RFC 9921: CBOR Object Signing and Encryption (COSE) Header Parameter for Timestamp Tokens as Defined in RFC 3161

H. Birkholz,
T. Fossati,
M. Riechert

Proposed Standard

Abstract

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

Status of This Memo

This is an Internet Standards Track document.¶

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9921.¶

Copyright Notice

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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

1. Introduction

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

This document defines two new CBOR Object Signing and Encryption (COSE) [RFC9052] header parameters that carry the TimeStampToken (TST) output [RFC3161], thus allowing existing and widely deployed trust infrastructure to be used with COSE structures used for signing (COSE_Sign and COSE_Sign1).¶

1.1. Use Cases

This section discusses two use cases, each representing one of the two modes of use defined in Section 2. As the security characteristics of the two cases differ, care must be taken when choosing the appropriate mode for a given application. See Section 5.1 for a discussion on the security of the implementations.¶

The primary 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 Time Stamping Authority (TSA) [RFC3161] 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 primary usage scenario motivates the "COSE, Then Timestamp" mode described in Section 2.1.¶

The second use case is new. It 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 and to avoid 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 has 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 new usage scenario motivates the "Timestamp, Then COSE" mode defined in Section 2.2.¶

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 COSE Signed Message 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.¶

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 COSE Signed Message is then built as follows:¶

The obtained timestamp token is added to the protected headers.¶
The original datum becomes the payload of the COSE Signed Message.¶

Figure 2: Timestamp, Then COSE (TTC)

3. Timestamp Tokens per RFC 3161: COSE Header Parameters

The two modes described in Sections 2.2 and 2.1 use different inputs into the timestamping machinery and consequently create different kinds of bindings 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 TST [RFC3161] wrapped in a CBOR byte string (Major type 2).¶

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

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 Sections 3.1.1 and 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¶

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¶

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 TST [RFC3161] 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 Message. 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 MessageImprint [RFC3161]. However, this may not be possible if the timestamp requester and the COSE message signer are different entities.¶

4. Timestamp Processing

Timestamp tokens [RFC3161] use Cryptographic Message Syntax (CMS) as the signature envelope format. [RFC5652] provides details about signature verification, and [RFC3161] provides 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 [RFC9052]. 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 that 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 mode 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.¶

In CTT mode, an attacker could manipulate the unprotected header by removing or replacing the timestamp. To avoid that, the COSE Signed Message 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 is out of scope for this document.¶

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.¶

5.1. Avoiding Semantic Confusion

CTT mode and TTC mode have different semantic meanings. An implementation must ensure that the contents of the CTT and TTC 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.¶

Implementers MUST clearly differentiate between TSA timestamps [RFC3161] 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 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 TSA timestamps [RFC3161] explicitly covering signature data for determining signature validity timing.¶

6. IANA Considerations

IANA has allocated the COSE header parameters defined in Table 1 in the "COSE Header Parameters" registry [IANA.cose_header-parameters] as follows:¶

Table 1: New COSE Header Parameters
Name	Label	Value Type	Value Registry	Description	Reference
`3161-ttc`	269	`bstr`	-	Timestamp Token per [RFC3161]: Timestamp, Then COSE	RFC 9921, Section 3.2
`3161-ctt`	270	`bstr`	-	Timestamp Token per [RFC3161]: COSE, Then Timestamp	RFC 9921, Section 3.1

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/info/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/info/rfc3161>.
[RFC5652]: Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, DOI 10.17487/RFC5652, September 2009, <https://www.rfc-editor.org/info/rfc5652>.
[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>.

Appendix A. Examples

A.1. TTC

The payload¶

'This is the content.'

is hashed using SHA-256 to create the following 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 TSA.¶

A TimeStampResp containing the following TST is returned:¶

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 12096870
              GeneralizedTime 29/08/2025 07:45:46 GMT
              BOOLEAN TRUE
[...]

