CBOR Object Signing and Encryption (COSE): Countersignatures
draft-ietf-cose-countersign-10

COSE Working Group                                             J. Schaad
Internet-Draft                                            August Cellars
Updates: 9052 (if approved)                              R. Housley, Ed.
Intended status: Standards Track                          Vigil Security
Expires: 24 March 2023                                 20 September 2022

      CBOR Object Signing and Encryption (COSE): Countersignatures
                     draft-ietf-cose-countersign-10

Abstract

   Concise Binary Object Representation (CBOR) is a data format designed
   for small code size and small message size.  CBOR Object Signing and
   Encryption (COSE) defines a set of security services for CBOR.  This
   document defines a countersignature algorithm along with the needed
   header parameters and CBOR tags for COSE.  This document updates RFC
   9052.

Status of This Memo

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

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   This Internet-Draft will expire on 24 March 2023.

Copyright Notice

   Copyright (c) 2022 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 (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Terminology  . . . . . . . . . . . . . . . .   4
     1.2.  CBOR Grammar  . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Document Terminology  . . . . . . . . . . . . . . . . . .   4
   2.  Countersignature Header Parameters  . . . . . . . . . . . . .   5
   3.  Version 2 Countersignatures . . . . . . . . . . . . . . . . .   6
     3.1.  Full Countersignatures  . . . . . . . . . . . . . . . . .   7
     3.2.  Abbreviated Countersignatures . . . . . . . . . . . . . .   8
     3.3.  Signing and Verification Process  . . . . . . . . . . . .   8
   4.  CBOR Encoding Restrictions  . . . . . . . . . . . . . . . . .  10
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  CBOR Tag Assignment . . . . . . . . . . . . . . . . . . .  11
     5.2.  COSE Header Parameters Registry . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Author's Versions . . . . . . . . . . . . . . . . . . . .  12
     7.2.  COSE Testing Library  . . . . . . . . . . . . . . . . . .  13
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  15
     A.1.  Use of Early Code Points  . . . . . . . . . . . . . . . .  16
     A.2.  Examples of Signed Messages . . . . . . . . . . . . . . .  16
       A.2.1.  Countersignature  . . . . . . . . . . . . . . . . . .  16
     A.3.  Examples of Signed1 Messages  . . . . . . . . . . . . . .  17
       A.3.1.  Countersignature  . . . . . . . . . . . . . . . . . .  17
     A.4.  Examples of Enveloped Messages  . . . . . . . . . . . . .  18
       A.4.1.  Countersignature on Encrypted Content . . . . . . . .  18
     A.5.  Examples of Encrypted Messages  . . . . . . . . . . . . .  19
       A.5.1.  Countersignature on Encrypted Content . . . . . . . .  20
     A.6.  Examples of MACed Messages  . . . . . . . . . . . . . . .  20
       A.6.1.  Countersignature on MAC Content . . . . . . . . . . .  20
     A.7.  Examples of MAC0 Messages . . . . . . . . . . . . . . . .  21
       A.7.1.  Countersignature on MAC0 Content  . . . . . . . . . .  21
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

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1.  Introduction

   There has been an increased focus on small, constrained devices that
   make up the Internet of Things (IoT).  One of the standards that has
   come out of this process is "Concise Binary Object Representation
   (CBOR)" [RFC8949].  CBOR extended the data model of the JavaScript
   Object Notation (JSON) [STD90] by allowing for binary data, among
   other changes.  CBOR has been adopted by several of the IETF working
   groups dealing with the IoT world as their method of encoding data
   structures.  CBOR was designed specifically to be small in terms of
   both messages transported and implementation size and to have a
   schema-free decoder.  A need exists to provide message security
   services for IoT, and using CBOR as the message-encoding format makes
   sense.

   A countersignature is a second signature that confirms the primary
   signature.  During the process of advancing COSE to Internet
   Standard, it was noticed that the description of the security
   properties of countersignatures was incorrect for the COSE_Sign1
   structure in Section 4.5 of [RFC8152].  To remedy this situation, the
   working group decided to remove all of the countersignature text from
   [RFC9052], which obsoletes [RFC8152].  This document defines a new
   countersignature with the desired security properties.

