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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 RFC 8152

COSE Working Group                                             J. Schaad
Internet-Draft                                            August Cellars
Intended status: Informational                              July 5, 2015
Expires: January 6, 2016


                      CBOR Encoded Message Syntax
                         draft-ietf-cose-msg-01

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 [1].  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 January 6, 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Design changes from JOSE  . . . . . . . . . . . . . . . .   4
     1.2.  Requirements Terminology  . . . . . . . . . . . . . . . .   4
     1.3.  CBOR Grammar  . . . . . . . . . . . . . . . . . . . . . .   4
     1.4.  CBOR Related Terminology  . . . . . . . . . . . . . . . .   5
     1.5.  Mandatory to Implement Algorithms . . . . . . . . . . . .   5
   2.  The COSE_MSG structure  . . . . . . . . . . . . . . . . . . .   6
   3.  Header Parameters . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  COSE Headers  . . . . . . . . . . . . . . . . . . . . . .  10
   4.  Signing Structure . . . . . . . . . . . . . . . . . . . . . .  13
   5.  Encryption object . . . . . . . . . . . . . . . . . . . . . .  16
     5.1.  Key Management Methods  . . . . . . . . . . . . . . . . .  17
     5.2.  Encryption Algorithm for AEAD algorithms  . . . . . . . .  17
     5.3.  Encryption algorithm for AE algorithms  . . . . . . . . .  18
   6.  MAC objects . . . . . . . . . . . . . . . . . . . . . . . . .  19
   7.  Key Structure . . . . . . . . . . . . . . . . . . . . . . . .  21
     7.1.  COSE Key Map Labels . . . . . . . . . . . . . . . . . . .  21
   8.  CBOR Encoder Restrictions . . . . . . . . . . . . . . . . . .  24
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
     9.1.  CBOR Tag assignment . . . . . . . . . . . . . . . . . . .  24
     9.2.  COSE Object Labels Registry . . . . . . . . . . . . . . .  25
     9.3.  COSE Header Label Table . . . . . . . . . . . . . . . . .  25
     9.4.  COSE Header Algorithm Label Table . . . . . . . . . . . .  26
     9.5.  COSE Algorithm Registry . . . . . . . . . . . . . . . . .  26
     9.6.  COSE Key Map Registry . . . . . . . . . . . . . . . . . .  27
     9.7.  COSE Key Parameter Registry . . . . . . . . . . . . . . .  28
     9.8.  Media Type Registration . . . . . . . . . . . . . . . . .  28
       9.8.1.  COSE Security Message . . . . . . . . . . . . . . . .  28
       9.8.2.  COSE Key media type . . . . . . . . . . . . . . . . .  30
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  32
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  32
     11.2.  Informative References . . . . . . . . . . . . . . . . .  33
   Appendix A.  AEAD and AE algorithms . . . . . . . . . . . . . . .  34
   Appendix B.  Three Levels of Recipient Information  . . . . . . .  35
   Appendix C.  Examples . . . . . . . . . . . . . . . . . . . . . .  37



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     C.1.  Examples of MAC messages  . . . . . . . . . . . . . . . .  38
       C.1.1.  Shared Secret Direct MAC  . . . . . . . . . . . . . .  38
       C.1.2.  ECDH Direct MAC . . . . . . . . . . . . . . . . . . .  38
       C.1.3.  Wrapped MAC . . . . . . . . . . . . . . . . . . . . .  39
       C.1.4.  Multi-recipient MAC message . . . . . . . . . . . . .  40
     C.2.  Examples of Encrypted Messages  . . . . . . . . . . . . .  41
       C.2.1.  Direct ECDH . . . . . . . . . . . . . . . . . . . . .  41
     C.3.  Examples of Signed Message  . . . . . . . . . . . . . . .  42
       C.3.1.  Single Signature  . . . . . . . . . . . . . . . . . .  42
       C.3.2.  Multiple Signers  . . . . . . . . . . . . . . . . . .  43
   Appendix D.  COSE Header Algorithm Label Table  . . . . . . . . .  44
   Appendix E.  Document Updates . . . . . . . . . . . . . . . . . .  45
     E.1.  Version -00 to -01  . . . . . . . . . . . . . . . . . . .  45
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  46

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).  This standard extends 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 to do their encoding of data
   structures.  CBOR was designed specifically to be both small in terms
   of messages transport and implementation size.  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 JavaScript Object Notation (JSON) documents and then to encode
   the results using the JSON format [RFC7159].  This document does the
   same work for use with the Concise Binary Object Representation
   (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  The author did not always agree with some of the decisions made by
      the JOSE working group.  Many of these decisions have been re-
      examined, and where it seems to the author to be superior or
      simpler, replaced.



