COSE Header parameter for RFC 3161 Time-Stamp Tokens
draft-ietf-cose-tsa-tst-header-parameter-06

COSE                                                         H. Birkholz
Internet-Draft                                            Fraunhofer SIT
Intended status: Standards Track                              T. Fossati
Expires: 14 December 2025                                         Linaro
                                                             M. Riechert
                                                               Microsoft
                                                            12 June 2025

          COSE Header parameter for RFC 3161 Time-Stamp Tokens
              draft-ietf-cose-tsa-tst-header-parameter-06

Abstract

   This document defines two CBOR Signing And Encrypted (COSE) header
   parameters for incorporating RFC 3161-based timestamping into COSE
   message structures (COSE_Sign and COSE_Sign1).  This enables the use
   of established RFC 3161 timestamping infrastructure in COSE-based
   protocols.

About This Document

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

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-cose-tsa-tst-header-
   parameter/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-scitt/draft-birkholz-cose-tsa-tst-header-
   parameter.

Status of This Memo

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   This Internet-Draft will expire on 14 December 2025.

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Copyright Notice

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Notation . . . . . . . . . . . . . . . . . .   4
   2.  Modes of Use  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  COSE then Timestamp (CTT) . . . . . . . . . . . . . . . .   4
     2.2.  Timestamp then COSE (TTC) . . . . . . . . . . . . . . . .   5
   3.  RFC 3161 Time-Stamp Tokens COSE Header Parameters . . . . . .   6
     3.1.  3161-ctt  . . . . . . . . . . . . . . . . . . . . . . . .   6
       3.1.1.  MessageImprint Computation for COSE_Sign1 . . . . . .   7
       3.1.2.  MessageImprint Computation for COSE_Sign  . . . . . .   8
     3.2.  3161-ttc  . . . . . . . . . . . . . . . . . . . . . . . .   9
   4.  Timestamp Processing  . . . . . . . . . . . . . . . . . . . .  10
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .  12
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  13
     A.1.  TTC . . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     A.2.  CTT . . . . . . . . . . . . . . . . . . . . . . . . . . .  18
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  24
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

1.  Introduction

   RFC 3161 [RFC3161] provides a method to timestamp a message digest to
   prove that it was created before a given time.

   This document defines two new CBOR Object Signing and Encryption
   (COSE) [STD96] header parameters that carry the TimestampToken (TST)
   output of RFC 3161, thus allowing existing and widely deployed trust
   infrastructure to be used with COSE structures used for signing
   (COSE_Sign and COSE_Sign1).

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1.1.  Use Cases

   This section discusses two use cases, each representing one of the
   two modes of use defined in Section 2.

   The first use case is that of "long-term signatures", i.e.,
   signatures that can still be verified even after the signing
   certificate has expired.  This can address situations where it is
   important to prevent subsequent denial by the signer or to verify
   signatures made using (very) short-term certificates.  To achieve
   this, the document signer acquires a fresh TST for the document's
   signature from a trusted TSA and concatenates it with the document.
   Later, when a relying party verifies the signed document and its
   associated TST, they can be certain that the document was signed _at
   least_ at the time specified by the TSA, and that the signing
   certificate was valid at the time the signature was made.

   This usage scenario motivates the "COSE then Timestamp" mode
   described in Section 2.1.

   The second use case is the notarization of a signed document by
   registering it with a transparency service.  This is common practice
   for ensuring the accountability and auditability of issued documents,
   which are typically referred to as "statements" in this context.  It
   is also common practice to only register the signed parts of a
   statement (the "signed statement" portion) with a transparency
   service, in order to reduce the complexity of consistency checks at a
   later stage, as well as avoiding the need to retrieve or reconstruct
   unsigned parts.  Once the signed parts of a document have been
   registered in the append-only log at a transparency service, the log
   entry cannot be changed.  In order to avoid losing the TST during the
   registration process, the TST must be included in the signed
   statement.  To achieve this, the issuer acquires a TST from a TSA,
   includes it in the to-be-signed part of the statement so that the
   resulting signed statement includes the TST, and then registers the
   signed parts (rendering it a "transparent statement").  Later on, a
   relying party consuming the transparent statement including the TST
   can be certain that the statement was signed by the issuer _at least_
   at the time specified by the TSA.  If the issuer's signing key has
   expired (or been compromised), the authenticity of the statement can
   be ascertained by ensuring that no revocation information was made
   public before the time asserted by the issuer and registered at the
   transparency service.

