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Versions: (draft-schaad-cose-rfc8152bis-struct) 00 01 02 03 04 05 06

COSE Working Group                                             J. Schaad
Internet-Draft                                            August Cellars
Obsoletes8152 (if approved)                              August 18, 2019
Intended status: Standards Track
Expires: February 19, 2020


   CBOR Object Signing and Encryption (COSE): Structures and Process
                  draft-ietf-cose-rfc8152bis-struct-06

Abstract

   Concise Binary Object Representation (CBOR) is a data format designed
   for small code size and small message size.  There is a need for the
   ability to have basic security services defined for this data format.
   This document defines the CBOR Object Signing and Encryption (COSE)
   protocol.  This specification describes how to create and process
   signatures, message authentication codes, and encryption using CBOR
   for serialization.  This specification additionally describes how to
   represent cryptographic keys using CBOR.

   This document along with [I-D.ietf-cose-rfc8152bis-algs] obsoletes
   RFC8152.

Contributing to this document

   This note is to be removed before publishing as an RFC.

   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-rfc8152bis.  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 https://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."



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   This Internet-Draft will expire on February 19, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://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.  Changes from RFC8152  . . . . . . . . . . . . . . . . . .   6
     1.3.  Requirements Terminology  . . . . . . . . . . . . . . . .   6
     1.4.  CBOR Grammar  . . . . . . . . . . . . . . . . . . . . . .   6
     1.5.  CBOR-Related Terminology  . . . . . . . . . . . . . . . .   8
     1.6.  Document Terminology  . . . . . . . . . . . . . . . . . .   8
   2.  Basic COSE Structure  . . . . . . . . . . . . . . . . . . . .   9
   3.  Header Parameters . . . . . . . . . . . . . . . . . . . . . .  13
     3.1.  Common COSE Headers Parameters  . . . . . . . . . . . . .  15
   4.  Signing Objects . . . . . . . . . . . . . . . . . . . . . . .  19
     4.1.  Signing with One or More Signers  . . . . . . . . . . . .  19
     4.2.  Signing with One Signer . . . . . . . . . . . . . . . . .  21
     4.3.  Externally Supplied Data  . . . . . . . . . . . . . . . .  22
     4.4.  Signing and Verification Process  . . . . . . . . . . . .  23
   5.  Counter Signatures  . . . . . . . . . . . . . . . . . . . . .  25
     5.1.  Full Countersignatures  . . . . . . . . . . . . . . . . .  25
     5.2.  Abbreviated Countersignatures . . . . . . . . . . . . . .  26
   6.  Encryption Objects  . . . . . . . . . . . . . . . . . . . . .  27
     6.1.  Enveloped COSE Structure  . . . . . . . . . . . . . . . .  27
       6.1.1.  Content Key Distribution Methods  . . . . . . . . . .  29
     6.2.  Single Recipient Encrypted  . . . . . . . . . . . . . . .  29
     6.3.  How to Encrypt and Decrypt for AEAD Algorithms  . . . . .  30
     6.4.  How to Encrypt and Decrypt for AE Algorithms  . . . . . .  32
   7.  MAC Objects . . . . . . . . . . . . . . . . . . . . . . . . .  34
     7.1.  MACed Message with Recipients . . . . . . . . . . . . . .  34
     7.2.  MACed Messages with Implicit Key  . . . . . . . . . . . .  35
     7.3.  How to Compute and Verify a MAC . . . . . . . . . . . . .  36
   8.  Key Objects . . . . . . . . . . . . . . . . . . . . . . . . .  38
     8.1.  COSE Key Common Parameters  . . . . . . . . . . . . . . .  38



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   9.  Taxonomy of Algorithms used by COSE . . . . . . . . . . . . .  41
     9.1.  Signature Algorithms  . . . . . . . . . . . . . . . . . .  42
     9.2.  Message Authentication Code (MAC) Algorithms  . . . . . .  43
     9.3.  Content Encryption Algorithms . . . . . . . . . . . . . .  43
     9.4.  Key Derivation Functions (KDFs) . . . . . . . . . . . . .  44
     9.5.  Content Key Distribution Methods  . . . . . . . . . . . .  44
       9.5.1.  Direct Encryption . . . . . . . . . . . . . . . . . .  45
       9.5.2.  Key Wrap  . . . . . . . . . . . . . . . . . . . . . .  45
       9.5.3.  Key Transport . . . . . . . . . . . . . . . . . . . .  46
       9.5.4.  Direct Key Agreement  . . . . . . . . . . . . . . . .  46
       9.5.5.  Key Agreement with Key Wrap . . . . . . . . . . . . .  47
   10. CBOR Encoding Restrictions  . . . . . . . . . . . . . . . . .  48
   11. Application Profiling Considerations  . . . . . . . . . . . .  48
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  49
     12.1.  CBOR Tag Assignment  . . . . . . . . . . . . . . . . . .  50
     12.2.  COSE Header Parameters Registry  . . . . . . . . . . . .  50
     12.3.  COSE Header Algorithm Parameters Registry  . . . . . . .  50
     12.4.  COSE Key Common Parameters Registry  . . . . . . . . . .  50
     12.5.  Media Type Registrations . . . . . . . . . . . . . . . .  51
       12.5.1.  COSE Security Message  . . . . . . . . . . . . . . .  51
       12.5.2.  COSE Key Media Type  . . . . . . . . . . . . . . . .  52
     12.6.  CoAP Content-Formats Registry  . . . . . . . . . . . . .  54
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  54
   14. Implementation Status . . . . . . . . . . . . . . . . . . . .  56
     14.1.  Author's Versions  . . . . . . . . . . . . . . . . . . .  56
     14.2.  JavaScript Version . . . . . . . . . . . . . . . . . . .  57
     14.3.  Python Version . . . . . . . . . . . . . . . . . . . . .  57
     14.4.  COSE Testing Library . . . . . . . . . . . . . . . . . .  58
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  58
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  58
     15.2.  Informative References . . . . . . . . . . . . . . . . .  59
   Appendix A.  Guidelines for External Data Authentication of
           Algorithms  . . . . . . . . . . . . . . . . . . . . . . .  62
   Appendix B.  Two Layers of Recipient Information  . . . . . . . .  65
   Appendix C.  Examples . . . . . . . . . . . . . . . . . . . . . .  67
     C.1.  Examples of Signed Messages . . . . . . . . . . . . . . .  67
       C.1.1.  Single Signature  . . . . . . . . . . . . . . . . . .  67
       C.1.2.  Multiple Signers  . . . . . . . . . . . . . . . . . .  68
       C.1.3.  Counter Signature . . . . . . . . . . . . . . . . . .  69
       C.1.4.  Signature with Criticality  . . . . . . . . . . . . .  70
     C.2.  Single Signer Examples  . . . . . . . . . . . . . . . . .  71
       C.2.1.  Single ECDSA Signature  . . . . . . . . . . . . . . .  71
     C.3.  Examples of Enveloped Messages  . . . . . . . . . . . . .  72
       C.3.1.  Direct ECDH . . . . . . . . . . . . . . . . . . . . .  72
       C.3.2.  Direct Plus Key Derivation  . . . . . . . . . . . . .  73
       C.3.3.  Counter Signature on Encrypted Content  . . . . . . .  74
       C.3.4.  Encrypted Content with External Data  . . . . . . . .  75
     C.4.  Examples of Encrypted Messages  . . . . . . . . . . . . .  76



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       C.4.1.  Simple Encrypted Message  . . . . . . . . . . . . . .  76
       C.4.2.  Encrypted Message with a Partial IV . . . . . . . . .  77
     C.5.  Examples of MACed Messages  . . . . . . . . . . . . . . .  77
       C.5.1.  Shared Secret Direct MAC  . . . . . . . . . . . . . .  77
       C.5.2.  ECDH Direct MAC . . . . . . . . . . . . . . . . . . .  78
       C.5.3.  Wrapped MAC . . . . . . . . . . . . . . . . . . . . .  79
       C.5.4.  Multi-Recipient MACed Message . . . . . . . . . . . .  80
     C.6.  Examples of MAC0 Messages . . . . . . . . . . . . . . . .  81
       C.6.1.  Shared Secret Direct MAC  . . . . . . . . . . . . . .  81
     C.7.  COSE Keys . . . . . . . . . . . . . . . . . . . . . . . .  82
       C.7.1.  Public Keys . . . . . . . . . . . . . . . . . . . . .  82
       C.7.2.  Private Keys  . . . . . . . . . . . . . . . . . . . .  83
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  85
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  86

1.  Introduction

   There has been an increased focus on small, constrained devices that
   make up the Internet of Things (IoT).  One of the standards that has
   come out of this process is "Concise Binary Object Representation
   (CBOR)" [RFC7049].  CBOR extended the data model of the JavaScript
   Object Notation (JSON) [RFC8259] by allowing for binary data, among
   other changes.  CBOR has been 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 and be 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 that specified how to
   process encryption, signatures, and Message Authentication Code (MAC)
   operations and how to encode keys using JSON.  This document along
   with [I-D.ietf-cose-rfc8152bis-algs] defines the CBOR Object Signing
   and Encryption (COSE) standard, which does the same thing for the
   CBOR encoding format.  While there is a strong attempt to keep the
   flavor of the original JSON Object Signing and Encryption (JOSE)
   documents, two considerations are taken into account:

   *  CBOR has capabilities that are not present in JSON and are
      appropriate to use.  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.

   *  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.



