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

COSE Working Group                                             J. Schaad
Internet-Draft                                            August Cellars
Intended status: Informational                               B. Campbell
Expires: February 29, 2016                                 Ping Identity
                                                         August 28, 2015


                      CBOR Encoded Message Syntax
                         draft-ietf-cose-msg-04

Abstract

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

Contributing to this document

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

Status of This Memo

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

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

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

   This Internet-Draft will expire on February 29, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Design changes from JOSE  . . . . . . . . . . . . . . . .   5
     1.2.  Requirements Terminology  . . . . . . . . . . . . . . . .   5
     1.3.  CBOR Grammar  . . . . . . . . . . . . . . . . . . . . . .   5
     1.4.  CBOR Related Terminology  . . . . . . . . . . . . . . . .   6
     1.5.  Document Terminology  . . . . . . . . . . . . . . . . . .   6
     1.6.  Mandatory to Implement Algorithms . . . . . . . . . . . .   7
   2.  The COSE_MSG structure  . . . . . . . . . . . . . . . . . . .   7
   3.  Header Parameters . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  Common COSE Headers Parameters  . . . . . . . . . . . . .  10
   4.  Signing Structure . . . . . . . . . . . . . . . . . . . . . .  13
     4.1.  Externally Supplied Data  . . . . . . . . . . . . . . . .  14
     4.2.  Signing and Verification Process  . . . . . . . . . . . .  15
   5.  Encryption object . . . . . . . . . . . . . . . . . . . . . .  16
     5.1.  Recipient Algorithm Classes . . . . . . . . . . . . . . .  17
     5.2.  Encryption Algorithm for AEAD algorithms  . . . . . . . .  18
     5.3.  Encryption algorithm for AE algorithms  . . . . . . . . .  19
   6.  MAC objects . . . . . . . . . . . . . . . . . . . . . . . . .  19
   7.  Key Structure . . . . . . . . . . . . . . . . . . . . . . . .  21
     7.1.  COSE Key Common Parameters  . . . . . . . . . . . . . . .  22
   8.  Signature Algorithms  . . . . . . . . . . . . . . . . . . . .  24
     8.1.  ECDSA . . . . . . . . . . . . . . . . . . . . . . . . . .  25
       8.1.1.  Security Considerations . . . . . . . . . . . . . . .  26
     8.2.  RSASSA-PSS  . . . . . . . . . . . . . . . . . . . . . . .  27
       8.2.1.  Security Considerations . . . . . . . . . . . . . . .  27
   9.  Message Authentication (MAC) Algorithms . . . . . . . . . . .  28
     9.1.  Hash-based Message Authentication Codes (HMAC)  . . . . .  28
       9.1.1.  Security Considerations . . . . . . . . . . . . . . .  29
     9.2.  AES Message Authentication Code (AES-CBC-MAC) . . . . . .  29
       9.2.1.  Security Considerations . . . . . . . . . . . . . . .  30
   10. Content Encryption Algorithms . . . . . . . . . . . . . . . .  30
     10.1.  AES GCM  . . . . . . . . . . . . . . . . . . . . . . . .  31
       10.1.1.  Security Considerations  . . . . . . . . . . . . . .  31
     10.2.  AES CCM  . . . . . . . . . . . . . . . . . . . . . . . .  32
       10.2.1.  Security Considerations  . . . . . . . . . . . . . .  34
     10.3.  ChaCha20 and Poly1305  . . . . . . . . . . . . . . . . .  35



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       10.3.1.  Security Considerations  . . . . . . . . . . . . . .  35
   11. Key Derivation Functions (KDF)  . . . . . . . . . . . . . . .  36
     11.1.  HMAC-based Extract-and-Expand Key Derivation Function
            (HKDF) . . . . . . . . . . . . . . . . . . . . . . . . .  36
     11.2.  Context Information Structure  . . . . . . . . . . . . .  38
   12. Recipient Algorithm Classes . . . . . . . . . . . . . . . . .  42
     12.1.  Direct Encryption  . . . . . . . . . . . . . . . . . . .  42
       12.1.1.  Direct Key . . . . . . . . . . . . . . . . . . . . .  42
       12.1.2.  Direct Key with KDF  . . . . . . . . . . . . . . . .  43
     12.2.  Key Wrapping . . . . . . . . . . . . . . . . . . . . . .  44
       12.2.1.  AES Key Wrapping . . . . . . . . . . . . . . . . . .  45
     12.3.  Key Encryption . . . . . . . . . . . . . . . . . . . . .  45
       12.3.1.  RSAES-OAEP . . . . . . . . . . . . . . . . . . . . .  46
     12.4.  Direct Key Agreement . . . . . . . . . . . . . . . . . .  46
       12.4.1.  ECDH . . . . . . . . . . . . . . . . . . . . . . . .  48
     12.5.  Key Agreement with KDF . . . . . . . . . . . . . . . . .  51
       12.5.1.  ECDH . . . . . . . . . . . . . . . . . . . . . . . .  51
   13. Keys  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  51
     13.1.  Elliptic Curve Keys  . . . . . . . . . . . . . . . . . .  52
       13.1.1.  Single Coordinate Curves . . . . . . . . . . . . . .  53
       13.1.2.  Double Coordinate Curves . . . . . . . . . . . . . .  54
     13.2.  RSA Keys . . . . . . . . . . . . . . . . . . . . . . . .  55
     13.3.  Symmetric Keys . . . . . . . . . . . . . . . . . . . . .  56
   14. CBOR Encoder Restrictions . . . . . . . . . . . . . . . . . .  57
   15. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  57
     15.1.  CBOR Tag assignment  . . . . . . . . . . . . . . . . . .  57
     15.2.  COSE Header Parameter Registry . . . . . . . . . . . . .  58
     15.3.  COSE Header Algorithm Label Table  . . . . . . . . . . .  58
     15.4.  COSE Algorithm Registry  . . . . . . . . . . . . . . . .  59
     15.5.  COSE Key Common Parameter Registry . . . . . . . . . . .  60
     15.6.  COSE Key Type Parameter Registry . . . . . . . . . . . .  61
     15.7.  COSE Elliptic Curve Registry . . . . . . . . . . . . . .  61
     15.8.  Media Type Registration  . . . . . . . . . . . . . . . .  62
       15.8.1.  COSE Security Message  . . . . . . . . . . . . . . .  62
       15.8.2.  COSE Key media type  . . . . . . . . . . . . . . . .  64
   16. Security Considerations . . . . . . . . . . . . . . . . . . .  66
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  66
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  66
     17.2.  Informative References . . . . . . . . . . . . . . . . .  66
   Appendix A.  Three Levels of Recipient Information  . . . . . . .  69
   Appendix B.  Examples . . . . . . . . . . . . . . . . . . . . . .  70
     B.1.  Examples of MAC messages  . . . . . . . . . . . . . . . .  71
       B.1.1.  Shared Secret Direct MAC  . . . . . . . . . . . . . .  71
       B.1.2.  ECDH Direct MAC . . . . . . . . . . . . . . . . . . .  72
       B.1.3.  Wrapped MAC . . . . . . . . . . . . . . . . . . . . .  73
       B.1.4.  Multi-recipient MAC message . . . . . . . . . . . . .  74
     B.2.  Examples of Encrypted Messages  . . . . . . . . . . . . .  76
       B.2.1.  Direct ECDH . . . . . . . . . . . . . . . . . . . . .  76



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       B.2.2.  Direct plus Key Derivation  . . . . . . . . . . . . .  76
     B.3.  Examples of Signed Message  . . . . . . . . . . . . . . .  77
       B.3.1.  Single Signature  . . . . . . . . . . . . . . . . . .  77
       B.3.2.  Multiple Signers  . . . . . . . . . . . . . . . . . .  78
     B.4.  COSE Keys . . . . . . . . . . . . . . . . . . . . . . . .  79
       B.4.1.  Public Keys . . . . . . . . . . . . . . . . . . . . .  79
       B.4.2.  Private Keys  . . . . . . . . . . . . . . . . . . . .  81
   Appendix C.  Document Updates . . . . . . . . . . . . . . . . . .  83
     C.1.  Version -03 to -04  . . . . . . . . . . . . . . . . . . .  84
     C.2.  Version -02 to -03  . . . . . . . . . . . . . . . . . . .  84
     C.3.  Version -02 to -03  . . . . . . . . . . . . . . . . . . .  84
     C.4.  Version -01 to -2 . . . . . . . . . . . . . . . . . . . .  84
     C.5.  Version -00 to -01  . . . . . . . . . . . . . . . . . . .  85
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  86

1.  Introduction

   There has been an increased focus on the small, constrained devices
   that make up the Internet of Things (IOT).  One of the standards that
   has come of of this process is the Concise Binary Object
   Representation (CBOR).  CBOR extended the data model of the
   JavaScript Object Notation (JSON) by allowing for binary data among
   other changes.  CBOR is being adopted by several of the IETF working
   groups dealing with the IOT world as their encoding of data
   structures.  CBOR was designed specifically to be both small in terms
   of messages transport and implementation size as well having a schema
   free decoder.  A need exists to provide basic 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] that defined how to perform
   encryption, signatures and message authentication (MAC) operations
   for JSON documents and then to encode the results using the JSON
   format [RFC7159].  This document does the same work for use with the
   CBOR [RFC7049] document format.  While there is a strong attempt to
   keep the flavor of the original JOSE documents, two considerations
   are taken into account:

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

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



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1.1.  Design changes from JOSE

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

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

   o  Recipient processing has been made more uniform.  A recipient
      structure is required for all recipients rather than only for
      some.

   o  MAC messages are separated from signed messages.

   o  MAC messages have the ability to do use all recipient algorithms
      on the MAC authentication key.

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

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

   o  Remove the flattened mode of encoding.  Forcing the use of an
      array of recipients at all times forces the message size to be two
      bytes larger, but one gets a corresponding decrease in the
      implementation size that should compensate for this.  [CREF1]

1.2.  Requirements Terminology

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

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

1.3.  CBOR Grammar

   There currently is no standard CBOR grammar available for use by
   specifications.  While we describe the CBOR structures in prose, they
   are agumented in the text by the use of the CBOR Data Definition
   Language (CDDL) [I-D.greevenbosch-appsawg-cbor-cddl].  The use of
   CDDL is intended to be explanitory.  In the event of a conflict
   between the text and the CDDL grammar, the text is authorative.




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   (Problems may be introduced at a later point because the CDDL grammar
   is not yet fixed.)

   CDDL productions that together define the grammar are interspersed in
   the document like this:


   start = COSE_MSG / COSE_Key / COSE_KeySet

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


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

1.4.  CBOR Related Terminology

   In JSON, maps are called objects and only have one kind of map key: a
   string.  In COSE, we use both strings and integers (both negative and
   non-negative integers) as map keys, as well as data items to identify
   specific choices.  The integers (both positive and negative) are used
   for compactness of encoding and easy comparison.  (Generally, in this
   document the value zero is going to be reserved and not used.)  Since
   the work "key" is mainly used in its other meaning, as a
   cryptographic key, we use the term "label" for this usage of either
   an integer or a string to identify map keys and choose data items.

   The CDLL grammar that defines a type that represents a label is given
   below:


   label = int / tstr
   values = any

1.5.  Document Terminology

   In this document we use the following terminology: [CREF2]

   Byte is a synonym for octet.

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





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1.6.  Mandatory to Implement Algorithms

   One of the issues that needs to be addressed is a requirement that a
   standard specify a set of algorithms that are required to be
   implemented.  [CREF3] This is done to promote interoperability as it
   provides a minimal set of algorithms that all devices can be sure
   will exist at both ends.  However, we have elected not to specify a
   set of mandatory algorithms in this document.

   It is expected that COSE is going to be used in a wide variety of
   applications and on a wide variety of devices.  Many of the
   constrained devices are going to be setup to used a small fixed set
   of algorithms, and this set of algorithms may not match those
   available on a device.  We therefore have deferred to the application
   protocols the decision of what to specify for mandatory algorithms.