   This usage scenario motivates the "Timestamp then COSE" mode defined
   in Section 2.2.

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1.2.  Requirements Notation

   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.

2.  Modes of Use

   There are two different modes of composing COSE protection and
   timestamping, motivated by the usage scenarios discussed above.

   The diagrams in this section illustrate the processing flow of the
   specified modes.  For simplicity, only the COSE_Sign1 processing is
   shown.  Similar diagrams for COSE_Sign can be derived by allowing
   multiple private-key parallelogram boxes and replacing the label
   [signature] with [signatures].

2.1.  COSE then Timestamp (CTT)

   Figure 1 shows the case where the signature(s) field of the signed
   COSE object is digested and submitted to a TSA to be timestamped.
   The obtained timestamp token is then added back as an unprotected
   header into the same COSE object.

   This mode is utilized when a record of the timing of the signature
   operation is desired.

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   .--------.                                     .-----.
   | Signer |                                     | TSA |
   +--------+----------------------------------.  +-----+-------------.
   |   .-------------. .-----------. .-------. |  |   .-------------. |
   |  / private-key /  | protected | | datum | |  |  / private-key /  |
   | '-----+-------'   '---+-------' '---+---' |  | '------+------'   |
   |       |               |             |     |  |        |          |
   |       +---------------+------------'      |  |        |          |
   |       |                                   |  |       .+.         |
   |       v                  .---------.      |  |      | L | Clock  |
   |  .---------.    .----.   | Message |      |  |       '+'         |
   | |   Sign1   +->+ hash +->+ Imprint +-------->|        |          |
   |  '-+-------'    '----'   '---------'      |  |        v          |
   |    |                      .-------.       |  |    .---------.    |
   |    |                      | nonce +--------->|   | timestamp |   |
   |    |                      '-------'       |  |    '---------'    |
   '----|--------------------------------------'  '---------+---------'
        |                                 .-------------.   |
        +-------------+-----------+       | unprotected |   |
        |             |            |      |   .-----.   |   |
   [protected]    [payload]   [signature] |   | TST |<-----'
        |             |            |      |   '-----'   |
        |             v            v      '------+------'
        |     .-------+------------+-----.       |
         '--->+     rfc3161-ctt COSE     +<-----'
              '--------------------------'

                    Figure 1: COSE, then Timestamp (CTT)

   In this context, timestamp tokens are similar to a countersignature
   made by the TSA.

2.2.  Timestamp then COSE (TTC)

   Figure 2 shows the case where a datum is first digested and submitted
   to a TSA to be timestamped.

   This mode is used to wrap the signed document and its timestamp
   together in an immutable payload.

   A signed COSE message is then built as follows:

   *  The obtained timestamp token is added to the protected headers,

   *  The original datum becomes the payload of the signed COSE message.

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   .--------.                                     .-----.
   | Signer |                                     | TSA |
   +--------+----------------------------------.  +-----+-------------.
   |   .-------------.          .-------.      |  |   .-------------. |
   |  / private-key /           | nonce +-------->+  / private-key /  |
   | '-+-----------'            '-------'      |  | '------+------'   |
   |   |                       .---------.     |  |        |          |
   |   |  .-------.   .----.   | Message |     |  |        |          |
   |   |  + datum +->+ hash +->+ Imprint +------->+       .+.         |
   |   |  '-+-----'   '----'   '---------'     |  |      | L | Clock  |
   |   |    |                                  |  |       '+'         |
   |   |    |               .-------------.    |  |        |          |
   |   v    v               |  protected  |    |  |        v          |
   |  .-------.             |   .-----.   |    |  |    .---------.    |
   | |  Sign1  +<-----------+   | TST |<----------+   | timestamp |   |
   |  '-+-----'             |   '-----'   |    |  |    '---------'    |
   |    |                   '-------------'    |  |                   |
   '----|--------------------------------------'  '-------------------'
        |                                 .-------------.
        +-------------+-----------+       | unprotected |
        |             |            |      |   .-----.   |
   [protected]    [payload]   [signature] |   | ... |   |
        |             |            |      |   '-----'   |
        |             v            v      '------+------'
        |     .-------+------------+-----.       |
         '--->+     rfc3161-ttc COSE     +<-----'
              '--------------------------'