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   This document contains:

   *  The description of the structure for the CBOR objects which are
      transmitted over the wire.  Two objects are defined for
      encryption, signing and message authentication.  One object is
      defined for transporting keys and one for transporting groups of
      keys.

   *  The procedures used to build the inputs to the cryptographic
      functions required for each of the structures.

   *  A starting set of attributes that apply to the different security
      objects.

   This document does not contain the rules and procedures for using
   specific cryptographic algorithms.  Details on specific algorithms
   can be found in [I-D.ietf-cose-rfc8152bis-algs] and [RFC8230].
   Details for additional algorithms are expected to be defined in
   future documents.

   One feature that is present in CMS [RFC5652] that is not present in
   this standard is a digest structure.  This omission is deliberate.
   It is better for the structure to be defined in each document as
   different protocols will want to include a different set of fields as
   part of the structure.  While an algorithm identifier and the digesst
   value are going to be common to all applications, the two values may
   not always be adjacent as the algorithm could be defined once with
   multiple values.  Applications may additionally want to define
   additional data fields as part of the stucture.  A common structure
   is going to include a URI or other pointer to where the data that is
   being hashed is kept, allowing this to be application specific.

1.1.  Design Changes from JOSE

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

   *  Signed messages distinguish between the protected and unprotected
      parameters that relate to the content from those that relate to
      the signature.

   *  MACed messages are separated from signed messages.

   *  MACed messages have the ability to use the same set of recipient
      algorithms as enveloped messages for obtaining the MAC
      authentication key.




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

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

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

1.2.  Changes from RFC8152

   *  Split the orignal document into this document and
      [I-D.ietf-cose-rfc8152bis-algs].

   *  Add some text describing why there is no digest structure defined
      by COSE.

   *  Rearrange the text around counter signatures and define a CBOR Tag
      for a standalone countersignature.

1.3.  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 BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.4.  CBOR Grammar

   There was not a standard CBOR grammar available when COSE was
   originally written.  For that reason the CBOR structures defined here
   are described in prose.  Since that time CBOR Data Definition
   Language (CDDL) [RFC8610] has been published as an RFC.  The CBOR
   grammar presented in this document is compatible with CDDL.

   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 CBOR
   Data Definition Language (CDDL) [RFC8610].  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).




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      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).

   Two syntaxes from CDDL appear in this document as shorthand.  These
   are:

      FOO / BAR -- indicates that either FOO or BAR can appear here.

      [+ FOO] -- indicates that the type FOO appears one or more times
      in an array.

   Two of the constraints defined by CDDL are also used in this
   document.  These are:

      type1 .cbor type2 -- indicates that the contents of type1, usually
      bstr, contains a value of type2.

      type1 .size integer -- indicates that the contents of type1 is
      integer bytes long

   As well as the prose description, a version of a CBOR grammar is
   presented in CDDL.  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 >
   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 defined to make the tool quieter:
   Internal_Types = Sig_structure / Enc_structure / MAC_structure



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   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.

1.5.  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.  The inclusion of strings allows for an
   additional range of short encoded values to be used as well.  Since
   the word "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 COSE map that is not a string or an
   integer is an error.  Applications can either fail processing or
   process messages by ignoring incorrect labels; however, they MUST NOT
   create messages with incorrect labels.

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

   label = int / tstr
   values = any

1.6.  Document Terminology

   In this document, we use the following terminology:

   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].

   Authenticated Encryption (AE) [RFC5116] algorithms are those
   encryption algorithms that provide an authentication check of the
   contents algorithm with the encryption service.

   Authenticated Encryption with Associated Data (AEAD) [RFC5116]
   algorithms provide the same content authentication service as AE
   algorithms, but they additionally provide for authentication of non-
   encrypted data as well.

   Context is used throughout the document to represent information that
   is not part of the COSE message.  Information which is part of the
   context can come from several different sources including: Protocol



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   interactions, associated key structures and program configuration.
   The context to use can be implicit, identified using the 'kid
   context' header field defined in [RFC8613], or identified by a
   protocol specific identifier.  Context should generally be included
   in the cryptographic configuration, for more details see Section 4.3.

2.  Basic COSE Structure

   The COSE object structure is designed so that there can be a large
   amount of common code when parsing and processing the different types
   of security messages.  All of the message structures are built on the
   CBOR array type.  The first three elements of the array always
   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 plaintext
       or the ciphertext as appropriate.  The content may be detached
       (i.e. transported separately from the COSE structure), but the
       location is still used.  The content is wrapped in a bstr when
       present and is a nil value when detached.

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

   COSE messages are built using the concept of layers to separate
   different types of cryptographic concepts.  As an example of how this
   works, consider the COSE_Encrypt message (Section 6.1).  This message
   type is broken into two layers: the content layer and the recipient
   layer.  In the content layer, the plaintext is encrypted and
   information about the encrypted message is placed.  In the recipient
   layer, the content encryption key (CEK) is encrypted and information
   about how it is encrypted for each recipient is placed.  A single
   layer version of the encryption message COSE_Encrypt0 (Section 6.2)
   is provided for cases where the CEK is pre-shared.

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

   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.  Messages with a
       CBOR tag are known in this specification as tagged messages,
       while those without the CBOR tag are known as untagged messages.




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       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
       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-Format Option can be used to identify the message
       content.  The CoAP Content-Format values can be found in Table 2.
       The CBOR tag for the message structure is not required as each
       security message is uniquely identified.



































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     +------+------------------+-----------------------+-------------+
     | CBOR | cose-type        | Data Item             | Semantics   |
     | Tag  |                  |                       |             |
     +======+==================+=======================+=============+
     | 98   | cose-sign        | COSE_Sign             | COSE Signed |
     |      |                  |                       | Data Object |
     +------+------------------+-----------------------+-------------+
     | 18   | cose-sign1       | COSE_Sign1            | COSE Single |
     |      |                  |                       | Signer Data |
     |      |                  |                       | Object      |
     +------+------------------+-----------------------+-------------+
     | 96   | cose-encrypt     | COSE_Encrypt          | COSE        |
     |      |                  |                       | Encrypted   |
     |      |                  |                       | Data Object |
     +------+------------------+-----------------------+-------------+
     | 16   | cose-encrypt0    | COSE_Encrypt0         | COSE Single |
     |      |                  |                       | Recipient   |
     |      |                  |                       | Encrypted   |
     |      |                  |                       | Data Object |
     +------+------------------+-----------------------+-------------+
     | 97   | cose-mac         | COSE_Mac              | COSE MACed  |
     |      |                  |                       | Data Object |
     +------+------------------+-----------------------+-------------+
     | 17   | cose-mac0        | COSE_Mac0             | COSE Mac w/ |
     |      |                  |                       | o           |
     |      |                  |                       | Recipients  |
     |      |                  |                       | Object      |
     +------+------------------+-----------------------+-------------+
     | TBD0 | cose-countersign | COSE_Countersignature | COSE        |
     |      |                  |                       | standalone  |
     |      |                  |                       | counter     |
     |      |                  |                       | signature   |
     +------+------------------+-----------------------+-------------+

                    Table 1: COSE Message Identification
















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        +---------------------------+----------+-----+------------+
        | Media Type                | Encoding | ID  | Reference  |
        +===========================+==========+=====+============+
        | application/cose; cose-   |          | 98  | [[THIS     |
        | type="cose-sign"          |          |     | DOCUMENT]] |
        +---------------------------+----------+-----+------------+
        | application/cose; cose-   |          | 18  | [[THIS     |
        | type="cose-sign1"         |          |     | DOCUMENT]] |
        +---------------------------+----------+-----+------------+
        | application/cose; cose-   |          | 96  | [[THIS     |
        | type="cose-encrypt"       |          |     | DOCUMENT]] |
        +---------------------------+----------+-----+------------+
        | application/cose; cose-   |          | 16  | [[THIS     |
        | type="cose-encrypt0"      |          |     | DOCUMENT]] |
        +---------------------------+----------+-----+------------+
        | application/cose; cose-   |          | 97  | [[THIS     |
        | type="cose-mac"           |          |     | DOCUMENT]] |
        +---------------------------+----------+-----+------------+
        | application/cose; cose-   |          | 17  | [[THIS     |
        | type="cose-mac0"          |          |     | DOCUMENT]] |
        +---------------------------+----------+-----+------------+
        | application/cose-key      |          | 101 | [[THIS     |
        |                           |          |     | DOCUMENT]] |
        +---------------------------+----------+-----+------------+
        | application/cose-key-set  |          | 102 | [[THIS     |
        |                           |          |     | DOCUMENT]] |
        +---------------------------+----------+-----+------------+

                   Table 2: CoAP Content-Formats for COSE

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

   COSE_Messages = COSE_Untagged_Message / COSE_Tagged_Message

   COSE_Untagged_Message = COSE_Sign / COSE_Sign1 /
       COSE_Encrypt / COSE_Encrypt0 /
       COSE_Mac / COSE_Mac0 / COSE_Countersignature

   COSE_Tagged_Message = COSE_Sign_Tagged / COSE_Sign1_Tagged /
       COSE_Encrypt_Tagged / COSE_Encrypt0_Tagged /
       COSE_Mac_Tagged / COSE_Mac0_Tagged / COSE_Countersignature_Tagged








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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 are 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 is left empty.

   Both buckets are implemented as CBOR maps.  The map key is a 'label'
   (Section 1.5).  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 have been divided into several
   sections including 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 12.2).

