CBOR Object Signing and Encryption (COSE): Countersignatures
draft-ietf-cose-countersign-01

Document Type Active Internet-Draft (cose WG)
Authors Jim Schaad  , Russ Housley 
Last updated 2020-10-07
Stream IETF
Intended RFC status (None)
Formats plain text html xml pdf htmlized (tools) htmlized bibtex
Stream WG state WG Document
Document shepherd No shepherd assigned
IESG IESG state I-D Exists
Consensus Boilerplate Unknown
Telechat date
Responsible AD (None)
Send notices to (None)
COSE Working Group                                             J. Schaad
Internet-Draft                                            August Cellars
Intended status: Standards Track                         R. Housley, Ed.
Expires: 10 April 2021                                    Vigil Security
                                                          7 October 2020

      CBOR Object Signing and Encryption (COSE): Countersignatures
                     draft-ietf-cose-countersign-01

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.

Contributing to this document

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

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

Status of This Memo

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

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

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

   This Internet-Draft will expire on 10 April 2021.

Schaad & Housley          Expires 10 April 2021                 [Page 1]
Internet-Draft               COSE Structure                 October 2020

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Terminology  . . . . . . . . . . . . . . . .   3
     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 . . . . . . . . . . . . . . . . . . .  10
     5.2.  COSE Header Parameters Registry . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .  13
     7.1.  Author's Versions . . . . . . . . . . . . . . . . . . . .  14
     7.2.  COSE Testing Library  . . . . . . . . . . . . . . . . . .  14
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  16
     A.1.  Examples of Signed Messages . . . . . . . . . . . . . . .  17
       A.1.1.  Countersignature  . . . . . . . . . . . . . . . . . .  17
     A.2.  Examples of Enveloped Messages  . . . . . . . . . . . . .  18
       A.2.1.  Countersignature on Encrypted Content . . . . . . . .  18
     A.3.  Examples of Encrypted Messages  . . . . . . . . . . . . .  19
     A.4.  Examples of MACed Messages  . . . . . . . . . . . . . . .  19
     A.5.  Examples of MAC0 Messages . . . . . . . . . . . . . . . .  20
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

Schaad & Housley          Expires 10 April 2021                 [Page 2]
Internet-Draft               COSE Structure                 October 2020

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)" [I-D.ietf-cbor-7049bis].  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 encoding of
   data structures.  CBOR was designed specifically both to be small in
   terms of messages transported and implementation size and to be a
   schema-free decoder.  A need exists to provide message security
   services for IoT, and using CBOR as the message-encoding format makes
   sense.

   During the process of advancing COSE to an Internet Standard, it was
   noticed the description of the security properties of
   countersignatures was incorrect for the COSE_Sign1 structure.  Since
   the security properties that were described, those of a true
   countersignature, were those that the working group desired, the
   decision was made to remove all of the countersignature text from
   [I-D.ietf-cose-rfc8152bis-struct] and create a new document to both
   deprecate the old countersignature algorithm and to define a new one
   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 is more aggressive about the set of values included in the
   countersignature computation so that the cryptographic computed
   values is included.  The exception to this is the COSE_Signature
   structure where there is no cryptographic computed value.

   The new algorithm is designed to produce the same countersignature
   value in those cases where the cryptographic computed 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.

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.

Schaad & Housley          Expires 10 April 2021                 [Page 3]
Internet-Draft               COSE Structure                 October 2020

1.2.  CBOR Grammar

   CBOR grammar in the document is presented using 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].

   Context is used throughout the document to represent information that
   is not part of the COSE message.  Information which is part of the
   context can come from several different sources including: Protocol
   interactions, associated key structures, and program 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 [I-D.ietf-cose-rfc8152bis-struct].

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

Schaad & Housley          Expires 10 April 2021                 [Page 4]
Internet-Draft               COSE Structure                 October 2020

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: COSE_Sign1, COSE_Signature, COSE_Encrypt,
      COSE_recipient, COSE_Encrypt0, COSE_Mac, and COSE_Mac0.  Details
      on 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

   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; header parameters required in specific maps are discussed
   above.

         Generic_Headers /= (
         ? TBD10 => COSE_Countersignature / [+COSE_Countersignature]
         ; V2 Countersignature
         ? TBD11 => COSE_Countersignature0  ; V2 Countersignature0
         )

Schaad & Housley          Expires 10 April 2021                 [Page 5]
Internet-Draft               COSE Structure                 October 2020

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.  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.  It needs to be noted that 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-groupcomm-bis].

   COSE supports two different forms for countersignatures.  Full
   countersignatures use the structure COSE_Countersignature.  This is
   same structure as COSE_Signature and thus it 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 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.

