octet string as defined in [SEC1].  Leading zero
      octets MUST be preserved.

   y  contains either the sign bit or the value of y coordinate for the
      EC point.  When encoding the value y, the integer is converted to
      an octet string (as defined in [SEC1]) and encoded as a CBOR bstr.
      Leading zero octets MUST be preserved.  The compressed point
      encoding is also supported.  Compute the sign bit as laid out in
      the Elliptic-Curve-Point-to-Octet-String Conversion function of
      [SEC1].  If the sign bit is zero, then encode y as a CBOR false
      value, otherwise encode y as a CBOR true value.  The encoding of
      the infinity point is not supported.

   d  contains the private key.

   For public keys, it is REQUIRED that 'crv', 'x' and 'y' be present in
   the structure.  For private keys, it is REQUIRED that 'crv' and 'd'



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   be present in the structure.  For private keys, it is RECOMMENDED
   that 'x' and 'y' also be present, but they can be recomputed from the
   required elements and omitting them saves on space.

   +------+-------+-------+---------+----------------------------------+
   | name | key   | value | type    | description                      |
   |      | type  |       |         |                                  |
   +------+-------+-------+---------+----------------------------------+
   | crv  | 2     | -1    | int /   | EC Curve identifier - Taken from |
   |      |       |       | tstr    | the COSE Curve Registry          |
   |      |       |       |         |                                  |
   | x    | 2     | -2    | bstr    | X Coordinate                     |
   |      |       |       |         |                                  |
   | y    | 2     | -3    | bstr /  | Y Coordinate                     |
   |      |       |       | bool    |                                  |
   |      |       |       |         |                                  |
   | d    | 2     | -4    | bstr    | Private key                      |
   +------+-------+-------+---------+----------------------------------+

                        Table 21: EC Key Parameters

13.2.  Symmetric Keys

   Occasionally it is required that a symmetric key be transported
   between entities.  This key structure allows for that to happen.

   For symmetric keys, the 'kty' member is set to 3 (Symmetric).  The
   member that is defined for this key type is:

   k  contains the value of the key.

   This key structure contains only private key information, care must
   be taken that it is never transmitted accidentally.  For public keys,
   there are no required fields.  For private keys, it is REQUIRED that
   'k' be present in the structure.

             +------+----------+-------+------+-------------+
             | name | key type | value | type | description |
             +------+----------+-------+------+-------------+
             | k    | 4        | -1    | bstr | Key Value   |
             +------+----------+-------+------+-------------+

                    Table 22: Symmetric Key Parameters








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14.  CBOR Encoder Restrictions

   There has been an attempt to limit the number of places where the
   document needs to impose restrictions on how the CBOR Encoder needs
   to work.  We have managed to narrow it down to the following
   restrictions:

   o  The restriction applies to the encoding the Sig_structure, the
      Enc_structure, and the MAC_structure.

   o  The rules for Canonical CBOR (Section 3.9 of RFC 7049) MUST be
      used in these locations.  The main rule that needs to be enforced
      is that all lengths in these structures MUST be encoded such that
      they are encoded using definite lengths and the minimum length
      encoding is used.

   o  Applications MUST NOT generate messages with the same label used
      twice as a key in a single map.  Applications MUST NOT parse and
      process messages with the same label used twice as a key in a
      single map.  Applications can enforce the parse and process
      requirement by using parsers that will fail the parse step or by
      using parsers that will pass all keys to the application and the
      application can perform the check for duplicate keys.

15.  Application Profiling Considerations

   This document is designed to provide a set of security services, but
   not to provide implementation requirements for specific usage.  The
   interoperability requirements are provided for how each of the
   individual services are used and how the algorithms are to be used
   for interoperability.  The requirements about which algorithms and
   which services are needed is deferred to each application.

   Applications are therefore intended to profile the usage of this
   document.  This section provides a set of guidelines and topics that
   applications need to consider when using this document.

   o  Applications need to determine the set of messages defined in this
      document that they will be using.  The set of messages corresponds
      fairly directly to the set of security services that are needed
      and to the security levels needed.

   o  Applications may define new header parameters for a specific
      purpose.  Applications will often times select specific header
      parameters to use or not to use.  For example, an application
      would normally state a preference for using either the IV or the
      partial IV parameter.  If the partial IV parameter is specified,




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      then the application would also need to define how the fixed
      portion of the IV would be determined.

   o  When applications use externally defined authenticated data, they
      need to define how that data is encoded.  This document assumes
      that the data will be provided as a byte stream.  More information
      can be found in Section 4.3.

   o  Applications need to determine the set of security algorithms that
      are to be used.  When selecting the algorithms to be used as the
      mandatory to implement set, consideration should be given to
      choosing different types of algorithms when two are chosen for a
      specific purpose.  An example of this would be choosing HMAC-
      SHA512 and AES-CMAC as different MAC algorithms, the construction
      is vastly different between these two algorithms.  This means that
      a weakening of one algorithm would be unlikely to lead to a
      weakening of the other algorithms.  Of course, these algorithms do
      not provide the same level of security and thus may not be
      comparable for the desired security functionality.

   o  Applications may need to provide some type of negotiation or
      discovery method if multiple algorithms or message structures are
      permitted.  The method can be as simple as requiring
      preconfiguration of the set of algorithms to providing a discovery
      method built into the protocol.  S/MIME provided a number of
      different ways to approach the problem that applications could
      follow:

      *  Advertising in the message (S/MIME capabilities) [RFC5751].

      *  Advertising in the certificate (capabilities extension)
         [RFC4262].

      *  Minimum requirements for the S/MIME, which have been updated
         over time [RFC2633][RFC5751].

16.  IANA Considerations

16.1.  CBOR Tag assignment

   It is requested that IANA assign the following tags from the "Concise
   Binary Object Representation (CBOR) Tags" registry.  It is requested
   that the tags for COSE_Sign1, COSE_Encrypted and COSE_Mac0 be
   assigned in the 1 to 23 value range (i.e. one byte long when
   encoded).  It is requested that the rest of the tags be assigned in
   the 24 to 255 value range (i.e. two bytes long when encoded).

   The tags to be assigned are in table Table 1.



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16.2.  COSE Header Parameter Registry

   It is requested that IANA create a new registry entitled "COSE Header
   Parameters".  The registry is to be created as Expert Review
   Required.  Expert review guidelines are provided in Section 16.10

   The columns of the registry are:

   name  The name is present to make it easier to refer to and discuss
      the registration entry.  The value is not used in the protocol.
      Names are to be unique in the table.

   label  This is the value used for the label.  The label can be either
      an integer or a string.  Registration in the table is based on the
      value of the label requested.  Integer values between 1 and 255
      and strings of length 1 are designated as Standards Track Document
      required.  Integer values from 256 to 65535 and strings of length
      2 are designated as Specification Required.  Integer values of
      greater than 65535 and strings of length greater than 2 are
      designated as first come, first served.  Integer values in the
      range -1 to -65536 are delegated to the "COSE Header Algorithm
      Label" registry.  Integer values beyond -65536 are marked as
      private use.

   value  This contains the CBOR type for the value portion of the
      label.

   value registry  This contains a pointer to the registry used to
      contain values where the set is limited.

   description  This contains a brief description of the header field.

   specification  This contains a pointer to the specification defining
      the header field (where public).

   The initial contents of the registry can be found in Table 2.  The
   specification column for all rows in that table should be this
   document.

   Additionally, the label of 0 is to be marked as 'Reserved'.