   The problem with the previous countersignature algorithm was that the
   cryptographically computed value was not always included.  The
   initial assumption that the cryptographic value was in the third slot
   of the array was known not to be true at the time, but in the case of
   the MAC structures this was not deemed to be an issue.  The new
   algorithm defined in this document requires the inclusion of more
   values for the countersignature computation.  The exception to this
   is the COSE_Signature structure where there is no cryptographic
   computed value.

   The new algorithm defined in this document is designed to produce the
   same countersignature value in those cases where the computed
   cryptographic value was already included.  This means that for those
   structures the only thing that would need to be done is to change the
   value of the header parameter.

   With the publication of this document, implementers are encouraged to
   migrate uses of the previous countersignature algorithm to the one
   specified in this document.  In particular, uses of
   "CounterSignature" will migrate to "CounterSignatureV2", and uses of
   "CounterSignature0" will migrate to "CounterSignature0V2".  In
   addition, verification of "CounterSignature" must be supported by new
   implementations to remain compatible with senders that adhere to
   [RFC8152], which assumes that all implementations will understand

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   "CounterSignature" as header parameter label 7.  Likewise,
   verification of "CounterSignature0" may be supported for further
   compatibility.

1.1.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.2.  CBOR Grammar

   CBOR grammar in this document uses the CBOR Data Definition Language
   (CDDL) [RFC8610].

   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.)

         //sourcecode[@type='cddl']/text()

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

         start = COSE_Countersignature_Tagged / Internal_Types

         ; This is defined to make the tool quieter:
         Internal_Types = Countersign_structure / COSE_Countersignature0

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

1.3.  Document Terminology

   In this document, we use the following terminology:

   "Byte" is a synonym for "octet".

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

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   "Context" is used throughout the document to represent information
   that is not part of the COSE message.  Information which is part of
   the context can come from different sources including: protocol
   interactions, associated key structures, and application
   configuration.  The context to use can be implicit, identified using
   the "kid context" header parameter defined in [RFC8613], or
   identified by a protocol-specific identifier.  Context should
   generally be included in the cryptographic construction; for more
   details see Section 4.3 of [RFC9052].

   The term "byte string" is used for sequences of bytes, while the term
   "text string" is used for sequences of characters.

2.  Countersignature Header Parameters

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

   The set of header parameters defined in this section are:

   V2 countersignature:  This header parameter holds one or more
      countersignature values.  Countersignatures provide a method of
      having a second party sign some data.  The countersignature header
      parameter can occur as an unprotected attribute in any of the
      following structures that are defined in [RFC9052]: COSE_Sign1,
      COSE_Signature, COSE_Encrypt, COSE_recipient, COSE_Encrypt0,
      COSE_Mac, and COSE_Mac0.  Details of version 2 countersignatures
      are found in Section 3.

   +===========+=====+========================+==========+=============+
   |Name       |Label|Value Type              | Value    | Description |
   |           |     |                        | Registry |             |
   +===========+=====+========================+==========+=============+
   |counter    |TBD10|COSE_Countersignature / |          | V2 counter  |
   |signature  |     |[+ COSE_Countersignature|          | signature   |
   |version 2  |     |]                       |          | attribute   |
   +-----------+-----+------------------------+----------+-------------+
   |counter    |TBD11|COSE_Countersignature0  |          | Abbreviated |
   |signature 0|     |                        |          | Counter     |
   |version 2  |     |                        |          | signature   |
   |           |     |                        |          | vesion 2    |
   +-----------+-----+------------------------+----------+-------------+

                     Table 1: Common Header Parameters

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

         CountersignatureV2_header = (
             TBD10 => COSE_Countersignature / [+COSE_Countersignature]
         )

         Countersignature0V2_header = (
             TBD11 => COSE_Countersignature0
         )