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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
      attributes that are for the content and the signature.

   o  Key management has been made to be more uniform.  All key
      management techniques are represented as a recipient rather than
      only have some of them be so.

   o  MAC messages are separated from signed messages.

   o  MAC messages have the ability to do key management 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.  In this document, we use the grammar defined in the
   CBOR data definition language (CDDL)
   [I-D.greevenbosch-appsawg-cbor-cddl].

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



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   start = COSE_MSG

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


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

   NOTE: At some point we need to make some decisions about how we are
   using CDDL in this document.  Since this draft has not been moving
   forward at a great rate, changing all references on it to
   informational is a good idea.  On the other hand, having some type of
   syntax that can be examined by a machine to do syntax checking is a
   big win.  The build system for this draft is currently using the
   latest version of CDDL to check that the syntax of the examples is
   correct.  Doing this has found problems in both the syntax checker,
   the syntax and the examples.

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 choice data items.


   label = int / tstr

1.5.  Mandatory to Implement Algorithms

   One of the standard issues that is specified in IETF cryptographic
   algorithms is a requirement that a standard specify a set of minimal
   algorithms that are required to be implemented.  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



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   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)

   o  Advertising in the certificate (capabilities extension)

   o  Minimum requirements for the S/MIME which have been updated over
      time

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].  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.

   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 map as the base
   structure.  Items in the map at the top level are identified by a
   label.  This document defines a number of labels in the IANA "COSE
   Object Labels Registry" (defined in Section 9.2).

   The set of labels present in a security object is not restricted to
   those defined in this document.  However, it is not recommended that



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   additional fields be added to a structure unless this is going to be
   done in a closed environment.  When new fields need to be added, it
   is recommended that a new message type be created so that processing
   of the field can be ensured.  Using an older structure with a new
   field means that any security properties of the new field will not be
   enforced.  Before a new field is added at the outer level, strong
   consideration needs to be given to defining a new header field and
   placing it into the protected headers.  Applications should make a
   determination if non-standardized fields are going to be permitted.
   It is suggested that libraries allow for an option to fail parsing if
   non-standardized fields exist, this is especially true if they do not
   allow for access to the fields in other ways.

   A field 'msg_type' is defined to distinguish between the different
   structures when they appear as part of a COSE_MSG object.  [CREF2]
   [CREF3] This field is indexed by an integer value 1, the values
   defined in this document are:

      0 - Reserved.

      1 - Signed Message.

      2 - Encrypted Message

      3 - Authenticated Message (MACed message)

   Implementations MUST be prepared to find an integer under this label
   that does not correspond to the values 1 to 3.  If this is found then
   the client MUST stop attempting to parse 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.

   NOTE: Is there any reason to allow for a marker of a COSE_Key
   structure and allow it to be a COSE_MSG?  Doing so does allow for a
   security risk, but may simplify the code.  [CREF4]

   The CDDL grammar that corresponds to the above is:











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   COSE_MSG = COSE_Sign /
       COSE_encrypt /
       COSE_mac

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

   ; msg_type values
   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
   maps.  We use an integer to distinguish between the different
   security message types.  By searching for the integer under the label
   identified by msg_type (which is in turn an integer), one can
   determine which security message is being used and thus what syntax
   is for the rest of the elements in the map.

   +-------------+--------+--------------------------------------------+
   | name        | number | comments                                   |
   +-------------+--------+--------------------------------------------+
   | msg_type    | 1      | Occurs only in top level messages          |
   |             |        |                                            |
   | protected   | 2      | Occurs in all structures                   |
   |             |        |                                            |
   | unprotected | 3      | Occurs in all structures                   |
   |             |        |                                            |
   | payload     | 4      | Contains the content of the structure      |
   |             |        |                                            |
   | signatures  | 5      | For COSE_Sign - array of signatures        |
   |             |        |                                            |
   | signature   | 6      | For COSE_signature only                    |
   |             |        |                                            |
   | ciphertext  | 4      | TODO: Should we reuse the same as payload  |
   |             |        | or not?                                    |
   |             |        |                                            |
   | recipients  | 9      | For COSE_encrypt and COSE_mac              |
   |             |        |                                            |
   | tag         | 10     | For COSE_mac only                          |
   +-------------+--------+--------------------------------------------+

                         Table 1: COSE Map Labels

   The CDDL grammar that provides the label values is:





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   ; message_labels
   msg_type=1
   protected=2
   unprotected=3
   payload=4
   signatures=5
   signature=6
   ciphertext=4
   recipients=9
   tag=10


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 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 present for 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 element
   is not 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 range 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 tables of labels can be found in
   Table 2.