   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 as a
   timestamp, a time point at which the signature exists.  When done on
   a COSE_Sign, this is the same as applying a second signature to the
   payload and adding a parallel signature as a new COSE_Signature is
   the preferred method.  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 big difference
   between attesting to the encrypted data as opposed to attesting to
   the unencrypted data.  If the latter is what is desired, then one

Schaad & Housley          Expires 10 April 2021                 [Page 6]
Internet-Draft               COSE Structure                 October 2020

   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
   is 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 it is used in.  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 = #6.9999(COSE_Countersignature)
   COSE_Countersignature = COSE_Signature

   The details of the fields of a countersignature can be found in
   Section 4.1 of [I-D.ietf-cose-rfc8152bis-struct].

   An example of a countersignature on a signature can be found in
   Appendix A.1.1.  An example of a countersignature in an encryption
   object can be found in Appendix A.2.1.

   It should be noted that only a signature algorithm with appendix (see
   Section 8 of [I-D.ietf-cose-rfc8152bis-struct]) 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.

Schaad & Housley          Expires 10 April 2021                 [Page 7]
Internet-Draft               COSE Structure                 October 2020

3.2.  Abbreviated Countersignatures

   Abbreviated countersignatures were designed primarily to deal with
   the problem of encrypted group messaging, but where it is required to
   know who originated the message.  The objective was to keep the
   countersignature as small as possible while still providing the
   needed security.  For abbreviated countersignatures, there is no
   provision for any protected attributes related to the signing
   operation.  Instead, the parameters for computing or verifying the
   abbreviated countersignature are 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.  The attribute is defined below.

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 countersignature target
   structure, 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 it's cryptographic functions finalized before the
   computing the signature.  The fields of the Countersign_structure in
   order are:

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

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

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

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

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

Schaad & Housley          Expires 10 April 2021                 [Page 8]
Internet-Draft               COSE Structure                 October 2020

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

   3.  The 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.

   4.  The externally supplied data from the application encoded in a
       bstr type.  If this field is not supplied, it defaults to a zero-
       length byte string.  (See Section 4.3 of
       [I-D.ietf-cose-rfc8152bis-struct] for application guidance on
       constructing this field.)

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

   6.  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:

   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).

Schaad & Housley          Expires 10 April 2021                 [Page 9]
Internet-Draft               COSE Structure                 October 2020

   4.  Place the resulting signature value in the correct location.
       This is the 'signature' field of the COSE_Countersignature
       structure.  This is the value of the Countersignature0 attribute.

   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

   In order to always regenerate the same byte string for the "to be
   signed" value, the deterministic encoding rules defined in
   Section 4.2 of [I-D.ietf-cbor-7049bis].  These rules match the ones
   laid out in Section 11 of [I-D.ietf-cose-rfc8152bis-struct].

5.  IANA Considerations

   The registries and registrations listed below were created during
   processing of RFC 8152 [RFC8152].  The majority of the actions are to
   update the references to point to this document.

5.1.  CBOR Tag Assignment

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

   *  Tag: TBD0

   *  Data Item: COSE_Countersignature

   *  Semantics: COSE standalone V2 countersignature

   *  Reference: [[this document]]

Schaad & Housley          Expires 10 April 2021                [Page 10]
Internet-Draft               COSE Structure                 October 2020

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.

   +=================+=====+======================+========+================+
   |Name             |Label|Value Type            |Value   |Description     |
   |                 |     |                      |Registry|                |
   +=================+=====+======================+========+================+
   |counter signature|TBD10|COSE_Countersignature |        |V2              |
   |version 2        |     |/ [+                  |        |countersignature|
   |                 |     |COSE_Countersignature |        |attribute       |
   |                 |     |]                     |        |                |
   +-----------------+-----+----------------------+--------+----------------+
   |Countersignature0|TBD11|COSE_Countersignature0|        |Abbreviated     |
   |version 2        |     |                      |        |Counter         |
   |                 |     |                      |        |signature vesion|
   |                 |     |                      |        |2               |
   +-----------------+-----+----------------------+--------+----------------+

                   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

   TODO - review and trim as needed.

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

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

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

Schaad & Housley          Expires 10 April 2021                [Page 11]
Internet-Draft               COSE Structure                 October 2020

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

   *  Several of the algorithms in [I-D.ietf-cose-rfc8152bis-algs] have
      limits on the number of times that a key can be used without
      leaking information about the key.

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

   Although it has been mentioned before, the use of a single key for
   multiple algorithms has been demonstrated in some cases to leak
   information about that key, provide the opportunity for attackers to
   forge integrity tags, or gain information about encrypted content.
   Binding a key to a single algorithm prevents these problems.  Key
   creators and key consumers are strongly encouraged not only to create
   new keys for each different algorithm, but to include that selection
   of algorithm in any distribution of key material and strictly enforce
   the matching of algorithms in the key structure to algorithms in the
   message structure.  In addition to checking that algorithms are
   correct, the key form needs to be checked as well.  Do not use an
   'EC2' key where an 'OKP' key is expected.

