Internet-Draft TST Header February 2024
Birkholz, et al. Expires 27 August 2024 [Page]
Intended Status:
Standards Track
H. Birkholz
Fraunhofer SIT
T. Fossati
M. Riechert

COSE Header parameter for RFC 3161 Time-Stamp Tokens


RFC 3161 provides a method for timestamping a message digest to prove that the message was created before a given time. This document defines a CBOR Signing And Encrypted (COSE) header parameter that can be used to combine COSE message structures used for signing (i.e., COSE_Sign and COSE_Sign1) with existing RFC 3161-based timestamping infrastructure.

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This Internet-Draft will expire on 27 August 2024.

Table of Contents

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 a new COSE [STD96] header parameter that carries 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).

1.1. 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.

2.1. Timestamp then COSE (TTC)

Figure 1 shows the case where a datum is first digested and submitted to a TSA to be timestamped.

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.

payload Sig_structure COSE_Sign/COSE_Sign1 TSA TST
Figure 1: Timestamp, then COSE (TTC)

The message imprint sent to the TSA (Section 2.4 of [RFC3161]) MUST be the hash of the payload field of the COSE signed object.

2.2. COSE then Timestamp (CTT)

Figure 2 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.

COSE_Sign/COSE_Sign1 TST signatures/signature TSA
Figure 2: COSE, then Timestamp (CTT)

In this context, timestamp tokens are similar to a countersignature [RFC9338] made by the TSA.

3. RFC 3161 Time-Stamp Tokens COSE Header Parameters

The two modes described in Section 2.1 and Section 2.2 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-ttc

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

The 3161-ttc protected header is defined as follows:

The content of the byte string are the bytes of the DER-encoded RFC 3161 TimeStampToken structure.

3.2. 3161-ctt

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

The message imprint sent in the request to the TSA MUST be either:

  • the hash of the signature field of the COSE_Sign1 message.

  • the hash of the 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.

The 3161-ctt unprotected header is defined as follows:

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 message imprint that was sent to the TSA. The hash algorithm is contained in the message imprint structure, together with the hash itself.

As part of the signature verification, the receiver MUST make sure that the message imprint in the embedded timestamp token matches either the payload or the signature fields, depending on the mode of use.

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

The security considerations made in [RFC3161] as well as those of [RFC9338] apply.

6. IANA Considerations

IANA is requested to add the two COSE header parameters described in Section 3 to the "COSE Header Parameters" subregistry of the [IANA.cose] registry.

7. References

7.1. Normative References

IANA, "CBOR Object Signing and Encryption (COSE)", <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
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, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, DOI 10.17487/RFC5652, , <>.
Schaad, J., "CBOR Object Signing and Encryption (COSE): Structures and Process", STD 96, RFC 9052, DOI 10.17487/RFC9052, , <>.

7.2. Informative References

Schaad, J., "CBOR Object Signing and Encryption (COSE): Countersignatures", STD 96, RFC 9338, DOI 10.17487/RFC9338, , <>.

Authors' Addresses

Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
64295 Darmstadt
Thomas Fossati
Maik Riechert
United Kingdom