   Since the set of algorithms in an environment of constrained devices
   may depend on what the set of devices are and how long they have been
   in operation, we want to highlight that application protocols will
   need to specify some type of discovery method of algorithm
   capabilities.  The discovery method may 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:

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

   o  Advertising in the certificate (capabilities extension) [RFC4262]

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

2.  The COSE_MSG structure

   The COSE_MSG structure is a top level CBOR object that corresponds to
   the DataContent type in the Cryptographic Message Syntax (CMS)
   [RFC5652].  [CREF4] This structure allows for a top level message to
   be sent that could be any of the different security services.  The
   security service is identified within the message.

   The COSE_Tagged_MSG CBOR type takes the COSE_MSG and prepends a CBOR
   tag of TBD1 to the encoding of COSE_MSG.  By having both a tagged and
   untagged version of the COSE_MSG structure, it becomes easy to either
   use COSE_MSG as a top level object or embedded in another object.
   The tagged version allows for a method of placing the COSE_MSG
   structure into a choice, using a consistent tag value to determine
   that this is a COSE object.




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   The existence of the COSE_MSG and COSE_Tagged_MSG CBOR data types are
   not intended to prevent protocols from using the individual security
   primitives directly.  Where only a single service is required, that
   structure can be used directly.

   Each of the top-level security objects use a CBOR array as the base
   structure.

   A field 'msg_type' is defined to distinguish between the different
   structures when they appear as part of a COSE_MSG object.  [CREF5]
   [CREF6]

      0 - Reserved.

      1 - Signed Message.

      2 - Encrypted Message

      3 - Authenticated Message (MACed message)

   Implementations MUST be prepared to find an integer in this field
   that does not correspond to the values 1 to 3.  If this is found then
   the client MUST stop attempting to process the structure and fail.
   The value of 0 is reserved and not to be used.  If the value of 0 is
   found, then clients MUST fail processing the structure.
   Implementations need to recognize that the set of values might be
   extended at a later date, but they should not provide a security
   service based on guesses of what is there.

   The CDDL grammar that corresponds to the above is:

   COSE_MSG = COSE_Sign /
       COSE_encrypt /
       COSE_mac

   COSE_Tagged_MSG = #6.999(COSE_MSG)   ; Replace 999 with TBD1

   ; msg_type values
   msg_type_reserved=0
   msg_type_signed=1
   msg_type_encrypted=2
   msg_type_mac=3

   The top level of each of the COSE message structures are encoded as
   arrays.






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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 in this
   document except for keys.  While these buckets can be present, they
   may not all be usable in all instances.  For example, while the
   protected bucket is defined as part of recipient structures, most of
   the algorithms that are used for recipients do not provide the
   necessary functionality to provide the needed protection and thus the
   bucket should not be used.

   Both buckets are implemented as CBOR maps.  The map key is a 'label'
   (Section 1.4).  The value portion is dependent on the definition for
   the label.  Both maps use the same set of label/value pairs.  The
   integer and string values for labels has been divided into several
   sections with a standard range, a private range, and a range that is
   dependent on the algorithm selected.  The defined labels can be found
   in the 'COSE Header Parameters' IANA registry (Section 15.2).

   Two buckets are provided for each layer: [CREF7]

   protected  contains parameters about the current layer that are to be
      cryptographically protected.  This bucket MUST be empty if it is
      not going to be included in a cryptographic computation.  This
      bucket is encoded in the message as a binary object.  This value
      is obtained by CBOR encoding the protected map and wrapping it in
      a bstr object.  Senders SHOULD encode an empty protected map as a
      zero length binary object (it is shorter).  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 finesses the problem of all parties needing to be
      able to do a common canonical encoding.

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

   Only parameters that deal with the current layer are to be placed at
   that layer.  As an example of this, the parameter 'content type'
   describes the content of the message being carried in the message.
   As such this parameter is placed only the the content layer and is
   not placed in the recipient or signature layers.  In principle, one



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   should be able to process any given layer without reference to any
   other layer.  (The only data that should need to cross layers is the
   cryptographic key.)

   The presence of both buckets is required.  The parameters that go
   into the buckets come from the IANA "COSE Header Parameters"
   (Section 15.2).  Some common parameters are defined in the next
   section, but a number of parameters are defined throughout this
   document.

   The CDDL fragment that describes the two buckets is:

   header_map = {+ label => any }

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

3.1.  Common COSE Headers Parameters

   This section defines a set of common header parameters.  A summary of
   those parameters can be found in Table 1.  This table should be
   consulted to determine the value of label used as well as the type of
   the value.

   The set of header parameters defined in this section are:

   alg  This parameter is used to indicate the algorithm used for the
      security processing.  This parameter MUST be present at each level
      of a signed, encrypted or authenticated message.  The value is
      taken from the 'COSE Algorithm Registry' (see Section 15.3).

   crit  This parameter is used to ensure that applications will take
      appropriate action based on the values found.  The parameter is
      used to indicate which protected header labels an application that
      is processing a message is required to understand.  The value is
      an array of COSE Header Labels.  When present, this parameter MUST
      be placed in the protected header bucket.


      *  Integer labels in the range of 0 to 10 SHOULD be omitted.

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



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         higher end will deal with more optional algorithm specific
         items.)

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

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

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

   nonce  This parameter holds either a nonce or Initialization Vector
      value.  The value can be used either as a counter value for a
      protocol or as an IV for an algorithm.

   +----------+-------+----------+-----------------+-------------------+
   | name     | label | value    | value registry  | description       |
   |          |       | type     |                 |                   |
   +----------+-------+----------+-----------------+-------------------+
   | alg      | 1     | int /    | COSE Algorithm  | Integers are      |
   |          |       | tstr     | Registry        | taken from table  |
   |          |       |          |                 | --POINT TO        |
   |          |       |          |                 | REGISTRY--        |
   |          |       |          |                 |                   |
   | crit     | 2     | [+       | COSE Header     | integer values    |
   |          |       | label]   | Label Registry  | are from  --      |
   |          |       |          |                 | POINT TO REGISTRY |
   |          |       |          |                 | --                |
   |          |       |          |                 |                   |
   | content  | 3     | tstr /   | CoAP Content-   | Value is either a |
   | type     |       | int      | Formats or      | Media Type or an  |



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   |          |       |          | Media Types     | integer from the  |
   |          |       |          | registry        | CoAP Content      |
   |          |       |          |                 | Format registry   |
   |          |       |          |                 |                   |
   | jku      | *     | tstr     |                 | URL to COSE key   |
   |          |       |          |                 | object            |
   |          |       |          |                 |                   |
   | jwk      | *     | COSE_Key |                 | contains a COSE   |
   |          |       |          |                 | key not a JWK key |
   |          |       |          |                 |                   |
   | kid      | 4     | bstr     |                 | key identifier    |
   |          |       |          |                 |                   |
   | nonce    | 5     | bstr     |                 | Nonce or          |
   |          |       |          |                 | Initialization    |
   |          |       |          |                 | Vector (IV)       |
   |          |       |          |                 |                   |
   | x5c      | *     | bstr*    |                 | X.509 Certificate |
   |          |       |          |                 | Chain             |
   |          |       |          |                 |                   |
   | x5t      | *     | bstr     |                 | SHA-1 thumbprint  |
   |          |       |          |                 | of key            |
   |          |       |          |                 |                   |
   | x5t#S256 | *     | bstr     |                 | SHA-256           |
   |          |       |          |                 | thumbprint of key |
   |          |       |          |                 |                   |
   | x5u      | *     | tstr     |                 | URL for X.509     |
   |          |       |          |                 | certificate       |
   |          |       |          |                 |                   |
   | zip      | *     | int /    |                 | Integers are      |
   |          |       | tstr     |                 | taken from the    |
   |          |       |          |                 | table --POINT TO  |
   |          |       |          |                 | REGISTRY--        |
   +----------+-------+----------+-----------------+-------------------+

                     Table 1: Common Header Parameters

   OPEN ISSUES:

   1.  Which of the following items do we want to have standardized in
       this document: jku, jwk, x5c, x5t, x5t#S256, x5u, zip

   2.  I am currently torn on the question "Should epk and iv/nonce be
       algorithm specific or generic headers?"  They are really specific
       to an algorithm and can potentially be defined in different ways
       for different algorithms.  As an example, it would make sense to
       defined nonce for CCM and GCM modes that can have the leading
       zero bytes stripped, while for other algorithms this might be
       undesirable.



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   3.  We might want to define some additional items.  What are they?  A
       possible example would be a sequence number as this might be
       common.  On the other hand, this is the type of things that is
       frequently used as the nonce in some places and thus should not
       be used in the same way.  Other items might be challenge/response
       fields for freshness as these are likely to be common.

4.  Signing Structure

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

   When more than one signature is present, the successful validation of
   one signature associated with a given signer is usually treated as a
   successful signature by that signer.  However, there are some
   application environments where other rules are needed.  An
   application that employs a rule other than one valid signature for
   each signer must specify those rules.  Also, where simple matching of
   the signer identifier is not sufficient to determine whether the
   signatures were generated by the same signer, the application
   specification must describe how to determine which signatures were
   generated by the same signer.  Support of different communities of
   recipients is the primary reason that signers choose to include more
   than one signature.  For example, the COSE_Sign structure might
   include signatures generated with the RSA signature algorithm and
   with the Elliptic Curve Digital Signature Algorithm (ECDSA) signature
   algorithm.  This allows recipients to verify the signature associated
   with one algorithm or the other.  (The original source of this text
   is [RFC5652].)  More detailed information on multiple signature
   evaluation can be found in [RFC5752].

   The CDDL grammar for a signature message is:

   COSE_Sign = [
       msg_type: msg_type_signed,
       Headers,
       payload : bstr,
       signatures : [+ COSE_signature]
   ]




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   The fields in the array have the following semantics:

   msg_type  identifies this as providing the signed security service.
      The value MUST be msg_type_signed (1).

   protected  is described in Section 3.

   unprotected  is described in Section 3.

   payload  contains the serialized content to be signed.  If the
      payload is not present in the message, the application is required
      to supply the payload separately.  The payload is wrapped in a
      bstr to ensure that it is transported without changes.  If the
      payload is transported separately, it is the responsibility of the
      application to ensure that it will be transported without changes.

   signatures  is an array of signature items.  Each of these items uses
      the COSE_signature structure for its representation.

   The CDDL grammar structure for a signature is:

   COSE_signature =  [
       Headers,
       signature : bstr
   ]

   The fields in the array have the following semantics:

   protected  is described in Section 3.

   unprotected  is described in Section 3.

   signature  contains the computed signature value.

4.1.  Externally Supplied Data

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




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   modified in transit.  This is the purpose of the externally supplied
   data field.

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

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

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

4.2.  Signing and Verification Process

   The COSE structure used to create the byte stream to be signed uses
   the following CDDL grammar structure:

   Sig_structure = [
       body_protected: bstr,
       sign_protected: bstr,
       external_aad: bstr,
       payload: bstr
   ]

   How to compute a signature:

   1.  Create a Sig_structure object and populate it with the
       appropriate fields.  For body_protected and sign_protected, if
       the fields are not present in their corresponding maps, an bstr
       of length zero is used.

   2.  If the application has supplied external additional authenticated
       data to be included in the computation, then it is placed in the
       'external_aad' field.  If no data was supplied, then a zero
       length binary value is used.

   3.  Create the value ToBeSigned by encoding the Sig_structure to a
       byte string.




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   4.  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).

   5.  Place the resulting signature value in the 'signature' field of
       the map.

   How to verify a signature:

   1.  Create a Sig_structure object and populate it with the
       appropriate fields.  For body_protected and sign_protected, if
       the fields are not present in their corresponding maps, an bstr
       of length zero is used.

   2.  If the application has supplied external additional authenticated
       data to be included in the computation, then it is placed in the
       'external_aad' field.  If no data was supplied, then a zero
       length binary value is used.

