COSE Header parameter for RFC 3161 Time-Stamp Tokens

Introduction RFC 3161 provides a method to timestamp a message digest to prove that it was created before a given time. This document defines a new COSE 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).

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 when, and only when, they appear in all capitals, as shown here.

Modes of use There are two different modes of composing COSE protection and timestamping.

Timestamp then COSE 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.

Timestamp, then COSE | Sig_structure +---->| COSE_Sign/COSE_Sign1 | '----+----' '---------------' '----------------------' | ^ | .---. | | | | .-----. | '--->| TSA +---->| TST +---' | | '-----' '---' ]]>

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

COSE, then Timestamp | TSA +---' | | '---' ]]>

RFC 3161 Time-Stamp Tokens COSE Header Parameter To carry RFC 3161 timestamp tokens in COSE signed messages, a new COSE header parameter, rfc3161-tst, is defined as follows:

Name: rfc3161-tst
Label: TBD
Value Type: bstr
Value Registry: none
Description: RFC 3161 timestamp token
Reference: RFCthis

The content of the byte string are the bytes of the DER-encoded RFC 3161 TimeStampToken structure. When used as described in , the message imprint sent to the TSA () MUST be the hash of the payload field of the COSE signed object. When used as described in , the message imprint sent in the request to the TSA MUST be either:

the hash of the signature field of the COSE_Sign1.
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. RFC 3161 timestamp tokens use CMS as signature envelope format. provides the details about signature verification, and provides the details specific to timestamp token validation. The payload of the signed timestamp token is the TSTInfo structure defined in , 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.. Guidance is illustrated in via an example that shows how timestamp tokens can be used during signature verification of a timestamped message when using X.509 certificates.

Security Considerations The security considerations made in as well as those of apply.

IANA Considerations IANA is requested to add the COSE Header parameter described in to the "COSE Header Parameters" of the registry.