3.  Version 2 Countersignatures

   A countersignature is normally defined as a second signature that
   confirms a primary signature.  A normal example of a countersignature
   is the signature that a notary public places on a document as
   witnessing that you have signed the document.  A notary typically
   includes a timestamp to indicate when notarization occurs; however,
   such a timestamp has not yet been defined for COSE.  A timestamp,
   once defined in a future document, might be included as a protected
   header parameter.  Thus applying a countersignature to either the
   COSE_Signature or COSE_Sign1 objects match this traditional
   definition.  This document extends the context of a countersignature
   to allow it to be applied to all of the security structures defined.
   The countersignature needs to be treated as a separate operation from
   the initial operation even if it is applied by the same user as is
   done in [I-D.ietf-core-oscore-groupcomm].

   COSE supports two different forms for countersignatures.  Full
   countersignatures use the structure COSE_Countersignature, which has
   the same structure as COSE_Signature.  Thus, full countersignatures
   can have protected and unprotected attributes, including chained
   countersignatures.  Abbreviated countersignatures use the structure
   COSE_Countersignature0.  This structure only contains the signature
   value and nothing else.  The structures cannot be converted between
   each other; as the signature computation includes a parameter
   identifying which structure is being used, the converted structure
   will fail signature validation.

   The version 2 countersignature changes the algorithm used for
   computing the signature from the original version that is specified
   Section 4.5 of [RFC8152].  The new version now includes the
   cryptographic material generated for all of the structures rather
   than just for a subset.

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   COSE was designed for uniformity in how the data structures are
   specified.  One result of this is that for COSE one can expand the
   concept of countersignatures beyond just the idea of signing a
   signature to being able to sign most of the structures without having
   to create a new signing layer.  When creating a countersignature, one
   needs to be clear about the security properties that result.  When
   done on a COSE_Signature or COSE_Sign1, the normal countersignature
   semantics are preserved.  That is, the countersignature makes a
   statement about the existence of a signature and, when used with a
   yet-to-be-specified timestamp, a point in time at which the signature
   exists.  When done on a COSE_Mac or COSE_Mac0, the payload is
   included as well as the MAC value.  When done on a COSE_Encrypt or
   COSE_Encrypt0, the existence of the encrypted data is attested to.
   It should be noted that there is a distinction between attesting to
   the encrypted data as opposed to attesting to the unencrypted data.
   If the latter is what is desired, then one needs to apply a signature
   to the data and then encrypt that.  It is always possible to
   construct cases where the use of two different keys will appear to
   result in a successful decryption (the tag check success), but which
   produce two completely different plaintexts.  This situation is not
   detectable by a countersignature on the encrypted data.

3.1.  Full Countersignatures

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

   The full countersignature structure can be encoded as either tagged
   or untagged depending on the context.  A tagged COSE_Countersignature
   structure is identified by the CBOR tag TBD0.  The countersignature
   structure is the same as that used for a signer on a signed object.
   The CDDL fragment for full countersignatures is:

         COSE_Countersignature_Tagged = #6.TBD0(COSE_Countersignature)
         COSE_Countersignature = COSE_Signature

   The details of the fields of a countersignature can be found in
   Section 4.1 of [RFC9052].

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   An example of a countersignature on a signature can be found in
   Appendix A.2.1.  An example of a countersignature in an encryption
   object can be found in Appendix A.4.1.

   It should be noted that only a signature algorithm with appendix (see
   Section 8.1 of [RFC9052]) can be used for countersignatures.  This is
   because the body should be able to be processed without having to
   evaluate the countersignature, and this is not possible for signature
   schemes with message recovery.

3.2.  Abbreviated Countersignatures

   Abbreviated countersignatures support encrypted group messaging,
   where identification of the message originator is required, but there
   is a desire to keep the countersignature as small as possible.  For
   abbreviated countersignatures, there is no provision for any
   protected attributes related to the signing operation.  That is, the
   parameters for computing or verifying the abbreviated
   countersignature are provided by the same context used to describe
   the encryption, signature, or MAC processing.