16.3.  COSE Header Algorithm Label Table

   It is requested that IANA create a new registry entitled "COSE Header
   Algorithm Labels".  The registry is to be created as Expert Review
   Required.  Expert review guidelines are provided in Section 16.10

   The columns of the registry are:



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   name  The name is present to make it easier to refer to and discuss
      the registration entry.  The value is not used in the protocol.

   algorithm  The algorithm(s) that this registry entry is used for.
      This value is taken from the "COSE Algorithm Value" registry.
      Multiple algorithms can be specified in this entry.  For the
      table, the algorithm, label pair MUST be unique.

   label  This is the value used for the label.  The label is an integer
      in the range of -1 to -65536.

   value  This contains the CBOR type for the value portion of the
      label.

   value registry  This contains a pointer to the registry used to
      contain values where the set is limited.

   description  This contains a brief description of the header field.

   specification  This contains a pointer to the specification defining
      the header field (where public).

   The initial contents of the registry can be found in Table 12,
   Table 13, and Table 18.  The specification column for all rows in
   that table should be this document.

16.4.  COSE Algorithm Registry

   It is requested that IANA create a new registry entitled "COSE
   Algorithm Registry".  The registry is to be created as Expert Review
   Required.  Expert review guidelines are provided in Section 16.10

   The columns of the registry are:

   value  The value to be used to identify this algorithm.  Algorithm
      values MUST be unique.  The value can be a positive integer, a
      negative integer or a string.  Integer values between -256 and 255
      and strings of length 1 are designated as Standards Track Document
      required.  Integer values from -65536 to 65535 and strings of
      length 2 are designated as Specification Required.  Integer values
      of greater than 65535 and strings of length greater than 2 are
      designated as first come, first served.  Integer values beyond
      -65536 are marked as private use.

   description  A short description of the algorithm.

   specification  A document where the algorithm is defined (if publicly
      available).



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   The initial contents of the registry can be found in Table 9,
   Table 8, Table 10, Table 5, Table 6, Table 7, Table 14, Table 15,
   Table 16, and Table 17.  The specification column for all rows in
   that table should be this document.

   NOTE: The assignment of algorithm identifiers in this document was
   done so that positive numbers were used for the first level objects
   (COSE_Sign, COSE_Sign1, COSE_Enveloped, COSE_Encrypted, COSE_Mac and
   COSE_Mac0).  Negative numbers were used for second level objects
   (COSE_Signature and COSE_recipient).  Expert reviewers should
   consider this practice, but are not expected to be restricted by this
   precedent.

16.5.  COSE Key Common Parameter Registry

   It is requested that IANA create a new registry entitled "COSE Key
   Common Parameter" Registry.  The registry is to be created as Expert
   Review Required.  Expert review guidelines are provided in
   Section 16.10

   The columns of the registry are:

   name  This is a descriptive name that enables easier reference to the
      item.  It is not used in the encoding.

   label  The value to be used to identify this algorithm.  Key map
      labels MUST be unique.  The label can be a positive integer, a
      negative integer or a string.  Integer values between 0 and 255
      and strings of length 1 are designated as Standards Track Document
      required.  Integer values from 256 to 65535 and strings of length
      2 are designated as Specification Required.  Integer values of
      greater than 65535 and strings of length greater than 2 are
      designated as first come, first served.  Integer values in the
      range -1 to -65536 are used for key parameters specific to a
      single algorithm delegated to the "COSE Key Type Parameter Label"
      registry.  Integer values beyond -65536 are marked as private use.

   CBOR Type  This field contains the CBOR type for the field

   registry  This field denotes the registry that values come from, if
      one exists.

   description  This field contains a brief description for the field

   specification  This contains a pointer to the public specification
      for the field if one exists





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   This registry will be initially populated by the values in
   Section 7.1.  The specification column for all of these entries will
   be this document.

16.6.  COSE Key Type Parameter Registry

   It is requested that IANA create a new registry "COSE Key Type
   Parameters".  The registry is to be created as Expert Review
   Required.  Expert review guidelines are provided in Section 16.10

   The columns of the table are:

   key type  This field contains a descriptive string of a key type.
      This should be a value that is in the COSE General Values table
      and is placed in the 'kty' field of a COSE Key structure.

   name  This is a descriptive name that enables easier reference to the
      item.  It is not used in the encoding.

   label  The label is to be unique for every value of key type.  The
      range of values is from -256 to -1.  Labels are expected to be
      reused for different keys.

   CBOR type  This field contains the CBOR type for the field

   description  This field contains a brief description for the field

   specification  This contains a pointer to the public specification
      for the field if one exists

   This registry will be initially populated by the values in Table 21
   and Table 22.  The specification column for all of these entries will
   be this document.

16.7.  COSE Elliptic Curve Registry

   It is requested that IANA create a new registry "COSE Elliptic Curve
   Parameters".  The registry is to be created as Expert Review
   Required.  Expert review guidelines are provided in Section 16.10

   The columns of the table are:

   name  This is a descriptive name that enables easier reference to the
      item.  It is not used in the encoding.

   value  This is the value used to identify the curve.  These values
      MUST be unique.  The integer values from -256 to 255 are
      designated as Standards Track Document Required.  The integer



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      values from 256 to 65535 and -65536 to -257 are designated as
      Specification Required.  Integer values over 65535 are designated
      as first come, first served.  Integer values less than -65536 are
      marked as private use.

   key type  This designates the key type(s) that can be used with this
      curve.

   description  This field contains a brief description of the curve.

   specification  This contains a pointer to the public specification
      for the curve if one exists.

   This registry will be initially populated by the values in Table 19.
   The specification column for all of these entries will be this
   document.

16.8.  Media Type Registrations

16.8.1.  COSE Security Message

   This section registers the "application/cose" media type in the
   "Media Types" registry.  These media types are used to indicate that
   the content is a COSE_MSG.

      Type name: application

      Subtype name: cose

      Required parameters: N/A

      Optional parameters: cose-type

      Encoding considerations: binary

      Security considerations: See the Security Considerations section
      of RFC TBD.

      Interoperability considerations: N/A

      Published specification: RFC TBD

      Applications that use this media type: To be identified

      Fragment identifier considerations: N/A

      Additional information:




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      *  Magic number(s): N/A

      *  File extension(s): cbor

      *  Macintosh file type code(s): N/A

      Person & email address to contact for further information:
      iesg@ietf.org

      Intended usage: COMMON

      Restrictions on usage: N/A

      Author: Jim Schaad, ietf@augustcellars.com

      Change Controller: IESG

      Provisional registration?  No

16.8.2.  COSE Key media type

   This section registers the "application/cose-key+cbor" and
   "application/cose-key-set+cbor" media types in the "Media Types"
   registry.  These media types are used to indicate, respectively, that
   content is a COSE_Key or COSE_KeySet object.

      Type name: application

      Subtype name: cose-key+cbor

      Required parameters: N/A

      Optional parameters: N/A

      Encoding considerations: binary

      Security considerations: See the Security Considerations section
      of RFC TBD.

      Interoperability considerations: N/A

      Published specification: RFC TBD

      Applications that use this media type: To be identified

      Fragment identifier considerations: N/A

      Additional information:



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      *  Magic number(s): N/A

      *  File extension(s): cbor

      *  Macintosh file type code(s): N/A

      Person & email address to contact for further information:
      iesg@ietf.org

      Intended usage: COMMON

      Restrictions on usage: N/A

      Author: Jim Schaad, ietf@augustcellars.com

      Change Controller: IESG

      Provisional registration?  No

      Type name: application

      Subtype name: cose-key-set+cbor

      Required parameters: N/A

      Optional parameters: N/A

      Encoding considerations: binary

      Security considerations: See the Security Considerations section
      of RFC TBD.

      Interoperability considerations: N/A

      Published specification: RFC TBD

      Applications that use this media type: To be identified

      Fragment identifier considerations: N/A

      Additional information:

      *  Magic number(s): N/A

      *  File extension(s): cbor

      *  Macintosh file type code(s): N/A




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      Person & email address to contact for further information:
      iesg@ietf.org

      Intended usage: COMMON

      Restrictions on usage: N/A

      Author: Jim Schaad, ietf@augustcellars.com

      Change Controller: IESG

      Provisional registration?  No

16.9.  CoAP Content Format Registrations

   This section registers a set of content formats for CoAP.  ID
   assignment in the 24-255 range is requested.