                    Figure 2: Timestamp, then COSE (TTC)

3.  RFC 3161 Time-Stamp Tokens COSE Header Parameters

   The two modes described in Section 2.2 and Section 2.1 use different
   inputs into the timestamping machinery, and consequently create
   different kinds of binding between COSE and TST.  To clearly separate
   their semantics two different COSE header parameters are defined as
   described in the following subsections.

3.1.  3161-ctt

   The 3161-ctt COSE _unprotected_ header parameter MUST be used for the
   mode described in Section 2.1.

   The 3161-ctt unprotected header parameter contains a DER-encoded
   RFC3161 TimeStampToken wrapped in a CBOR byte string (Major type 2).

   The MessageImprint sent in the request to the TSA MUST be:

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   *  the hash of the CBOR-encoded signature field of the COSE_Sign1
      message, or

   *  the hash of the CBOR-encoded signatures field of the COSE_Sign
      message.

   In either case, to minimize dependencies, the hash algorithm SHOULD
   be the same as the algorithm used for signing the COSE message.  This
   may not be possible if the timestamp token has been obtained outside
   the processing context in which the COSE object is assembled.

   Refer to Section 3.1.1 and Section 3.1.2 for concrete examples of
   MessageImprint computation.

3.1.1.  MessageImprint Computation for COSE_Sign1

   The following illustrates how MessageImprint is computed using a
   sample COSE_Sign1 message.

   Given the COSE_Sign1 message

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

   the bstr-wrapped signature

   58 40                                     # bytes(64)
      8eb33e4ca31d1c465ab05aac34cc6b23
      d58fef5c083106c4d25a91aef0b0117e
      2af9a291aa32e14ab834dc56ed2a2234
      44547e01f11d3b0916e5a4c345cacb36

   (including the heading bytes 0x5840) is used as input for computing
   the MessageImprint.

   When using SHA-256, the resulting MessageImprint is

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   SEQUENCE {
     SEQUENCE {
       OBJECT IDENTIFIER sha-256 (2 16 840 1 101 3 4 2 1)
       NULL
       }
     OCTET STRING
       44 C2 41 9D 13 1D 53 D5 55 84 B5 DD 33 B7 88 C2
       4E 55 1C 6D 44 B1 AF C8 B2 B8 5E 69 54 76 3B 4E
     }

3.1.2.  MessageImprint Computation for COSE_Sign

   The following illustrates how MessageImprint is computed using a
   sample COSE_Sign message.

   Given the COSE_Sign message

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

   the signatures array

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   81                                        # array(1)
      83                                     # array(3)
         43                                  # bytes(3)
            a10126
         a1                                  # map(1)
            04                               # unsigned(4)
            42                               # bytes(2)
               3131                          # "11"
         58 40                               # bytes(64)
            e2aeafd40d69d19dfe6e52077c5d7ff4
            e408282cbefb5d06cbf414af2e19d982
            ac45ac98b8544c908b4507de1e90b717
            c3d34816fe926a2b98f53afd2fa0f30a

   is used as input for computing the MessageImprint.

   When using SHA-256, the resulting MessageImprint is

   SEQUENCE {
     SEQUENCE {
       OBJECT IDENTIFIER sha-256 (2 16 840 1 101 3 4 2 1)
       NULL
       }
     OCTET STRING
       80 3F AD A2 91 2D 6B 7A 83 3A 27 BD 96 1C C0 5B
       C1 CC 16 47 59 B1 C5 6F 7A A7 71 E4 E2 15 26 F7
     }

3.2.  3161-ttc

   The 3161-ttc COSE _protected_ header parameter MUST be used for the
   mode described in Section 2.2.

   The 3161-ttc protected header parameter contains a DER-encoded
   RFC3161 TimeStampToken wrapped in a CBOR byte string (Major type 2).