   The two buckets are:

   protected:    Contains parameters about the current layer that are
                 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 a zero-length map as a
                 zero-length byte string rather than as a zero-length
                 map (encoded as h'a0').  The zero-length binary
                 encoding is preferred because it is both shorter and
                 the version used in the serialization structures for
                 cryptographic computation.  After encoding the map, the
                 value is wrapped in the binary object.  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 string.)  This avoids the
                 problem of all parties needing to be able to do a
                 common canonical encoding.




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   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"
   registry (Section 12.2).  Some common parameters are defined in the
   next section.

   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 verify that the same label does not
   occur in both the protected and unprotected headers.  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 that 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 that
   represents the map of common headers.

   Headers = (
       protected : empty_or_serialized_map,
       unprotected : header_map
   )

   header_map = {
       Generic_Headers,
       * label => values
   }

   empty_or_serialized_map = bstr .cbor header_map / bstr .size 0






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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 3.  This table should be
   consulted to determine the value of label and 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 authenticated where the ability to do so
                       exists.  This support is provided by AEAD
                       algorithms or construction (COSE_Sign,
                       COSE_Sign1, COSE_Mac, and COSE_Mac0).  This
                       authentication can be done either by placing the
                       parameter in the protected header bucket or as
                       part of the externally supplied data.  The value
                       is taken from the "COSE Algorithms" registry (see
                       [COSE.Algorithms]).

   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 7 SHOULD
                          be omitted.

                       *  Integer labels in the range -1 to -128 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.  Integer
                          labels in the range -129 to -65536 SHOULD be
                          included as these would be less common
                          parameters that might not be generally
                          supported.

                       *  Labels for parameters required for an



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                          application MAY be omitted.  Applications
                          should have a statement if the label can be
                          omitted.

                       The header parameter values indicated by 'crit'
                       can be processed by either the security library
                       code or 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 [COAP.Formats].
                       Text values following the syntax of "<type-
                       name>/<subtype-name>" where <type-name> and
                       <subtype-name> are defined in Section 4.2 of
                       [RFC6838].  Leading and trailing whitespace is
                       also omitted.  Textual content values along with
                       parameters and subparameters can be located using
                       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, so all of the keys
                       associated with this 'kid' may need to be
                       checked.  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.  This is not a security-
                       critical field.  For this reason, it can be
                       placed in the unprotected headers bucket.

   IV:                 This parameter holds the Initialization Vector
                       (IV) value.  For some symmetric encryption
                       algorithms, this may be referred to as a nonce.
                       The IV can be placed in the unprotected header as



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                       modifying the IV will cause the decryption to
                       yield plaintext that is readily detectable as
                       garbled.

   Partial IV:         This parameter holds a part of the IV value.
                       When using the COSE_Encrypt0 structure, a portion
                       of the IV can be part of the context associated
                       with the key (Context IV) while a portion can be
                       changed with each message (Parital IV).  This
                       field is used to carry a value that causes the IV
                       to be changed for each message.  The Parital IV
                       can be placed in the unprotected header as
                       modifying the value will cause the decryption to
                       yield plaintext that is readily detectable as
                       garbled.  The 'Initialization Vector' and
                       'Partial Initialization Vector' parameters MUST
                       NOT both 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.

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

   counter signature:  This parameter holds one or more counter
                       signature values.  Counter signatures provide a
                       method of having a second party sign some data.
                       The counter signature parameter can occur as an
                       unprotected attribute in any of the following
                       structures: COSE_Sign1, COSE_Signature,
                       COSE_Encrypt, COSE_recipient, COSE_Encrypt0,
                       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 counter signatures
                       are found in Section 5.












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   +---------+-----+----------------+-----------------+----------------+
   |Name     |Label| Value Type     | Value Registry  | Description    |
   +=========+=====+================+=================+================+
   |alg      |1    | int / tstr     | COSE Algorithms | Cryptographic  |
   |         |     |                | registry        | algorithm to use |
   +---------+-----+----------------+-----------------+----------------+
   |crit     |2    | [+ label]      | COSE Header     | Critical headers |
   |         |     |                | Parameters      | to be understood |
   |         |     |                | registry        |                |
   +---------+-----+----------------+-----------------+----------------+
   |content  |3    | tstr / uint    | CoAP Content-   | Content type of |
   |type     |     |                | Formats or Media | the payload    |
   |         |     |                | Types registries |                |
   +---------+-----+----------------+-----------------+----------------+
   |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      |
   +---------+-----+----------------+-----------------+----------------+

                     Table 3: 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.

   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
   )






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4.  Signing Objects

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

4.1.  Signing with One or More Signers

   The COSE_Sign structure allows for one or more signatures to be
   applied to a message payload.  Parameters relating to the content and
   parameters relating to the signature are 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
   and counter signatures.

   RFC 5652 indicates that:

   |  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 for 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 Edwards-curve Digital Signature Algorithm (EdDSA)
   [RFC8032] and with the Elliptic Curve Digital Signature Algorithm
   (ECDSA) [DSS].  This allows recipients to verify the signature
   associated with one algorithm or the other.  More-detailed
   information on multiple signature evaluations can be found in
   [RFC5752].

   The signature structure can be encoded as either tagged or untagged
   depending on the context it will be used in.  A tagged COSE_Sign
   structure is identified by the CBOR tag 98.  The CDDL fragment that
   represents this is:




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   COSE_Sign_Tagged = #6.98(COSE_Sign)

   A COSE Signed Message is defined in 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
   Section c.1.

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

   protected:    This is as described in Section 3.

   unprotected:  This is as described in Section 3.

   payload:      This field 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 ("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 a message recovery
                 algorithm is used (Section 9.1), 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:   This field is an array of signatures.  Each signature
                 is represented as a COSE_Signature structure.

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

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





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

   protected:    This is as described in Section 3.

   unprotected:  This is as described in Section 3.

   signature:    This field contains the computed signature value.  The
                 type of the field is a bstr.  Algorithms MUST specify
                 padding if the signature value is not a multiple of 8
                 bits.

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

   COSE_Signature =  [
       Headers,
       signature : bstr
   ]

4.2.  Signing with One Signer

   The COSE_Sign1 signature structure is used when only one signature is
   going to be placed on a message.  The parameters dealing with the
   content and the signature are placed in the same pair of buckets
   rather than having the separation of COSE_Sign.

   The structure can be encoded as either tagged or untagged depending
   on the context it will be used in.  A tagged COSE_Sign1 structure is
   identified by the CBOR tag 18.  The CDDL fragment that represents
   this is:

   COSE_Sign1_Tagged = #6.18(COSE_Sign1)

   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_Sign1 messages can be found in
   Section c.2.

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

   protected:    This is as described in Section 3.

   unprotected:  This is as described in Section 3.

   payload:      This is as described in Section 4.1.




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

   The CDDL fragment that 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 offered in the COSE document is the ability for
   applications to provide additional data to be authenticated, 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.  Examples of data that can be placed in this location
   would be the CoAP code or CoAP options.  If the data is in the header
   section, then it is available for proxies to help in performing its
   operations.  For example, the Accept Option can be used by a proxy to
   determine if an appropriate value is in the proxy's cache.  But the
   sender can cause a failure at the server if a proxy, or an attacker,
   changes the set of accept values by including the field in the
   application supplied data.

   This document describes the process for using a byte array of
   externally supplied authenticated data; 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:

   *  If multiple items are included, applications need to ensure that
      the same byte string cannot produced if there are different
      inputs.  This would occur by appending the strings 'AB' and 'CDE'
      or by appending the strings 'ABC' and 'DE'.  This is usually
      addressed by making fields a fixed width and/or encoding the
      length of the field as part of the output.  Using options from
      CoAP [RFC7252] as an example, these fields use a TLV structure so
      they can be concatenated without any problems.

   *  If multiple items are included, an 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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   *  Applications need to ensure that the byte string is going to be
      the same on both sides.  Using options from CoAP might give a
      problem if the same relative numbering is kept.  An intermediate
      node could insert or remove an option, changing how the relative
      number is done.  An application would need to specify that the
      relative number must be re-encoded to be relative only to the
      options that are in the external data.

4.4.  Signing and Verification Process

   In order to create a signature, a well-defined byte string is needed.
   The Sig_structure is used to create the canonical form.  This signing
   and verification process takes in the body information (COSE_Sign or
   COSE_Sign1), the signer information (COSE_Signature), and the
   application data (external source).  A Sig_structure is a CBOR array.
   The fields of the Sig_structure 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.

          "CounterSignature0" for signatures used as CounterSignature0
          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 and CounterSignature0 attributes.

   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 that describes the above text is:



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   Sig_structure = [
       context : "Signature" / "Signature1" / "CounterSignature" /
                 "CounterSignature0",
       body_protected : empty_or_serialized_map,
       ? sign_protected : empty_or_serialized_map,
       external_aad : bstr,
       payload : bstr
   ]

   How to compute a signature:

   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 10.

   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 correct location.
       This is the 'signature' field of the COSE_Signature, COSE_Sign1
       or COSE_Countersignature structures.  This is the value of the
       Countersignature0 attribute.

   The steps for verifying a signature are:

   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 10.

   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, the application
   performs the appropriate checks to ensure that the key is correctly
   paired with the signing identity and that the signing identity is
   authorized before performing actions.