   +----------------------------------+----------+-------+-------------+
   | Media Type                       | Encoding | ID    | Reference   |
   +----------------------------------+----------+-------+-------------+
   | application/cose; cose-type      |          | TBD10 | [This       |
   | ="cose-sign"                     |          |       | Document]   |
   |                                  |          |       |             |
   | application/cose; cose-type      |          | TBD11 | [This       |
   | ="cose-sign1"                    |          |       | Document]   |
   |                                  |          |       |             |
   | application/cose; cose-type      |          | TBD12 | [This       |
   | ="cose-enveloped"                |          |       | Document]   |
   |                                  |          |       |             |
   | application/cose; cose-type      |          | TBD13 | [This       |
   | ="cose-encrypted"                |          |       | Document]   |
   |                                  |          |       |             |
   | application/cose; cose-type      |          | TBD14 | [This       |
   | ="cose-mac"                      |          |       | Document]   |
   |                                  |          |       |             |
   | application/cose; cose-type      |          | TBD15 | [This       |
   | ="cose-mac0"                     |          |       | Document]   |
   |                                  |          |       |             |
   | application/cose-key             |          | TBD16 | [This       |
   |                                  |          |       | Document]   |
   |                                  |          |       |             |
   | application/cose-key-set         |          | TBD17 | [This       |
   |                                  |          |       | Document    |
   +----------------------------------+----------+-------+-------------+

                                 Table 23




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16.10.  Expert Review Instructions

   All of the IANA registries established in this document are defined
   as expert review.  This section gives some general guidelines for
   what the experts should be looking for, but they are being designated
   as experts for a reason so they should be given substantial latitude.

   Expert reviewers should take into consideration the following points:

   o  Point squatting should be discouraged.  Reviewers are encouraged
      to get sufficient information for registration requests to ensure
      that the usage is not going to duplicate one that is already
      registered and that the point is likely to be used in deployments.
      The zones tagged as private use are intended for testing purposes
      and closed environments, code points in other ranges should not be
      assigned for testing.

   o  Specifications are required for the standards track range of point
      assignment.  Specifications should exist for specification
      required ranges, but early assignment before a specification is
      available is considered to be permissible.  Specifications are
      needed for the first-come, first-serve range if they are expected
      to be used outside of closed environments in an inoperable way.
      When specifications are not provided, the description provided
      needs to have sufficient information to identify what point is
      being used for.

   o  Experts should take into account the expected usage of fields when
      approving point assignment.  The fact that there is a range for
      standards track documents does not mean that a standards track
      document cannot have points assigned outside of that range.  Some
      of the ranges are restricted in range, items which are not
      expected to be common or are not expected to be used in restricted
      environments should be assigned to values which will encode to
      longer byte strings.

   o  When algorithms are registered, vanity registrations should be
      discouraged.  One way to do this is to require applications to
      provide additional documentation on security analysis of
      algorithms.  Another thing that should be considered is to request
      for an opinion on the algorithm from the Cryptographic Forum
      Research Group.  Algorithms which do not meet the security
      requirements of the community and the messages structures should
      not be registered.







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17.  Security Considerations

   There are a number of security considerations that need to be taken
   into account by implementers of this specification.  The security
   considerations that are specific to an individual algorithm are
   placed next to the description of the algorithm.  While some
   considerations have been highlighted here, additional considerations
   may be found in the documents listed in the references.

   Implementations need to protect the private key for any individuals.
   There are some cases in this document that need to be highlighted on
   this issue.

   o  Using the same key for two different algorithms can leak
      information about the key.  It is therefore recommended that keys
      be restricted to a single algorithm.

   o  Use of 'direct' as a recipient algorithm combined with a second
      recipient algorithm, either directly in a separate message,
      exposes the direct key to the second recipient.

   o  Several of the algorithms in this document have limits on the
      number of times that a key can be used without leaking information
      about the key.

   The use of ECDH and direct plus KDF (with no key wrap) will not
   directly lead to the private key being leaked, the one way function
   of the KDF will prevent that.  There is however a different issue
   that needs to be addressed.  Having two recipients, requires that the
   CEK be shared between two recipients.  The second recipient therefore
   has a CEK that was derived from material that can be used for the
   weak proof of origin.  The second recipient could create a message
   using the same CEK and send it to the first recipient, the first
   recipient would, for either static-static ECDH or direct plus KDF,
   make an assumption that the CEK could be used for proof of origin
   even though it is from the wrong entity.  If the key wrap step is
   added, then no proof of origin is implied and thus is not an issue.

   Although it has been mentioned before, the use of a single key for
   multiple algorithms has been demonstrated in some cases to leak
   information about a key, provide for attackers to forge integrity
   tags, or gain information about encrypted content.  Binding a key to
   a single algorithm prevents these problems.  Key creators and key
   consumers are strongly encourged not only to create new keys for each
   different algorithm, but to include that selection of algorithm in
   any distribution of key material and strictly enforce the matching of
   algorithms in the key structure to algorithms in the message
   structure.  In addition to checking that algorithms are correct, the



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   key form needs to be checked as well.  Do not use an 'EC2' key where
   an 'oct' key is expected.

   Before using a key for transmission, or before acting on information
   recieved, a trust decision on a key needs to be made.  Is the data or
   action something that the entity associated with the key has a right
   to see or a right to request.  A number of factors are associated
   with this trust decision.  Some of the ones that are highlighted here
   are:

   o  What are the permissions associated with the key owner?

   o  Is the cryptographic algorithm acceptable in the current context?

   o  Have the restrictions associated with the key, such as algorithm
      or freshness, been checked and are correct?

   o  Is the request something that is reasonable given the current
      state of the application?

   o  Have any security considerations that are part of the message been
      enforced?  (As specified by the application or crit parameter.)

   One area that has been starting to get exposure is doing traffic
   analysis of encrypted messages based on the length of the message.
   This specification does not provide for a uniform method of providing
   padding as part of the message structure.  An observer can
   distinguish between two different strings (for example 'YES' and
   'NO') based on length for all of the content encryption algorithms
   that are defined in this document.  This means that it is up to
   applications to document how content padding is to be done in order
   to prevent or discourage such analysis.  (For example the strings
   could be defined as 'YES' and 'NO '.)

18.  Acknowledgments

   This document is a product of the COSE working group of the IETF.

19.  References

19.1.  Normative References

   [AES-GCM]  Dworkin, M., "NIST Special Publication 800-38D:
              Recommendation for Block Cipher Modes of Operation:
              Galois/Counter Mode (GCM) and GMAC.", Nov 2007.

   [DSS]      U.S. National Institute of Standards and Technology,
              "Digital Signature Standard (DSS)", July 2013.



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   [MAC]      NiST, N., "FIPS PUB 113: Computer Data Authentication",
              May 1985.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <http://www.rfc-editor.org/info/rfc2104>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3394]  Schaad, J. and R. Housley, "Advanced Encryption Standard
              (AES) Key Wrap Algorithm", RFC 3394, DOI 10.17487/RFC3394,
              September 2002, <http://www.rfc-editor.org/info/rfc3394>.

   [RFC3610]  Whiting, D., Housley, R., and N. Ferguson, "Counter with
              CBC-MAC (CCM)", RFC 3610, DOI 10.17487/RFC3610, September
              2003, <http://www.rfc-editor.org/info/rfc3610>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <http://www.rfc-editor.org/info/rfc5869>.

   [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
              Curve Cryptography Algorithms", RFC 6090,
              DOI 10.17487/RFC6090, February 2011,
              <http://www.rfc-editor.org/info/rfc6090>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <http://www.rfc-editor.org/info/rfc7049>.

   [RFC7539]  Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
              Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015,
              <http://www.rfc-editor.org/info/rfc7539>.

   [SEC1]     Standards for Efficient Cryptography Group, "SEC 1:
              Elliptic Curve Cryptography", May 2009.

