| Internet-Draft | TST Header | March 2023 |
| Birkholz & Riechert | Expires 14 September 2023 | [Page] |
RFC 3161 provides a method to time-stamp a message digest to prove that it was created before a given time. This document defines how signatures of CBOR Signing And Encrypted (COSE) message structures can be time-stamped using RFC 3161 along with the needed header parameter to carry the corresponding time-stamp.¶
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Useful new COSE [STD96] header member that is the TST output of RFC 3161.¶
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.¶
The use of RFC 3161 Time-Stamp Tokens, often in combination with X.509 certificates, allows for an existing trust infrastructure to be used with COSE.¶
The new COSE header parameter for carrying time-stamp tokens is defined as:¶
The content of the byte string are the bytes of the DER-encoded RFC 3161 TimeStampToken structure. This matches the content of the equivalent header attribute defined in [RFC3161] for Cryptographic Message Syntax (CMS, see [STD70]) envelopes.¶
This header parameter allows for a single time-stamp token or multiple time-stamp tokens to be carried in the message. If a single time-stamp token is conveyed, it is placed in a CBOR byte string. If multiple time-stamp tokens are conveyed, a CBOR array of byte strings is used, with each time-stamp token being in its own byte string.¶
Given that time-stamp tokens in this context are similar to a countersignature [RFC9338], the header parameter can be included in the unprotected header of a COSE envelope.¶
When sending a request to an RFC 3161 Time Stamping Authority (TSA, see [RFC3161]) to obtain a time-stamp token, then the so-called message imprint (Section 2.4 of [RFC3161]) of the request MUST be the hash of the bytes within the byte string of the signature field of the COSE structure to be time-stamped. The hash algorithm does not have to match the algorithm used for signing the COSE message.¶
RFC 3161 time-stamp tokens use CMS as signature envelope format. [STD70] illustrates details of signature verification and [RFC3161] details specific to time-stamp token validation. The payload of the signed time-stamp token is a TSTInfo structure as defined in [RFC3161] and contains the message imprint that was sent to the TSA. As part of validation, the message imprint MUST be matched to the hash of the bytes within the byte string of the signature field of the time-stamped COSE structure. The hash algorithm is contained in the message imprint structure, together with the hash itself.¶
Appendix B of RFC 3161 provides an example of how time-stamp tokens can be used during signature verification of a time-stamped message when using X.509 certificates.¶
TBD¶
Similar security considerations as described in RFC 3161 apply.¶
TBD¶
IANA is requested to register the new COSE Header parameter described in section TBD in the "COSE Header Parameters" registry.¶