   3.  Create the value ToBeSigned by encoding the Sig_structure to a
       byte string.

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

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

5.  Encryption object

   The encryption 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 parameters associated with the content can be
   authenticated by the content encryption algorithm.  The parameters
   associated with the recipient can be authenticated by the recipient
   algorithm (when the algorithm supports it).  Examples of parameters
   about the content are the type of the content, when the content was
   created, and the content encryption algorithm.  Examples of
   parameters about the recipient are the recipients key identifier, the
   recipient encryption algorithm.

   In COSE, the same techniques and structures for encrypting both the
   plain text and the keys used to protect the text.  This is different
   from the approach used by both [RFC5652] and [RFC7516] where



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   different structures are used for the content layer and for the
   recipient layer.

   The CDDL grammar structure for encryption is:

   COSE_encrypt = [
       msg_type: msg_type_encrypted,
       COSE_encrypt_fields
   ]

   COSE_encrypt_fields = (
       Headers,
       ciphertext: bstr,
       ? recipients: [+[COSE_encrypt_fields]]
   )

   Description of the fields:

   msg_type  identifies this as providing the encrypted security
      service.  The value MUST be msg_type_encrypted (2).

   protected  is described in Section 3.

   unprotected  is described in Section 3.

   ciphertext  contains the encrypted plain text.  If the ciphertext is
      to be transported independently of the control information about
      the encryption process (i.e. detached content) then the field is
      omitted.

   recipients  contains the recipient information.  It is required that
      at least one recipient MUST be present for the content encryption
      layer.

5.1.  Recipient Algorithm Classes

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

   none:  The CEK is the same as as the identified previously
      distributed symmetric key or derived from a previously distributed
      secret.




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

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

   key transport:  the CEK is encrypted in the recipient's public key

   passwords:  the CEK is encrypted in a key-encryption key that is
      derived from a password.

5.2.  Encryption Algorithm for AEAD algorithms

   The encryption algorithm for AEAD algorithms is fairly simple.  In
   order to get a consistent encoding of the data to be authenticated,
   the Enc_structure is used to have canonical form of the AAD.

   Enc_structure = [
       protected: bstr,
       external_aad: bstr
   ]


   1.  Copy the protected header field from the message to be sent.

   2.  If the application has supplied external additional authenticated
       data to be included in the computation, then it is placed in the
       'external_aad' field.  If no data was supplied, then a zero
       length binary value is used.  (See Section 4.1 for application
       guidance on constructing this field.)

   3.  Encode the Enc_structure using a CBOR Canonical encoding
       Section 14 to get the AAD value.

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

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

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




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       Other:  The key is randomly generated.

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

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

5.3.  Encryption algorithm for AE algorithms

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

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

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

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

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

       Other:  The key is randomly generated.

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

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

6.  MAC objects

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





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

   COSE_mac = [
      msg_type: msg_type_mac,
      Headers,
      payload: bstr,
      tag: bstr,
      recipients: [+[COSE_encrypt_fields]]
   ]


   Field descriptions:

   msg_type  identifies this as providing the encrypted security
      service.  The value MUST be msg_type_mac (3).

   protected  is described in Section 3.

   unprotected  is described in Section 3.

   payload  contains the serialized content to be MACed.  If the payload
      is not present in the message, the application is required to
      supply the payload separately.  The payload is wrapped in a bstr
      to ensure that it is transported without changes, if the payload
      is transported separately it is the responsibility of the
      application to ensure that it will be transported without changes.

   tag  contains the MAC value.

   recipients  contains the recipient information.  See the description
      under COSE_Encryption for more info.

   MAC_structure = [
        protected: bstr,
        external_aad: bstr,
        payload: bstr
   ]



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   How to compute a MAC:

   1.  Create a MAC_structure and copy the protected and payload fields
       from the COSE_mac structure.

   2.  If the application has supplied external authenticated data,
       encode it as a binary value and place in the MAC_structure.  If
       there is no external authenticated data, then use a zero length
       'bstr'.  (See Section 4.1 for application guidance on
       constructing this field.)

   3.  Encode the MAC_structure using a canonical CBOR encoder.  The
       resulting bytes is the value to compute the MAC on.

   4.  Compute the MAC and place the result in the 'tag' field of the
       COSE_mac structure.

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

7.  Key Structure

   A COSE Key structure is built on a CBOR map object.  The set of
   common parameters that can appear in a COSE Key can be found in the
   IANA registry 'COSE Key Common Parameter Registry' (Section 15.5).
   Additional parameters defined for specific key types can be found in
   the IANA registry 'COSE Key Type Parameters' (Section 15.6).

   A COSE Key Set uses a CBOR array object as it's underlying type.  The
   values of the array elements are COSE Keys.  A Key Set MUST have at
   least one element in the array.  [CREF8]

   The CDDL grammar describing a COSE_Key and COSE_KeySet is: [CREF9]

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

   COSE_KeySet = [+COSE_Key]

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







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7.1.  COSE Key Common Parameters

   This document defines a set of common parameters for a COSE Key
   object.  Table 2 provides a summary of the parameters defined in this
   section.  There are also a set of parameters that are defined for a
   specific key type.  Key type specific parameters can be found in
   Section 13.

   +----------+-------+-------------+------------+---------------------+
   | name     | label | CBOR type   | registry   | description         |
   +----------+-------+-------------+------------+---------------------+
   | kty      | 1     | tstr / int  | COSE       | Identification of   |
   |          |       |             | General    | the key type        |
   |          |       |             | Values     |                     |
   |          |       |             |            |                     |
   | key_ops  | 4     | [*          |            | Restrict set of     |
   |          |       | (tstr/int)] |            | permissible         |
   |          |       |             |            | operations          |
   |          |       |             |            |                     |
   | alg      | 3     | tstr / int  | COSE       | Key usage           |
   |          |       |             | Algorithm  | restriction to this |
   |          |       |             | Values     | algorithm           |
   |          |       |             |            |                     |
   | kid      | 2     | bstr        |            | Key Identification  |
   |          |       |             |            | value - match to    |
   |          |       |             |            | kid in message      |
   |          |       |             |            |                     |
   | x5u      | *     | tstr        |            |                     |
   |          |       |             |            |                     |
   | x5c      | *     | bstr*       |            |                     |
   |          |       |             |            |                     |
   | x5t      | *     | bstr        |            |                     |
   |          |       |             |            |                     |
   | x5t#S256 | *     | bstr        |            |                     |
   |          |       |             |            |                     |
   | use      | *     | tstr        |            | deprecated - don't  |
   |          |       |             |            | use                 |
   +----------+-------+-------------+------------+---------------------+

                          Table 2: 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 can be found in Table 20.  This
      parameter MUST be present in a key object.

   alg:  This parameter is used to restrict the algorithms that are to
      be used with this key.  If this parameter is present in the key



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

   kid:  This parameter is used to give an identifier for a key.  The
      identifier is not structured and can be anything from a user
      provided string to a value computed on the public portion of the
      key.  This field is intended for matching against a 'kid'
      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 3.

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

                       Table 3: Key Operation Values

   The following provides a CDDL fragment which duplicates the
   assignment labels from Table 2 and Table 3.





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

   ;key_ops values
   key_ops_sign=1
   key_ops_verify=2
   key_ops_encrypt=3
   key_ops_decrypt=4
   key_ops_wrap=5
   key_ops_unwrap=6
   key_ops_agree=7

8.  Signature Algorithms

   There are two basic signature algorithm structures that can be used.
   The first is the common signature with appendix.  In this structure,
   the message content is processed and a signature is produced, the
   signature is called the appendix.  This is the message structure used
   by our common algorithms such as ECDSA and RSASSA-PSS.  (In fact the
   SSA in RSASSA-PSS stands for Signature Scheme with Appendix.)  The
   basic structure becomes:


   signature = Sign(message content, key)

   valid = Verification(message content, key, signature)


   The second is a signature with message recovery.  (An example of such
   an algorithm is [PVSig].)  In this structure, the message content is
   processed, but part of is included in the signature.  Moving bytes of
   the message content into the signature allows for an effectively
   smaller signature, the signature size is still potentially large, but
   the message content is shrunk.  This has implications for systems
   implementing these algoritms and for applications that use them.  The
   first is that the message content is not fully available until after
   a signature has been validated.  Until that point the part of the
   message contained inside of the signature is unrecoverable.  The
   second is that the security analysis of the strength of the signature
   is very much based on the structure of the message content.  Messages
   which are highly predictable require additional randomness to be
   supplied as part of the signature process, in the worst case it
   becomes the same as doing a signature with appendix.  Thirdly, in the
   event that multple signatures are applied to a message, all of the




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   signature algorithms are going to be required to consume the same
   number of bytes of message content.


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

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


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

8.1.  ECDSA

   ECDSA [DSS] defines a signature algorithm using ECC.

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

   The algorithms defined in this document can be found in Table 4.

              +-------+-------+---------+------------------+
              | name  | value | hash    | description      |
              +-------+-------+---------+------------------+
              | ES256 | -7    | SHA-256 | ECDSA w/ SHA-256 |
              |       |       |         |                  |
              | ES384 | -8    | SHA-384 | ECDSA w/ SHA-384 |
              |       |       |         |                  |
              | ES512 | -9    | SHA-512 | ECDSA w/ SHA-512 |
              +-------+-------+---------+------------------+

                      Table 4: ECDSA Algorithm Values

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

   The signature algorithm results in a pair of integers (R, S).  These
   integers will be of the same order as length of the key used for the
   signature process.  The signature is encoded by converting the
   integers into byte strings of the same length as the key size.  The
   length is rounded up to the nearest byte and is left padded with zero



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   bits to get to the correct length.  The two integers are then
   concatenated together to form a byte string that is the resulting
   signature.

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

8.1.1.  Security Considerations

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

   System which have poor random number generation can leak their keys
   by signing two different messages with the same value of 'k'.
   [RFC6979] provides a method to deal with this problem by making 'k'
   be deterministic based on the message content rather than randomly
   generated.  Applications which specify ECDSA should evaluate the
   ability to get good random number generation and require this when it
   is not possible.  Note: Use of this technique a good idea even when
   good random number generation exists.  Doing so both reduces the
   possiblity of having the same value of 'k' in two signature
   operations, but allows for reproducable signature values which helps
   testing.

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

   o  Changing the curve used to validate the signature: If one changes
      the curve used to validate the signature, then potentially one
      could have a two messages with the same signature each computed
      under a different curve.  The only requirement on the new curve is
      that it's order be the same as the old one and it be acceptable to
      the client.  An example would be to change from using the curve
      P-256 to using Curve25519.  (Both are 256 bit curves.)  We current
      do not have any way to deal with this version of the attack except
      to restrict the overall set of curves that can be used.

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




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8.2.  RSASSA-PSS

   The RSASSA-PSS signature algorithm is defined in [RFC3447].

   The RSASSA-PSS signature algorithm is parametized with a hash
   function (h), a mask generation function (mgf) and a salt length
   (sLen).  For this specification, the mask generation function is
   fixed to be MGF1 as defined in [RFC3447].  It has been recommended
   that the same hash function be used for hashing the data as well as
   in the mask generation function, for this specification we following
   this recommendation.  The salt length is the same length as the hash
   function output.

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

     +-------+-------+---------+-------------+-----------------------+
     | name  | value | hash    | salt length | description           |
     +-------+-------+---------+-------------+-----------------------+
     | PS256 | -26   | SHA-256 | 32          | RSASSA-PSS w/ SHA-256 |
     |       |       |         |             |                       |
     | PS384 | -27   | SHA-384 | 48          | RSASSA-PSS w/ SHA-384 |
     |       |       |         |             |                       |
     | PS512 | -28   | SHA-512 | 64          | RSASSA-PSS w/ SHA-512 |
     +-------+-------+---------+-------------+-----------------------+

                   Table 5: RSASSA-PSS Algorithm Values

8.2.1.  Security Considerations

   In addition to needing to worry about keys that are too small to
   provide the required security, there are issues with keys that are
   too large.  Denial of service attacks have been mounted with overly
   large keys.  This has the potential to consume resources with
   potentially bad keys.  There are two reasonable ways to address this
   attack.  First, a key should not be used for a cryptographic
   operation until it has been matched back to an authorized user.  This
   approach means that no cryptography would be done except for
   authorized users.  Second, applications can impose maximum as well as
   minimum length requirements on keys.  This limits the resources
   consumed even if the matching is not performed until the cryptography
   has been done.