   Two buckets are provided for each layer: [CREF5]

   protected  contains attributes about the layer that are to be
      cryptographically protected.  This bucket MUST NOT be used if it
      is not going to be included in a cryptographic computation.

   unprotected  contains attributes about the layer that are not
      cryptographically protected.

   Both of the buckets are optional and are omitted if there are no
   items contained in the map.  The CDDL fragment that describes the two
   buckets is:





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   header_map = {+ label => any }

   Headers = (
       ? protected => bstr,
       ? unprotected => header_map
   )

3.1.  COSE Headers

   The set of header fields defined in this document are:

   alg  This field is used to indicate the algorithm used for the
      security processing.  This field MUST be present at each level of
      a signed, encrypted or authenticated message.  This field using
      the integer '1' for the label.  The value is taken from the 'COSE
      Algorithm Registry' (see Section 9.4).

   crit  This field is used to ensure that applications will take
      appropriate action based on the values found.  The field is used
      to indicate which protected header labels an application that is
      processing a message is required to understand.  This field uses
      the integer '2' for the label.  The value is an array of COSE
      Header Labels.  When present, this 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 header 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 field is processed.

   cty  This field is used to indicate the content type of the data in
      the payload or ciphertext fields.  The field uses the integer of
      '3' for the label.  The value can be either an integer or a
      string.  [CREF6] Integers are from the XXXXX[CREF7] IANA registry
      table.  Strings are from the IANA 'mime-content types' registry.
      Applications SHOULD provide this field if the content structure is
      potentially ambiguous.

   kid  This field one of the ways that can be used to find the key to
      be used.  This value can be matched against the 'kid' field in a
      COSE_Key structure.  Applications MUST NOT assume that 'kid'
      values are unique.  There may be more than one key with the same



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      'kid' value, it may be required that all of the keys need to be
      checked to find the correct one.  This field uses the integer
      value of '4' for the label.  The value of field is the CBOR 'bstr'
      type.  The internal structure of 'kid' 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 normally be
      placed in the unprotected headers bucket.

   nonce  This field holds either a nonce or Initialization Vector
      value.  This value can be used either as a counter value for a
      protocol or as an IV for an algorithm.  TODO: Talk about zero
      extending the value in some cases.

   This table contains a list of all of the parameters for use in
   signature and encryption message types defined by the JOSE document
   set.  In the table is the data value type to be used for CBOR as well
   as the integer value that can be used as a replacement for the name
   in order to further decrease the size of the sent item.
































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   +----------+-------+----------+-------------+-----------------------+
   | name     | label | value    | registry    | description           |
   +----------+-------+----------+-------------+-----------------------+
   | alg      | 1     | int /    | COSE        | Integers are taken    |
   |          |       | tstr     | Algorithm   | from table --POINT TO |
   |          |       |          | Registry    | REGISTRY--            |
   |          |       |          |             |                       |
   | crit     | 2     | [+       | COSE Header | integer values are    |
   |          |       | label]   | Label       | from this table.      |
   |          |       |          | Registry    |                       |
   |          |       |          |             |                       |
   | cty      | 3     | tstr /   | media-types | Value is either a     |
   |          |       | int      | registry    | media-type or an      |
   |          |       |          |             | integer from the      |
   |          |       |          |             | media-type registry   |
   |          |       |          |             |                       |
   | jku      | *     | tstr     |             | URL to COSE key       |
   |          |       |          |             | object                |
   |          |       |          |             |                       |
   | jwk      | *     | COSE_Key |             | contains a COSE key   |
   |          |       |          |             | not a JWK key         |
   |          |       |          |             |                       |
   | kid      | 4     | bstr     |             | key identifier        |
   |          |       |          |             |                       |
   | 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 taken    |
   |          |       | tstr     |             | from the table        |
   |          |       |          |             | --POINT TO REGISTRY-- |
   |          |       |          |             |                       |
   | nonce    | 5     | bstr     |             | Nonce or              |
   |          |       |          |             | Initialization Vector |
   |          |       |          |             | (IV)                  |
   +----------+-------+----------+-------------+-----------------------+

                          Table 2: Header Labels

   OPEN ISSUES:



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   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.