   The MessageImprint sent to the TSA (Section 2.4 of [RFC3161]) MUST be
   the hash of the payload of the COSE signed object.  This does not
   include the bstr-wrapping, only the payload bytes.  (For an example,
   see Appendix A.1.)

   To minimize dependencies, the hash algorithm used for signing the
   COSE message SHOULD be the same as the algorithm used in the RFC3161
   MessageImprint.  However, this may not be possible if the timestamp
   requester and the COSE message signer are different entities.

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4.  Timestamp Processing

   RFC 3161 timestamp tokens use CMS as signature envelope format.
   [STD70] provides the details about signature verification, and
   [RFC3161] provides the details specific to timestamp token
   validation.  The payload of the signed timestamp token is the TSTInfo
   structure defined in [RFC3161], which contains the MessageImprint
   that was sent to the TSA.  The hash algorithm is contained in the
   MessageImprint structure, together with the hash itself.

   As part of the signature verification, the receiver MUST make sure
   that the MessageImprint in the embedded timestamp token matches a
   hash of either the payload, signature, or signature fields, depending
   on the mode of use and type of COSE structure.

   Appendix B of [RFC3161] provides an example that illustrates how
   timestamp tokens can be used to verify signatures of a timestamped
   message when utilizing X.509 certificates.

5.  Security Considerations

   Please review the Security Considerations section in [RFC3161]; these
   considerations apply to this document as well.

   Also review the Security Considerations section in [STD96].  These
   considerations apply to this document as well, particularly with
   regard to the need for implementations to protect private key
   material.  Additionally, solutions based on the COSE header
   parameters defined in this document must be able to report
   compromised keys promptly.

   The following scenario assumes an attacker can manipulate the clocks
   on the COSE signer and its relying parties, but not the TSA.  It is
   also assumed that the TSA is a trusted third party, so the attacker
   cannot impersonate the TSA and create valid timestamp tokens.  In
   such a setting, any tampering with the COSE signer's clock does not
   have an impact because, once the timestamp is obtained from the TSA,
   it becomes the only reliable source of time.  However, in both CTT
   and TTC mode, a denial of service can occur if the attacker can
   adjust the relying party's clock so that the CMS validation fails.
   This could disrupt the timestamp validation.

   Implementers MUST clearly differentiate between RFC 3161 TSA
   timestamps proving the existence of payload data at an earlier point
   in time (TTC) and timestamps explicitly providing evidence of the
   existence of the cryptographic signature (CTT).  Failure to clearly
   distinguish between these timestamp semantics can result in
   vulnerabilities, such as incorrectly accepting signatures created

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   after key revocation based on older payload-only timestamps.
   Validators must not interpret protected-header payload timestamps as
   proof of signature creation time and should rely exclusively on RFC
   3161 TSA timestamps explicitly covering signature data for
   determining signature validity timing.

   In CTT mode, an attacker could manipulate the unprotected header by
   removing or replacing the timestamp.  To avoid that, the signed COSE
   object should be integrity protected during transit and at rest.

   In TTC mode, the TSA is given an opaque identifier (a cryptographic
   hash value) for the payload.  While this means that the content of
   the payload is not directly revealed, to prevent comparison with
   known payloads or disclosure of identical payloads being used over
   time, the payload would need to be armored, e.g., with a nonce that
   is shared with the recipient of the header parameter but not the TSA.
   Such a mechanism can be employed inside the ones described in this
   specification, but is out of scope for this document.

   CTT and TTC modes have different semantic meanings.  An
   implementation must ensure that the contents of the CTT and TCC
   headers are interpreted according to their specific semantics.  In
   particular, symmetric to the signature and assembly mechanics, each
   mode has its own separate verification algorithm.

   The resolution, accuracy, and precision of the TSA clock, as well as
   the expected latency introduced by round trips to and from the TSA
   must be taken into account when implementing solutions based on the
   COSE header parameters defined in this document.

6.  IANA Considerations

   IANA is requested to add the COSE header parameters defined in
   Table 1 to the "COSE Header Parameters" registry
   [IANA.cose_header-parameters].

