COSE H. Birkholz Internet-Draft Fraunhofer SIT Intended status: Standards Track T. Fossati Expires: 14 March 2025 Linaro M. Riechert Microsoft 10 September 2024 COSE Header parameter for RFC 3161 Time-Stamp Tokens draft-ietf-cose-tsa-tst-header-parameter-03 Abstract This document defines a CBOR Signing And Encrypted (COSE) header parameter for incorporating RFC 3161-based timestamping into COSE message structures (COSE_Sign and COSE_Sign1). This enables the use of established RFC 3161 timestamping infrastructure to prove the creation time of a message. 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 14 March 2025. Birkholz, et al. Expires 14 March 2025 [Page 1] Internet-Draft TST Header September 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. Use Cases . . . . . . . . . . . . . . . . . . . . . . . 2 1.2. Requirements Notation . . . . . . . . . . . . . . . . . . 3 2. Modes of Use . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Timestamp then COSE (TTC) . . . . . . . . . . . . . . . . 4 2.2. COSE then Timestamp (CTT) . . . . . . . . . . . . . . . . 5 3. RFC 3161 Time-Stamp Tokens COSE Header Parameters . . . . . . 6 3.1. 3161-ttc . . . . . . . . . . . . . . . . . . . . . . . . 6 3.2. 3161-ctt . . . . . . . . . . . . . . . . . . . . . . . . 7 4. Timestamp Processing . . . . . . . . . . . . . . . . . . . . 7 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 7. Normative References . . . . . . . . . . . . . . . . . . . . 9 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 9 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 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 two new CBOR Object Signing and Encryption (COSE) [STD96] header parameters that carry 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. Use Cases This section discusses two use cases, each representing one of the two modes of use defined in Section 2. Birkholz, et al. Expires 14 March 2025 [Page 2] Internet-Draft TST Header September 2024 A first use case is a digital document signed alongside a trustworthy timestamp. This is a common case in legal contracts. In such scenario, the document signer wants to reinforce the claim that the document existed on a specific date. To achieve this, the document signer acquires a fresh TST for the document from a TSA, combines it with the document, and then signs the bundle. Later on, a relying party consuming the signed bundle can be certain that the document existed _at least_ at the time specified by the TSA. The relying party does not have to trust the signer's clock, which may have been maliciously altered or simply inaccurate. This usage scenario motivates the "Timestamp then COSE" mode defined in Section 2.1. A second use case is the notarization of a signed document by registering it at a Transparency Service. This is common for accountability and auditability of issued documents. Once a document is registered at a Transparency Service's append-only log, its log entry cannot be changed. In certain cases, the registration policy of a Transparency Service may add a trustworthy timestamp to the signed document. This is done to lock the signature to a specific point in time. To achieve this, the Transparency Service acquires a TST from a TSA, bundles it alongside the signed document, and then registers it. A relying party that wants to ascertain the authenticity of the document after the signing key has been compromised, can do so by making sure that no revocation information has been made public before the time asserted in the TST. This usage scenario motivates the "COSE then Timestamp" mode described 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. Birkholz, et al. Expires 14 March 2025 [Page 3] Internet-Draft TST Header September 2024 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 boxes and replacing the label [signature] with [signatures]. 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. This mode is utilized when the signature should also be performed over the timestamp to provide an immutable timestamp. 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. 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. Birkholz, et al. Expires 14 March 2025 [Page 4] Internet-Draft TST Header September 2024 .--------. .-----. | Signer | | TSA | +--------+----------------------------------. +-----+-------------. | .-------------. .-------. | | .-------------. | | / private-key / | nonce +-------->+ / private-key / | | '-+-----------' '-------' | | '------+------' | | | .---------. | | | | | | .-------. .----. | message | | | | | | | + datum +->+ hash +->+ Imprint +------->+ .+. | | | '-+-----' '----' '---------' | | | L | Clock | | | | | | '+' | | | | .