   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 attributes
   about the content and attributes about the signature to be carried
   along with the signature itself.  These attributes may be
   authenticated by the signature, or just present.  Examples of
   attributes about the content would be the type of content, when the
   content was created, and who created the content.  Examples of
   attributes about the signature would be the algorithm and key used to
   create the signature, when the signature was created, and counter-
   signatures.

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




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

   The CDDL grammar for a signature message is:

   COSE_Sign = {
       msg_type => msg_type_signed,
       Headers,
       ? payload => bstr,
       signatures => [+ COSE_signature]
   }

   The fields is the structure have the following semantics:

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

   protected  contains attributes about the payload that are to be
      protected by the signature.  An example of such an attribute would
      be the content type ('cty') attribute.  The content is a CBOR map
      of attributes that is encoded to a byte stream.  This field MUST
      NOT contain attributes about the signature, even if those
      attributes are common across multiple signatures.  The labels in
      this map are typically taken from Table 2.

   unprotected  contains attributes about the payload that are not
      protected by the signature.  An example of such an attribute would
      be the content type ('cty') attribute.  This field MUST NOT
      contain attributes about a signature, even if the attributes are
      common across multiple signatures.  The labels in this map are
      typically taken from Table 2.

   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.

   We use the values in Table 1 as the labels in the COSE_Sign map.
   While other labels can be present in the map, it is not generally a
   recommended practice.  The other labels can be either of integer or
   string type, use of other types SHOULD be treated as an error.

   The CDDL grammar structure for a signature is:



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   COSE_signature =  {
       Headers,
       signature => bstr
   }

   The fields in the structure have the following semantics:

   protected  contains additional information to be authenticated by the
      signature.  The field holds data about the signature operation.
      The field MUST NOT hold attributes about the payload being signed.
      The content is a CBOR map of attributes that is encoded to a byte
      stream.  At least one of protected and unprotected MUST be
      present.

   unprotected  contains attributes about the signature that are not
      protected by the signature.  This field MUST NOT contain
      attributes about the payload being signed.  At least one of
      protected and unprotected MUST be present.

   signature  contains the computed signature value.

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

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

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

   How to verify a signature:



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

   3.  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

   In this section we describe the structure and methods to be used when
   doing an encryption in COSE.  In COSE, we use 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 different structures are used for the
   plain text and for the different key management techniques.

   One of the byproducts of using the same technique for encrypting and
   encoding both the content and the keys using the various key
   management techniques, is a requirement that all of the key
   management techniques use an Authenticated Encryption (AE) algorithm.
   (For the purpose of this document we use a slightly loose definition
   of AE algorithms.)  When encrypting the plain text, it is normal to
   use an Authenticated Encryption with Additional Data (AEAD)
   algorithm.  For key management, either AE or AEAD algorithms can be
   used.  See Appendix A for more details about the different types of
   algorithms.  [CREF8]

   The CDDL grammar structure for encryption is:













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   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  contains the information about the plain text or
      encryption process that is to be integrity protected.  The field
      is encoded in CBOR as a 'bstr'.  The contents of the protected
      field is a CBOR map of the protected data names and values.  The
      map is CBOR encoded before placing it into the bstr.  Only values
      associated with the current cipher text are to be placed in this
      location even if the value would apply to multiple recipient
      structures.

   unprotected  contains information about the plain text that is not
      integrity protected.  Only values associated with the current
      cipher text are to be placed in this location even if the value
      would apply to multiple recipient structures.

   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.  Key Management Methods

   This section has moved.  Still need to make some small comments here.

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.



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   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.

   3.  Encode the Enc_structure using a CBOR Canonical encoding
       Section 8 to get the AAD value.

   4.  Determine the encryption key.  This step is dependent on the key
       management method 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.

   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 key
       management method being used: For:



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       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.  JOSE used a variant of the
   signature structure for doing MAC operations and it is restricted to
   using a single pre-shared secret to do the authentication.  [CREF9]
   This document allows for the use of all of the same methods of key
   management as are allowed for encryption.

   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 of the key management
   methods 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 key management to verify who sent it.  The key
   management modes that support this are ones that either use a pre-
   shared secret, or do static-static key agreement.  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}]
   }




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   Field descriptions:

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

   protected  contains attributes about the payload that are to be
      protected by the MAC.  An example of such an attribute would be
      the content type ('cty') attribute.  The content is a CBOR map of
      attributes that is encoded to a byte stream.  This field MUST NOT
      contain attributes about the recipient, even if those attributes
      are common across multiple recipients.  At least one of protected
      and unprotected MUST be present.

   unprotected  contains attributes about the payload that are not
      protected by the MAC.  An example of such an attribute would be
      the content type ('cty') attribute.  This field MUST NOT contain
      attributes about a recipient, even if the attributes are common
      across multiple recipients.  At least one of protected and
      unprotected MUST be present.