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   +==========+=======+=======+==========+=============+===========+
   | Name     | Label | Value | Value    | Description | Reference |
   |          |       | Type  | Registry |             |           |
   +==========+=======+=======+==========+=============+===========+
   | 3161-ttc | TBD1  | bstr  | -        | RFC 3161    | RFCthis,  |
   |          |       |       |          | timestamp   | Section   |
   |          |       |       |          | token:      | 3.2       |
   |          |       |       |          | Timestamp   |           |
   |          |       |       |          | then COSE   |           |
   +----------+-------+-------+----------+-------------+-----------+
   | 3161-ctt | TBD2  | bstr  | -        | RFC 3161    | RFCthis,  |
   |          |       |       |          | timestamp   | Section   |
   |          |       |       |          | token: COSE | 3.1       |
   |          |       |       |          | then        |           |
   |          |       |       |          | Timestamp   |           |
   +----------+-------+-------+----------+-------------+-----------+

                  Table 1: New COSE Header Parameters

7.  Normative References

   [IANA.cose_header-parameters]
              IANA, "COSE Header Parameters",
              <https://www.iana.org/assignments/cose>.

   [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/rfc/rfc2119>.

   [RFC3161]  Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
              "Internet X.509 Public Key Infrastructure Time-Stamp
              Protocol (TSP)", RFC 3161, DOI 10.17487/RFC3161, August
              2001, <https://www.rfc-editor.org/rfc/rfc3161>.

   [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/rfc/rfc8174>.

   [STD70]    Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/rfc/rfc5652>.

   [STD96]    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/rfc/rfc9052>.

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Appendix A.  Examples

A.1.  TTC

   The payload

   This is the content.

   is hashed using SHA-256 to create the TimeStampReq object

   SEQUENCE {
     INTEGER 1
     SEQUENCE {
       SEQUENCE {
         OBJECT IDENTIFIER sha-256 (2 16 840 1 101 3 4 2 1)
         NULL
         }
       OCTET STRING
         09 E6 38 D4 AA 95 FD 72 71 86 62 03 59 53 03 BC
         E2 32 F4 62 A9 4D 38 E3 93 77 3C D3 AA E3 F6 B0
       }
     BOOLEAN TRUE
     }

   which is sent to the Time Stamping Authority.

   A TimeStampResp is returned which contains the TimeStampToken

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   SEQUENCE {
     OBJECT IDENTIFIER signedData (1 2 840 113549 1 7 2)
     [0] {
       SEQUENCE {
         INTEGER 3
         SET {
           SEQUENCE {
             OBJECT IDENTIFIER sha-512 (2 16 840 1 101 3 4 2 3)
             NULL
             }
           }
         SEQUENCE {
           OBJECT IDENTIFIER tSTInfo (1 2 840 113549 1 9 16 1 4)
           [0] {
             OCTET STRING, encapsulates {
               SEQUENCE {
                 INTEGER 1
                 OBJECT IDENTIFIER '1 2 3 4 1'
                 SEQUENCE {
                   SEQUENCE {
                     OBJECT IDENTIFIER sha-256 (2 16 840 1 101 3 4 2 1)
                     NULL
                     }
                   OCTET STRING
                     09 E6 38 D4 AA 95 FD 72 71 86 62 03 59 53 03 BC
                     E2 32 F4 62 A9 4D 38 E3 93 77 3C D3 AA E3 F6 B0
                   }
                 INTEGER 85048992
                 GeneralizedTime 18/01/2025 11:20:06 GMT
                 BOOLEAN TRUE
   [...]

   The contents of the TimeStampToken are bstr-wrapped and added to the
   protected headers bucket which is then signed alongside the original
   payload to obtain the COSE_Sign1 object

   =============== NOTE: '\' line wrapping per RFC 8792 ================

   18([
           <<{1: -7, 258: h'\
   3082154906092a864886f70d010702a082153a30821536020103310f300d06096086\
   48016503040203050030820184060b2a864886f70d0109100104a08201730482016f\
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   6c79207369676e7320646f63756d656e747320616e642074696d65207374616d7020\

Birkholz, et al.        Expires 14 December 2025               [Page 14]
Internet-Draft                 TST Header                      June 2025