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

18([
        <<{1: -7, 269: h'3082154906092a864886f70d010702a082153a308215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d0e041604146e760b7b4e4f9ce160ca6d2ce927a2a294b37737301f0603551d230418
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206c030160603551d250101ff040c300a06082b06010505070308306306082b060105
0507010104573055302a06082b06010505073002861e687474703a2f2f7777772e667
265657473612e6f72672f7473612e637274302706082b06010505073001861b687474
703a2f2f7777772e667265657473612e6f72673a3235363030370603551d1f0430302
e302ca02aa0288626687474703a2f2f7777772e667265657473612e6f72672f63726c
2f726f6f745f63612e63726c3081c60603551d200481be3081bb3081b80601003081b
2303306082b060105050702011627687474703a2f2f7777772e667265657473612e6f
72672f667265657473615f6370732e68746d6c303206082b060105050702011626687
474703a2f2f7777772e667265657473612e6f72672f667265657473615f6370732e70
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4696d657374616d70696e6720536f6674776172652061732061205365727669636520
285361615329300d06092a864886f70d01010d05000382020100a5c944e2c6fac0a14
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0e43eaa81f849a6c5fe3fe8b5ea23275f687f2052e502ea6c30762a668cce07871dd8
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1c0c42bb4c898c13df41c7f6573b4fc56515971a610a7b0d2857c8225a9fb204eacec
a2e8971aa1af87886a2ae3c72fe0a0aae842980a77bef16b92115458090d982b59466
03764e75a0ad3d11454b9986f678b9ab6afe8497033ae3abfd4eb43b7bc9dee688159
49e6481582a82e785277f2282107efe390200e0508acb8ea82ea2505276f3c9da2a3d
3b4ad38bbf8842bda36fc2448291f558dc02dd1e0308207ff308205e7a00302010202
0900c1e986160da8e980300d06092a864886f70d01010d05003081953111300f06035
5040a130846726565205453413110300e060355040b1307526f6f7420434131183016
0603550403130f7777772e667265657473612e6f72673122302006092a864886f70d0
109011613627573696c657a617340676d61696c2e636f6d3112301006035504071309
577565727a62757267310f300d0603550408130642617965726e310b3009060355040
613024445301e170d3136303331333031353231335a170d3431303330373031353231
335a3081953111300f060355040a130846726565205453413110300e060355040b130
7526f6f74204341311830160603550403130f7777772e667265657473612e6f726731
22302006092a864886f70d0109011613627573696c657a617340676d61696c2e636f6
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65726e310b300906035504061302444530820222300d06092a864886f70d010101050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'}>>,
        {4: '11'},
        'This is the content.',
        h'f5f0f27964f178dcb2254b30fdfdc48abc4499beaea7cb80f4004f30403
f13a44bcca24fc61c5d71d3823bac04b923011dc7d31de35df1aefcd5a8ec5fe0fe6e
'
    ])

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¶

SEQUENCE {
  INTEGER 1
  SEQUENCE {
    SEQUENCE {
      OBJECT IDENTIFIER sha-256 (2 16 840 1 101 3 4 2 1)
      NULL
      }
    OCTET STRING
      DD 94 71 EF E7 43 C4 05 13 35 DF 8F 6D 28 82 F3
      BA DC 38 77 00 F7 ED 3F 70 91 67 2A 3E EA F7 C8
    }
  BOOLEAN TRUE
  }

which is sent to the TSA.¶

A TimeStampResp containing the following TST is returned:¶

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
                  DD 94 71 EF E7 43 C4 05 13 35 DF 8F 6D 28 82 F3
                  BA DC 38 77 00 F7 ED 3F 70 91 67 2A 3E EA F7 C8
                }
              INTEGER 12100074
              GeneralizedTime 29/08/2025 07:53:00 GMT
              BOOLEAN TRUE
[...]