19.2.  Informative References








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   [I-D.greevenbosch-appsawg-cbor-cddl]
              Vigano, C. and H. Birkholz, "CBOR data definition language
              (CDDL): a notational convention to express CBOR data
              structures", draft-greevenbosch-appsawg-cbor-cddl-07 (work
              in progress), October 2015.

   [PVSig]    Brown, D. and D. Johnson, "Formal Security Proofs for a
              Signature Scheme with Partial Message Recover", February
              2000.

   [RFC2633]  Ramsdell, B., Ed., "S/MIME Version 3 Message
              Specification", RFC 2633, DOI 10.17487/RFC2633, June 1999,
              <http://www.rfc-editor.org/info/rfc2633>.

   [RFC2898]  Kaliski, B., "PKCS #5: Password-Based Cryptography
              Specification Version 2.0", RFC 2898,
              DOI 10.17487/RFC2898, September 2000,
              <http://www.rfc-editor.org/info/rfc2898>.

   [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February
              2003, <http://www.rfc-editor.org/info/rfc3447>.

   [RFC4231]  Nystrom, M., "Identifiers and Test Vectors for HMAC-SHA-
              224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512",
              RFC 4231, DOI 10.17487/RFC4231, December 2005,
              <http://www.rfc-editor.org/info/rfc4231>.

   [RFC4262]  Santesson, S., "X.509 Certificate Extension for Secure/
              Multipurpose Internet Mail Extensions (S/MIME)
              Capabilities", RFC 4262, DOI 10.17487/RFC4262, December
              2005, <http://www.rfc-editor.org/info/rfc4262>.

   [RFC4493]  Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
              AES-CMAC Algorithm", RFC 4493, DOI 10.17487/RFC4493, June
              2006, <http://www.rfc-editor.org/info/rfc4493>.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <http://www.rfc-editor.org/info/rfc4949>.

   [RFC5480]  Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
              "Elliptic Curve Cryptography Subject Public Key
              Information", RFC 5480, DOI 10.17487/RFC5480, March 2009,
              <http://www.rfc-editor.org/info/rfc5480>.





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   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <http://www.rfc-editor.org/info/rfc5652>.

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, DOI 10.17487/RFC5751, January
              2010, <http://www.rfc-editor.org/info/rfc5751>.

   [RFC5752]  Turner, S. and J. Schaad, "Multiple Signatures in
              Cryptographic Message Syntax (CMS)", RFC 5752,
              DOI 10.17487/RFC5752, January 2010,
              <http://www.rfc-editor.org/info/rfc5752>.

   [RFC5990]  Randall, J., Kaliski, B., Brainard, J., and S. Turner,
              "Use of the RSA-KEM Key Transport Algorithm in the
              Cryptographic Message Syntax (CMS)", RFC 5990,
              DOI 10.17487/RFC5990, September 2010,
              <http://www.rfc-editor.org/info/rfc5990>.

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, DOI 10.17487/RFC6151, March 2011,
              <http://www.rfc-editor.org/info/rfc6151>.

   [RFC6979]  Pornin, T., "Deterministic Usage of the Digital Signature
              Algorithm (DSA) and Elliptic Curve Digital Signature
              Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
              2013, <http://www.rfc-editor.org/info/rfc6979>.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <http://www.rfc-editor.org/info/rfc7159>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <http://www.rfc-editor.org/info/rfc7252>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <http://www.rfc-editor.org/info/rfc7515>.

   [RFC7516]  Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
              RFC 7516, DOI 10.17487/RFC7516, May 2015,
              <http://www.rfc-editor.org/info/rfc7516>.





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   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517,
              DOI 10.17487/RFC7517, May 2015,
              <http://www.rfc-editor.org/info/rfc7517>.

   [RFC7518]  Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
              DOI 10.17487/RFC7518, May 2015,
              <http://www.rfc-editor.org/info/rfc7518>.

   [SP800-56A]
              Barker, E., Chen, L., Roginsky, A., and M. Smid, "NIST
              Special Publication 800-56A: Recommendation for Pair-Wise
              Key Establishment Schemes Using Discrete Logarithm
              Cryptography", May 2013.

Appendix A.  Making Mandatory Algorithm Header Optional

   There has been a minority of the working group who have expressed a
   strong desire to relax the rule that the algorithm identifier be
   required to appear in each level of a COSE mesage.  There are two
   basic reasons that have been advanced to support this position.
   First, the resulting message will be smaller if the algorithm
   identifier is omitted from the most common messages in a CoAP
   environment.  Second, there is a potential bug that will arise if
   full checking is not done correctly between the different places that
   an algorithm identifier could be placed.  (The message itself, an
   application statement, the key structure that the sender possesses
   and the key structure the recipient possesses.)

   This appendix lays out how such a change can be made and the details
   that an application needs to specify in order to use this option.
   Two different sets of details are specified: Those needed to omit an
   algorithm identifier and those needed to use a variant on the counter
   signature attribute which contains no attributes about itself.

A.1.  Algorithm Identification

   In this section are laid out three sets of recommendations.  The
   first set of recommendations apply to having an implicit algorithm
   identified for a single layer of a COSE message.  The second set of
   recommendations apply to having multiple implicit algorithm
   identified for multiple layers of a COSE message.  The third set of
   recommendations apply to having implicit algorithms for multiple COSE
   message constructs.

   RFC 2119 language is deliberately not used here, this specification
   can provide recommendations, but it cannot enforce them.





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   This set of recommendations applies to the case where an application
   is distributing a fixed algorithm along with the key information for
   use in a single COSE message object.  This normally applies to the
   smallest of the COSE messages, specifically COSE_Sign1, COSE_Mac0 and
   COSE_Encrypted, but could apply to the other structures as well.

   The following items should be taken into account:

   o  Applications need to list the set of COSE structures that implicit
      algorithms are to be used in.  Applications need to require that
      the receipt of an explicit algorithm identifier in one of these
      structures will lead to the message being rejected.  This
      requirement is stated so that there will never be a case where
      there is any ambiguity about the question of which algorithm
      should be used, the implicit or the explicit one.  This applies
      even if the transported algorithm is a protected attribute.  This
      applies even if the transported algorithm is the same as the
      implicit algorithm.

   o  Applications need to define the set of information that is to be
      considered to be part of a context when omitting algorithm
      identifiers.  At a minimum this would be the key identifier, the
      key, the algorithm and the COSE structures it can be used for.
      Applications should restrict the use of a single key to a single
      algorithm.  As noted for some of the algorithms in this document,
      the use of the same key in different related algorithms can lead
      to leakage of information about the key, leakage about the data or
      the ability to perform forgeries.

   o  In many cases applications which make the algorithm identifier
      will also want to make the context identifier implicit for the
      same reason.  That is omitting the context identifier will
      decrease the message size (potentially significantly depending on
      the length of the identifier).  Applications that do this will
      need to describe the circumstances where the context identifier is
      to be omitted and how the context identifier is to be inferred in
      these cases.  (Exhaustive search would normally not be considered
      to be acceptable.)  An example of how this can be done is to tie
      the context to a transaction identifier.  Both would be sent on
      the original message, but only the transaction identifier would
      need to be sent after that point as the context is tied into the
      transaction identifier.  Another way would be to associate a
      context with a network address.  All messages coming from a single
      network address can be assumed to be associated with a specific
      context.  (In this case the address would probably be distributed
      as part of the context.)





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   o  Applications cannot rely on key identifiers being unique unless
      they take significant efforts to ensure that they are computed in
      such a way as to create this guarantee.  Even when an application
      does this, the uniqueness might be violated if the application is
      run in different contexts (i.e. with a different security
      coordinator) or if the system the application runs on combines
      security contexts from different applications together into a
      single store.

   o  Applications should continue the practice of protecting the
      algorithm identifier.  Since this is not done by placing it in the
      protected attributes field, applications should define an
      application specific external data structure which includes this
      value.  This external data field can be used as such for content
      encryption, MAC and signature algorithms.  It can be used in the
      SuppPrivInfo field for those algorithms which use a KDF function
      to derive a key value.  Applications may also want to protect
      other information that is part of the context structure as well.
      It should be noted that those fields, such as the key or a base IV
      are already protected by virtue of being used in the cryptogrpahic
      computation and do not need to be included in the external data
      field.