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5.  Counter Signatures

   COSE supports two different forms for counter signatures.  Full
   countersignatures use the structure COSE_Countersign.  This is same
   structure as COSE_Signature and thus it can have protected
   attributes, chained countersignatures and information about
   identifying the key.  Abbreviated countersignatures use the structure
   COSE_Countersign1.  This structure only contains the signature value
   and nothing else.  The structures cannot be converted between each
   other; as the signature computation includes a parameter identifying
   which structure is being used, the converted structure will fail
   signature validation.

   COSE was designed for uniformity in how the data strutures are
   specified.  One result of this is that for COSE one can expand the
   concept of countersignatures beyond just the idea of signing a
   signature to being able to sign most of the structures without having
   to create a new signing layer.  When creating a countersignature, one
   needs to be clear about the security properties that result.  When
   done on a COSE_Signature, the normal countersignature semantics are
   preserved.  That is the countersignature makes a statement about the
   existence of a signature and, when used as a timestamp, a time point
   at which the signature exists.  When done on a COSE_Mac or a
   COSE_Mac0, one effectively upgrades the MAC operation to a sginature
   operation.  When done on a COSE_Encrypt or COSE_Encrypt0, the
   existence of the encrypted data is attested to.  It should be noted
   that there is a big difference between attesting to the encrypted
   data as opposed to attesting to the unencrypted data.  If the latter
   is what is desired, then one needs to apply a signature to the data
   and then encrypt that.  It is always possible to construct cases
   where the decryption is successful, while providing completely
   different answers by using a different key.  This situation is not
   detectable by a countersignature on the encrypted data.

5.1.  Full Countersignatures

   The COSE_Countersignature structure allows for the same set of
   capabilities of a COSE_Signature.  This means that all of the
   capabilities of a signature are duplicated with this structure.
   Specifically, the countersigner does not need to be related to the
   producer of what is being counter signed as key and algorithm
   identification can be placed in the countersignature attributes.
   This also means that the countersignature can itself be
   countersigned.  This is a feature required by protocols such as long-
   term archiving services.  More information on how this is used can be
   found in the evidence record syntax described in [RFC4998].





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   The full countersignature structure can be encoded as either a tagged
   or untagged depending on the context it is used in.  A tagged
   COSE_Countersign structure is identified by the CBOR tag TBD0.  The
   CDDL fragment for full countersignatures is:

   COSE_CounterSignature_Tagged = #6.98(COSE_CounterSignature)
   COSE_CounterSignature = COSE_Signature

   The details of the fields of a countersignature can be found in
   Section 4.1.  The process of creating and validating abbreviated
   countersignatures is defined in Section 4.4.

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

   It should be noted that only a signature algorithm with appendix (see
   Section 9.1) 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.

5.2.  Abbreviated Countersignatures

   Abbreviated countersignatures were designed primarily to deal with
   the problem of having group encrypted messaging, but still needing to
   know who originated the message.  The objective was to keep the
   countersignature as small as possible while still providing the
   needed security.  For abbreviated countersignatures, there is no
   provision for any protected attributes related to the signing
   operation.  Instead, the parameters for computing or verifying the
   abbreviated countersignature are inferred from the same context used
   to describe the encryption, signature, or MAC processing.

   The byte string representing the signature value is placed in the
   CounterSignature0 attribute.  This attribute is then encoded as an
   unprotected header.  The attribute is defined below.

   The process of creating and validating abbreviated countersignatures
   is defined in Section 4.4.











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   +-------------------+-------+------------+-------+------------------+
   | Name              | Label | Value      | Value | Description      |
   |                   |       | Type       |       |                  |
   +===================+=======+============+=======+==================+
   | CounterSignature0 | 9     | bstr       |       | Abbreviated      |
   |                   |       |            |       | Countersignature |
   +-------------------+-------+------------+-------+------------------+

              Table 4: Header Parameter for CounterSignature0

6.  Encryption Objects

   COSE supports two different encryption structures.  COSE_Encrypt0 is
   used when a recipient structure is not needed because the key to be
   used is known implicitly.  COSE_Encrypt is used the rest of the time.
   This includes cases where there are multiple recipients or a
   recipient algorithm other than direct (i.e. pre-shared secret) is
   used.

6.1.  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 and the recipient's
   encryption algorithm.

   The same techniques and nearly the same structure is used for
   encrypting both the plaintext and the keys.  This is different from
   the approach used by both "Cryptographic Message Syntax (CMS)"
   [RFC5652] and "JSON Web Encryption (JWE)" [RFC7516] where different
   structures are used for the content layer and for the recipient
   layer.  Two structures are defined: COSE_Encrypt to hold the
   encrypted content and COSE_recipient to hold the encrypted keys for
   recipients.  Examples of encrypted messages can be found in
   Section c.3.

   The COSE_Encrypt structure can be encoded as either tagged or
   untagged depending on the context it will be used in.  A tagged
   COSE_Encrypt structure is identified by the CBOR tag 96.  The CDDL
   fragment that represents this is:




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   COSE_Encrypt_Tagged = #6.96(COSE_Encrypt)

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

   protected:    This is as described in Section 3.

   unprotected:  This is as described in Section 3.

   ciphertext:   This field contains the ciphertext 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 nil value.

   recipients:   This field 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_Encrypt = [
       Headers,
       ciphertext : bstr / nil,
       recipients : [+COSE_recipient]
   ]

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

   protected:    This is as described in Section 3.

   unprotected:  This is as described in Section 3.

   ciphertext:   This field contains the encrypted key encoded as a
                 bstr.  All encoded keys are symmetric keys; the binary
                 value of the key is the content.  If there is not an
                 encrypted key, then this field is encoded as a nil
                 value.

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

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



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

6.1.1.  Content Key Distribution Methods

   An 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 depend on which class the recipient algorithm
   falls into.  Specific details on each of the classes can be found in
   Section 9.5.  A short summary of the five content key distribution
   methods is:

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

   symmetric key-encryption keys (KEK):  The CEK is encrypted using a
      previously distributed symmetric KEK.  Also known as key wrap.

   key agreement:  The recipient's public key and a sender's private key
      are used to generate a pairwise secret, a Key Derivation Function
      (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.

   passwords:  The CEK is encrypted in a KEK that is derived from a
      password.  As of when this document was published, no password
      algorithms have been defined.

6.2.  Single Recipient Encrypted

   The COSE_Encrypt0 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.

   Examples of encrypted messages can be found in Section c.3.

   The COSE_Encrypt0 structure can be encoded as either tagged or
   untagged depending on the context it will be used in.  A tagged




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   COSE_Encrypt0 structure is identified by the CBOR tag 16.  The CDDL
   fragment that represents this is:

   COSE_Encrypt0_Tagged = #6.16(COSE_Encrypt0)

   The COSE_Encrypt0 structure is a CBOR array.  The fields of the array
   in order are:

   protected:    This is as described in Section 3.

   unprotected:  This is as described in Section 3.

   ciphertext:   This is as described in Section 6.1.

   The CDDL fragment for COSE_Encrypt0 that corresponds to the above
   text is:

   COSE_Encrypt0 = [
       Headers,
       ciphertext : bstr / nil,
   ]

6.3.  How to Encrypt and Decrypt for AEAD Algorithms

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

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

          "Encrypt0" for the content encryption of a COSE_Encrypt0 data
          structure.

          "Encrypt" for the first layer of a COSE_Encrypt data structure
          (i.e., for content encryption).

          "Enc_Recipient" for a recipient encoding to be placed in an
          COSE_Encrypt data structure.

          "Mac_Recipient" for a recipient encoding to be placed in a
          MACed 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 that describes the above text is:

   Enc_structure = [
       context : "Encrypt" / "Encrypt0" / "Enc_Recipient" /
           "Mac_Recipient" / "Rec_Recipient",
       protected : empty_or_serialized_map,
       external_aad : bstr
   ]

   How to encrypt a message:

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

   2.  Encode the Enc_structure to a byte string (Additional
       Authenticated Data (AAD)), using the encoding described in
       Section 10.

   3.  Determine the encryption key (K).  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 layer.  Examples are key transport keys
          (Section 9.5.3), key wrap keys (Section 9.5.2), or pre-shared
          secrets.

       Direct Encryption 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.)  Examples of these
          algorithms are found in Sections 6.1.2 and 6.3 of
          [I-D.ietf-cose-rfc8152bis-algs].

       Other:  The key is randomly or pseudorandomly generated.

   4.  Call the encryption algorithm with K (the encryption key), P (the




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       plaintext), and AAD.  Place the returned ciphertext into the
       'ciphertext' field of the structure.

   5.  For recipients of the message, recursively perform the encryption
       algorithm for that recipient, using K (the encryption key) as the
       plaintext.

   How to decrypt a message:

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

   2.  Encode the Enc_structure to a byte string (AAD), using the
       encoding described in Section 10.

   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 layer.  Examples are key transport keys
          (Section 9.5.3), key wrap keys (Section 9.5.2), or pre-shared
          secrets.

       Direct Encryption 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 one of
          the recipient structures.

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

6.4.  How to Encrypt and Decrypt for AE Algorithms

   How to encrypt a message:

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

   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:




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       No Recipients:  The key to be used is determined by the algorithm
          and key at the current layer.  Examples are key transport keys
          (Section 9.5.3), key wrap keys (Section 9.5.2), or pre-shared
          secrets.

       Direct Encryption 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.)  Examples of these
          algorithms are found in Sections 6.1.2 and 6.3 of
          [I-D.ietf-cose-rfc8152bis-algs].

       Other:  The key is randomly generated.