   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
   ]

   How to compute a MAC:

   1.  Create a MAC_structure and copy the protected and payload
       elements 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'.




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   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

   There are only a few changes between JOSE and COSE for how keys are
   formatted.  As with JOSE, COSE uses a map to contain the elements of
   a key.  Those values, which in JOSE are base64url encoded because
   they are binary values, are encoded as bstr values in COSE.

   For COSE we use the same set of fields that were defined in
   [RFC7517].  [CREF10] [CREF11]

   COSE_Key = {
       kty => tstr / int,
       ? key_ops => [+ tstr / int ],
       ? alg => tstr / int,
       ? kid => bstr,
       * label => values
   }

   COSE_KeySet = [+COSE_Key]

   The element "kty" is a required element in a COSE_Key map.  All other
   elements are optional and not all of the elements listed in [RFC7517]
   or [RFC7518] have been listed here even though they can all appear in
   a COSE_Key map.

7.1.  COSE Key Map Labels

   This document defines a set of common map elements for a COSE Key
   object.  Table 3 provides a summary of the elements defined in this
   section.  There are also a set of map elements that are defined for a
   specific key type.












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   +----------+-------+-------------+------------+---------------------+
   | name     | label | CBOR type   | registry   | description         |
   +----------+-------+-------------+------------+---------------------+
   | kty      | 1     | tstr / int  | COSE       | Identification of   |
   |          |       |             | General    | the key type        |
   |          |       |             | Values     |                     |
   |          |       |             |            |                     |
   | key_ops  | 4     | [*          |            | 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 3: Key Map Labels

   kty:  This field is used to identify the family of keys for this
      structure, and thus the set of fields to be found.

   alg:  This field is used to restrict the algorithms that are to be
      used with this key.  If this field is present in the key
      structure, the application MUST verify that this algorithm matches
      the algorithm for which the key is being used.  If the 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 field is used to give an identifier for a key.  The
      identifier is not structured and can be anything from a user



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      provided string to a value computed on the public portion of the
      key.  This field is intended for matching against a 'kid' field in
      a message in order to filter down the set of keys that need to be
      checked.

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

   Only the 'kty' field MUST be present in a key object.  All other
   members may be omitted if their behavior is not needed.

   +---------+-------+-------------------------------------------------+
   | 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 4: Key Operation Values

   The following provides a CDDL fragment which duplicates the
   assignment labels from Table 3 and Table 4.












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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.  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.

9.  IANA Considerations

9.1.  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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9.2.  COSE Object Labels Registry

   It is requested that IANA create a new registry entitled "COSE Object
   Labels Registry".  [CREF12]

   This table is initially populated by the table in Table 1.

9.3.  COSE Header Label Table

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

   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 2.  The
   specification column for all rows in that table should be this
   document.

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





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   NOTE: Need to review the range assignments.  It does not necessarily
   make sense as specification required uses 1 byte positive integers
   and 2 byte strings.

9.4.  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:

   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 Appendix D.  The
   specification column for all rows in that table should be this
   document.

9.5.  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



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      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.

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

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

9.6.  COSE Key Map Registry

   It is requested that IANA create a new registry entitled "COSE Key
   Map 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



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

9.7.  COSE Key Parameter Registry

   It is requested that IANA create a new registry "COSE Key
   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 --Multiple
   Tables--.  The specification column for all of these entries will be
   this document.

9.8.  Media Type Registration

9.8.1.  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



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      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

      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



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      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

9.8.2.  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



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      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

      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




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      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

10.  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.

11.  References

11.1.  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.




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11.2.  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.

   [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.

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

   [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.

   [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.



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   [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.

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

   [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.

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

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

   [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.  AEAD and AE algorithms

   The set of encryption algorithms that can be used with this
   specification is restricted to authenticated encryption (AE) and
   authenticated encryption with additional data (AEAD) algorithms.
   This means that there is a strong check that the data decrypted by



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   the recipient is the same as what was encrypted by the sender.
   Encryption modes such as counter have no check on this at all.  The
   CBC encryption mode had a weak check that the data is correct, given
   a random key and random data, the CBC padding check will pass one out
   of 256 times.  There have been several times that a normal encryption
   mode has been combined with an integrity check to provide a content
   encryption mode that does provide the necessary authentication.  AES-
   GCM [AES-GCM], AES-CCM [RFC3610], AES-CBC-HMAC
   [I-D.mcgrew-aead-aes-cbc-hmac-sha2] are examples of these composite
   modes.