   The second case is having multiple implicit algorithm identifiers
   specified for a multiple layer COSE message.  An example of how this
   would work is that the encryption context that an application
   specifies contains a content encryption algorithm, a key wrap
   algorithm, a key identifier, and a shared secret.  The sender would
   then omit sending the algorithm identifier at both the content layer
   and the recipient layer leaving only the key identifier in situations
   where it could not be implied.

   The following additional items need to be taken into consideration:

   o  Applications that want to support this will need to define a
      structure that allows for, and clearly identifies, both the COSE
      structure to be used with a given key and the structure and
      algorithm to be used for the secondary layer.  The key for the
      secondary layer is computed normally in the recipient layer.

   o

   The third case is having multiple implicit algorithm identifiers, but
   targeted at potentially unrelated layers or different COSE messages.
   There are a number of different scenarios where this might be
   applicable.  Some of these scenarios are:





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   o  Two contexts are distributed as a pair.  Each of the contexts is
      for use with a COSE_Encrypt message.  Each context will consist of
      distinct secret keys and IVs and potentially even different
      algorithms.  One context is for sending messages from party A to
      party B, the second context is for sending messages from party B
      to party A.  This means that there is no chance for a reflection
      attack to occur as each party uses different secret keys to send
      its messages, a message that is reflected back to it would fail to
      decrypt.

   o  Two contexts are distributed as a pair.  The first context is used
      for encryption of the message, the second context is used to place
      a counter signature on the message.  The intention is that the
      second context can be distributed to other entities independently
      of the first context.  This allows these entities to validate that
      the message came from an individual without being able to decrypt
      the message and see the content.

   o  Two contexts are distributed as a pair.  The first context
      contains a key for dealing with MAC messages, the second context
      contains a key for dealing with encrypted messages.  This allows
      for a unified distribution of keys to participants for different
      types of messages which have different keys, but where the keys
      may be used in coordinated manner.

   For these cases, the following items need to be considered:

   o  Applications need to ...

A.2.  Counter Signature Without Headers

   TBD

   o  No parameter for counter sig

   o  Define to be signed structure

   o  id how key is decided

   o  external data struture includes alg id

   o  bind key in distribution

   o  single alg of key structure

   o  uniques of kid not real

   o  very specialized for small size



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   o  kid can be either implied OR show as kid of what is counter signed

Appendix B.  Three Levels of Recipient Information

   All of the currently defined recipient algorithms classes only use
   two levels of the COSE_Enveloped structure.  The first level is the
   message content and the second level is the content key encryption.
   However, if one uses a recipient algorithm such as RSA-KEM (see
   Appendix A of RSA-KEM [RFC5990], then it make sense to have three
   levels of the COSE_Enveloped structure.

   These levels would be:

   o  Level 0: The content encryption level.  This level contains the
      payload of the message.

   o  Level 1: The encryption of the CEK by a KEK.

   o  Level 2: The encryption of a long random secret using an RSA key
      and a key derivation function to convert that secret into the KEK.

   This is an example of what a triple layer message would look like.
   The message has the following layers:

   o  Level 0: Has a content encrypted with AES-GCM using a 128-bit key.

   o  Level 1: Uses the AES Key wrap algorithm with a 128-bit key.

   o  Level 2: Uses ECDH Ephemeral-Static direct to generate the level 1
      key.

   In effect this example is a decomposed version of using the ECDH-
   ES+A128KW algorithm.

   Size of binary file is 184 bytes
















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   992(
     [
       / protected / h'a10101' / {
           \ alg \ 1:1 \ AES-GCM 128 \
         } / ,
       / unprotected / {
         / iv / 5:h'02d1f7e6f26c43d4868d87ce'
       },
       / ciphertext / h'64f84d913ba60a76070a9a48f26e97e863e28529bf9be9d
   e3bea1788f681200d875242f6',
       / recipients / [
         [
           / protected / h'',
           / unprotected / {
             / alg / 1:-3 / A128KW /
           },
           / ciphertext / h'f4b117264ab6d4d1476e0204bb15db58c5834461e83
   5e884',
           / recipients / [
             [
               / protected / h'a1013818' / {
                   \ alg \ 1:-25 \ ECDH-ES + HKDF-256 \
                 } / ,
               / unprotected / {
                 / ephemeral / -1:{
                   / kty / 1:2,
                   / crv / -1:1,
                   / x / -2:h'b2add44368ea6d641f9ca9af308b4079aeb519f11
   e9b8a55a600b21233e86e68',
                   / y / -3:false
                 },
                 / kid / 4:'meriadoc.brandybuck@buckland.example'
               },
               / ciphertext / h''
             ]
           ]
         ]
       ]
     ]
   )

Appendix C.  Examples

   This appendix includes a set of examples that show the different
   features and message types that have been defined in this document.
   To make the examples easier to read, they are presented using the
   extended CBOR diagnostic notation (defined in
   [I-D.greevenbosch-appsawg-cbor-cddl]) rather than as a binary dump.



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   A GITHUB project has been created at https://github.com/cose-wg/
   Examples that contains not only the examples presented in this
   document, but a more complete set of testing examples as well.  Each
   example is found in a JSON file that contains the inputs used to
   create the example, some of the intermediate values that can be used
   in debugging the example and the output of the example presented in
   both a hex and a CBOR diagnostic notation format.  Some of the
   examples at the site are designed failure testing cases, these are
   clearly marked as such in the JSON file.  If errors in the examples
   in this document are found, the examples on github will be updated
   and a note to that effect will be placed in the JSON file.

   As noted, the examples are presented using the CBOR's diagnostic
   notation.  A ruby based tool exists that can convert between the
   diagnostic notation and binary.  This tool can be installed with the
   command line:

   gem install cbor-diag

   The diagnostic notation can be converted into binary files using the
   following command line:

   diag2cbor < inputfile > outputfile

   The examples can be extracted from the XML version of this document
   via an XPath expression as all of the artwork is tagged with the
   attribute type='CBORdiag'.  (Depending on the XPath evaluator one is
   using, it may be necessary to deal with &gt; as an entity.)

   //artwork[@type='CDDL']/text()

C.1.  Examples of Signed Message

C.1.1.  Single Signature

   This example uses the following:

   o  Signature Algorithm: ECDSA w/ SHA-256, Curve P-256-1

   Size of binary file is 104 bytes











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   991(
     [
       / protected / h'',
       / unprotected / {},
       / payload / 'This is the content.',
       / signatures / [
         [
           / protected / h'a10126' / {
               \ alg \ 1:-7 \ ECDSA 256 \
             } / ,
           / unprotected / {
             / kid / 4:'11'
           },
           / signature / h'eae868ecc176883766c5dc5ba5b8dca25dab3c2e56a5
   51ce5705b793914348e14eea4aee6e0c9f09db4ef3ddeca8f3506cd1a98a8fb64327
   be470355c9657ce0'
         ]
       ]
     ]
   )

C.1.2.  Multiple Signers

   This example uses the following:

   o  Signature Algorithm: ECDSA w/ SHA-256, Curve P-256-1

   o  Signature Algorithm: ECDSA w/ SHA-512, Curve P-521

   Size of binary file is 278 bytes





















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   991(
     [
       / protected / h'',
       / unprotected / {},
       / payload / 'This is the content.',
       / signatures / [
         [
           / protected / h'a10126' / {
               \ alg \ 1:-7 \ ECDSA 256 \
             } / ,
           / unprotected / {
             / kid / 4:'11'
           },
           / signature / h'0dc1c5e62719d8f3cce1468b7c881eee6a8088b46bf8
   36ae956dd38fe93199199951a6a5e02a24aed5edde3509748366b1c539aaef7dea34
   f2cd618fe19fe55d'
         ],
         [
           / protected / h'a1013823' / {
               \ alg \ 1:-36
             } / ,
           / unprotected / {
             / kid / 4:'bilbo.baggins@hobbiton.example'
           },
           / signature / h'012ce5b1dfe8b5aa6eaa09a54c58a84ad0900e4fdf27
   59ec22d1c861cccd75c7e1c4025a2da35e512fc2874d6ac8fd862d09ad07ed2deac2
   97b897561e04a8d42476017c11a4a34e26c570c9eff22c1dc84d56cdf6e03ed34bc9
   e934c5fdf676c7948d79e97dfe161730217c57748aadb364a0207cee811e9dde65ae
   37942e8a8348cc91'
         ]
       ]
     ]
   )

C.1.3.  Counter Signature

   This example uses the following:

   o  Signature Algorithm: ECDSA w/ SHA-256, Curve P-256-1

   o  The same parameters are used for both the signature and the
      counter signature.