   There is a theoretical hash substitution attack that can be mounted
   against RSASSA-PSS.  However, the requirement that the same hash
   function be used consistently for all operations is an effective
   mitigation against it.  Unlike ECDSA, hash functions are not
   truncated so that the full hash value is always signed.  The internal
   padding structure of RSASSA-PSS means that one needs to have multiple



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   collisions between the two hash functions in order to be successful
   in producing a forgery based on changing the hash function.  This is
   highly unlikely.

9.  Message Authentication (MAC) Algorithms

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

   MACs are designed in the same basic structure 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 basic structure becomes:


   tag = MAC_Create(message content, key)

   valid = MAC_Verify(message content, key, tag)


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

9.1.  Hash-based Message Authentication Codes (HMAC)

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

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




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

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

                      Table 6: HMAC Algorithm Values

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

9.1.1.  Security Considerations

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

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

   AES-CBC-MAC is defined in [MAC].

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








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   +-------------+-------+----------+----------+-----------------------+
   | name        | value | key      | tag      | description           |
   |             |       | length   | length   |                       |
   +-------------+-------+----------+----------+-----------------------+
   | AES-MAC     | *     | 128      | 64       | AES-MAC 128 bit key,  |
   | 128/64      |       |          |          | 64-bit tag            |
   |             |       |          |          |                       |
   | AES-MAC     | *     | 256      | 64       | AES-MAC 256 bit key,  |
   | 256/64      |       |          |          | 64-bit tag            |
   |             |       |          |          |                       |
   | AES-MAC     | *     | 128      | 128      | AES-MAC 128 bit key,  |
   | 128/128     |       |          |          | 128-bit tag           |
   |             |       |          |          |                       |
   | AES-MAC     | *     | 256      | 128      | AES-MAC 256 bit key,  |
   | 256/128     |       |          |          | 128-bit tag           |
   +-------------+-------+----------+----------+-----------------------+

                     Table 7: AES-MAC Algorithm Values

9.2.1.  Security Considerations

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

   o  A single key must only be used for messages of a fixed and known
      length.  If this is not the case, an attacker will be able to
      generated a message with a valid tag given two message, tag pairs.
      This can be addressed by using different keys for different length
      messages.  (CMAC mode also addresses this issue.)

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

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

10.  Content Encryption Algorithms

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

   We restrict the set of legal content encryption algorithms to those
   which support authentication both of the content and additional data.
   The encryption process will generate some type of authentication



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   value, but that value may be either explicit or implicit in terms of
   the algorithm definition.  For simplicity sake, the authentication
   code will normally be defined as being appended to the cipher text
   stream.  The basic structure becomes:


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

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


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

10.1.  AES GCM

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

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

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

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

                   Table 8: Algorithm Value for AES-GCM

10.1.1.  Security Considerations

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

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





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

10.2.  AES CCM

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

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

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

   We define a matrix of algorithms in this document over the values of
   L and M.  Constrained devices are usually operating in situations
   where they use short messages and want to avoid doing recipient
   specific cryptographic operations.  This favors smaller values of M
   and larger values of L.  Less constrained devices do will want to be
   able to user larger messages and are more willing to generate new
   keys for every operation.  This favors larger values of M and smaller
   values of L.  (The use of a large nonce means that random generation
   of both the key and the nonce will decrease the chances of repeating
   the pair on two different messages.)

   The following values are used for L:

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

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



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   The following values are used for M:

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

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










































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

                   Table 9: Algorithm Values for AES-CCM

10.2.1.  Security Considerations

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

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




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

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

10.3.  ChaCha20 and Poly1305

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

   The ChaCha20/Poly1305 AEAD construction defined in [RFC7539] has no
   parameterization.  It takes a 256-bit key and an a 96-bit nonce as
   well as the plain text and additional data as inputs and produces the
   cipher text as an option.  We define one algorithm identifier for
   this algorithm in Table 10.

     +-------------------+-------+----------------------------------+
     | name              | value | description                      |
     +-------------------+-------+----------------------------------+
     | ChaCha20/Poly1305 | 11    | ChaCha20/Poly1305 w/ 256-bit key |
     +-------------------+-------+----------------------------------+

                   Table 10: Algorithm Value for AES-GCM

10.3.1.  Security Considerations

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







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11.  Key Derivation Functions (KDF)

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

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

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

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

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

   Many functions are going to handle the first two type of secrets
   differently.  The KDF function defined in Section 11.1 can use
   different underlying constructions if the secret is uniformly random
   than if the secret is not uniformly random.  This is reflected in the
   set of algorithms defined for HKDF.

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

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

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

   The HKDF algorithm is defined to take a number of inputs These inputs
   are:

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

      salt - an optional public value that is used to change the
      generation process.  If specified, the salt is carried using the



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      'salt' algorithm parameter.  While [RFC5869] suggests that the
      length of the salt be the same as the length of the underlying
      hash value, any amount of salt will improve the security as
      different key values will be generated.  A parameter to carry the
      salt is defined in Table 12.  This parameter is protected by being
      included in the key computation and does not need to be separately
      authenticated.

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

      context information - Information that describes the context in
      which the resulting value will be used.  Making this information
      specific to the context that the material is going to be used
      ensures that the resulting material will always be unique.  The
      context structure used is encoded into the algorithm identifier.

      hash function - The underlying hash function to be used in the
      HKDF algorithm.  The hash function is encoded into the HKDF
      algorithm selection.

   HKDF is defined to use HMAC as the underlying PRF.  However, it is
   possible to use other functions in the same construct to provide a
   different KDF function that may be more appropriate in the
   constrained world.  Specifically, one can use AES-CBC-MAC as the PRF
   for the expand step, but not for the extract step.  When using a good
   random shared secret of the correct length, the extract step can be
   skipped.  The extract cannot be skipped if the secret is not
   uniformly random, for example if it is the result of a ECDH key
   agreement step.

   The algorithms defined in this document are found in Table 11




















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   +-------------+-------------+----------+----------------------------+
   | name        | hash        | Skip     | context                    |
   |             |             | extract  |                            |
   +-------------+-------------+----------+----------------------------+
   | HKDF        | SHA-256     | no       | XXX                        |
   | SHA-256     |             |          |                            |
   |             |             |          |                            |
   | HKDF        | SHA-512     | no       | XXX                        |
   | SHA-512     |             |          |                            |
   |             |             |          |                            |
   | HKDF AES-   | AES-CBC-128 | yes      | HKDF using AES-MAC as the  |
   | MAC-128     |             |          | PRF w/ 128-bit key         |
   |             |             |          |                            |
   | HKDF AES-   | AES-CBC-128 | yes      | HKDF using AES-MAC as the  |
   | MAC-256     |             |          | PRF w/ 256-bit key         |
   +-------------+-------------+----------+----------------------------+

                         Table 11: HKDF algorithms

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

                    Table 12: HKDF Algorithm Parameters

11.2.  Context Information Structure

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

   The context information structure refers to PartyU and PartyV as the
   two parties which are doing the key derivation.  Unless the
   application protocol defines differently, we assign PartyU to the
   entity that is creating the message and PartyV to the entity that is
   receiving the message.  This is because we are assuming a set of
   standalone store and forward messaging processes.  In [SP800-56A],
   PartyU is the initiator and PartyV is the responder.  The
   specification is written with the idea of on-line protocols rather
   than store and forward protocols as the main consumer.




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   Application protocols are free to define the roles differently.  For
   example, they could assign the PartyU role to the entity that
   initiates the connection and allow directly sending multiple messages
   over the connection in both directions without changing the role
   information.

   Using the PartyU and PartyV fields is the easiest way to get
   different keys in each direction.  The use of a transaction
   identifier, either in one of the supplemental fields or as the salt
   if one is using HKDF, ensures that a unique key is generated for each
   set of transactions.  Combining nonce fields with the transaction
   identifier provides a method so that a different key is used for each
   message in each direction.

   We encode the context specific information using a CBOR array type.
   For the fields that we define an algorithm parameter, the details of
   the parameters can be found in Table 13.  The fields in the array
   are:

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

   PartyUInfo  This field holds information about party U.  The
      ParytUInfo structure is divided into three pieces:

      identity  This contains the identity information for party U.  The
         identities can be assigned in one of two manners.  Firstly, a
         protocol can assign identities based on roles.  For example,
         the roles of "client" and "server" may be assigned to different
         entities in the protocol.  Each entity would then use the
         correct label for the data they they send or receive.  The
         second way is for a protocol to assign identities is to use a
         name based on a naming system (i.e.  DNS, X.509 names).
         We define an algorithm parameter 'PartyU identity' that can be
         used to carry identity information in the message.  However,
         identity information is often known as part of the protocol and
         can thus be inferred rather than made explicit.  If identity
         information is carried in the message, applications SHOULD have
         a way of validating the supplied identity information.  The
         identity information does not need to be specified and can be
         left as absent.



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         The identity value supplied will be integrity checked as part
         of the key derivation process.  If the identity string is
         wrong, then the wrong key will be created.

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

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

   PartyVInfo  M00TODO: Copy down from PartyUInfo when that text is
      ready.

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

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

      protected  This field contains the protected parameter field.

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

   SuppPrivInfo  This field contains private information that is
      mutually known information.  An example of this information would
      be a pre-existing shared secret.  The field is optional and will
      only be present if the application defines a structure for this
      information.  Applications that define this SHOULD use CBOR to
      encode the data so that types and lengths are correctly include.






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

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

                  Table 13: Context Algorithm Parameters








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12.  Recipient Algorithm Classes

   Recipient algorithms can be defined into a number of different
   classes.  COSE has the ability to support many classes of recipient
   algorithms.  In this section, a number of classes are listed and then
   a set of algorithms are specified for each of the classes.  The names
   of the recipient algorithm classes used here are the same as are
   defined in [RFC7517].  Other specifications use different terms for
   the recipient algorithm classes or do not support some of our
   recipient algorithm classes.

12.1.  Direct Encryption

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

   The COSE_encrypt structure for the recipient is organized as follows:

   o  The 'protected' field MUST be a zero length item if it is not used
      in the computation of the content key.

   o  The 'alg' parameter MUST be present.

   o  A parameter identifying the shared secret SHOULD be present.

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

   o  The 'recipients' field MUST be absent.

12.1.1.  Direct Key

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

   When this algorithm is used, the protected field MUST be zero length.

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

                           Table 14: Direct Key




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12.1.1.1.  Security Considerations

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

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

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

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

12.1.2.  Direct Key with KDF

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

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

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

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




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

                           Table 15: Direct Key

12.1.2.1.  Security Considerations

   The shared secret need to have some method to be regularly updated
   over time.  The shared secret is forming the basis of trust, although
   not used directly it should still be subject to scheduled rotation.

12.2.  Key Wrapping

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

   The COSE_encrypt structure for the recipient is organized as follows:

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

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

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



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

12.2.1.  AES Key Wrapping

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

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

                  Table 16: AES Key Wrap Algorithm Values

12.2.1.1.  Security Considerations for AES-KW

   The shared secret need to have some method to be regularly updated
   over time.  The shared secret is forming the basis of trust, although
   not used directly it should still be subject to scheduled rotation.

12.3.  Key Encryption

   Key Encryption mode is also called key transport mode in some
   standards.  Key Encryption mode differs from Key Wrap mode in that it
   uses an asymmetric encryption algorithm rather than a symmetric
   encryption algorithm to protect the key.  This document defines one
   Key Encryption mode algorithm.