   The CDDL fragment for the abbreviated countersignatures is:

         COSE_Countersignature0 = bstr

   The byte string representing the signature value is placed in the
   Countersignature0 attribute.  This attribute is then encoded as an
   unprotected header parameter.

3.3.  Signing and Verification Process

   In order to create a signature, a well-defined byte string is needed.
   The Countersign_structure is used to create the canonical form.  This
   signing and verification process takes in the countersignature target
   structure (COSE_Signature, COSE_Sign1, COSE_Sign, COSE_Mac,
   COSE_Mac0, COSE_Encrypt, or COSE_Encrypt0), the signer information
   (COSE_Signature), and the application data (external source).  A
   Countersign_structure is a CBOR array.  The target structure of the
   countersignature needs to have all of its cryptographic functions
   finalized before the computing the signature.  The fields of the
   Countersign_structure in order are:

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

      "CounterSignature" for countersignatures using the
      COSE_Countersignature structure when other_fields is absent.

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      "CounterSignature0" for countersignatures using the
      COSE_Countersignature0 structure when other_fields is absent.

      "CounterSignatureV2" for countersignatures using the
      COSE_Countersignature structure when other_fields is present.

      "CounterSignature0V2" for countersignatures using the
      COSE_Countersignature0 structure when other_fields is present.

   body_protected:  The serialized protected attributes from the target
      structure encoded in a bstr type.  If there are no protected
      attributes, a zero-length byte string is used.

   sign_protected:  The serialized protected attributes from the signer
      structure encoded in a bstr type.  If there are no protected
      attributes, a zero-length byte string is used.  This field is
      omitted for the Countersignature0V2 attribute.

   external_aad:  The externally supplied additional authenticated data
      (AAD) from the application is encoded in a bstr type.  If this
      field is not supplied, it defaults to a zero-length byte string.
      (See Section 4.3 of [RFC9052] for application guidance on
      constructing this field.)

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

   other_fields:  If there are only two bstr fields in the target
      structure, this field is omitted.  The field is an array of all
      bstr fields after the second.  As an example, this would be an
      array of one element for the COSE_Sign1 structure containing the
      signature value.

   The CDDL fragment that describes the above text is:

         Countersign_structure = [
           context : "CounterSignature" / "CounterSignature0" /
                     "CounterSignatureV2" / "CounterSignature0V2" /,
           body_protected : empty_or_serialized_map,
           ? sign_protected : empty_or_serialized_map,
           external_aad : bstr,
           payload : bstr,
           ? other_fields : [ + bstr ]
         ]

   How to compute a countersignature:

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

   2.  Create the value ToBeSigned by encoding the Countersign_structure
       to a byte string, using the encoding described in Section 4.

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

   4.  Place the resulting signature value in the correct location.
       This is the "signature" field of the COSE_Countersignature
       structure for full countersignatures (see Section 3.1).  This is
       the value of the Countersignature0 attribute for abbreviated
       countersignatures (see Section 3.2).

   The steps for verifying a countersignature are:

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

   2.  Create the value ToBeSigned by encoding the Countersign_structure
       to a byte string, using the encoding described in Section 4.

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

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

4.  CBOR Encoding Restrictions

   The deterministic encoding rules are defined in Section 4.2 of
   [RFC8949].  These rules are further narrowed in Section 9 of
   [RFC9052].  The narrowed deterministic encoding rules MUST be used to
   ensure that all implementations generate the same byte string for the
   "to be signed" value.

5.  IANA Considerations

   The registries and registrations listed below were created during
   processing of [RFC8152].  The majority of the actions are to update
   the references to point to this document.

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5.1.  CBOR Tag Assignment

   IANA is requested to assign a new tag for the CounterSignature type
   in the "CBOR Tags" registry.

   *  Tag: TBD0

   *  Data Item: COSE_Countersignature

   *  Semantics: COSE standalone V2 countersignature

   *  Reference: [[this document]]

5.2.  COSE Header Parameters Registry

   IANA created a registry titled "COSE Header Parameters" as part of
   processing [RFC8152].