   Size of binary file is 181 bytes







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   991(
     [
       / protected / h'',
       / unprotected / {
         / countersign / 7:[
           / protected / h'a10126' / {
               \ alg \ 1:-7 \ ECDSA 256 \
             } / ,
           / unprotected / {
             / kid / 4:'11'
           },
           / signature / h'c9d3402485aa585cee3efc69b14496c0b00714584b26
   0f8e05764b7dbc70ae2be52a463555fc78e8da59bf8b3af281e739741dbac0b6f56a
   4b03ef23cb93b1e1'
         ]
       },
       / payload / 'This is the content.',
       / signatures / [
         [
           / protected / h'a10126' / {
               \ alg \ 1:-7 \ ECDSA 256 \
             } / ,
           / unprotected / {
             / kid / 4:'11'
           },
           / signature / h'eae868ecc176883766c5dc5ba5b8dca25dab3c2e56a5
   51ce5705b793914348e14eea4aee6e0c9f09db4ef3ddeca8f3506cd1a98a8fb64327
   be470355c9657ce0'
         ]
       ]
     ]
   )

C.1.4.  Signature w/ Operation Time and Criticality

   This example uses the following:

   o  Signature Algorithm: ECDSA w/ SHA-256, Curve P-256-1

   o  There is an operation time of 2014-02-14T12:00Z

   o  There is a criticality marker on the "reserved" header parameter

   Size of binary file is 132 bytes







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   991(
     [
       / protected / h'a2687265736572766564f40281687265736572766564' /
   {
           "reserved":false,
           \ crit \ 2:[
             "reserved"
           ]
         } / ,
       / unprotected / {},
       / payload / 'This is the content.',
       / signatures / [
         [
           / protected / h'a20126081a56bffbc0' / {
               \ alg \ 1:-7 \ ECDSA 256 \,
               8:1455422400
             } / ,
           / unprotected / {
             / kid / 4:'11'
           },
           / signature / h'eae868ecc176883766c5dc5ba5b8dca25dab3c2e56a5
   51ce5705b793914348e150d023101a60dddbf0c11f6cdaf5708e12925c67dbb5d1db
   d16b2474483e367b'
         ]
       ]
     ]
   )

C.2.  Single Signer Examples

C.2.1.  Single ECDSA signature

   This example uses the following:

   o  Signature Algorithm: ECDSA w/ SHA-256, Curve P-256-1

   Size of binary file is 100 bytes














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   997(
     [
       / protected / h'a10126' / {
           \ alg \ 1:-7 \ ECDSA 256 \
         } / ,
       / unprotected / {
         / kid / 4:'11'
       },
       / payload / 'This is the content.',
       h'eae868ecc176883766c5dc5ba5b8dca25dab3c2e56a551ce5705b793914348
   e19f43d6c6ba654472da301b645b293c9ba939295b97c4bdb847782bff384c5794'
     ]
   )

C.3.  Examples of Enveloped Messages

C.3.1.  Direct ECDH

   This example uses the following:

   o  CEK: AES-GCM w/ 128-bit key

   o  Recipient class: ECDH Ephemeral-Static, Curve P-256

   Size of binary file is 152 bytes


























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   992(
     [
       / protected / h'a10101' / {
           \ alg \ 1:1 \ AES-GCM 128 \
         } / ,
       / unprotected / {
         / iv / 5:h'c9cf4df2fe6c632bf7886413'
       },
       / ciphertext / h'40970cd7ab5fbd10f505bf7a86e6fc0a99a31224b3b5895
   c9fc7892ba138233e0e65af84',
       / recipients / [
         [
           / protected / h'a1013818' / {
               \ alg \ 1:-25 \ ECDH-ES + HKDF-256 \
             } / ,
           / unprotected / {
             / ephemeral / -1:{
               / kty / 1:2,
               / crv / -1:1,
               / x / -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbf
   bf054e1c7b4d91d6280',
               / y / -3:true
             },
             / kid / 4:'meriadoc.brandybuck@buckland.example'
           },
           / ciphertext / h''
         ]
       ]
     ]
   )

C.3.2.  Direct plus Key Derivation

   This example uses the following:

   o  CEK: AES-CCM w/128-bit key, truncate the tag to 64 bits

   o  Recipient class: Use HKDF on a shared secret with the following
      implicit fields as part of the context.

      *  salt: "aabbccddeeffgghh"

      *  APU identity: "lighting-client"

      *  APV identity: "lighting-server"

      *  Supplementary Public Other: "Encryption Example 02"




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   Size of binary file is 92 bytes

   992(
     [
       / protected / h'a1010a' / {
           \ alg \ 1:10 \ AES-CCM-16-64-128 \
         } / ,
       / unprotected / {
         / iv / 5:h'89f52f65a1c580933b5261a76c'
       },
       / ciphertext / h'89bedc91e9909346a8fe87834445679ee12b2c953cbb685
   25aa7675f',
       / recipients / [
         [
           / protected / h'a10129' / {
               \ alg \ 1:-10
             } / ,
           / unprotected / {
             / salt / -20:'aabbccddeeffgghh',
             / kid / 4:'our-secret'
           },
           / ciphertext / h''
         ]
       ]
     ]
   )

C.3.3.  Counter Signature on Encrypted Content

   This example uses the following:

   o  CEK: AES-GCM w/ 128-bit key

   o  Recipient class: ECDH Ephemeral-Static, Curve P-256

   Size of binary file is 327 bytes















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   992(
     [
       / protected / h'a10101' / {
           \ alg \ 1:1 \ AES-GCM 128 \
         } / ,
       / unprotected / {
         / iv / 5:h'c9cf4df2fe6c632bf7886413',
         / countersign / 7:[
           / protected / h'a1013823' / {
               \ alg \ 1:-36
             } / ,
           / unprotected / {
             / kid / 4:'bilbo.baggins@hobbiton.example'
           },
           / signature / h'00aa98cbfd382610a375d046a275f30266e8d0faacb9
   069fde06e37825ae7825419c474f416ded0c8e3e7b55bff68f2a704135bdf99186f6
   6659461c8cf929cc7fb3013ac242342ddd8443c6292a1f8c78c5985aa7d86f34c0f1
   ba0b3dee5f4b59737b230da980886137da6f2ca79cc5c40ee89b771c71cdb1ee966e
   cfc7d4b2cdc1410a'
         ]
       },
       / ciphertext / h'40970cd7ab5fbd10f505bf7a86e6fc0a99a31224b3b5895
   c9fc7892ba138233e0e65af84',
       / recipients / [
         [
           / protected / h'a1013818' / {
               \ alg \ 1:-25 \ ECDH-ES + HKDF-256 \
             } / ,
           / unprotected / {
             / ephemeral / -1:{
               / kty / 1:2,
               / crv / -1:1,
               / x / -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbf
   bf054e1c7b4d91d6280',
               / y / -3:true
             },
             / kid / 4:'meriadoc.brandybuck@buckland.example'
           },
           / ciphertext / h''
         ]
       ]
     ]
   )