   7265717565737473206d616465207573696e672074686520667265657473612e6f72\
   67206f6e6c696e65207365727669636573311830160603550403130f7777772e6672\
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   672f667265657473615f6370732e706466304706082b06010505070202303b1a3946\

Birkholz, et al.        Expires 14 December 2025               [Page 15]
Internet-Draft                 TST Header                      June 2025

   72656554534120747275737465642074696d657374616d70696e6720536f66747761\
   72652061732061205365727669636520285361615329300d06092a864886f70d0101\
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   fa550d8c346651434cf7e7b3a76c95af7ae6a497a1819ba481983081953111300f06\

Birkholz, et al.        Expires 14 December 2025               [Page 16]
Internet-Draft                 TST Header                      June 2025

   0355040a130846726565205453413110300e060355040b1307526f6f742043413118\
   30160603550403130f7777772e667265657473612e6f72673122302006092a864886\
   f70d0109011613627573696c657a617340676d61696c2e636f6d3112301006035504\
   071309577565727a62757267310f300d0603550408130642617965726e310b300906\
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   028761f0f2eaab1525bc489525d6ed34b34ae00a7ce34ceefaa6df08026047e470e3\

Birkholz, et al.        Expires 14 December 2025               [Page 17]
Internet-Draft                 TST Header                      June 2025

   09d0507832b65dad717287dcef8c250d7d7ddf677dd3a6c267c2d29c86e04653ce84\
   f7376c2434e2e85ec0eeaf2031a5f8cb4025f13b67c3ed4062af46000dbb1e3b5699\
   d14cb309c8cdabb736651b76957cb4392f9e2452a88233936e39bd23dae37eeee3de\
   4733a1ce2545324deb8a2203eed8264e3d657e60479cc08fa93916c266dcd1027daa\
   1afd091bde8bea923d92b6e17615eb9f83210c4f2b6fcecb918cfa638a75679aa3df\
   b5f959edc50923ff70c0d45a647a714f01ea48d803f68bb5081c97a57dcbd00c15d9\
   44ba3a89e126bec18b9f49c0225cab0c9e9a9b24de43e5e767b7512a525d909a52e5\
   cb2d79f5221d4f056e60dafb1dcc6e46f6dd1bb553d8caa37ee6add7c1dc70796766\
                                         d126e88b37d69fec915aa3dd65'}>>,
           {4: '11'},
           'This is the content.',
           h'\
   1b512caa05005b7a2329c1b92cc5447de3a387acc2537ec579d26d38c5be8740ed85\
           b8d3888630cc080b5eaaad12c029cde6117599565e63ca8485e927958682'
       ])

A.2.  CTT

   Starting with the following COSE_Sign1 object

   18(
     [
       / protected h'a10126' / << {
           / alg / 1:-7 / ECDSA 256 /
         } >>,
       / unprotected / {
         / kid / 4:'11'
       },
       / payload / 'This is the content.',
       / signature / h'8eb33e4ca31d1c465ab05aac34cc6b23d58fef5c083106c4d
   25a91aef0b0117e2af9a291aa32e14ab834dc56ed2a223444547e01f11d3b0916e5a4
   c345cacb36'
     ]
   )

   The CBOR-encoded signature field is hashed using SHA-256 to create
   the following TimeStampReq object

Birkholz, et al.        Expires 14 December 2025               [Page 18]
Internet-Draft                 TST Header                      June 2025

   SEQUENCE {
     INTEGER 1
     SEQUENCE {
       SEQUENCE {
         OBJECT IDENTIFIER sha-256 (2 16 840 1 101 3 4 2 1)
         NULL
         }
       OCTET STRING
         44 C2 41 9D 13 1D 53 D5 55 84 B5 DD 33 B7 88 C2
         4E 55 1C 6D 44 B1 AF C8 B2 B8 5E 69 54 76 3B 4E
       }
     BOOLEAN TRUE
     }

   which is sent to the Time Stamping Authority.