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

   5.  For recipients of the message, recursively perform the encryption
       algorithm for that recipient, using K (the encryption key) as the
       plaintext.

   How to decrypt a message:

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

   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 layer.  Examples are key transport keys
          (Section 9.5.3), key wrap keys (Section 9.5.2), or pre-shared
          secrets.

       Direct Encryption 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.)  Examples of these
          algorithms are found in Sections 6.1.2 and 6.3 of
          [I-D.ietf-cose-rfc8152bis-algs].




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       Other:  The key is determined by decoding and decrypting one of
          the recipient structures.

   4.  Call the decryption algorithm with K (the decryption key to use)
       and C (the ciphertext).

7.  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,
   and a recipient algorithm of other than direct.

   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.

   When using MAC operations, there are two modes in which they 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 (SS) 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 the sender
   assumes that there are only two parties involved and that you did not
   send the message to yourself.)  The origination property can be
   obtained with both of the MAC message structures.

7.1.  MACed Message with Recipients

   The multiple recipient MACed message uses two structures: the
   COSE_Mac structure defined in this section for carrying the body and
   the COSE_recipient structure (Section 6.1) to hold the key used for
   the MAC computation.  Examples of MACed messages can be found in
   Section c.5.

   The MAC structure can be encoded as either tagged or untagged
   depending on the context it will be used in.  A tagged COSE_Mac
   structure is identified by the CBOR tag 97.  The CDDL fragment that
   represents this is:

   COSE_Mac_Tagged = #6.97(COSE_Mac)




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

   protected:    This is as described in Section 3.

   unprotected:  This is as described in Section 3.

   payload:      This field 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 nil CBOR value is placed in this location, and
                 it is the responsibility of the application to ensure
                 that it will be transported without changes.

   tag:          This field contains the MAC value.

   recipients:   This is as described in Section 6.1.

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

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

7.2.  MACed 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 MACed message uses the COSE_Mac0 structure defined in this
   section for carrying the body.  Examples of MACed messages with an
   implicit key can be found in Section c.6.

   The MAC structure can be encoded as either tagged or untagged
   depending on the context it will be used in.  A tagged COSE_Mac0
   structure is identified by the CBOR tag 17.  The CDDL fragment that
   represents this is:

   COSE_Mac0_Tagged = #6.17(COSE_Mac0)




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

   protected:    This is as described in Section 3.

   unprotected:  This is as described in Section 3.

   payload:      This is as described in Section 7.1.

   tag:          This field contains the MAC value.

   The CDDL fragment that corresponds to the above text is:

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

7.3.  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.
   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.  The protected attributes from the application encoded as 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.)

   4.  The payload to be MACed 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:









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   MAC_structure = [
        context : "MAC" / "MAC0",
        protected : empty_or_serialized_map,
        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.  Create the value ToBeMaced by encoding the MAC_structure to a
       byte string, using the encoding described in Section 10.

   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 or
       COSE_Mac0 structure.

   5.  For COSE_Mac structures, encrypt and encode the MAC key for each
       recipient of the message.

   The steps to verify a MAC are:

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

   2.  Create the value ToBeMaced by encoding the MAC_structure to a
       byte string, using the encoding described in Section 10.

   3.  For COSE_Mac structures, 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 or
       COSE_Mac0 structure.









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8.  Key Objects

   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 "COSE Key Common Parameters" registry (Section 12.4).
   Additional parameters defined for specific key types can be found in
   the IANA "COSE Key Type Parameters" registry ([COSE.KeyParameters]).

   A COSE Key Set uses a CBOR array object as its underlying type.  The
   values of the array elements are COSE Keys.  A COSE Key Set MUST have
   at least one element in the array.  Examples of COSE Key Sets can be
   found in Section c.7.

   Each element in a COSE Key Set MUST be processed independently.  If
   one element in a COSE Key Set is either malformed or uses a key that
   is not understood by an application, that key is ignored and the
   other keys are processed normally.

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

   The CDDL grammar describing COSE_Key and COSE_KeySet is:

   COSE_Key = {
       1 => tstr / int,          ; kty
       ? 2 => bstr,              ; kid
       ? 3 => tstr / int,        ; alg
       ? 4 => [+ (tstr / int) ], ; key_ops
       ? 5 => bstr,              ; Base IV
       * label => values
   }

   COSE_KeySet = [+COSE_Key]

8.1.  COSE Key Common Parameters

   This document defines a set of common parameters for a COSE Key
   object.  Table 5 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
   [I-D.ietf-cose-rfc8152bis-algs].











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    +---------+-------+-------------+------------+--------------------+
    | Name    | Label | CBOR Type   | Value      | Description        |
    |         |       |             | Registry   |                    |
    +=========+=======+=============+============+====================+
    | kty     | 1     | tstr / int  | COSE Key   | Identification of  |
    |         |       |             | Types      | the key type       |
    +---------+-------+-------------+------------+--------------------+
    | kid     | 2     | bstr        |            | Key identification |
    |         |       |             |            | value -- match to  |
    |         |       |             |            | kid in message     |
    +---------+-------+-------------+------------+--------------------+
    | alg     | 3     | tstr / int  | COSE       | Key usage          |
    |         |       |             | Algorithms | restriction to     |
    |         |       |             |            | this algorithm     |
    +---------+-------+-------------+------------+--------------------+
    | key_ops | 4     | [+ (tstr/   |            | Restrict set of    |
    |         |       | int)]       |            | permissible        |
    |         |       |             |            | operations         |
    +---------+-------+-------------+------------+--------------------+
    | Base IV | 5     | bstr        |            | Base IV to be xor- |
    |         |       |             |            | ed with Partial    |
    |         |       |             |            | IVs                |
    +---------+-------+-------------+------------+--------------------+

                          Table 5: 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 [COSE.KeyTypes].  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 algorithm that is
             used with the 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



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             portion of the key.  This field is intended for matching
             against a 'kid' 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 6.  Algorithms define the values
             of key ops that are permitted to appear and are required
             for specific operations.  The set of values matches that in
             [RFC7517] and [W3C.WebCrypto].

   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 Partial IV.

             Extreme care needs to be taken when using a Base IV in an
             application.  Many encryption algorithms lose security if
             the same IV is used twice.

             If different keys are derived for each sender, using the
             same Base IV with Partial IVs starting at zero is likely to
             ensure that the IV would not be used twice for a single
             key.  If different keys are derived for each sender,
             starting at the same Base IV is likely to satisfy this
             condition.  If the same key is used for multiple senders,
             then the application needs to provide for a method of
             dividing the IV space up between the senders.  This could
             be done by providing a different base point to start from
             or a different Partial IV to start with and restricting the
             number of messages to be sent before rekeying.



















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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 wrap encryption.     |
    | key     |       |                                              |
    +---------+-------+----------------------------------------------+
    | unwrap  | 6     | The key is used for key wrap decryption.     |
    | key     |       | Requires 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 not to be  |
    | bits    |       | used as a key.  Requires private key fields. |
    +---------+-------+----------------------------------------------+
    | MAC     | 9     | The key is used for creating MACs.           |
    | create  |       |                                              |
    +---------+-------+----------------------------------------------+
    | MAC     | 10    | The key is used for validating MACs.         |
    | verify  |       |                                              |
    +---------+-------+----------------------------------------------+

                      Table 6: Key Operation Values

9.  Taxonomy of Algorithms used by COSE

   In this section, a taxonomy of the different algorithm types that can
   be used in COSE is laid out.  This taxonomy should not be considered
   to be exhaustive as there are new algorithm structures that could be
   found or are not known to the author.









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9.1.  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 the RSA
   Probabilistic Signature Scheme (RSASSA-PSS).  (In fact, the SSA in
   RSASSA-PSS stands for Signature Scheme with Appendix.)

   The signature functions for this scheme are:

   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
   that 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 the 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)

   Signature algorithms are used with the COSE_Signature and COSE_Sign1
   structures.  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




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   the effective size reduction that is possible.  Implementations will
   need to keep this in mind for later possible integration.

9.2.  Message Authentication Code (MAC) Algorithms

   Message Authentication Codes (MACs) provide data authentication and
   integrity protection.  They provide either no or very limited data
   origination.  A MAC, for example, cannot 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., a Hash-based Message
   Authentication Code (HMAC)).  [I-D.ietf-cose-rfc8152bis-algs] defines
   a MAC algorithm using each of these constructions.

   MAC algorithms are used in the COSE_Mac and COSE_Mac0 structures.

9.3.  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
   CEK 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's
   sake, the authentication code will normally be defined as being
   appended to the ciphertext stream.  The encryption functions are:

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

   valid, message content = Decrypt(ciphertext, key, additional data)



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   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.

   These algorithms are used in COSE_Encrypt and COSE_Encrypt0.

9.4.  Key Derivation Functions (KDFs)

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

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

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

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

   General KDFs work well with the first type of secret, can do
   reasonably well with the second type of secret, and generally do
   poorly with the last type of secret.  Functions like PBES2 [RFC8018]
   need to be used for non-random secrets.

   The same KDF can be set up to deal with the first two types of
   secrets in a different way.  The KDF defined in section 5.1 of
   [I-D.ietf-cose-rfc8152bis-algs] is such a function.  This is
   reflected in the set of algorithms defined around the HMAC-based
   Extract-and-Expand Key Derivation Function (HKDF).

   When using KDFs, 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.

9.5.  Content Key Distribution Methods

   Content key distribution methods (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.  The names of the recipient algorithm
   classes used here are the same as those defined in [RFC7516].  Other
   specifications use different terms for the recipient algorithm
   classes or do not support some of the recipient algorithm classes.