   IANA is requested to register the following new items in the
   registry.  For these entries, the Value Registry column will be blank
   and the Reference column will be [[This Document]].

   +===================+=====+========================+================+
   | Name              |Label|Value Type              |Description     |
   +===================+=====+========================+================+
   | Countersignature  |TBD10|COSE_Countersignature / |V2              |
   | version 2         |     |[+ COSE_Countersignature|countersignature|
   |                   |     |]                       |attribute       |
   +-------------------+-----+------------------------+----------------+
   | Countersignature0 |TBD11|COSE_Countersignature0  |V2 Abbreviated  |
   | version 2         |     |                        |Countersignature|
   +-------------------+-----+------------------------+----------------+

                   Table 2: New Common Header Parameters

   IANA is requested to modify the Description field for "counter
   signature" and "CounterSignature0" to include the words "(Deprecated
   by [[This Document]])".

6.  Security Considerations

   Please review the Security Consideration in [RFC9052]; these
   considerations apply to this document as well, especially the need
   for implementations to protect private key material.

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   When either COSE_Encrypt and COSE_Mac is used and more than two
   parties share the key, data origin authentication is not provided.
   Any party that knows the message-authentication key can compute a
   valid authentication tag; therefore, the contents could originate
   from any one of the parties that share the key.

   Countersignatures of COSE_Encrypt and COSE_Mac with short
   authentication tags do not provide the security properties associated
   with the same algorithm used in COSE_Sign.  To provide 128-bit
   security against collision attacks, the tag length MUST be at least
   256-bits.  A countersignature of a COSE_Mac with AES-MAC (using a
   128-bit key or larger) provides at most 64 bits of integrity
   protection.  Similarly, a countersignature of a COSE_Encrypt with
   AES-CCM-16-64-128 provides at most 32 bits of integrity protection.

7.  Implementation Status

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

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

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

7.1.  Author's Versions

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

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

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   *  Primary Maintainer: Jim Schaad

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

   *  Cryptography: The Java and C# libraries use Bouncy Castle to
      provide the required cryptography.

   *  Coverage: Both implementations can produce and consume both the
      old and new countersignatures.

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

   *  Licensing: Revised BSD License

7.2.  COSE Testing Library

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

   *  Primary Maintainer: Jim Schaad

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

   *  Coverage: An attempt has been made to have test cases for every
      message type and algorithm in the document.  However, examples
      dealing with countersignatures, and ECDH with Curve25519 and
      Goldilocks are missing.

   *  Licensing: Public Domain

8.  References

8.1.  Normative References

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

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

   [RFC9052]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", STD 96, RFC 9052,
              DOI 10.17487/RFC9052, August 2022,
              <https://www.rfc-editor.org/info/rfc9052>.

8.2.  Informative References

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

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

   [STD90]    Turner, S., "EST (Enrollment over Secure Transport)
              Extensions", RFC 8295, January 2018.

              <https://www.rfc-editor.org/info/std90>

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

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

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

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

   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., Mattsson, J. P.,
              and J. Park, "Group OSCORE - Secure Group Communication
              for CoAP", Work in Progress, Internet-Draft, draft-ietf-
              core-oscore-groupcomm-15, 5 September 2022,
              <https://www.ietf.org/archive/id/draft-ietf-core-oscore-
              groupcomm-15.txt>.

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

   [CBORDIAG] Bormann, C., "CBOR diagnostic utilities",
              <https://github.com/cabo/cbor-diag>.

Appendix A.  Examples

   This appendix includes a set of examples that show the different
   features and message types that have been defined in this document.
   To make the examples easier to read, they are presented using the
   extended CBOR diagnostic notation (defined in [RFC8610]) rather than
   as a binary dump.

   The examples are presented using the CBOR's diagnostic notation.  A
   Ruby-based tool exists [CBORDIAG] that can convert between the
   diagnostic notation and binary.  The referenced webpage includes
   installation instructions.