   A TimeStampResp is returned which contains the following
   TimeStampToken

Birkholz, et al.        Expires 14 December 2025               [Page 19]
Internet-Draft                 TST Header                      June 2025

   SEQUENCE {
     OBJECT IDENTIFIER signedData (1 2 840 113549 1 7 2)
     [0] {
       SEQUENCE {
         INTEGER 3
         SET {
           SEQUENCE {
             OBJECT IDENTIFIER sha-512 (2 16 840 1 101 3 4 2 3)
             NULL
             }
           }
         SEQUENCE {
           OBJECT IDENTIFIER tSTInfo (1 2 840 113549 1 9 16 1 4)
           [0] {
             OCTET STRING, encapsulates {
               SEQUENCE {
                 INTEGER 1
                 OBJECT IDENTIFIER '1 2 3 4 1'
                 SEQUENCE {
                   SEQUENCE {
                     OBJECT IDENTIFIER sha-256 (2 16 840 1 101 3 4 2 1)
                     NULL
                     }
                   OCTET STRING
                     44 C2 41 9D 13 1D 53 D5 55 84 B5 DD 33 B7 88 C2
                     4E 55 1C 6D 44 B1 AF C8 B2 B8 5E 69 54 76 3B 4E
                   }
                 INTEGER 84895155
                 GeneralizedTime 17/01/2025 18:29:13 GMT
                 BOOLEAN TRUE
   [...]

   The contents of the TimeStampToken are bstr-wrapped and added to the
   unprotected headers bucket in the original COSE_Sign1 object to
   obtain the following

   =============== NOTE: '\' line wrapping per RFC 8792 ================

   18(
     [
       / protected h'a10126' / << {
           / alg / 1:-7 / ECDSA 256 /
         } >>,
       / unprotected / {
         / 3161-ctt / 259 : h'\
   3082154906092a864886f70d010702a082153a30821536020103310f300d06096086\
   48016503040203050030820184060b2a864886f70d0109100104a08201730482016f\
   3082016b02010106042a0304013031300d06096086480165030402010500042044c2\

Birkholz, et al.        Expires 14 December 2025               [Page 20]
Internet-Draft                 TST Header                      June 2025

   419d131d53d55584b5dd33b788c24e551c6d44b1afc8b2b85e6954763b4e0204050f\
   65b3180f32303235303131373138323931335a0101ffa0820111a482010d30820109\
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Birkholz, et al.        Expires 14 December 2025               [Page 21]
Internet-Draft                 TST Header                      June 2025

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Birkholz, et al.        Expires 14 December 2025               [Page 22]
Internet-Draft                 TST Header                      June 2025

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Birkholz, et al.        Expires 14 December 2025               [Page 23]
Internet-Draft                 TST Header                      June 2025

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                                            ca81532e740cb0824ca501504b',
         / kid / 4:'11'
       },
       / payload / 'This is the content.',
       / signature / h'8eb33e4ca31d1c465ab05aac34cc6b23d58fef5c083106c4
   d25a91aef0b0117e2af9a291aa32e14ab834dc56ed2a223444547e01f11d3b0916e5
   a4c345cacb36'
     ]
   )

Acknowledgments

   The editors would like to thank Alexey Melnikov, Carl Wallace,
   Carsten Bormann, Deb Cooley, Éric Vyncke, Francesca Palombini,
   Leonard Rosenthol, Linda Dunbar, Michael B.  Jones, Michael Prorock,
   Mike Bishop, Mohamed Boucadair, Orie Steele, Roman Danyliw, Shuping
   Peng, Stefan Santesson, Steve Lasker, and Yingzhen Qu for their
   reviews and comments.

Contributors

   Carsten Bormann
   Email: cabo@tzi.org

   Carsten contributed part of the security considerations.

   Orie Steele
   Email: orie@transmute.industries

   Orie contributed an improved version of the diagrams.

Authors' Addresses

Birkholz, et al.        Expires 14 December 2025               [Page 24]
Internet-Draft                 TST Header                      June 2025

   Henk Birkholz
   Fraunhofer SIT
   Rheinstrasse 75
   64295 Darmstadt
   Germany
   Email: henk.birkholz@sit.fraunhofer.de

   Thomas Fossati
   Linaro
   Email: thomas.fossati@linaro.org

   Maik Riechert
   Microsoft
   United Kingdom
   Email: Maik.Riechert@microsoft.com

Birkholz, et al.        Expires 14 December 2025               [Page 25]