   PKCS v1.5 RSA key transport does not qualify as an AE algorithm.
   There are only three bytes in the encoding that can be checked as
   having decrypted correctly, the rest of the content can only be
   probabilistically checked as having decrypted correctly.  For this
   reason, PKCS v1.5 RSA key transport MUST NOT be used with this
   specification.  RSA-OAEP was designed to have the necessary checks
   that that content correctly decrypted and does qualify as an AE
   algorithm.

   When dealing with authenticated encryption algorithms, there is
   always some type of value that needs to be checked to see if the
   authentication level has passed.  This authentication value may be:

   o  A separately generated tag computed by both the encrypter and
      decrypter and then compared by the decryptor.  This tag value may
      be either placed at the end of the cipher text (the decision we
      made) or kept separately (the decision made by the JOSE working
      group).  This is the approach followed by AES-GCM [AES-GCM] and
      AES-CCM [RFC3610].

   o  A fixed value that is part of the encoded plain text.  This is the
      approach followed by the AES key wrap algorithm [RFC3394].

   o  A computed value is included as part of the encoded plain text.
      The computed value is then checked by the decryptor using the same
      computation path.  This is the approach followed by RSAES-OAEP
      [RFC3447].

Appendix B.  Three Levels of Recipient Information

   All of the currently defined Key Management methods 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 key management technique such as RSA-KEM (see
   Appendix A of RSA-KEM [RFC5990], then it make sense to have three
   levels of the COSE_Encrypt structure.




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   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 3: Uses ECDH Ephemeral-Static direct to generate the level 1
      key.

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





























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   {
     1: 2,
     2: h'a10101',
     3: {
       -1: h'02d1f7e6f26c43d4868d87ce'
     },
     4: h'64f84d913ba60a76070a9a48f26e97e863e285295a44320878caceb076
   3a334806857c67',
     9: [
       {
         3: {
           1: -3
         },
         4: h'5a15dbf5b282ecb31a6074ee3815c252405dd7583e078188',
         9: [
           {
             3: {
               1: "ECDH-ES",
               5: h'6d65726961646f632e6272616e64796275636b406275636b
   6c616e642e6578616d706c65',
               4: {
                 1: 1,
                 -1: 4,
                 -2: h'b2add44368ea6d641f9ca9af308b4079aeb519f11e9b8
   a55a600b21233e86e68',
                 -3: h'1a2cf118b9ee6895c8f415b686d4ca1cef362d4a7630a
   31ef6019c0c56d33de0'
               }
             }
           }
         ]
       }
     ]
   }

Appendix C.  Examples

   The examples can be found at https://github.com/cose-wg/Examples.  I
   am currently still in the process of getting the 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.  [CREF14] Using
   the Ruby based CBOR diagnostic tools at ????, the diagnostic notation



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   can be converted into binary files using the following command line:
   (install command is?...)


            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'.

C.1.  Examples of MAC messages

C.1.1.  Shared Secret Direct MAC

   This example users the following:

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

   o  Key management: direct shared secret

   o  File name: Mac-04

   {
     1: 3,
     2: h'a1016f4145532d434d41432d3235362f3634',
     4: h'546869732069732074686520636f6e74656e742e',
     10: h'd9afa663dd740848',
     9: [
       {
         3: {
           1: -6,
           5: h'6f75722d736563726574'
         }
       }
     ]
   }

C.1.2.  ECDH Direct MAC

   This example uses the following:

   o  MAC: HMAC w/SHA-256, 256-bit key [CREF15]

   o  Key management: ECDH key agreement, two static keys, HKDF w/
      context structure





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   {
     1: 3,
     2: h'a10104',
     4: h'546869732069732074686520636f6e74656e742e',
     10: h'2ba937ca03d76c3dbad30cfcbaeef586f9c0f9ba616ad67e9205d3857
   6ad9930',
     9: [
       {
         3: {
           1: "ECDH-SS",
           5: h'6d65726961646f632e6272616e64796275636b406275636b6c61
   6e642e6578616d706c65',
           "spk": {
             "kid": "peregrin.took@tuckborough.example"
           },
           "apu": h'4d8553e7e74f3c6a3a9dd3ef286a8195cbf8a23d19558ccf
   ec7d34b824f42d92bd06bd2c7f0271f0214e141fb779ae2856abf585a58368b01
   7e7f2a9e5ce4db5'
         }
       }
     ]
   }