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9.5.1.  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 CEK.  When direct encryption mode is used, it
   MUST be the only mode used on the message.

   The COSE_Recipient structure for the recipient is organized as
   follows:

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

   *  The 'alg' parameter MUST be present.

   *  A parameter identifying the shared secret SHOULD be present.

   *  The 'ciphertext' field MUST be a zero-length item.

   *  The 'recipients' field MUST be absent.

9.5.2.  Key Wrap

   In key wrap 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 wrap algorithms for COSE are AE
   algorithms.  Key wrap 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 that has some degree of organization and may in
   fact be repeating the same content.  The use of key wrap loses the
   weak data origination that is provided by the direct encryption
   algorithms.

   The COSE_Encrypt structure for the recipient is organized as follows:

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

   *  The 'recipients' field is normally absent, but can be used.
      Applications MUST deal with a recipient field being present that
      has an unsupported algorthms, 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.

   *  The plaintext to be encrypted is the key from next layer down
      (usually the content layer).




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

9.5.3.  Key Transport

   Key transport mode is also called key encryption mode in some
   standards.  Key transport mode differs from key wrap mode in that it
   uses an asymmetric encryption algorithm rather than a symmetric
   encryption algorithm to protect the key.  A set of key transport
   algorithms are defined in [RFC8230].

   When using a key transport algorithm, the COSE_Encrypt structure for
   the recipient is organized as follows:

   *  The 'protected' field MUST be absent.

   *  The plaintext to be encrypted is the key from the next layer down
      (usually the content layer).

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

9.5.4.  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.
   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 online 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 object.

   Two variants of DH that are supported are:

      Ephemeral-Static (ES) 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.





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      Static-Static (SS) DH: where a static key is used for both the
      sender and the recipient.  The use of static keys allows for the
      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_Encrypt structure for the recipient is organized as follows:

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

   *  The headers SHOULD identify the sender's key for the static-static
      versions and MUST contain the sender's ephemeral key for the
      ephemeral-static versions.

9.5.5.  Key Agreement with Key Wrap

   Key Agreement with Key Wrap uses a randomly generated CEK.  The CEK
   is then encrypted using a key wrap algorithm and a key derived from
   the shared secret computed by the key agreement algorithm.  The
   function for this would be:

   encryptedKey = KeyWrap(KDF(DH-Shared, context), CEK)

   The COSE_Encrypt structure for the recipient is organized as follows:

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

   *  The plaintext to be encrypted is the key from the next layer down
      (usually the content layer).

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

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









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10.  CBOR Encoding 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:

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

   *  Encoding MUST be done using definite lengths and values MUST be
      the minimum possible length.  This means that the integer 1 is
      encoded as "0x01" and not "0x1801".

   *  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.

11.  Application Profiling Considerations

   This document is designed to provide a set of security services, but
   not impose algorithm 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 are deferred to each application.

   An example of a profile can be found in [RFC8613] where one was
   developed for carrying content in combination with CoAP headers.

   It is intended that a profile of this document be created that
   defines the interoperability requirements for that specific
   application.  This section provides a set of guidelines and topics
   that need to be considered when profiling this document.

   *  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.

   *  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



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      Partial IV parameter.  If the Partial IV parameter is specified,
      then the application also needs to define how the fixed portion of
      the IV is determined.

   *  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 string.  More information
      can be found in Section 4.3.

   *  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.

   *  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] (note that [RFC2633] has been
         obsoleted by [RFC5751]).

12.  IANA Considerations

   The registeries and registrations listed below were created during
   processing of RFC 8152 [RFC8152].  The only known action at this time
   is to update the references.








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12.1.  CBOR Tag Assignment

   IANA assigned tags in the "CBOR Tags" registry as part of processing
   [RFC8152].  IANA is requested to update the references from [RFC8152]
   to this document.

   IANA is requested to register a new tag for the CounterSignature
   type.

   *  Tag: TBD0

   *  Data Item: COSE_Signature

   *  Semantics: COSE standalone counter signature

   *  Reference: [[this document]]

12.2.  COSE Header Parameters Registry

   IANA created a registry titled "COSE Header Parameters" as part of
   processing [RFC8152].  The registry has been created to use the
   "Expert Review Required" registration procedure [RFC8126].

   IANA is requested to update the reference for entries in the table
   from [RFC8152] to this document.  This document does not update the
   expert review guidelines provided in [RFC8152].

12.3.  COSE Header Algorithm Parameters Registry

   IANA created a registry titled "COSE Header Algorithm Parameters" as
   part of processing [RFC8152].  The registry has been created to use
   the "Expert Review Required" registration procedure [RFC8126].

   IANA is requested to update the references from [RFC8152] to this
   document.  This document does not update the expert review guidelines
   provided in [RFC8152].

12.4.  COSE Key Common Parameters Registry

   IANA created a registry titled "COSE Key Common Parameters" as part
   of the processing of [RFC8152].  The registry has been created to use
   the "Expert Review Required" registration procedure [RFC8126].

   IANA is requested to update the reference for entries in the table
   from [RFC8152] to this document.  This document does not update the
   expert review guidelines provided in [RFC8152].





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12.5.  Media Type Registrations

12.5.1.  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 message.

      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 [[This Document]].

      Interoperability considerations: N/A

      Published specification: [[this document]]

      Applications that use this media type: IoT applications sending
      security content over HTTP(S) transports.

      Fragment identifier considerations: N/A

      Additional information:

      -  Deprecated alias names for this type: N/A

      -  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



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      Change Controller: IESG

      Provisional registration?  No

12.5.2.  COSE Key Media Type

   This section registers the 'application/cose-key' and 'application/
   cose-key-set' 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.

   The template for registering 'application/cose-key' is:

      Type name: application

      Subtype name: cose-key

      Required parameters: N/A

      Optional parameters: N/A

      Encoding considerations: binary

      Security considerations: See the Security Considerations section
      of [[This Document]].

      Interoperability considerations: N/A

      Published specification: [[this document]]

      Applications that use this media type: Distribution of COSE based
      keys for IoT applications.

      Fragment identifier considerations: N/A

      Additional information:

      -  Deprecated alias names for this type: N/A

      -  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




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      Intended usage: COMMON

      Restrictions on usage: N/A

      Author: Jim Schaad, ietf@augustcellars.com

      Change Controller: IESG

      Provisional registration?  No

   The template for registering 'application/cose-key-set' is:

      Type name: application

      Subtype name: cose-key-set

      Required parameters: N/A

      Optional parameters: N/A

      Encoding considerations: binary

      Security considerations: See the Security Considerations section
      of [[This Document]].

      Interoperability considerations: N/A

      Published specification: [[this document]]

      Applications that use this media type: Distribution of COSE based
      keys for IoT applications.

      Fragment identifier considerations: N/A

      Additional information:

      -  Deprecated alias names for this type: N/A

      -  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



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      Restrictions on usage: N/A

      Author: Jim Schaad, ietf@augustcellars.com

      Change Controller: IESG

      Provisional registration?  No

12.6.  CoAP Content-Formats Registry

   IANA added the following entries to the "CoAP Content-Formats"
   registry while processing [RFC8152].  IANA is requested to update the
   reference value from [RFC8152] to [[This Document]].

13.  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 material for any
   individuals.  There are some cases that need to be highlighted on
   this issue.

   *  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.

   *  Use of 'direct' as a recipient algorithm combined with a second
      recipient algorithm exposes the direct key to the second
      recipient.

   *  Several of the algorithms in [I-D.ietf-cose-rfc8152bis-algs] 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,



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   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 this 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 the opportunity 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 encouraged 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 key form needs to be checked as well.  Do not use an
   'EC2' key where an 'OKP' key is expected.

   Before using a key for transmission, or before acting on information
   received, 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:

   *  What are the permissions associated with the key owner?

   *  Is the cryptographic algorithm acceptable in the current context?

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

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

   *  Have any security considerations that are part of the message been
      enforced (as specified by the application or 'crit' parameter)?

   There are a large number of algorithms presented in
   [I-D.ietf-cose-rfc8152bis-algs] that use nonce values.  Nonces
   generally have some type of restriction on their values.  Generally a
   nonce needs to be a unique value either for a key or for some other
   conditions.  In all of these cases, there is no known requirement on
   the nonce being both unique and unpredictable; under these
   circumstances, it's reasonable to use a counter for creation of the
   nonce.  In cases where one wants the pattern of the nonce to be
   unpredictable as well as unique, one can use a key created for that
   purpose and encrypt the counter to produce the nonce value.




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   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 the length for all of the content encryption
   algorithms that are defined in [I-D.ietf-cose-rfc8152bis-algs]
   document.  This means that it is up to the 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 '.)

14.  Implementation Status

   This section is to be removed before publishing as an RFC.

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   According to [RFC7942], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

14.1.  Author's Versions

   There are three different implementations that have been created by
   the author of the document both to create the examples that are
   included in the document and to validate the structures and
   methodology used in the design of COSE.

   *  Implementation Location: https://github.com/cose-wg

   *  Primary Maintainer: Jim Schaad





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   *  Languages: There are three different languages that are currently
      supported: Java, C# and C.

   *  Cryptography: The Java and C# libraries use Bouncy Castle to
      provide the required cryptography.  The C version uses OPENSSL
      Version 1.0 for the cryptography.