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C.3.4.  Encrypted Content with External Data

   This example uses the following:

   o  CEK: AES-GCM w/ 128-bit key

   o  Recipient class: ECDH static-Static, Curve P-256 with AES Key Wrap

   o  Externally Supplied AAD: h'0011bbcc22dd44ee55ff660077'

   Size of binary file is 174 bytes

   992(
     [
       / protected / h'a10101' / {
           \ alg \ 1:1 \ AES-GCM 128 \
         } / ,
       / unprotected / {
         / iv / 5:h'02d1f7e6f26c43d4868d87ce'
       },
       / ciphertext / h'64f84d913ba60a76070a9a48f26e97e863e2852951f6f24
   9e6c3616233a911748a80be95',
       / recipients / [
         [
           / protected / h'a101381f' / {
               \ alg \ 1:-32 \ ECHD-SS+A128KW \
             } / ,
           / unprotected / {
             / static kid / -3:'peregrin.took@tuckborough.example',
             / kid / 4:'meriadoc.brandybuck@buckland.example',
             / U nonce / -22:h'0101'
           },
           / ciphertext / h'59463342fd2193f30daeb1ebb2dc7310b56cee0939d
   d6692'
         ]
       ]
     ]
   )

C.4.  Examples of Encrypted Messages

C.4.1.  Simple Encrypted Message

   This example uses the following:

   o  CEK: AES-CCM w/ 128-bit key and a 64-bit tag

   Size of binary file is 54 bytes



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   993(
     [
       / protected / h'a1010a' / {
           \ alg \ 1:10 \ AES-CCM-16-64-128 \
         } / ,
       / unprotected / {
         / iv / 5:h'89f52f65a1c580933b5261a78c'
       },
       / ciphertext / h'5974e1b99a3a4cc09a659aa2e9e7fff161d38ce74693c90
   dcda22121'
     ]
   )

C.4.2.  Encrypted Message w/ a Partial IV

   This example uses the following:

   o  CEK: AES-CCM w/ 128-bit key and a 64-bit tag

   o  Prefix for IV is 89F52F65A1C580933B52

   Size of binary file is 43 bytes

   993(
     [
       / protected / h'a1010a' / {
           \ alg \ 1:10 \ AES-CCM-16-64-128 \
         } / ,
       / unprotected / {
         / partial iv / 6:h'61a7'
       },
       / ciphertext / h'252a8911d465c125b6764739700f0141ed09192d2e16ce9
   e579fea11'
     ]
   )

C.5.  Examples of MAC messages

C.5.1.  Shared Secret Direct MAC

   This example users the following:

   o  MAC: AES-CMAC, 256-bit key, truncated to 64 bits

   o  Recipient class: direct shared secret

   Size of binary file is 58 bytes




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   994(
     [
       / protected / h'a1010f' / {
           \ alg \ 1:15 \ AES-CBC-MAC-256//64 \
         } / ,
       / unprotected / {},
       / payload / 'This is the content.',
       / tag / h'9e1226ba1f81b848',
       / recipients / [
         [
           / protected / h'',
           / unprotected / {
             / alg / 1:-6 / direct /,
             / kid / 4:'our-secret'
           },
           / ciphertext / h''
         ]
       ]
     ]
   )

C.5.2.  ECDH Direct MAC

   This example uses the following:

   o  MAC: HMAC w/SHA-256, 256-bit key

   o  Recipient class: ECDH key agreement, two static keys, HKDF w/
      context structure

   Size of binary file is 215 bytes




















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   994(
     [
       / protected / h'a10105' / {
           \ alg \ 1:5 \ HMAC 256//256 \
         } / ,
       / unprotected / {},
       / payload / 'This is the content.',
       / tag / h'42cf68ae1253948c500dff27da3904342625a23e914f7aa545dcf6
   629519f18e',
       / recipients / [
         [
           / protected / h'a101381a' / {
               \ alg \ 1:-27 \ ECDH-SS + HKDF-256 \
             } / ,
           / unprotected / {
             / static kid / -3:'peregrin.took@tuckborough.example',
             / kid / 4:'meriadoc.brandybuck@buckland.example',
             / U nonce / -22:h'4d8553e7e74f3c6a3a9dd3ef286a8195cbf8a23d
   19558ccfec7d34b824f42d92bd06bd2c7f0271f0214e141fb779ae2856abf585a583
   68b017e7f2a9e5ce4db5'
           },
           / ciphertext / h''
         ]
       ]
     ]
   )

C.5.3.  Wrapped MAC

   This example uses the following:

   o  MAC: AES-MAC, 128-bit key, truncated to 64 bits

   o  Recipient class: AES keywrap w/ a pre-shared 256-bit key

   Size of binary file is 110 bytes















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   994(
     [
       / protected / h'a1010e' / {
           \ alg \ 1:14 \ AES-CBC-MAC-128//64 \
         } / ,
       / unprotected / {},
       / payload / 'This is the content.',
       / tag / h'36f5afaf0bab5d43',
       / recipients / [
         [
           / protected / h'',
           / unprotected / {
             / alg / 1:-5 / A256KW /,
             / kid / 4:'018c0ae5-4d9b-471b-bfd6-eef314bc7037'
           },
           / ciphertext / h'711ab0dc2fc4585dce27effa6781c8093eba906f227
   b6eb0'
         ]
       ]
     ]
   )

C.5.4.  Multi-recipient MAC message

   This example uses the following:

   o  MAC: HMAC w/ SHA-256, 128-bit key

   o  Recipient class: Uses three different methods

      1.  ECDH Ephemeral-Static, Curve P-521, AES-Key Wrap w/ 128-bit
          key

      2.  AES-Key Wrap w/ 256-bit key

   Size of binary file is 310 bytes















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   994(
     [
       / protected / h'a10105' / {
           \ alg \ 1:5 \ HMAC 256//256 \
         } / ,
       / unprotected / {},
       / payload / 'This is the content.',
       / tag / h'bf48235e809b5c42e995f2b7d5fa13620e7ed834e337f6aa43df16
   1e49e9323e',
       / recipients / [
         [
           / protected / h'a101381c' / {
               \ alg \ 1:-29 \ ECHD-ES+A128KW \
             } / ,
           / unprotected / {
             / ephemeral / -1:{
               / kty / 1:2,
               / crv / -1:3,
               / x / -2:h'0043b12669acac3fd27898ffba0bcd2e6c366d53bc4db
   71f909a759304acfb5e18cdc7ba0b13ff8c7636271a6924b1ac63c02688075b55ef2
   d613574e7dc242f79c3',
               / y / -3:true
             },
             / kid / 4:'bilbo.baggins@hobbiton.example'
           },
           / ciphertext / h'c07072310285bbd3f0675774418138e14388ed47a4a
   81219d42a8bfbe3a5559c19de83435d21c6bc'
         ],
         [
           / protected / h'',
           / unprotected / {
             / alg / 1:-5 / A256KW /,
             / kid / 4:'018c0ae5-4d9b-471b-bfd6-eef314bc7037'
           },
           / ciphertext / h'0b2c7cfce04e98276342d6476a7723c090dfdd15f9a
   518e7736549e998370695e6d6a83b4ae507bb'
         ]
       ]
     ]
   )

C.6.  Examples of MAC0 messages

C.6.1.  Shared Secret Direct MAC

   This example users the following:

   o  MAC: AES-CMAC, 256-bit key, truncated to 64 bits



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   o  Recipient class: direct shared secret

   Size of binary file is 39 bytes

   996(
     [
       / protected / h'a1010f' / {
           \ alg \ 1:15 \ AES-CBC-MAC-256//64 \
         } / ,
       / unprotected / {},
       / payload / 'This is the content.',
       / tag / h'726043745027214f'
     ]
   )

   Note that this example uses the same inputs as Appendix C.5.1.