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

   *  What are the permissions associated with the key owner?

   *  Is the cryptographic algorithm acceptable in the current context?

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

Schaad & Housley          Expires 10 April 2021                [Page 12]
Internet-Draft               COSE Structure                 October 2020

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

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

   One area that has been getting exposure is traffic analysis of
   encrypted messages based on the length of the message.  This
   specification does not provide for a uniform method of providing
   padding as part of the message structure.  An observer can
   distinguish between two different messages (for example, 'YES' and
   'NO') based on the length for all of the content encryption
   algorithms that are defined in [I-D.ietf-cose-rfc8152bis-algs]
   document.  This means that it is up to the applications to document
   how content padding is to be done in order to prevent or discourage
   such analysis.  (For example, the text strings could be defined as
   'YES' and 'NO '.)

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".

Schaad & Housley          Expires 10 April 2021                [Page 13]
Internet-Draft               COSE Structure                 October 2020

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

   *  Primary Maintainer: Jim Schaad

   *  Languages: There are three 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.  Currently examples
      dealing with countersignatures, and ECDH with Curve25519 and
      Goldilocks are missing.

Schaad & Housley          Expires 10 April 2021                [Page 14]
Internet-Draft               COSE Structure                 October 2020

   *  Licensing: Public Domain

8.  References

8.1.  Normative References

   [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>.

   [I-D.ietf-cbor-7049bis]
              Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", Work in Progress, Internet-Draft,
              draft-ietf-cbor-7049bis-16, 30 September 2020,
              <https://tools.ietf.org/html/draft-ietf-cbor-7049bis-16>.

   [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>.

   [I-D.ietf-cose-rfc8152bis-algs]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", Work in Progress, Internet-Draft,
              draft-ietf-cose-rfc8152bis-algs-12, 24 September 2020,
              <https://tools.ietf.org/html/draft-ietf-cose-rfc8152bis-
              algs-12>.

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]    Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259, December 2017.

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

Schaad & Housley          Expires 10 April 2021                [Page 15]
Internet-Draft               COSE Structure                 October 2020

   [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>.

   [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-groupcomm-bis]
              Dijk, E., Wang, C., and M. Tiloca, "Group Communication
              for the Constrained Application Protocol (CoAP)", Work in
              Progress, Internet-Draft, draft-ietf-core-groupcomm-bis-
              01, 13 July 2020, <https://tools.ietf.org/html/draft-ietf-
              core-groupcomm-bis-01>.

   [I-D.ietf-cose-rfc8152bis-struct]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", Work in Progress, Internet-Draft,
              draft-ietf-cose-rfc8152bis-struct-13, 4 September 2020,
              <https://tools.ietf.org/html/draft-ietf-cose-rfc8152bis-
              struct-13>.

   [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>.

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.

   A GitHub project has been created at <https://github.com/cose-wg/
   Examples> that contains not only the examples presented in this
   document, but a more complete set of testing examples as well.  Each
   example is found in a JSON file that contains the inputs used to
   create the example, some of the intermediate values that can be used
   in debugging the example and the output of the example presented both
   as a hex dump and in CBOR diagnostic notation format.  Some of the

Schaad & Housley          Expires 10 April 2021                [Page 16]
Internet-Draft               COSE Structure                 October 2020

   examples at the site are designed failure testing cases; these are
   clearly marked as such in the JSON file.  If errors in the examples
   in this document are found, the examples on GitHub will be updated,
   and a note to that effect will be placed in the JSON file.

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

   gem install cbor-diag

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

   diag2cbor.rb < inputfile > outputfile

   The examples can be extracted from the XML version of this document
   via an XPath expression as all of the 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()

A.1.  Examples of Signed Messages

A.1.1.  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

Schaad & Housley          Expires 10 April 2021                [Page 17]
Internet-Draft               COSE Structure                 October 2020

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

A.2.  Examples of Enveloped Messages

A.2.1.  Countersignature on Encrypted Content

   This example uses the following:

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

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

   Size of binary file is 326 bytes

Schaad & Housley          Expires 10 April 2021                [Page 18]
Internet-Draft               COSE Structure                 October 2020

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

A.3.  Examples of Encrypted Messages

A.4.  Examples of MACed Messages

Schaad & Housley          Expires 10 April 2021                [Page 19]
Internet-Draft               COSE Structure                 October 2020

A.5.  Examples of MAC0 Messages

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.

   {{{ 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

   Jim Schaad
   August Cellars

   Email: ietf@augustcellars.com

   Russ Housley (editor)
   Vigil Security, LLC

   Email: housley@vigilsec.com

Schaad & Housley          Expires 10 April 2021                [Page 20]