   *  Coverage: The C version currently does not have full countersign
      support.  The other two versions do.  They do have support to
      allow for implicit algorithm support as they allow for the
      application to set attributes that are not to be sent in the
      message.

   *  Testing: All of the examples in the example library are generated
      by the C# library and then validated using the Java and C
      libraries.  All three libraries have tests to allow for the
      creating of the same messages that are in the example library
      followed by validating them.  These are not compared against the
      example library.  The Java and C# libraries have unit testing
      included.  Not all of the MUST statements in the document have
      been implemented as part of the libraries.  One such statement is
      the requirement that unique labels be present.

   *  Licensing: Revised BSD License

14.2.  JavaScript Version

   *  Implementation Location: https://github.com/erdtman/cose-js

   *  Primary Maintainer: Samuel Erdtman

   *  Languages: JavaScript

   *  Cryptography: TBD

   *  Coverage: Full Encrypt, Signature and MAC objects are supported.

   *  Testing: Basic testing against the common example library.

   *  Licensing: Apache License 2.0

14.3.  Python Version

   *  Implementation Location: https://github.com/TimothyClaeys/COSE-
      PYTHON

   *  Primary Maintainer: Timothy Claeys




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   *  Languages: Python

   *  Cryptography: pyecdsak, crypto python libraries

   *  Coverage: TBD

   *  Testing: Basic testing plus running against the common example
      library.

   *  Licensing: BSD 3-Clause License

14.4.  COSE Testing Library

   *  Implementation Location: https://github.com/cose-wg/Examples

   *  Primary Maintainer: Jim Schaad

   *  Description: A set of tests for the COSE library is provided as
      part of the implementation effort.  Both success and fail tests
      have been provided.  All of the examples in this document are part
      of this example set.

   *  Coverage: An attempt has been made to have test cases for every
      message type and algorithm in the document.  Currently examples
      dealing with counter signatures, and ECDH with Curve24459 and
      Goldilocks are missing.

   *  Licensing: Public Domain

15.  References

15.1.  Normative References

   [COAP.Formats]
              IANA, "CoAP Content-Formats", August 2019,
              <https://www.iana.org/assignments/core-parameters/core-
              parameters.xhtml#content-formats>.

   [COSE.Algorithms]
              IANA, "COSE Algorithms", August 2019,
              <https://www.iana.org/assignments/cose/
              cose.xhtml#algorithms>.

   [COSE.KeyParameters]
              IANA, "COSE Key Parameters", August 2019,
              <https://www.iana.org/assignments/cose/cose.xhtml#key-
              common-parameters>.




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   [COSE.KeyTypes]
              IANA, "COSE Key Types", August 2019,
              <https://www.iana.org/assignments/cose/cose.xhtml#key-
              type>.

   [DSS]      National Institute of Standards and Technology, "Digital
              Signature Standard (DSS)", DOI 10.6028/NIST.FIPS.186-4,
              FIPS PUB 186-4, July 2013,
              <http://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.186-4.pdf>.

   [I-D.ietf-cose-rfc8152bis-algs]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", draft-ietf-cose-rfc8152bis-algs-03
              (work in progress), June 10, 2019,
              <https://www.ietf.org/archive/id/draft-ietf-cose-
              rfc8152bis-algs-03>.

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

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

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

15.2.  Informative References

   [PVSig]    Brown, D. and D. Johnson, "Formal Security Proofs for a
              Signature Scheme with Partial Message Recovery",
              DOI 10.1007/3-540-45353-9_11, LNCS Volume 2020, June 2000,
              <https://doi.org/10.1007/3-540-45353-9_11>.

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





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

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

   [RFC4998]  Gondrom, T., Brandner, R., and U. Pordesch, "Evidence
              Record Syntax (ERS)", RFC 4998, DOI 10.17487/RFC4998,
              August 2007, <https://www.rfc-editor.org/info/rfc4998>.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <https://www.rfc-editor.org/info/rfc5116>.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://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, <https://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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc5990>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

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





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

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

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

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

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

   [RFC8018]  Moriarty, K., Ed., Kaliski, B., and A. Rusch, "PKCS #5:
              Password-Based Cryptography Specification Version 2.1",
              RFC 8018, DOI 10.17487/RFC8018, January 2017,
              <https://www.rfc-editor.org/info/rfc8018>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,
              <https://www.rfc-editor.org/info/rfc8152>.

   [RFC8230]  Jones, M., "Using RSA Algorithms with CBOR Object Signing
              and Encryption (COSE) Messages", RFC 8230,
              DOI 10.17487/RFC8230, September 2017,
              <https://www.rfc-editor.org/info/rfc8230>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and



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              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.

   [W3C.WebCrypto]
              Watson, M., "Web Cryptography API", W3C Recommendation,
              January 2017, <https://www.w3.org/TR/WebCryptoAPI/>.

Appendix A.  Guidelines for External Data Authentication of Algorithms

   During development of COSE, the requirement that the algorithm
   identifier be located in the protected attributes was relaxed from a
   must to a should.  There were 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 that contains no attributes about itself.

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

   The key words from [RFC2119] are deliberately not used here.  This
   specification can provide recommendations, but it cannot enforce
   them.

   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 object.  This normally applies to the smallest
   of the COSE objects, specifically COSE_Sign1, COSE_Mac0, and
   COSE_Encrypt0, but could apply to the other structures as well.




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   The following items should be taken into account:

   *  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 identifier is a protected
      attribute.  This applies even if the transported algorithm is the
      same as the implicit algorithm.

   *  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 (if
      needed), the key, the algorithm, and the COSE structure it is used
      with.  Applications should restrict the use of a single key to a
      single algorithm.  As noted for some of the algorithms in
      [I-D.ietf-cose-rfc8152bis-algs], 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.

   *  In many cases, applications that make the algorithm identifier
      implicit 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.  (An exhaustive search over all of the
      keys 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 normally be distributed as part
      of the context.)

   *  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 context



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      provider) or if the system combines the security contexts from
      different applications together into a single store.

   *  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 that 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 that use a KDF 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 protected by virtue of being used in the cryptographic
      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 object.  An example of how this
   would work is the encryption context that an application specifies,
   which contains a content encryption algorithm, a key wrap algorithm,
   a key identifier, and a shared secret.  The sender omits sending the
   algorithm identifier for both the content layer and the recipient
   layer leaving only the key identifier.  The receiver then uses the
   key identifier to get the implicit algorithm identifiers.

   The following additional items need to be taken into consideration:

   *  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 as normal from the recipient layer.

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

   *  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, and 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.



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   *  Two contexts are distributed as a pair.  The first context is used
      for encryption of the message, and 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.

   *  Two contexts are distributed as a pair.  The first context
      contains a key for dealing with MACed messages, and 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 that have different keys, but where
      the keys may be used in a coordinated manner.

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

   *  Applications need to ensure that the multiple contexts stay
      associated.  If one of the contexts is invalidated for any reason,
      all of the contexts associated with it should also be invalidated.

Appendix B.  Two Layers of Recipient Information

   All of the currently defined recipient algorithm classes only use two
   layers of the COSE_Encrypt structure.  The first layer is the message
   content, and the second layer is the content key encryption.
   However, if one uses a recipient algorithm such as the RSA Key
   Encapsulation Mechanism (RSA-KEM) (see Appendix A of RSA-KEM
   [RFC5990]), then it makes sense to have three layers of the
   COSE_Encrypt structure.

   These layers would be:

   *  Layer 0: The content encryption layer.  This layer contains the
      payload of the message.

   *  Layer 1: The encryption of the CEK by a KEK.

   *  Layer 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:

   *  Layer 0: Has a content encrypted with AES-GCM using a 128-bit key.

   *  Layer 1: Uses the AES Key Wrap algorithm with a 128-bit key.



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   *  Layer 2: Uses ECDH Ephemeral-Static direct to generate the layer 1
      key.

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

   Size of binary file is 183 bytes

   96(
     [
       / protected / h'a10101' / {
           \ alg \ 1:1 \ AES-GCM 128 \
         } / ,
       / unprotected / {
         / iv / 5:h'02d1f7e6f26c43d4868d87ce'
       },
       / ciphertext / h'64f84d913ba60a76070a9a48f26e97e863e2852948658f0
   811139868826e89218a75715b',
       / recipients / [
         [
           / protected / h'',
           / unprotected / {
             / alg / 1:-3 / A128KW /
           },
           / ciphertext / h'dbd43c4e9d719c27c6275c67d628d493f090593db82
   18f11',
           / 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''
             ]
           ]
         ]
       ]
     ]
   )



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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 [RFC8610]) rather than
   as a binary dump.