C.1.3.  Wrapped MAC

   This example uses the following:

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

   o  Key management: AES keywrap w/ a pre-shared 256-bit key

   {
     1: 3,
     2: h'a1016e4145532d3132382d4d41432d3634',
     4: h'546869732069732074686520636f6e74656e742e',
     10: h'6d1fa77b2dd9146a',
     9: [
       {
         3: {
           1: -5,
           5: h'30313863306165352d346439622d343731622d626664362d6565
   66333134626337303337'
         },
         4: h'711ab0dc2fc4585dce27effa6781c8093eba906f227b6eb0'
       }
     ]
   }




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C.1.4.  Multi-recipient MAC message

   This example uses the following:

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

   o  Key management: 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

   {
     1: 3,
     2: h'a10104',
     4: h'546869732069732074686520636f6e74656e742e',
     10: h'7aaa6e74546873061f0a7de21ff0c0658d401a68da738dd8937486519
   83ce1d0',
     9: [
       {
         3: {
           1: "ECDH-ES+A128KW",
           5: h'62696c626f2e62616767696e7340686f626269746f6e2e657861
   6d706c65',
           4: {
             1: 1,
             -1: 5,
             -2: h'43b12669acac3fd27898ffba0bcd2e6c366d53bc4db71f909
   a759304acfb5e18cdc7ba0b13ff8c7636271a6924b1ac63c02688075b55ef2d61
   3574e7dc242f79c3',
             -3: h'812dd694f4ef32b11014d74010a954689c6b6e8785b333d1a
   b44f22b9d1091ae8fc8ae40b687e5cfbe7ee6f8b47918a07bb04e9f5b1a51a334
   a16bc09777434113'
           }
         },
         4: h'1b120c848c7f2f8943e402cbdbdb58efb281753af4169c70d0126c
   0d16436277160821790ef4fe3f'
       },
       {
         3: {
           1: -2,
           5: h'62696c626f2e62616767696e7340686f626269746f6e2e657861
   6d706c65'
         },
         4: h'46c4f88069b650909a891e84013614cd58a3668f88fa18f3852940



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   a20b35098591d3aacf91c125a2595cda7bee75a490579f0e2f20fd6bc956623bf
   de3029c318f82c426dac3463b261c981ab18b72fe9409412e5c7f2d8f2b5abaf7
   80df6a282db033b3a863fa957408b81741878f466dcc437006ca21407181a016c
   a608ca8208bd3c5a1ddc828531e30b89a67ec6bb97b0c3c3c92036c0cb84aa0f0
   ce8c3e4a215d173bfa668f116ca9f1177505afb7629a9b0b5e096e81d37900e06
   f561a32b6bc993fc6d0cb5d4bb81b74e6ffb0958dac7227c2eb8856303d989f93
   b4a051830706a4c44e8314ec846022eab727e16ada628f12ee7978855550249cc
   b58'
       },
       {
         3: {
           1: -5,
           5: h'30313863306165352d346439622d343731622d626664362d6565
   66333134626337303337'
         },
         4: h'0b2c7cfce04e98276342d6476a7723c090dfdd15f9a518e7736549
   e998370695e6d6a83b4ae507bb'
       }
     ]
   }

C.2.  Examples of Encrypted Messages

C.2.1.  Direct ECDH

   This example uses the following:

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

   o  Key managment: ECDH Ephemeral-Static, Curve P-256





















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   {
     1: 2,
     2: h'a10101',
     3: {
       -1: h'c9cf4df2fe6c632bf7886413'
     },
     4: h'45fce2814311024d3a479e7d3eed063850f3f0b9f3f948677e3ae9869b
   cf9ff4e1763812',
     9: [
       {
         3: {
           1: "ECDH-ES",
           5: h'6d65726961646f632e6272616e64796275636b406275636b6c61
   6e642e6578616d706c65',
           4: {
             1: 1,
             -1: 4,
             -2: h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf05
   4e1c7b4d91d6280',
             -3: h'f01400b089867804b8e9fc96c3932161f1934f4223069170d
   924b7e03bf822bb'
           }
         }
       }
     ]
   }

C.3.  Examples of Signed Message

C.3.1.  Single Signature

   This example uses the following:

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

















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   {
     1: 1,
     4: h'546869732069732074686520636f6e74656e742e',
     5: [
       {
         2: h'a20165505333383405581e62696c626f2e62616767696e7340686f
   626269746f6e2e6578616d706c65',
         6: h'1b22515f96fd798a331c7b156e90bfea7f558ec6de840e05a8e5f4
   b7be44ea1451c48517da7fd216c6143898673c232a96937ebcfb88264a58f5995
   82d89cf8a4f20ef35fbfcfd2aad46ad8b99ea6425367afd898de1b712d558b0d2
   49d6d180d0b1fb7256140ec3553556f3b5b95a49931a75998dfc23ca905efc7d8
   e04deeb92d5936c0824e535aa344396f73913d8a65de0010600270ae5df7f5c8d
   52ae525a7642d4c4ff9e219acaa52fd933df003be36b9e3c77ced37129d66745e
   2a42baa3d0b3f2675cd51ae8a64fd024d126be5396c91b9236fb5f8548d09881b
   b5d40a61c0d342bed9fe8058f36b8722b9e8465dc3b8bfa4f2fd138ce186b73e4
   082'
       }
     ]
   }

C.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
























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   {
     1: 1,
     4: h'546869732069732074686520636f6e74656e742e',
     5: [
       {
         2: h'a10129',
         3: {
           5: h'62696c626f2e62616767696e7340686f626269746f6e2e657861
   6d706c65'
         },
         6: h'028947ac3521f66f2506013007e2cd7b0cb09a209e76ab5b95f751
   eb63f5730f1672a282419c49b9653d742577fb6a6cea9ab2e1d4d5d9e786e2240
   4760663cc74a1c2c90160af92628e1ebbc3eeba552f757054b691ab17271396b7
   ff2d86c100b94a2fce0438c0b50ca70bcdd3074a0f8dc40c2e44e9b26e9093287
   b7245ee13171b28ea0f3e291c2cca64aa17f7094aee2be02b5fe5cd2cf343e18c
   eec0c763cb76a128df9a9cbfc37b835f6467d98d74505eee1dccc9e6ebf2405ea
   1329b41a33eeb13f1bbef3a272e42b3df96cdaea9016663e31ddff4603eb66a88
   5c583b53977c1fb9707550717d7387f84616a6670e27d4007b08879109aaf3720
   f33'
       },
       {
         3: {
           1: -9,
           5: h'62696c626f2e62616767696e7340686f626269746f6e2e657861
   6d706c65'
         },
         6: h'0195345953742c6725352a13cdc55402c895133525c9a3b16bb47d
   02ca5f57f8a34aebf47298c602a8feb1dd71d1936886f21029a4142abf38c3aa3
   94b3597c2f35c01987c801edc7022c8fddacbf25bc8794b9ffb7cb27f9f346ba4
   4db6f5c9b60406530f62b378c5da3e7e2259327f4e55f48271873496497724492
   d90ba67a4b65112'
       }
     ]
   }

Appendix D.  COSE Header Algorithm Label Table

   This section disappears when we make a decision on password based key
   management.

          +------+-----------+-------+-----------+-------------+
          | name | algorithm | label | CBOR type | description |
          +------+-----------+-------+-----------+-------------+
          | p2c  | PBE       | -1    | int       |             |
          |      |           |       |           |             |
          | p2s  | PBE       | -2    | bstr      |             |
          +------+-----------+-------+-----------+-------------+




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Appendix E.  Document Updates

E.1.  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  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  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 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?

[CREF3] JLS: Should we create an IANA registries for the values of
        msg_type?

[CREF4] JLS: OPEN ISSUE

[CREF5] 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?





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[CREF6] JLS: After looking at this, I am wondering if the type for this
        should be: [int int]/[int tstr] so that we can keep the major/
        minor difference of media-types.  This does cost a couple of
        bytes in the message.

[CREF7] JLS: Need to figure out how we are going to go about creating
        this registry -or are we going to modify the current mime-
        content table?

[CREF8] Ilari: I don't follow/understand this text

[CREF9] JLS: Should this sentence be removed?

[CREF10] JLS: Do we remove this line and just define them ourselves?

[CREF11] 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.

[CREF12] JLS: Finish the registration process.

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

[CREF14] JLS: Do we want to keep this as diagnostic notation or should
         we switch to having "binary" examples instead?

[CREF15] JLS: Need to examine how this is worked out.  In this case the
         length of the key to be used is implicit rather than explicit.
         This needs to be the case because a direct key could be any
         length, however it means that when the key is derived, there is
         currently nothing to state how long the derived key needs to
         be.

Author's Address

   Jim Schaad
   August Cellars

   Email: ietf@augustcellars.com









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