C.7.  COSE Keys

C.7.1.  Public Keys

   This is an example of a COSE Key set.  This example includes the
   public keys for all of the previous examples.

   In order the keys are:

   o  An EC key with a kid of "meriadoc.brandybuck@buckland.example"

   o  An EC key with a kid of "peregrin.took@tuckborough.example"

   o  An EC key with a kid of "bilbo.baggins@hobbiton.example"

   o  An EC key with a kid of "11"

   Size of binary file is 481 bytes
















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   [
     {
       -1:1,
       -2:h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de439c0
   8551d',
       -3:h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eecd008
   4d19c',
       1:2,
       2:'meriadoc.brandybuck@buckland.example'
     },
     {
       -1:1,
       -2:h'bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a86d6a
   09eff',
       -3:h'20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed28bbf
   c117e',
       1:2,
       2:'11'
     },
     {
       -1:3,
       -2:h'0072992cb3ac08ecf3e5c63dedec0d51a8c1f79ef2f82f94f3c737bf5de
   7986671eac625fe8257bbd0394644caaa3aaf8f27a4585fbbcad0f2457620085e5c8
   f42ad',
       -3:h'01dca6947bce88bc5790485ac97427342bc35f887d86d65a089377e247e
   60baa55e4e8501e2ada5724ac51d6909008033ebc10ac999b9d7f5cc2519f3fe1ea1
   d9475',
       1:2,
       2:'bilbo.baggins@hobbiton.example'
     },
     {
       -1:1,
       -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b4d91
   d6280',
       -3:h'f01400b089867804b8e9fc96c3932161f1934f4223069170d924b7e03bf
   822bb',
       1:2,
       2:'peregrin.took@tuckborough.example'
     }
   ]

C.7.2.  Private Keys

   This is an example of a COSE Key set.  This example includes the
   private keys for all of the previous examples.

   In order the keys are:




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   o  An EC key with a kid of "meriadoc.brandybuck@buckland.example"

   o  A shared-secret key with a kid of "our-secret"

   o  An EC key with a kid of "peregrin.took@tuckborough.example"

   o  A shared-secret key with a kid of "018c0ae5-4d9b-471b-
      bfd6-eef314bc7037"

   o  An EC key with a kid of "bilbo.baggins@hobbiton.example"

   o  An EC key with a kid of "11"

   Size of binary file is 816 bytes

   [
     {
       1:2,
       2:'meriadoc.brandybuck@buckland.example',
       -1:1,
       -2:h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de439c0
   8551d',
       -3:h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eecd008
   4d19c',
       -4:h'aff907c99f9ad3aae6c4cdf21122bce2bd68b5283e6907154ad911840fa
   208cf'
     },
     {
       1:2,
       2:'11',
       -1:1,
       -2:h'bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a86d6a
   09eff',
       -3:h'20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed28bbf
   c117e',
       -4:h'57c92077664146e876760c9520d054aa93c3afb04e306705db609030850
   7b4d3'
     },
     {
       1:2,
       2:'bilbo.baggins@hobbiton.example',
       -1:3,
       -2:h'0072992cb3ac08ecf3e5c63dedec0d51a8c1f79ef2f82f94f3c737bf5de
   7986671eac625fe8257bbd0394644caaa3aaf8f27a4585fbbcad0f2457620085e5c8
   f42ad',
       -3:h'01dca6947bce88bc5790485ac97427342bc35f887d86d65a089377e247e
   60baa55e4e8501e2ada5724ac51d6909008033ebc10ac999b9d7f5cc2519f3fe1ea1
   d9475',



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       -4:h'00085138ddabf5ca975f5860f91a08e91d6d5f9a76ad4018766a476680b
   55cd339e8ab6c72b5facdb2a2a50ac25bd086647dd3e2e6e99e84ca2c3609fdf177f
   eb26d'
     },
     {
       1:4,
       2:'our-secret',
       -1:h'849b57219dae48de646d07dbb533566e976686457c1491be3a76dcea6c4
   27188'
     },
     {
       1:2,
       -1:1,
       2:'peregrin.took@tuckborough.example',
       -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b4d91
   d6280',
       -3:h'f01400b089867804b8e9fc96c3932161f1934f4223069170d924b7e03bf
   822bb',
       -4:h'02d1f7e6f26c43d4868d87ceb2353161740aacf1f7163647984b522a848
   df1c3'
     },
     {
       1:4,
       2:'our-secret2',
       -1:h'849b5786457c1491be3a76dcea6c4271'
     },
     {
       1:4,
       2:'018c0ae5-4d9b-471b-bfd6-eef314bc7037',
       -1:h'849b57219dae48de646d07dbb533566e976686457c1491be3a76dcea6c4
   27188'
     }
   ]

Appendix D.  Document Updates

D.1.  Version -09 to -10

   o  Add more examples

   o  Revise Design changes

   o  Add context string for recursive recipient structures

   o  Change and assign some algorithm numbers






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D.2.  Version -08 to -09

   o  Integrate CDDL syntax into the text

   o  Define Expert review guidelines

   o  Expand application profiling guidelines

   o  Expand text around Partial IV

   o  Creation time becomes Operation time

   o  Add tagging for all structures so that they cannot be moved

   o  Add optional parameter to cose media type

   o  Add single signature and mac structures

D.3.  Version -07 to -08

   o  Redefine sequence number into a the Partial IV.

D.4.  Version -06 to -07

   o  Editorial Changes

   o  Make new IANA registries be Expert Review

D.5.  Version -05 to -06

   o  Remove new CFRG Elliptical Curve key agreement algorithms.

   o  Remove RSA algorithms

   o  Define a creation time and sequence number for discussions.

   o  Remove message type field from all structures.

   o  Define CBOR tagging for all structures with IANA registrations.

D.6.  Version -04 to -05

   o  Removed the jku, x5c, x5t, x5t#S256, x5u, and jwk headers.

   o  Add enveloped data vs encrypted data structures.

   o  Add counter signature parameter.




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D.7.  Version -03 to -04

   o  Change top level from map to array.

   o  Eliminate the term "key management" from the document.

   o  Point to content registries for the 'content type' attribute

   o  Push protected field into the KDF functions for recipients.

   o  Remove password based recipient information.

   o  Create EC Curve Registry.

D.8.  Version -02 to -03

   o  Make a pass over all of the algorithm text.

   o  Alter the CDDL so that Keys and KeySets are top level items and
      the key examples validate.

   o  Add sample key structures.

   o  Expand text on dealing with Externally Supplied Data.

   o  Update the examples to match some of the renumbering of fields.

D.9.  Version -02 to -03

   o  Add a set of straw man proposals for algorithms.  It is possible/
      expected that this text will be moved to a new document.

   o  Add a set of straw man proposals for key structures.  It is
      possible/expected that this text will be moved to a new document.

   o  Provide guidance on use of externally supplied authenticated data.

   o  Add external authenticated data to signing structure.

D.10.  Version -01 to -2

   o  Add first pass of algorithm information

   o  Add direct key derivation example.







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D.11.  Version -00 to -01

   o  Add note on where the document is being maintained and
      contributing notes.

   o  Put in proposal on MTI algorithms.

   o  Changed to use labels rather than keys when talking about what
      indexes a map.

   o  Moved nonce/IV to be a common header item.

   o  Expand section to discuss the common set of labels used in
      COSE_Key maps.

   o  Start marking element 0 in registries as reserved.

   o  Update examples.

Editorial Comments

[CREF1] JLS: I have not gone through the document to determine what
        needs to be here yet.  We mostly want to grab terms that are
        used in unusual ways or are not generally understood.

Author's Address

   Jim Schaad
   August Cellars

   Email: ietf@augustcellars.com




















Schaad                 Expires September 22, 2016             [Page 111]


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