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

   o  The 'protected' field MUST be absent.

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




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

12.3.1.  RSAES-OAEP

   RSAES-OAEP is an asymmetric key encryption algorithm.  The defintion
   of RSAEA-OAEP can be find in Section 7.1 of [RFC3447].  The algorithm
   is parameterized using a masking generation function (mgf), a hash
   function (h) and encoding parameters (P).  For the algorithm
   identifiers defined in this section:

   o  mgf is always set to MFG1 from [RFC3447] and uses the same hash
      function as h.

   o  P is always set to the empty octet string.

   Table 17 summarizes the rest of the values.

    +----------------------+-------+---------+-----------------------+
    | name                 | value | hash    | description           |
    +----------------------+-------+---------+-----------------------+
    | RSAES-OAEP w/SHA-256 | -25   | SHA-256 | RSAES OAEP w/ SHA-256 |
    |                      |       |         |                       |
    | RSAES-OAEP w/SHA-512 | -26   | SHA-512 | RSAES OAEP w/ SHA-512 |
    +----------------------+-------+---------+-----------------------+

                   Table 17: RSAES-OAEP Algorithm Values

12.3.1.1.  Security Considerations for RSAES-OAEP

   A key size of 2048 bits or larger MUST be used with these algorithms.
   This key size corresponds roughly to the same strength as provided by
   a 128-bit symmetric encryption algorithm.

   It is highly recommended that checks on the key length be done before
   starting a decryption operation.  One potential denial of service
   operation is to provide encrypted objects using either abnormally
   long or oddly sized RSA modulus values.  Implementations SHOULD be
   able to encrypt and decrypt with modulus between 2048 and 16K bits in
   length.  Applications can impose additional restrictions on the
   length of the modulus.

12.4.  Direct Key Agreement

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



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

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

   Two variants of DH that are easily supported are:

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

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

   In this specification, both variants are specified.  This has been
   done to provide the weak data origination option for use with MAC
   operations.

   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:

   o  The 'protected' field MUST be absent.

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

   o  The 'unprotected' field MUST contain the 'epk' parameter.







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12.4.1.  ECDH

   The basic mathematics for Elliptic Curve Diffie-Hellman can be found
   in [RFC6090].  Two new curves have been defined in
   [I-D.irtf-cfrg-curves].

   ECDH is parameterized by the following:

   o  Curve Type/Curve: The curve selected controls not only the size of
      the shared secret, but the mathematics for computing the shared
      secret.  The curve selected also controls how a point in the curve
      is represented and what happens for the identity points on the
      curve.  In this specification we allow for a number of different
      curves to be used.  The curves are defined in Table 21.
      Since the only the math is changed by changing the curve, the
      curve is not fixed for any of the algorithm identifiers we define,
      instead it is defined by the points used.

   o  Ephemeral-static or static-static: The key agreement process may
      be done using either a static or an ephemeral key at the senders
      side.  When using ephemeral keys, the sender MUST generate a new
      ephemeral key for every key agreement operation.  The ephemeral
      key is placed in in the 'ephemeral key' parameter and MUST be
      present for all algorithm identifiers which use ephemeral keys.
      When using static keys, the sender MUST either generate a new
      random value placed in either in the KDF parameters or the context
      structure.  For the KDF functions used, this means either in the
      'salt' parameter for HKDF (Table 12) or in in the 'PartyU nonce'
      parameter for the context struture (Table 13) MUST be present.
      (Both may be present if desired.)  The value in the parameter MUST
      be unique for the key pair being used.  It is acceptable to use a
      global counter which is incremented for every static-static
      operation and use the resulting value.  When using static keys,
      the static key needs to be identified to the recipient.  The
      static key can be identified either by providing the key ('static
      key') or by providing a key identifier for the static key ('static
      key id').  Both of these parameters are defined in Table 19

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

   o  Key Wrap algorithm: The key wrap algorithm can be 'none' if the
      result of the KDF is going to be used as the key directly.  This



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      option, along with static-static, should be used if knowledge
      about the sender is desired.  If 'none' is used then the content
      layer encryption algorithm size is value fed to the context
      structure.  Support is also provided for any of the key wrap
      algorithms defined in section Section 12.2.1.  If one of these
      options is used, the input key size to the key wrap algorithm is
      the value fed into the context structure as the key size.

   The set of algorithms direct ECDH defined in this document are found
   in Table 18.

   +-------------+------+-------+----------------+--------+------------+
   | name        | valu | KDF   | Ephemeral-     | Key    | descriptio |
   |             | e    |       | Static         | Wrap   | n          |
   +-------------+------+-------+----------------+--------+------------+
   | ECDH-ES +   | 50   | HKDF  | yes            | none   | ECDH ES w/ |
   | HKDF-256    |      | - SHA |                |        | HKDF -     |
   |             |      | -256  |                |        | generate   |
   |             |      |       |                |        | key        |
   |             |      |       |                |        | directly   |
   |             |      |       |                |        |            |
   | ECDH-ES +   | 51   | HKDF  | yes            | none   | ECDH ES w/ |
   | HKDF-512    |      | - SHA |                |        | HKDF -     |
   |             |      | -256  |                |        | generate   |
   |             |      |       |                |        | key        |
   |             |      |       |                |        | directly   |
   |             |      |       |                |        |            |
   | ECDH-SS +   | 52   | HKDF  | no             | none   | ECDH ES w/ |
   | HKDF-256    |      | - SHA |                |        | HKDF -     |
   |             |      | -256  |                |        | generate   |
   |             |      |       |                |        | key        |
   |             |      |       |                |        | directly   |
   |             |      |       |                |        |            |
   | ECDH-SS +   | 53   | HKDF  | no             | none   | ECDH ES w/ |
   | HKDF-512    |      | - SHA |                |        | HKDF -     |
   |             |      | -256  |                |        | generate   |
   |             |      |       |                |        | key        |
   |             |      |       |                |        | directly   |
   |             |      |       |                |        |            |
   | ECDH-       | 54   | HKDF  | yes            | A128KW | ECDH ES w/ |
   | ES+A128KW   |      | - SHA |                |        | Concat KDF |
   |             |      | -256  |                |        | and AES    |
   |             |      |       |                |        | Key wrap   |
   |             |      |       |                |        | w/ 128 bit |
   |             |      |       |                |        | key        |
   |             |      |       |                |        |            |
   | ECDH-       | 55   | HKDF  | yes            | A192KW | ECDH ES w/ |
   | ES+A192KW   |      | - SHA |                |        | Concat KDF |



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   |             |      | -256  |                |        | and AES    |
   |             |      |       |                |        | Key wrap   |
   |             |      |       |                |        | w/ 192 bit |
   |             |      |       |                |        | key        |
   |             |      |       |                |        |            |
   | ECDH-       | 56   | HKDF  | yes            | A256KW | ECDH ES w/ |
   | ES+A256KW   |      | - SHA |                |        | Concat KDF |
   |             |      | -256  |                |        | and AES    |
   |             |      |       |                |        | Key wrap   |
   |             |      |       |                |        | w/ 256 bit |
   |             |      |       |                |        | key        |
   |             |      |       |                |        |            |
   | ECDH-       | 57   | HKDF  | no             | A128KW | ECDH SS w/ |
   | SS+A128KW   |      | - SHA |                |        | Concat KDF |
   |             |      | -256  |                |        | and AES    |
   |             |      |       |                |        | Key wrap   |
   |             |      |       |                |        | w/ 128 bit |
   |             |      |       |                |        | key        |
   |             |      |       |                |        |            |
   | ECDH-       | 58   | HKDF  | no             | A192KW | ECDH SS w/ |
   | SS+A192KW   |      | - SHA |                |        | Concat KDF |
   |             |      | -256  |                |        | and AES    |
   |             |      |       |                |        | Key wrap   |
   |             |      |       |                |        | w/ 192 bit |
   |             |      |       |                |        | key        |
   |             |      |       |                |        |            |
   | ECDH-       | 59   | HKDF  | no             | A256KW | ECDH SS w/ |
   | SS+A256KW   |      | - SHA |                |        | Concat KDF |
   |             |      | -256  |                |        | and AES    |
   |             |      |       |                |        | Key wrap   |
   |             |      |       |                |        | w/ 256 bit |
   |             |      |       |                |        | key        |
   +-------------+------+-------+----------------+--------+------------+

                      Table 18: ECDH Algorithm Values
















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

                    Table 19: ECDH Algorithm Parameters

12.5.  Key Agreement with KDF

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

   The COSE_encrypt structure for the recipient is organized as follows:

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

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

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

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

12.5.1.  ECDH

   These algorithms are defined in Table 18.

13.  Keys

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

   For each of the key types, we define both public and private members.
   The public members are what is transmitted to others for their usage.



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   We define private members mainly for the purpose of archival of keys
   by individuals.  However, there are some circumstances where private
   keys may be distributed by various entities in a protocol.  Examples
   include: Entities which have poor random number generation.
   Centralized key creation for multi-cast type operations.  Protocols
   where a shared secret is used as a bearer token for authorization
   purposes.

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

    +-----------+-------+--------------------------------------------+
    | name      | value | description                                |
    +-----------+-------+--------------------------------------------+
    | EC1       | 1     | Elliptic Curve Keys w/ X Coordinate only   |
    |           |       |                                            |
    | EC2       | 2     | Elliptic Curve Keys w/ X,Y Coordinate pair |
    |           |       |                                            |
    | RSA       | 3     | RSA Keys                                   |
    |           |       |                                            |
    | Symmetric | 4     | Symmetric Keys                             |
    |           |       |                                            |
    | Reserved  | 0     | This value is reserved                     |
    +-----------+-------+--------------------------------------------+

                         Table 20: Key Type Values

13.1.  Elliptic Curve Keys

   Two different key structures are being defined for Elliptic Curve
   keys.  One version uses both an x and a y coordinate, potentially
   with point compression.  This is the traditional EC point
   representation that is used in [RFC5480].  The other version uses
   only the x coordinate as the y coordinate is either to be recomputed
   or not needed for the key agreement operation.  An example of this is
   Curve25519 [I-D.irtf-cfrg-curves].















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   +------------+----------+-------+-----------------------------------+
   | name       | key type | value | description                       |
   +------------+----------+-------+-----------------------------------+
   | P-256      | EC2      | 1     | NIST P-256 also known as          |
   |            |          |       | secp256r1                         |
   |            |          |       |                                   |
   | P-384      | EC2      | 2     | NIST P-384 also known as          |
   |            |          |       | secp384r1                         |
   |            |          |       |                                   |
   | P-521      | EC2      | 3     | NIST P-521 also known as          |
   |            |          |       | secp521r1                         |
   |            |          |       |                                   |
   | Curve25519 | EC1      | 1     | Curve 25519                       |
   |            |          |       |                                   |
   | Curve448   | EC1      | 2     | Curve 448                         |
   +------------+----------+-------+-----------------------------------+

                            Table 21: EC Curves

13.1.1.  Single Coordinate Curves

   One class of Elliptic Curve mathematics allows for a point to be
   completely defined using the curve and the x coordinate of the point
   on the curve.  The two curves that are initially setup to use is
   point format are Curve 25519 and Curve 448 which are defined in
   [I-D.irtf-cfrg-curves].

   For EC keys with only the x coordinates, the 'kty' member is set to 1
   (EC1).  The key parameters defined in this section are summarized in
   Table 22.  The members that are defined for this key type are:

   crv  contains an identifier of the curve to be used with the key.
      [CREF10] The curves defined in this document for this key type can
      be found in Table 21.  Other curves may be registered in the
      future and private curves can be used as well.

   x  contains the x coordinate for the EC point.  The octet string
      represents a little-endian encoding of x.

   d  contains the private key.