   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.rb < 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 Messages

C.1.1.  Single Signature

   This example uses the following:

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

   Size of binary file is 103 bytes



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   98(
     [
       / protected / h'',
       / unprotected / {},
       / payload / 'This is the content.',
       / signatures / [
         [
           / protected / h'a10126' / {
               \ alg \ 1:-7 \ ECDSA 256 \
             } / ,
           / unprotected / {
             / kid / 4:'11'
           },
           / signature / h'e2aeafd40d69d19dfe6e52077c5d7ff4e408282cbefb
   5d06cbf414af2e19d982ac45ac98b8544c908b4507de1e90b717c3d34816fe926a2b
   98f53afd2fa0f30a'
         ]
       ]
     ]
   )

C.1.2.  Multiple Signers

   This example uses the following:

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

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

   Size of binary file is 277 bytes





















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   98(
     [
       / protected / h'',
       / unprotected / {},
       / payload / 'This is the content.',
       / signatures / [
         [
           / protected / h'a10126' / {
               \ alg \ 1:-7 \ ECDSA 256 \
             } / ,
           / unprotected / {
             / kid / 4:'11'
           },
           / signature / h'e2aeafd40d69d19dfe6e52077c5d7ff4e408282cbefb
   5d06cbf414af2e19d982ac45ac98b8544c908b4507de1e90b717c3d34816fe926a2b
   98f53afd2fa0f30a'
         ],
         [
           / protected / h'a1013823' / {
               \ alg \ 1:-36
             } / ,
           / unprotected / {
             / kid / 4:'bilbo.baggins@hobbiton.example'
           },
           / signature / h'00a2d28a7c2bdb1587877420f65adf7d0b9a06635dd1
   de64bb62974c863f0b160dd2163734034e6ac003b01e8705524c5c4ca479a952f024
   7ee8cb0b4fb7397ba08d009e0c8bf482270cc5771aa143966e5a469a09f613488030
   c5b07ec6d722e3835adb5b2d8c44e95ffb13877dd2582866883535de3bb03d01753f
   83ab87bb4f7a0297'
         ]
       ]
     ]
   )

C.1.3.  Counter Signature

   This example uses the following:

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

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

   Size of binary file is 180 bytes







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   98(
     [
       / protected / h'',
       / unprotected / {
         / countersign / 7:[
           / protected / h'a10126' / {
               \ alg \ 1:-7 \ ECDSA 256 \
             } / ,
           / unprotected / {
             / kid / 4:'11'
           },
           / signature / h'5ac05e289d5d0e1b0a7f048a5d2b643813ded50bc9e4
   9220f4f7278f85f19d4a77d655c9d3b51e805a74b099e1e085aacd97fc29d72f887e
   8802bb6650cceb2c'
         ]
       },
       / payload / 'This is the content.',
       / signatures / [
         [
           / protected / h'a10126' / {
               \ alg \ 1:-7 \ ECDSA 256 \
             } / ,
           / unprotected / {
             / kid / 4:'11'
           },
           / signature / h'e2aeafd40d69d19dfe6e52077c5d7ff4e408282cbefb
   5d06cbf414af2e19d982ac45ac98b8544c908b4507de1e90b717c3d34816fe926a2b
   98f53afd2fa0f30a'
         ]
       ]
     ]
   )

C.1.4.  Signature with Criticality

   This example uses the following:

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

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

   Size of binary file is 125 bytes









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   98(
     [
       / protected / h'a2687265736572766564f40281687265736572766564' /
   {
           "reserved":false,
           \ crit \ 2:[
             "reserved"
           ]
         } / ,
       / unprotected / {},
       / payload / 'This is the content.',
       / signatures / [
         [
           / protected / h'a10126' / {
               \ alg \ 1:-7 \ ECDSA 256 \
             } / ,
           / unprotected / {
             / kid / 4:'11'
           },
           / signature / h'3fc54702aa56e1b2cb20284294c9106a63f91bac658d
   69351210a031d8fc7c5ff3e4be39445b1a3e83e1510d1aca2f2e8a7c081c7645042b
   18aba9d1fad1bd9c'
         ]
       ]
     ]
   )

C.2.  Single Signer Examples

C.2.1.  Single ECDSA Signature

   This example uses the following:

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

   Size of binary file is 98 bytes















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   18(
     [
       / protected / h'a10126' / {
           \ alg \ 1:-7 \ ECDSA 256 \
         } / ,
       / unprotected / {
         / kid / 4:'11'
       },
       / payload / 'This is the content.',
       / signature / h'8eb33e4ca31d1c465ab05aac34cc6b23d58fef5c083106c4
   d25a91aef0b0117e2af9a291aa32e14ab834dc56ed2a223444547e01f11d3b0916e5
   a4c345cacb36'
     ]
   )

C.3.  Examples of Enveloped Messages

C.3.1.  Direct ECDH

   This example uses the following:

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

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

   Size of binary file is 151 bytes

























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   96(
     [
       / protected / h'a10101' / {
           \ alg \ 1:1 \ AES-GCM 128 \
         } / ,
       / unprotected / {
         / iv / 5:h'c9cf4df2fe6c632bf7886413'
       },
       / ciphertext / h'7adbe2709ca818fb415f1e5df66f4e1a51053ba6d65a1a0
   c52a357da7a644b8070a151b0',
       / 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:

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

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

      -  salt: "aabbccddeeffgghh"

      -  PartyU identity: "lighting-client"

      -  PartyV identity: "lighting-server"

      -  Supplementary Public Other: "Encryption Example 02"




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   Size of binary file is 91 bytes

   96(
     [
       / protected / h'a1010a' / {
           \ alg \ 1:10 \ AES-CCM-16-64-128 \
         } / ,
       / unprotected / {
         / iv / 5:h'89f52f65a1c580933b5261a76c'
       },
       / ciphertext / h'753548a19b1307084ca7b2056924ed95f2e3b17006dfe93
   1b687b847',
       / 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:

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

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

   Size of binary file is 326 bytes















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   96(
     [
       / 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'00929663c8789bb28177ae28467e66377da12302d7f9
   594d2999afa5dfa531294f8896f2b6cdf1740014f4c7f1a358e3a6cf57f4ed6fb02f
   cf8f7aa989f5dfd07f0700a3a7d8f3c604ba70fa9411bd10c2591b483e1d2c31de00
   3183e434d8fba18f17a4c7e3dfa003ac1cf3d30d44d2533c4989d3ac38c38b71481c
   c3430c9d65e7ddff'
         ]
       },
       / ciphertext / h'7adbe2709ca818fb415f1e5df66f4e1a51053ba6d65a1a0
   c52a357da7a644b8070a151b0',
       / 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.3.4.  Encrypted Content with External Data

   This example uses the following:




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

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

   *  Externally Supplied AAD: h'0011bbcc22dd44ee55ff660077'

   Size of binary file is 173 bytes

   96(
     [
       / protected / h'a10101' / {
           \ alg \ 1:1 \ AES-GCM 128 \
         } / ,
       / unprotected / {
         / iv / 5:h'02d1f7e6f26c43d4868d87ce'
       },
       / ciphertext / h'64f84d913ba60a76070a9a48f26e97e863e28529d8f5335
   e5f0165eee976b4a5f6c6f09d',
       / 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'41e0d76f579dbd0d936a662d54d8582037de2e366fd
   e1c62'
         ]
       ]
     ]
   )

C.4.  Examples of Encrypted Messages

C.4.1.  Simple Encrypted Message

   This example uses the following:

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

   Size of binary file is 52 bytes







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   16(
     [
       / protected / h'a1010a' / {
           \ alg \ 1:10 \ AES-CCM-16-64-128 \
         } / ,
       / unprotected / {
         / iv / 5:h'89f52f65a1c580933b5261a78c'
       },
       / ciphertext / h'5974e1b99a3a4cc09a659aa2e9e7fff161d38ce71cb45ce
   460ffb569'
     ]
   )

C.4.2.  Encrypted Message with a Partial IV

   This example uses the following:

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

   *  Prefix for IV is 89F52F65A1C580933B52

   Size of binary file is 41 bytes

   16(
     [
       / protected / h'a1010a' / {
           \ alg \ 1:10 \ AES-CCM-16-64-128 \
         } / ,
       / unprotected / {
         / partial iv / 6:h'61a7'
       },
       / ciphertext / h'252a8911d465c125b6764739700f0141ed09192de139e05
   3bd09abca'
     ]
   )

C.5.  Examples of MACed Messages

C.5.1.  Shared Secret Direct MAC

   This example uses the following:

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

   *  Recipient class: direct shared secret

   Size of binary file is 57 bytes




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   97(
     [
       / 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:

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

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

   Size of binary file is 214 bytes




















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   97(
     [
       / protected / h'a10105' / {
           \ alg \ 1:5 \ HMAC 256//256 \
         } / ,
       / unprotected / {},
       / payload / 'This is the content.',
       / tag / h'81a03448acd3d305376eaa11fb3fe416a955be2cbe7ec96f012c99
   4bc3f16a41',
       / 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:

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

   *  Recipient class: AES Key Wrap w/ a pre-shared 256-bit key

   Size of binary file is 109 bytes















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   97(
     [
       / 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 MACed Message

   This example uses the following:

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

   *  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 309 bytes















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   97(
     [
       / 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'339bc4f79984cdc6b3e6ce5f315a4c7d2b0ac466fce
   a69e8c07dfbca5bb1f661bc5f8e0df9e3eff5'
         ],
         [
           / 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 uses the following:

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



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   *  Recipient class: direct shared secret

   Size of binary file is 37 bytes

   17(
     [
       / 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 Section 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:

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

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

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

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

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

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

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

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

   *  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'
     }
   ]

Acknowledgments

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

   The following individuals are to blame for getting me started on this
   project in the first place: Richard Barnes, Matt Miller, and Martin
   Thomson.

   The initial version of the specification was based to some degree on
   the outputs of the JOSE and S/MIME working groups.

   The following individuals provided input into the final form of the
   document: Carsten Bormann, John Bradley, Brain Campbell, Michael B.




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Internet-Draft               COSE Structure                  August 2019


   Jones, Ilari Liusvaara, Francesca Palombini, Ludwig Seitz, and Goran
   Selander.

Author's Address

   Jim Schaad
   August Cellars

   Email: ietf@augustcellars.com










































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