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

   diag2cbor.rb < inputfile > outputfile

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

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

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A.1.  Use of Early Code Points

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

   The examples in this Appendix use code points proposed for early
   allocation by IANA.  When IANA makes the allocation, these examples
   will be updated as needed.

A.2.  Examples of Signed Messages

A.2.1.  Countersignature

   This example uses the following:

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

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

   Size of binary file is 180 bytes

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

A.3.  Examples of Signed1 Messages

A.3.1.  Countersignature

   This example uses the following:

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

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

   Size of binary file is 275 bytes

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   18(
     [
       / protected h'A201260300' / << {
         / alg / 1:-7, / ECDSA 256 /
         / ctyp / 3:0
       } >>,
       / unprotected / {
         / kid / 4: "11",
         / countersign / 11: [
           / protected h'A1013823' / << {
             / alg / 1:-36 / ECDSA 512 /
           } >>,
           / unprotected / {
             / kid / 4: "bilbo.baggins@hobbiton.example"
           },
           / signature / h'01B1291B0E60A79C459A4A9184A0D393E034B34AF069
   A1CCA34F5A913AFFFF698002295FA9F8FCBFB6FDFF59132FC0C406E98754A98F1FBF
   E81C03095F481856BC470170227206FA5BEE3C0431C56A66824E7AAF692985952E31
   271434B2BA2E47A335C658B5E995AEB5D63CF2D0CED367D3E4CC8FFFD53B70D115BA
   A9E86961FBD1A5CF'
         ]
       },
       / payload / 'This is the content.',
       / signature / h'BB587D6B15F47BFD54D2CBFCECEF75451E92B08A514BD439
   FA3AA65C6AC92DF0D7328C4A47529B32ADD3DD1B4E940071C021E9A8F2641F1D8E3B
   053DDD65AE52'
     ]
   )

A.4.  Examples of Enveloped Messages

A.4.1.  Countersignature on Encrypted Content

   This example uses the following:

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

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

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

   Size of binary file is 326 bytes

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   96(
     [
       / protected h'a10101' / << {
           / alg / 1:1 / AES-GCM 128 /
         } >>,
       / unprotected / {
         / iv / 5:h'c9cf4df2fe6c632bf7886413',
         / countersign / 11:[
           / protected h'a1013823' / << {
               / alg / 1:-36 / ES512 /
             } >> ,
           / unprotected / {
             / kid / 4:'bilbo.baggins@hobbiton.example'
           },
           / signature / h'00929663c8789bb28177ae28467e66377da12302d7f9
   594d2999afa5dfa531294f8896f2b6cdf1740014f4c7f1a358e3a6cf57f4ed6fb02f
   cf8f7aa989f5dfd07f0700a3a7d8f3c604ba70fa9411bd10c2591b483e1d2c31de00
   3183e434d8fba18f17a4c7e3dfa003ac1cf3d30d44d2533c4989d3ac38c38b71481c
   c3430c9d65e7ddff'
         ]
       },
       / ciphertext / h'7adbe2709ca818fb415f1e5df66f4e1a51053ba6d65a1a0
   c52a357da7a644b8070a151b0',
       / recipients / [
         [
           / protected h'a1013818' / << {
               / alg / 1:-25 / ECDH-ES + HKDF-256 /
             } >> ,
           / unprotected / {
             / ephemeral / -1:{
               / kty / 1:2,
               / crv / -1:1,
               / x / -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbf
   bf054e1c7b4d91d6280',
               / y / -3:true
             },
             / kid / 4:'meriadoc.brandybuck@buckland.example'
           },
           / ciphertext / h''
         ]
       ]
     ]
   )

A.5.  Examples of Encrypted Messages

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A.5.1.  Countersignature on Encrypted Content

   This example uses the following:

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

   *  Countersignature algorithm: EdDSA with Curve Ed25519

   Size of binary file is 136 bytes

   16(
     [
       / protected h'A10101' / << {
         / alg / 1:1 / AES-GCM 128 /
       } >>,
       / unprotected / {
         / iv / 5: h'02D1F7E6F26C43D4868D87CE',
         / countersign / 11: [
           / protected h'A10127' / << {
             / alg / 1:-8 / EdDSA with Ed25519 /
           } >>,
           / unprotected / {
             / kid / 4: '11'
           },
           / signature / h'E10439154CC75C7A3A5391491F88651E0292FD0FE0E0
   2CF740547EAF6677B4A4040B8ECA16DB592881262F77B14C1A086C02268B17171CA1
   6BE4B8595F8C0A08'
         ]
       },
       / ciphertext / h'60973A94BB2898009EE52ECFD9AB1DD25867374B162E2C0
   3568B41F57C3CC16F9166250A'
     ]
   )

A.6.  Examples of MACed Messages

A.6.1.  Countersignature on MAC Content

   This example uses the following:

   *  MAC algorithm: HMAC/SHA-256 with 256-bit key

   *  Countersignature algorithm: EdDSA with Curve Ed25519

   Size of binary file is 159 bytes

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   97(
     [
       / protected h'A10105' / << {
         / alg / 1:5 / HS256 /
       } >>,
       / unprotected / {
         / countersign / 11: [
           / protected h'A10127' / << {
             / alg / 1:-8 / EdDSA /
           } >>,
           / unprotected / {
             / kid / 4: '11'
           },
           / signature / h'602566F4A311DC860740D2DF54D4864555E85BC036EA
   5A6CF7905B96E499C5F66B01C4997F6A20C37C37543ADEA1D705347D38A5B13594B2
   9583DD741F455101'
         ]
       },
       / payload / 'This is the content.',
       / tag / h'2BDCC89F058216B8A208DDC6D8B54AA91F48BD63484986565105C9
   AD5A6682F6',
       / recipients / [
         [
           / protected / h'',
           / unprotected / {
             / alg / 1: -6, / direct /
             / kid / 4: 'our-secret'
           },
           / ciphertext / h''
         ]
       ]
     ]
   )

A.7.  Examples of MAC0 Messages

A.7.1.  Countersignature on MAC0 Content

   This example uses the following:

   *  MAC algorithm: HMAC/SHA-256 with 256-bit key

   *  Countersignature algorithm: EdDSA with Curve Ed25519

   Size of binary file is 159 bytes

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   17(
     [
       / protected h'A10105' / << {
         / alg / 1:5 / HS256 /
       } >>,
       / unprotected / {
         / countersign / 11: [
           / protected h'A10127' / << {
             / alg / 1:-8 / EdDSA /
           } >>,
           / unprotected / {
             / kid / 4: '11'
           },
           / signature / h'968A315DF6B4F26362E11F4CFD2F2F4E76232F39657B
   F1598837FF9332CDDD7581E248116549451F81EF823DA5974F885B681D3D6E38FC41
   42D8F8E9E7DC8F0D'
         ]
       },
       / payload / 'This is the content.',
       / tag / h'A1A848D3471F9D61EE49018D244C824772F223AD4F935293F1789F
   C3A08D8C58'
     ]
   )

Acknowledgments

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

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

   Jim Schaad passed on 3 October 2020.  This document is primarily his
   work.  Russ Housley served as the document editor after Jim's
   untimely death, mostly helping with the approval and publication
   processes.  Jim deserves all credit for the technical content.

   Jim Schaad and Jonathan Hammell provided the examples in the
   Appendix.

   The reviews by Carsten Borman, Ben Kaduk, and Elwyn Davies greatly
   improved the clarity of the document.

   {{{ RFC EDITOR: Please remove Russ Housley as an author of this
   document at publication.  Jim Schaad should be listed as the sole
   author. }}}

Authors' Addresses

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   Jim Schaad
   August Cellars
   United States of America
   Email: ietf@augustcellars.com

   Russ Housley (editor)
   Vigil Security, LLC
   United States of America
   Email: housley@vigilsec.com

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