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





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

                        Table 22: EC Key Parameters

13.1.2.  Double Coordinate Curves

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

   For EC keys with both coordinates, the 'kty' member is set to 2
   (EC2).  The key parameters defined in this section are summarized in
   Table 23.  The members that are defined for this key type are:

   crv  contains an identifier of the curve to be used with the key.
      The curves defined in this document for this key type can be found
      in Table 21.  Other curves may be registered in the future and
      private curves can be used as well.

   x  contains the x coordinate for the EC point.  The integer is
      converted to an octet string as defined in [SEC1].  Zero octets
      MUST NOT be removed from the front of the octet string.  [CREF11]

   y  contains either the sign bit or the value of y coordinate for the
      EC point.  For the value, the integer is converted to an octet
      string as defined in [SEC1].  Zero octets MUST NOT be removed from
      the front of the octet string.  For the sign bit, the value is
      true if the value of y is positive.

   d  contains the private key.

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




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   that 'x' and 'y' also be present, but they can be recomputed from the
   required elements and omitting them saves on space.

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

                        Table 23: EC Key Parameters

13.2.  RSA Keys

   This document defines a key structure for both the public and private
   halves of RSA keys.  Together, an RSA public key and an RSA private
   key form an RSA key pair.  [CREF12]

   The document also provides support for the so-called "multi-prime"
   RSA where the modulus may have more than two prime factors.  The
   benefit of multi-prime RSA is lower computational cost for the
   decryption and signature primitives.  For a discussion on how multi-
   prime affects the security of RSA crypto-systems, the reader is
   referred to [MultiPrimeRSA].

   This document follows the naming convention of [RFC3447] for the
   naming of the fields of an RSA public or private key.  The table
   Table 24 provides a summary of the label values and the types
   associated with each of those labels.  The requirements for fields
   for RSA keys are as follows:

   o  For all keys, 'kty' MUST be present and MUST have a value of 3.

   o  For public keys, the fields 'n' and 'e' MUST be present.  All
      other fields defined in Table 24 MUST be absent.

   o  For private keys with two primes, the fields 'other', 'r_i', 'd_i'
      and 't_i' MUST be absent, all other fields MUST be present.





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   o  For private keys with more than two primes, all fields MUST be
      present.  For the third to nth primes, each of the primes is
      represented as a map containing the fields 'r_i', 'd_i' and 't_i'.
      The field 'other' is an array of those maps.

   +-------+----------+-------+-------+--------------------------------+
   | name  | key type | value | type  | description                    |
   +-------+----------+-------+-------+--------------------------------+
   | n     | 3        | -1    | bstr  | Modulus Parameter              |
   |       |          |       |       |                                |
   | e     | 3        | -2    | int   | Exponent Parameter             |
   |       |          |       |       |                                |
   | d     | 3        | -3    | bstr  | Private Exponent Parameter     |
   |       |          |       |       |                                |
   | p     | 3        | -4    | bstr  | First Prime Factor             |
   |       |          |       |       |                                |
   | q     | 3        | -5    | bstr  | Second Prime Factor            |
   |       |          |       |       |                                |
   | dP    | 3        | -6    | bstr  | First Factor CRT Exponent      |
   |       |          |       |       |                                |
   | dQ    | 3        | -7    | bstr  | Second Factor CRT Exponent     |
   |       |          |       |       |                                |
   | qInv  | 3        | -8    | bstr  | First CRT Coefficient          |
   |       |          |       |       |                                |
   | other | 3        | -9    | array | Other Primes Info              |
   |       |          |       |       |                                |
   | r_i   | 3        | -10   | bstr  | i-th factor, Prime Factor      |
   |       |          |       |       |                                |
   | d_i   | 3        | -11   | bstr  | i-th factor, Factor CRT        |
   |       |          |       |       | Exponent                       |
   |       |          |       |       |                                |
   | t_i   | 3        | -12   | bstr  | i-th factor, Factor CRT        |
   |       |          |       |       | Coefficient                    |
   +-------+----------+-------+-------+--------------------------------+

                       Table 24: RSA Key Parameters

13.3.  Symmetric Keys

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

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

   k  contains the value of the key.





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

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

                    Table 25: Symmetric Key Parameters

14.  CBOR Encoder Restrictions

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

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

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

   o  All parsers used SHOULD fail on both parsing and generation if the
      same label is used twice as a key for the same map.

15.  IANA Considerations

15.1.  CBOR Tag assignment

   It is requested that IANA assign a new tag from the "Concise Binary
   Object Representation (CBOR) Tags" registry.  It is requested that
   the tag be assigned in the 0 to 23 value range.

   Tag Value: TBD1

   Data Item: COSE_Msg

   Semantics: COSE security message.






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15.2.  COSE Header Parameter Registry

   It is requested that IANA create a new registry entitled "COSE Header
   Parameters".

   The columns of the registry are:

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

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

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

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

   description  This contains a brief description of the header field.

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

   The initial contents of the registry can be found in Table 1.  The
   specification column for all rows in that table should be this
   document.

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

15.3.  COSE Header Algorithm Label Table

   It is requested that IANA create a new registry entitled "COSE Header
   Algorithm Labels".

   The columns of the registry are:





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

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

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

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

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

   description  This contains a brief description of the header field.

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

   The initial contents of the registry can be found in: Table 12,
   Table 13, Table 19.  The specification column for all rows in that
   table should be this document.

15.4.  COSE Algorithm Registry

   It is requested that IANA create a new registry entitled "COSE
   Algorithm Registry".

   The columns of the registry are:

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

   description  A short description of the algorithm.





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

   The initial contents of the registry can be found in the following:
   Table 9, Table 8, Table 10, Table 4, Table 5, Table 6, Table 7,
   Table 14, Table 15, Table 16, Table 17, Table 18.  The specification
   column for all rows in that table should be this document.

15.5.  COSE Key Common Parameter Registry

   It is requested that IANA create a new registry entitled "COSE Key
   Common Parameter" Registry.

   The columns of the registry are:

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

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

   CBOR Type  This field contains the CBOR type for the field

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

   description  This field contains a brief description for the field

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

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








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15.6.  COSE Key Type Parameter Registry

   It is requested that IANA create a new registry "COSE Key Type
   Parameters".

   The columns of the table are:

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

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

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

   CBOR type  This field contains the CBOR type for the field

   description  This field contains a brief description for the field

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

   This registry will be initially populated by the values in Table 22,
   Table 23, Table 24, and Table 25.  The specification column for all
   of these entries will be this document.

15.7.  COSE Elliptic Curve Registry

   It is requested that IANA create a new registry "COSE Elliptic Curve
   Parameters".

   The columns of the table are:

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

   value  This is the value used to identify the curve.  These values
      MUST be unique.  The integer values from -256 to 255 are
      designated as Standards Track Document Required.  The the integer
      values from 256 to 65535 and -65536 to -257 are designated as
      Specification Required.  Integer values over 65535 are designated
      as first come first serve.  Integer values less than -65536 are
      marked as private use.





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

   description  This field contains a brief description of the curve.

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

   This registry will be initially populated by the values in Table 20.
   The specification column for all of these entries will be this
   document.

15.8.  Media Type Registration

15.8.1.  COSE Security Message

   This section registers the "application/cose" and "application/
   cose+cbor" media types in the "Media Types" registry.  [CREF13] These
   media types are used to indicate that the content is a COSE_MSG.

      Type name: application

      Subtype name: cose

      Required parameters: N/A

      Optional parameters: N/A

      Encoding considerations: binary

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

      Interoperability considerations: N/A

      Published specification: RFC TBD

      Applications that use this media type: To be identified

      Fragment identifier considerations: N/A

      Additional information:

      *  Magic number(s): N/A

      *  File extension(s): cbor

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



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

      Intended usage: COMMON

      Restrictions on usage: N/A

      Author: Jim Schaad, ietf@augustcellars.com

      Change Controller: IESG

      Provisional registration?  No

      Type name: application

      Subtype name: cose+cbor

      Required parameters: N/A

      Optional parameters: N/A

      Encoding considerations: binary

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

      Interoperability considerations: N/A

      Published specification: RFC TBD

      Applications that use this media type: To be identified

      Fragment identifier considerations: N/A

      Additional information:

      *  Magic number(s): N/A

      *  File extension(s): cbor

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

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

      Intended usage: COMMON

      Restrictions on usage: N/A



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      Author: Jim Schaad, ietf@augustcellars.com

      Change Controller: IESG

      Provisional registration?  No

15.8.2.  COSE Key media type

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

      Type name: application

      Subtype name: cose-key+cbor

      Required parameters: N/A

      Optional parameters: N/A

      Encoding considerations: binary

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

      Interoperability considerations: N/A

      Published specification: RFC TBD

      Applications that use this media type: To be identified

      Fragment identifier considerations: N/A

      Additional information:

      *  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



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      Author: Jim Schaad, ietf@augustcellars.com

      Change Controller: IESG

      Provisional registration?  No

      Type name: application

      Subtype name: cose-key-set+cbor

      Required parameters: N/A

      Optional parameters: N/A

      Encoding considerations: binary

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

      Interoperability considerations: N/A

      Published specification: RFC TBD

      Applications that use this media type: To be identified

      Fragment identifier considerations: N/A

      Additional information:

      *  Magic number(s): N/A

      *  File extension(s): cbor

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

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

      Intended usage: COMMON

      Restrictions on usage: N/A

      Author: Jim Schaad, ietf@augustcellars.com

      Change Controller: IESG

      Provisional registration?  No




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16.  Security Considerations

   There are security considerations:

   1.  Protect private keys

   2.  MAC messages with more than one recipient means one cannot figure
       out who sent the message

   3.  Use of direct key with other recipient structures hands the key
       to other recipients.

   4.  Use of direct ECDH direct encryption is easy for people to leak
       information on if there are other recipients in the message.

   5.  Considerations about protected vs unprotected header fields.

17.  References

17.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, October 2013.

17.2.  Informative References

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

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

   [I-D.greevenbosch-appsawg-cbor-cddl]
              Vigano, C., Birkholz, H., and R. Sun, "CBOR data
              definition language: a notational convention to express
              CBOR data structures.", draft-greevenbosch-appsawg-cbor-
              cddl-05 (work in progress), March 2015.

   [I-D.irtf-cfrg-curves]
              Langley, A. and R. Salz, "Elliptic Curves for Security",
              draft-irtf-cfrg-curves-02 (work in progress), March 2015.






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   [I-D.mcgrew-aead-aes-cbc-hmac-sha2]
              McGrew, D., Foley, J., and K. Paterson, "Authenticated
              Encryption with AES-CBC and HMAC-SHA", draft-mcgrew-aead-
              aes-cbc-hmac-sha2-05 (work in progress), July 2014.

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

   [MultiPrimeRSA]
              Hinek, M. and D. Cheriton, "On the Security of Multi-prime
              RSA", June 2006.

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

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104, February
              1997.

   [RFC2633]  Ramsdell, B., "S/MIME Version 3 Message Specification",
              RFC 2633, June 1999.

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

   [RFC3394]  Schaad, J. and R. Housley, "Advanced Encryption Standard
              (AES) Key Wrap Algorithm", RFC 3394, September 2002.

   [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, February 2003.

   [RFC3610]  Whiting, D., Housley, R., and N. Ferguson, "Counter with
              CBC-MAC (CCM)", RFC 3610, September 2003.

   [RFC4231]  Nystrom, M., "Identifiers and Test Vectors for HMAC-SHA-
              224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512", RFC
              4231, December 2005.

   [RFC4262]  Santesson, S., "X.509 Certificate Extension for Secure/
              Multipurpose Internet Mail Extensions (S/MIME)
              Capabilities", RFC 4262, December 2005.






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   [RFC5480]  Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
              "Elliptic Curve Cryptography Subject Public Key
              Information", RFC 5480, March 2009.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, September 2009.

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, January 2010.

   [RFC5752]  Turner, S. and J. Schaad, "Multiple Signatures in
              Cryptographic Message Syntax (CMS)", RFC 5752, January
              2010.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869, May 2010.