-------------. | | | | | v v | protected | | | v | | .-------. | .-----. | | | .---------. | | | Sign1 +<-----------+ | TST |<----------+ | timestamp | | | '-+-----' | '-----' | | | '---------' | | | '-------------' | | | '----|--------------------------------------' '-------------------' | .-------------. +-------------+-----------+ | unprotected | | | | | .-----. | [protected] [payload] [signature] | | ... | | | | | | '-----' | | v v '------+------' | .-------+------------+-----. | '--->+ rfc3161-ttc COSE +<-----' '--------------------------' Figure 1: Timestamp, then COSE (TCC) 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. This mode is utilized when a record of the timing of the signature operation is desired. Birkholz, et al. Expires 14 March 2025 [Page 5] Internet-Draft TST Header September 2024 .--------. .-----. | Signer | | TSA | +--------+----------------------------------. +-----+-------------. | .-------------. .-----------. .-------. | | .-------------. | | / private-key / | protected | | datum | | | / private-key / | | '-----+-------' '---+-------' '---+---' | | '------+------' | | | | | | | | | | +---------------+------------' | | | | | | | | .+. | | v .---------. | | | L | Clock | | .---------. .----. | message | | | '+' | | | Sign1 +->+ hash +->+ Imprint +-------->| | | | '-+-------' '----' '---------' | | v | | | .-------. | | .---------. | | | | nonce +--------->| | timestamp | | | | '-------' | | '---------' | '----|--------------------------------------' '---------+---------' | .-------------. | +-------------+-----------+ | unprotected | | | | | | .-----. | | [protected] [payload] [signature] | | TST |<-----' | | | | '-----' | | v v '------+------' | .-------+------------+-----. | '--->+ rfc3161-ctt COSE +<-----' '--------------------------' Figure 2: COSE, then Timestamp (CTT) In this context, timestamp tokens are similar to a countersignature 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 parameter contains a DER-encoded RFC3161 TimeStampToken wrapped in a CBOR byte string (Major type 2). Birkholz, et al. Expires 14 March 2025 [Page 6] Internet-Draft TST Header September 2024 To minimize dependencies, the hash algorithm used for signing the COSE message SHOULD be the same as the algorithm used in the RFC3161 MessageImprint. 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 parameter contains a DER-encoded RFC3161 TimeStampToken wrapped in a CBOR byte string (Major type 2). 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 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. Birkholz, et al. Expires 14 March 2025 [Page 7] Internet-Draft TST Header September 2024 Also review the Security Considerations section in [STD96]; these considerations apply to this document as well, especially the need for implementations to protect private key material. The following scenario assumes 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 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 signed COSE object 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 can be employed inside the ones described in this specification, but is out of scope for this document. 6. IANA Considerations IANA is requested to add the COSE header parameters defined in Table 1 to the "COSE Header Parameters" registry [IANA.cose_header-parameters]. +==========+=======+=======+==========+=================+===========+ | Name | Label | Value | Value | Description | Reference | | | | Type | Registry | | | +==========+=======+=======+==========+=================+===========+ | 3161-tcc | TBD1 | bstr | - | RFC 3161 | RFCthis, | | | | | | timestamp | Section | | | | | | token | 3.1 | +----------+-------+-------+----------+-----------------+-----------+ | 3161-ctt | TBD2 | bstr | - | RFC 3161 | RFCthis, | | | | | | timestamp | Section | | | | | | token | 3.2 | +----------+-------+-------+----------+-----------------+-----------+ Table 1: New COSE Header Parameters Birkholz, et al. Expires 14 March 2025 [Page 8] Internet-Draft TST Header September 2024 7. Normative References [IANA.cose_header-parameters] IANA, "COSE Header Parameters", . [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, . Acknowledgments The editors would like to thank Carl Wallace, Leonard Rosenthol, Michael B. Jones, Michael Prorock, Orie Steele, and Steve Lasker for their reviews and comments. Contributors Carsten Bormann Email: cabo@tzi.org Carsten contributed part of the security considerations. Orie Steele Email: orie@transmute.industries Orie contributed an improved version of the diagrams. Birkholz, et al. Expires 14 March 2025 [Page 9] Internet-Draft TST Header September 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 14 March 2025 [Page 10]