]> Fraunhofer SIT

Rheinstrasse 75 Darmstadt 64295 Germany henk.birkholz@sit.fraunhofer.de

Linaro

thomas.fossati@linaro.org

Microsoft

UK Maik.Riechert@microsoft.com

Security COSE Internet-Draft 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 Source for this draft and an issue tracker can be found at .

Introduction RFC 3161 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) 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).

 Use Cases This section discusses two use cases, each representing one of the two modes of use defined in . 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 . 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 .

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, motivated by the usage scenarios discussed above. 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].

Timestamp then COSE (TTC) 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 () MUST be the hash of the payload field of the COSE signed object.

Timestamp, then COSE (TCC) + / private-key / | | '-+-----------' '-------' | | '------+------' | | | .---------. | | | | | | .-------. .----. | message | | | | | | | + datum +->+ hash +->+ Imprint +------->+ .+. | | | '-+-----' '----' '---------' | | | L | Clock | | | | | | '+' | | | | .-------------. | | | | | v v | protected | | | v | | .-------. | .-----. | | | .---------. | | | Sign1 +<-----------+ | TST |<----------+ | timestamp | | | '-+-----' | '-----' | | | '---------' | | | '-------------' | | | '----|--------------------------------------' '-------------------' | .-------------. +-------------+-----------+ | unprotected | | | | | .-----. | [protected] [payload] [signature] | | ... | | | | | | '-----' | | v v '------+------' | .-------+------------+-----. | '--->+ rfc3161-ttc COSE +<-----' '--------------------------' ]]>

COSE then Timestamp (CTT) 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.

COSE, then Timestamp (CTT) + hash +->+ Imprint +-------->| | | | '-+-------' '----' '---------' | | v | | | .-------. | | .---------. | | | | nonce +--------->| | timestamp | | | | '-------' | | '---------' | '----|--------------------------------------' '---------+---------' | .-------------. | +-------------+-----------+ | unprotected | | | | | | .-----. | | [protected] [payload] [signature] | | TST |<-----' | | | | '-----' | | v v '------+------' | .-------+------------+-----. | '--->+ rfc3161-ctt COSE +<-----' '--------------------------' ]]>

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

RFC 3161 Time-Stamp Tokens COSE Header Parameters The two modes described in and 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.

3161-ttc The 3161-ttc COSE protected header parameter MUST be used for the mode described in . The 3161-ttc protected header parameter contains a DER-encoded RFC3161 TimeStampToken wrapped in a CBOR byte string (Major type 2). To minimize dependencies, the hash algorithm used for signing the COSE message SHOULD be the same as the algorithm used in the RFC3161 MessageImprint.

3161-ctt The 3161-ctt COSE unprotected header parameter MUST be used for the mode 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 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).

Timestamp Processing 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 a hash of either the payload, signature, or signature fields, depending on the mode of use and type of COSE structure. provides an example that illustrates how timestamp tokens can be used to verify signatures of a timestamped message when utilizing X.509 certificates.

Security Considerations Please review the Security Considerations section in ; these considerations apply to this document as well. Also review the Security Considerations section in ; 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.

IANA Considerations IANA is requested to add the COSE header parameters defined in to the "COSE Header Parameters" registry . New COSE Header Parameters

Name	Label	Value Type	Value Registry	Description	Reference
3161-tcc	TBD1	bstr	-	RFC 3161 timestamp token	RFCthis,
3161-ctt	TBD2	bstr	-	RFC 3161 timestamp token	RFCthis,

Normative References This document describes the Cryptographic Message Syntax (CMS). This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary message content. [STANDARDS-TRACK] This document describes the format of a request sent to a Time Stamping Authority (TSA) and of the response that is returned. It also establishes several security-relevant requirements for TSA operation, with regards to processing requests to generate responses. [STANDARDS-TRACK] Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines the CBOR Object Signing and Encryption (COSE) protocol. This specification describes how to create and process signatures, message authentication codes, and encryption using CBOR for serialization. This specification additionally describes how to represent cryptographic keys using CBOR. This document, along with RFC 9053, obsoletes RFC 8152. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. IANA

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

cabo@tzi.org

Carsten contributed part of the security considerations.

orie@transmute.industries

Orie contributed an improved version of the diagrams.