   [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, September
              2010.

   [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
              Curve Cryptography Algorithms", RFC 6090, February 2011.

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, March 2011.

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

   [RFC7159]  Bray, T., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, March 2014.

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

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, May 2015.

   [RFC7516]  Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
              RFC 7516, May 2015.



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   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517, May 2015.

   [RFC7518]  Jones, M., "JSON Web Algorithms (JWA)", RFC 7518, May
              2015.

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

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

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

Appendix A.  Three Levels of Recipient Information

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

   These levels would be:

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

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

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

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

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

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

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





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

   Size of binary file is 214 bytes

   [
     2,
     h'a10101',
     {
       5: h'02d1f7e6f26c43d4868d87ce'
     },
     h'64f84d913ba60a76070a9a48f26e97e863e285295a44320878caceb0763a3
   34806857c67',
     [
       [
         h'',
         {
           1: -3
         },
         h'5a15dbf5b282ecb31a6074ee3815c252405dd7583e078188',
         [
           [
             h'',
             {
               1: 50,
               4: h'6d65726961646f632e6272616e64796275636b406275636b
   6c616e642e6578616d706c65',
               -1: {
                 1: 2,
                 -1: 1,
                 -2: h'b2add44368ea6d641f9ca9af308b4079aeb519f11e9b8
   a55a600b21233e86e68',
                 -3: h'1a2cf118b9ee6895c8f415b686d4ca1cef362d4a7630a
   31ef6019c0c56d33de0'
               }
             },
             h''
           ]
         ]
       ]
     ]
   ]

Appendix B.  Examples

   The examples can be found at https://github.com/cose-wg/Examples.
   The file names in each section correspond the the same file names in
   the repository.  I am currently still in the process of getting the



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   examples up there along with some control information for people to
   be able to check and reproduce the examples.

   Examples may have some features that are in questions but not yet
   incorporated in the document.

   To make it easier to read, the examples are presented using the
   CBOR's diagnostic notation rather than a binary dump.  A ruby based
   tool exists to convert between a number of formats.  This tool can be
   installed with the command line:

           gem install cbor-diag

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


            diag2cbor < inputfile > outputfile


   The examples can be extracted from the XML version of this docuent
   via an XPath expression as all of the artwork is tagged with the
   attribute type='CBORdiag'.

B.1.  Examples of MAC messages

B.1.1.  Shared Secret Direct MAC

   This example users the following:

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

   o  Recipient class: direct shared secret

   o  File name: Mac-04

   Size of binary file is 71 bytes














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   [
     3,
     h'a1016f4145532d434d41432d3235362f3634',
     {
     },
     h'546869732069732074686520636f6e74656e742e',
     h'd9afa663dd740848',
     [
       [
         h'',
         {
           1: -6,
           4: h'6f75722d736563726574'
         },
         h''
       ]
     ]
   ]

B.1.2.  ECDH Direct MAC

   This example uses the following:

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

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

   Size of binary file is 215 bytes






















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   [
     3,
     h'a10104',
     {
     },
     h'546869732069732074686520636f6e74656e742e',
     h'2ba937ca03d76c3dbad30cfcbaeef586f9c0f9ba616ad67e9205d38576ad9
   930',
     [
       [
         h'',
         {
           1: 52,
           4: h'6d65726961646f632e6272616e64796275636b406275636b6c61
   6e642e6578616d706c65',
           -3: h'706572656772696e2e746f6f6b407475636b626f726f7567682
   e6578616d706c65',
           "apu": h'4d8553e7e74f3c6a3a9dd3ef286a8195cbf8a23d19558ccf
   ec7d34b824f42d92bd06bd2c7f0271f0214e141fb779ae2856abf585a58368b01
   7e7f2a9e5ce4db5'
         },
         h''
       ]
     ]
   ]

B.1.3.  Wrapped MAC

   This example uses the following:

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

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

   Size of binary file is 122 bytes
















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   [
     3,
     h'a1016e4145532d3132382d4d41432d3634',
     {
     },
     h'546869732069732074686520636f6e74656e742e',
     h'6d1fa77b2dd9146a',
     [
       [
         h'',
         {
           1: -5,
           4: h'30313863306165352d346439622d343731622d626664362d6565
   66333134626337303337'
         },
         h'711ab0dc2fc4585dce27effa6781c8093eba906f227b6eb0'
       ]
     ]
   ]

B.1.4.  Multi-recipient MAC message

   This example uses the following:

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

   o  Recipient class: Uses three different methods

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

      2.  RSA-OAEP w/ SHA-256

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

   Size of binary file is 670 bytes

   [
     3,
     h'a10104',
     {
     },
     h'546869732069732074686520636f6e74656e742e',
     h'7aaa6e74546873061f0a7de21ff0c0658d401a68da738dd893748651983ce
   1d0',
     [
       [
         h'',



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         {
           1: 55,
           4: h'62696c626f2e62616767696e7340686f626269746f6e2e657861
   6d706c65',
           -1: {
             1: 2,
             -1: 3,
             -2: h'43b12669acac3fd27898ffba0bcd2e6c366d53bc4db71f909
   a759304acfb5e18cdc7ba0b13ff8c7636271a6924b1ac63c02688075b55ef2d61
   3574e7dc242f79c3',
             -3: h'812dd694f4ef32b11014d74010a954689c6b6e8785b333d1a
   b44f22b9d1091ae8fc8ae40b687e5cfbe7ee6f8b47918a07bb04e9f5b1a51a334
   a16bc09777434113'
           }
         },
         h'f20ad9c96134f3c6be4f75e7101c0ecc5efa071ff20a87fd1ac285109
   41ee0376573e2b384b56b99'
       ],
       [
         h'',
         {
           1: -26,
           4: h'62696c626f2e62616767696e7340686f626269746f6e2e657861
   6d706c65'
         },
         h'46c4f88069b650909a891e84013614cd58a3668f88fa18f3852940a20
   b35098591d3aacf91c125a2595cda7bee75a490579f0e2f20fd6bc956623bfde3
   029c318f82c426dac3463b261c981ab18b72fe9409412e5c7f2d8f2b5abaf780d
   f6a282db033b3a863fa957408b81741878f466dcc437006ca21407181a016ca60
   8ca8208bd3c5a1ddc828531e30b89a67ec6bb97b0c3c3c92036c0cb84aa0f0ce8
   c3e4a215d173bfa668f116ca9f1177505afb7629a9b0b5e096e81d37900e06f56
   1a32b6bc993fc6d0cb5d4bb81b74e6ffb0958dac7227c2eb8856303d989f93b4a
   051830706a4c44e8314ec846022eab727e16ada628f12ee7978855550249ccb58
   '
       ],
       [
         h'',
         {
           1: -5,
           4: h'30313863306165352d346439622d343731622d626664362d6565
   66333134626337303337'
         },
         h'0b2c7cfce04e98276342d6476a7723c090dfdd15f9a518e7736549e99
   8370695e6d6a83b4ae507bb'
       ]
     ]
   ]




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B.2.  Examples of Encrypted Messages

B.2.1.  Direct ECDH

   This example uses the following:

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

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

   Size of binary file is 182 bytes

   [
     2,
     h'a10101',
     {
       5: h'c9cf4df2fe6c632bf7886413'
     },
     h'45fce2814311024d3a479e7d3eed063850f3f0b9f3f948677e3ae9869bcf9
   ff4e1763812',
     [
       [
         h'',
         {
           1: 50,
           4: h'6d65726961646f632e6272616e64796275636b406275636b6c61
   6e642e6578616d706c65',
           -1: {
             1: 2,
             -1: 1,
             -2: h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf05
   4e1c7b4d91d6280',
             -3: h'f01400b089867804b8e9fc96c3932161f1934f4223069170d
   924b7e03bf822bb'
           }
         },
         h''
       ]
     ]
   ]

B.2.2.  Direct plus Key Derivation

   This example uses the following:

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





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

      *  APU identity: "lighting-client"

      *  APV identity: "lighting-server"

      *  Supplimentary Public Other: "Encryption Example 02"

   Size of binary file is 95 bytes

   [
     2,
     h'a1010a',
     {
       5: h'89f52f65a1c580933b5261a7'
     },
     h'7b9dcfa42c4e1d3182c402dc18ef8b5637de4fb62cf1dd156ea6e6e0',
     [
       [
         h'',
         {
           1: "dir+kdf",
           4: h'6f75722d736563726574',
           -20: h'61616262636364646565666667676868'
         },
         h''
       ]
     ]
   ]

B.3.  Examples of Signed Message

B.3.1.  Single Signature

   This example uses the following:

   o  Signature Algorithm: RSA-PSS w/ SHA-384, MGF-1

   Size of binary file is 330 bytes











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   [
     1,
     h'',
     {
     },
     h'546869732069732074686520636f6e74656e742e',
     [
       [
         h'a20165505333383404581e62696c626f2e62616767696e7340686f626
   269746f6e2e6578616d706c65',
         {
         },
         h'6d9d88a90ef4d6d7c0079fb11a33c855e2274c773f358df43b68f7873
   eeda210692a61d70cd6a24ba0e3d82e359384be09faafea496bb0ed16f02091c4
   8c02f33574edab5b3e334bae68d19580021327cc131fbee38eb0b28289dbce118
   3f9067891b17fe752674b80437da02e9928ab7a155fef707b11d2bd38a71f224f
   53170480116d96cc3f7266487b268679a13cdedffa93252a550371acc19971369
   b58039056b308cc4e158bebe7c55db7874442d4321fd27f17dbb820ef19f43dcc
   16cd50ccdd1b7dfd7cdde239a9245af41d949cdbbf1337ca254af20eeb167a62d
   a5a51c83899c6f6e7c7e01dc3db21a250092a69fc635b74a2e54f5c98cb955d83
   '
       ]
     ]
   ]

B.3.2.  Multiple Signers

   This example uses the following:

   o  Signature Algorithm: RSA-PSS w/ SHA-256, MGF-1

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

   Size of binary file is 496 bytes

















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   [
     1,
     h'',
     {
     },
     h'546869732069732074686520636f6e74656e742e',
     [
       [
         h'a1013819',
         {
           4: h'62696c626f2e62616767696e7340686f626269746f6e2e657861
   6d706c65'
         },
         h'0ee972d931c7ab906e4bb71b80da0cc99c104fa53ebbf1f2cf7b668b9
   3d766d3d2da28299f074675bb0db3cd0792ba83050c23c96795d58f9c7d68f66a
   bbb8f35af8a0b5df369517b6db85e2cb62d852b666bc135c9022e46b538f78c26
   adc2668963e74a019de718254385bb9cb137926ad6a88d1ff70043f85e555fb57
   84107ce6e9de7c89c4fbadf8eca363a35f415f7a23523a8331b1aa2dfbac59a06
   3e4357bde8e53fe34195d59bcda37d2c604804fffe60362e81476436aaa677129
   f34b26639fc41b8e758e5edf273079c61b30130f0f83c57aa6856347e2556f718
   eaf79a1fee1397a4f0b16b1b34db946eaaff10c793e5d1e681cb21c4fd20c5fdf
   '
       ],
       [
         h'',
         {
           1: -9,
           4: h'62696c626f2e62616767696e7340686f626269746f6e2e657861
   6d706c65'
         },
         h'0118eaa7d62778b5a9525a583f06b115d80cd246bc930f0c2850588ee
   c85186b427026e096a076bfab738215f354be59f57643a7f6b2c92535cf3c37ee
   2746a908ab1dcc673a63f327d9eff852b874f7a98b6638c7054fdeeaa3dce6542
   4a21bd5dc728acedda7fcae6df6fc3298ff51ac911603a0f26d066935dccb85ea
   eb0ae6d0e6'
       ]
     ]
   ]

B.4.  COSE Keys

B.4.1.  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:




