COSE H. Birkholz Internet-Draft Fraunhofer SIT Intended status: Standards Track T. Fossati Expires: 27 August 2024 Linaro M. Riechert Microsoft 24 February 2024 COSE Header parameter for RFC 3161 Time-Stamp Tokens draft-ietf-cose-tsa-tst-header-parameter-02 Abstract 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. Discussion Venues This note is to be removed before publishing as an RFC. 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 This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 27 August 2024. Birkholz, et al. Expires 27 August 2024 [Page 1] Internet-Draft TST Header February 2024 Copyright Notice Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved. 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3 2. Modes of use . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Timestamp then COSE (TTC) . . . . . . . . . . . . . . . . 3 2.2. COSE then Timestamp (CTT) . . . . . . . . . . . . . . . . 3 3. RFC 3161 Time-Stamp Tokens COSE Header Parameters . . . . . . 4 3.1. 3161-ttc . . . . . . . . . . . . . . . . . . . . . . . . 4 3.2. 3161-ctt . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Timestamp Processing . . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 7.1. Normative References . . . . . . . . . . . . . . . . . . 6 7.2. Informative References . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 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). Birkholz, et al. Expires 27 August 2024 [Page 2] Internet-Draft TST Header February 2024 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. Birkholz, et al. Expires 27 August 2024 [Page 3] Internet-Draft TST Header February 2024 .----------------------. .-----. | COSE_Sign/COSE_Sign1 |<--------+ TST | '----+-----------------' '-----' | ^ v | .----------------------. | | 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: * Name: 3161-ttc * Label: TBD * Value Type: bstr * Value Registry: [IANA.cose] * Description: RFC 3161 timestamp token * Reference: Section 3.1 of RFCthis The content of the byte string are the bytes of the DER-encoded RFC 3161 TimeStampToken structure. Birkholz, et al. Expires 27 August 2024 [Page 4] Internet-Draft TST Header February 2024 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: * Name: 3161-ctt * Label: TBD * Value Type: bstr * Value Registry: [IANA.cose] * Description: RFC 3161 timestamp token * Reference: Section 3.2 of RFCthis 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. Birkholz, et al. Expires 27 August 2024 [Page 5] Internet-Draft TST Header February 2024 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.cose] IANA, "CBOR Object Signing and Encryption (COSE)", . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [STD70] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, DOI 10.17487/RFC5652, September 2009, . [STD96] Schaad, J., "CBOR Object Signing and Encryption (COSE): Structures and Process", STD 96, RFC 9052, DOI 10.17487/RFC9052, August 2022, . 7.2. Informative References [RFC9338] Schaad, J., "CBOR Object Signing and Encryption (COSE): Countersignatures", STD 96, RFC 9338, DOI 10.17487/RFC9338, December 2022, . Birkholz, et al. Expires 27 August 2024 [Page 6] Internet-Draft TST Header February 2024 Authors' Addresses 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 27 August 2024 [Page 7]