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

18(
  [
    / protected h'a10126' / << {
        / alg / 1:-7 / ECDSA 256 /
      } >>,
    / unprotected / {
      / 3161-ctt / 270 : h'3082154906092a864886f70d010702a082153a3082
1536020103310f300d0609608648016503040203050030820184060b2a864886f70d0
109100104a08201730482016f3082016b02010106042a0304013031300d0609608648
01650304020105000420dd9471efe743c4051335df8f6d2882f3badc387700f7ed3f7
091672a3eeaf7c8020400b8a1ea180f32303235303832393037353330305a0101ffa0
820111a482010d308201093111300f060355040a13084672656520545341310c300a0
60355040b130354534131763074060355040d136d5468697320636572746966696361
7465206469676974616c6c79207369676e7320646f63756d656e747320616e6420746
96d65207374616d70207265717565737473206d616465207573696e67207468652066
7265657473612e6f7267206f6e6c696e6520736572766963657331183016060355040
3130f7777772e667265657473612e6f72673122302006092a864886f70d0109011613
627573696c657a617340676d61696c2e636f6d3112301006035504071309577565727
a62757267310b3009060355040613024445310f300d0603550408130642617965726e
a082100830820801308205e9a003020102020900c1e986160da8e982300d06092a864
886f70d01010d05003081953111300f060355040a130846726565205453413110300e
060355040b1307526f6f74204341311830160603550403130f7777772e66726565747
3612e6f72673122302006092a864886f70d0109011613627573696c657a617340676d
61696c2e636f6d3112301006035504071309577565727a62757267310f300d0603550
408130642617965726e310b3009060355040613024445301e170d3136303331333031
353733395a170d3236303331313031353733395a308201093111300f060355040a130
84672656520545341310c300a060355040b130354534131763074060355040d136d54
686973206365727469666963617465206469676974616c6c79207369676e7320646f6
3756d656e747320616e642074696d65207374616d70207265717565737473206d6164
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669636573311830160603550403130f7777772e667265657473612e6f726731223020
06092a864886f70d0109011613627573696c657a617340676d61696c2e636f6d31123
01006035504071309577565727a62757267310b3009060355040613024445310f300d
0603550408130642617965726e30820222300d06092a864886f70d010101050003820
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21c65962548069dcdb7d06de181de0e9570d66f87220ce28b628ab55906f3ee0c210f
7051e8f4858af8b9a92d09e46af2d9cba5bfcfad168cdf604491a4b06603b114caf70
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050507010104573055302a06082b06010505073002861e687474703a2f2f7777772e6
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02e302ca02aa0288626687474703a2f2f7777772e667265657473612e6f72672f6372
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1b2303306082b060105050702011627687474703a2f2f7777772e667265657473612e
6f72672f667265657473615f6370732e68746d6c303206082b0601050507020116266
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40613024445301e170d3136303331333031353231335a170d34313033303730313532
31335a3081953111300f060355040a130846726565205453413110300e060355040b1
307526f6f74204341311830160603550403130f7777772e667265657473612e6f7267
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f6d3112301006035504071309577565727a62757267310f300d060355040813064261
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5000382020f003082020a0282020100b6028e0e3032f11110d964cda94b9d0278e194
2ae913aaa59907cda69793995bd9ac7e33bad9fe3704da1c01a98d21afe3f591a59d7
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c307157c722aa5b5dd0bfb2d6c38b1b93749c881ec60026d08951b3824bd71bacbce4
73aebd636f0b918b4a2c8ff4694f07457af2d6f1cf82554d1770fd79ff5d314dcd104
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2dd802c1672629e5c0052cb357dd29aafc43f615b3b1ff9d4e1ce08c71c73e1febb7d
c56a33621329e9ed6c230203010001a382024e3082024a300c0603551d13040530030
101ff300e0603551d0f0101ff0404030201c6301d0603551d0e04160414fa550d8c34
6651434cf7e7b3a76c95af7ae6a4973081ca0603551d230481c23081bf8014fa550d8
c346651434cf7e7b3a76c95af7ae6a497a1819ba481983081953111300f060355040a
130846726565205453413110300e060355040b1307526f6f742043413118301606035
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      / kid / 4:'11'
    },
    / payload / 'This is the content.',
    / signature / h'8eb33e4ca31d1c465ab05aac34cc6b23d58fef5c083106c4
d25a91aef0b0117e2af9a291aa32e14ab834dc56ed2a223444547e01f11d3b0916e5
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  ]
)

Acknowledgments

The authors 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.¶