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

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

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

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

   Size of binary file is 703 bytes

   [
     {
       -1: 1,
       -2: h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de4
   39c08551d',
       -3: h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eec
   d0084d19c',
       1: 2,
       2: h'6d65726961646f632e6272616e64796275636b406275636b6c616e64
   2e6578616d706c65'
     },
     {
       -1: 1,
       -2: h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b
   4d91d6280',
       -3: h'f01400b089867804b8e9fc96c3932161f1934f4223069170d924b7e
   03bf822bb',
       1: 2,
       2: h'706572656772696e2e746f6f6b407475636b626f726f7567682e6578
   616d706c65'
     },
     {
       -1: 3,
       -2: h'0072992cb3ac08ecf3e5c63dedec0d51a8c1f79ef2f82f94f3c737b
   f5de7986671eac625fe8257bbd0394644caaa3aaf8f27a4585fbbcad0f2457620
   085e5c8f42ad',
       -3: h'01dca6947bce88bc5790485ac97427342bc35f887d86d65a089377e
   247e60baa55e4e8501e2ada5724ac51d6909008033ebc10ac999b9d7f5cc2519f
   3fe1ea1d9475',
       1: 2,
       2: h'62696c626f2e62616767696e7340686f626269746f6e2e6578616d70
   6c65'
     },
     {
       -2: h'9f810fb4038273d02591e4073f31d2b6001b82cedb4d92f050165d4
   7cfcab8a3c41cb778ac7553793f8ef975768d1a2374d8712564c3bcd77b9ea434
   544899407cff0099920a931a24c4414852ab29bdb0a95c0653f36c60e60bf90b6
   258dda56f37047ba5c2d1d029af9c9d40bac7aa41c78a0dd1068add699e808fea



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   011ea1441d8a4f7bb4e97be39f55f1ddd44e9c4ba335159703d4d34b603e65147
   a4f23d6d3c0996c75edee846a82d190ae10783c961cf0387aed2106d2d0555b6f
   d937fad5535387e0ff72ffbe78941402b0b822ea2a74b6058c1dabf9b34a76cb6
   3b87faa2c6847b8e2837fff91186e6b1c14911cf989a89092a81ce601ddacd3f9
   cf',
       -1: h'010001',
       1: 3,
       2: h'62696c626f2e62616767696e7340686f626269746f6e2e6578616d70
   6c65'
     }
   ]

B.4.2.  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:

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

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

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

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

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

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

   Size of binary file is 1884 bytes

   [
     {
       1: 2,
       2: h'6d65726961646f632e6272616e64796275636b406275636b6c616e64
   2e6578616d706c65',
       -1: 1,
       -2: h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de4
   39c08551d',
       -3: h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eec
   d0084d19c',
       -4: h'aff907c99f9ad3aae6c4cdf21122bce2bd68b5283e6907154ad9118
   40fa208cf'
     },
     {



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       1: 4,
       2: h'6f75722d736563726574',
       -1: h'849b57219dae48de646d07dbb533566e976686457c1491be3a76dce
   a6c427188'
     },
     {
       1: 2,
       -1: 1,
       2: h'706572656772696e2e746f6f6b407475636b626f726f7567682e6578
   616d706c65',
       -2: h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b
   4d91d6280',
       -3: h'f01400b089867804b8e9fc96c3932161f1934f4223069170d924b7e
   03bf822bb',
       -4: h'02d1f7e6f26c43d4868d87ceb2353161740aacf1f7163647984b522
   a848df1c3'
     },
     {
       1: 4,
       2: h'30313863306165352d346439622d343731622d626664362d65656633
   3134626337303337',
       -1: h'849b57219dae48de646d07dbb533566e976686457c1491be3a76dce
   a6c427188'
     },
     {
       1: 2,
       2: h'62696c626f2e62616767696e7340686f626269746f6e2e6578616d70
   6c65',
       -1: 3,
       -2: h'0072992cb3ac08ecf3e5c63dedec0d51a8c1f79ef2f82f94f3c737b
   f5de7986671eac625fe8257bbd0394644caaa3aaf8f27a4585fbbcad0f2457620
   085e5c8f42ad',
       -3: h'01dca6947bce88bc5790485ac97427342bc35f887d86d65a089377e
   247e60baa55e4e8501e2ada5724ac51d6909008033ebc10ac999b9d7f5cc2519f
   3fe1ea1d9475',
       -4: h'00085138ddabf5ca975f5860f91a08e91d6d5f9a76ad4018766a476
   680b55cd339e8ab6c72b5facdb2a2a50ac25bd086647dd3e2e6e99e84ca2c3609
   fdf177feb26d'
     },
     {
       1: 3,
       2: h'62696c626f2e62616767696e7340686f626269746f6e2e6578616d70
   6c65',
       -2: h'9f810fb4038273d02591e4073f31d2b6001b82cedb4d92f050165d4
   7cfcab8a3c41cb778ac7553793f8ef975768d1a2374d8712564c3bcd77b9ea434
   544899407cff0099920a931a24c4414852ab29bdb0a95c0653f36c60e60bf90b6
   258dda56f37047ba5c2d1d029af9c9d40bac7aa41c78a0dd1068add699e808fea
   011ea1441d8a4f7bb4e97be39f55f1ddd44e9c4ba335159703d4d34b603e65147



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   a4f23d6d3c0996c75edee846a82d190ae10783c961cf0387aed2106d2d0555b6f
   d937fad5535387e0ff72ffbe78941402b0b822ea2a74b6058c1dabf9b34a76cb6
   3b87faa2c6847b8e2837fff91186e6b1c14911cf989a89092a81ce601ddacd3f9
   cf',
       -1: h'010001',
       -3: h'6d6502f41f84151228f24a467e1d19bb218fbcc34abd858db41fe29
   221fd936d1e4fe3b5abf23bf1e8999295f15d0d144c4b362ec3514bef2e25bbd0
   f80d62ae4c0c48c90ad49dd74c681dae10a4bbd81195d63bb0d03f00a64687e43
   aeb5ff8dab20d2d109ef16fa7677e2e8bfa8e7e42e72bd4160c3aa9688b00f9b3
   3059648316ed8c5016309074cc1332d81aa39ed389e8a9eab5844c414c704e05d
   90c5e2b85854ab5054ea5f83a84896c6a83cdac5edda1f8b3274f7d38e8039826
   8462a33ef9b525107c60ac8564c19cfe6e0e3775f242a1cafd3b9617d225dacf7
   4ce4f972976d61b057f82ff9870aea056aeee076c3df1cfc718d539c3a906b433
   c1',
       -4: h'dd297183f0f04d725c6fad3de51a17ca0402019e519c0bd9967a35c
   a11ed9d47b1fdfa7b019ffd9d168eec75fff9215f1907aeb5aa364c38c3016538
   56ea64f2bc3d251d00cd9d0dd9fbee2009abfd60ac986a5e36a4277afd53ec8c8
   4b2787c50cb7e9f909a7e1922933844b2b9a7747e8bc4eaef44996c3e9e99bfc6
   d4ab49',
       -5: h'b8a136761f9c4dfe84445e24e1efe3cbbf067cf61421a532a12489b
   81ce9dc2b9b937382aacea0ad3f1b47f72ed039b5319c169ad76a0f223de47ad4
   7aadcc3f5e6f30c38df251d3799bb69662afc2a5bb6a757953384cd6267bcf8c8
   c92e530156a01bf263cf7c117bd10fe85da91c47952a80675f76cc1de9545274b
   3ba457',
       -6: h'07c3d5bd792f26b8f62fe19843bbf7cbdafa2b0e60f526a15c1c2c5
   94ce9d7d4d596023e615f39ab53486f5af142d0fe22c5d7477f936a77afb913d1
   b7938139d88c190a7ca5bb76ea096361f294fc4f719fe4542c7cf4f9e77d13d81
   72ca0f85469e0a73f8f7d0feadbda64e71587a09a74d3d41fd47bc2862c515f9f
   5e8629',
       -7: h'08b0e60c676e87295cf68eebf38ac45159fba7343a3c5f3763e8816
   71e4d4fe4e99ce64a175a44ac031578acc5125e350e51c7aaa04b48cd16d6c385
   6f04f16166439bab08ea88398936f0406202de09c929b8bfee4fef260187c07c6
   03da5f63e7bcffb3c84903111b9ffabcb873f675d42abd02a0b6c9e2fa91d293d
   5c605f',
       -8: h'dcf8aabd740dd33c0c784fac06f6608b6f3d5cff57090177556a8fc
   cc2a7220429eff4ee828ebe35904a090b0c7f71da1060634d526cfe370af3e4d1
   5ef68a7beed931a423f157c175892cb1bbb434a0c386327e1ad8ac79a0d55aded
   d707d1c7f0c601541e9421ec5a02ae3149ea1e99129305eb19ae8ece2a3293f3f
   1a688e'
     }
   ]

Appendix C.  Document Updates








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C.1.  Version -03 to -04

   o  Change top level from map to array.

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

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

   o  Push protected field into the KDF functions for recipients.

   o  Remove password based recipient information.

   o  Create EC Curve Registry.

C.2.  Version -02 to -03

   o  Make a pass over all of the algorithm text.

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

   o  Add sample key structures.

   o  Expand text on dealing with Externally Supplied Data.

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

C.3.  Version -02 to -03

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

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

   o  Provide guidance on use of externally supplied authenticated data.

   o  Add external authenticated data to signing structure.

C.4.  Version -01 to -2

   o  Add first pass of algorithm information

   o  Add direct key derivation example.







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C.5.  Version -00 to -01

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

   o  Put in proposal on MTI algorithms.

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

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

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

   o  Start marking element 0 in registries as reserved.

   o  Update examples.

Editorial Comments

[CREF1] JLS: Need to check this list for correctness before publishing.

[CREF2] JLS: I have not gone through the document to determine what
        needs to be here yet.  We mostly want to grab terms which are
        used in unusual ways or are not generally understood.

[CREF3] JLS: It would be possible to extend this section to talk about
        those decisions which an application needs to think about rather
        than just talking about MTI algoithms.

[CREF4] Hannes: I would remove references to CMS and S/MIME since they
        are most likely only helpful to a very small audience.

[CREF5] JLS: I have moved msg_type into the individual structures.
        However, they would not be necessary in the cases where a) the
        security service is known and b) security libraries can setup to
        take individual structures.  Should they be moved back to just
        appearing if used in a COSE_MSG rather than on the individual
        structure?  This would make things shorter if one was using just
        a signed message because the msg_type field can be omitted as
        well as the COSE_Tagged_MSG tag field.  One the other hand, it
        will complicated the code if one is doing general purpose
        library type things.

[CREF6] JLS: Should we create an IANA registries for the values of
        msg_type?




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[CREF7] JLS: A completest version of this grammar would list the options
        available in the protected and unprotected headers.  Do we want
        to head that direction?

[CREF8] JLS: Is there a reason to assign a CBOR tag to identify keys
        and/or key sets?

[CREF9] JLS: We can really simplify the grammar for COSE_Key to be just
        the kty (the one required field) and the generic item.  The
        reason to do this is that it makes things simpler.  The reason
        not to do this says that we really need to add a lot more items
        so that a grammar check can be done that is more tightly
        enforced.

[CREF10] JLS: Do we create a registry for curves?  Is is the same
         registry for both EC1 and EC2?

[CREF11] JLS: Should we use the bignum encoding for x, y and d instead
         of bstr?

[CREF12] JLS: Looking at the CBOR specification, the bstr that we are
         looking in our table below should most likely be specified as
         big numbers rather than as binary strings.  This means that we
         would use the tag 6.2 instead.  From my reading of the
         specification, there is no difference in the encoded size of
         the resulting output.  The specification of bignum does
         explicitly allow for integers encoded with leading zeros.

[CREF13] JLS: Should we register both or just the cose+cbor one?

Authors' Addresses

   Jim Schaad
   August Cellars

   Email: ietf@augustcellars.com


   Brian Campbell
   Ping Identity

   Email: brian.d.campbell@gmail.com









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