]> 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 two CBOR Signing And Encrypted (COSE) header parameters 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. About This Document Status information for this document may be found at . 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, such as when a short-lived certificate is used for the signature, 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 expired (or 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 used to wrap the signed document and its timestamp together in an immutable payload. 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 (TTC) + / 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). The MessageImprint sent to the TSA () MUST be the hash of the payload of the COSE signed object. This does not include the bstr-wrapping, only the payload bytes. To minimize dependencies, the hash algorithm used for signing the COSE message SHOULD be the same as the algorithm used in the RFC3161 MessageImprint. However, this may not be possible if the timestamp requester and the COSE message signer are different entities.

3161-ctt The 3161-ctt COSE unprotected header parameter MUST be used for the mode described in . The 3161-ctt unprotected header parameter contains a DER-encoded RFC3161 TimeStampToken wrapped in a CBOR byte string (Major type 2). The MessageImprint sent in the request to the TSA MUST be:

• the hash of the CBOR-encoded signature field of the COSE_Sign1 message, or
• the hash of the CBOR-encoded 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. Refer to and for concrete examples of MessageImprint computation.

 MessageImprint Computation for COSE_Sign1 Using as an example the COSE_Sign1 message >, / unprotected / { / kid / 4:'11' }, / payload / 'This is the content.', / signature / h'8eb33e4ca31d1c465ab05aac34cc6b23d58fef5c083106c4 d25a91aef0b0117e2af9a291aa32e14ab834dc56ed2a223444547e01f11d3b0916e5 a4c345cacb36' ] ) ]]> then the bstr-wrapped signature (including the heading bytes 0x5840) is used as input for computing the MessageImprint. When using SHA-256, the resulting MessageImprint is

 MessageImprint Computation for COSE_Sign Using as an example the COSE_Sign message >, / unprotected / { / kid / 4:'11' }, / signature / h'e2aeafd40d69d19dfe6e52077c5d7ff4e408282cbefb 5d06cbf414af2e19d982ac45ac98b8544c908b4507de1e90b717c3d34816fe926a2b 98f53afd2fa0f30a' ] ] ] ) ]]> then the signatures array is used as input for computing the MessageImprint. When using SHA-256, the resulting MessageImprint is

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 MessageImprint that was sent to the TSA. The hash algorithm is contained in the MessageImprint structure, together with the hash itself. As part of the signature verification, the receiver MUST make sure that the MessageImprint 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. CTT and TTC modes have different semantic meanings. An implementation must ensure that the contents of the CTT and TCC headers are interpreted according to their specific semantics. In particular, symmetric to the signature and assembly mechanics, each mode has its own separate verification algorithm.

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-ttc	TBD1	bstr	-	RFC 3161 timestamp token: Timestamp then COSE	RFCthis,
3161-ctt	TBD2	bstr	-	RFC 3161 timestamp token: COSE then Timestamp	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

Examples

TTC The payload is hashed using SHA-256 to create the TimeStampReq object which is sent to the Time Stamping Authority. A TimeStampResp is returned which contains the TimeStampToken The contents of the TimeStampToken are bstr-wrapped and added to the protected headers bucket which is then signed alongside the original payload to obtain the COSE_Sign1 object >, {4: '11'}, 'This is the content.', h'\ 1b512caa05005b7a2329c1b92cc5447de3a387acc2537ec579d26d38c5be8740ed85\ b8d3888630cc080b5eaaad12c029cde6117599565e63ca8485e927958682' ]) ]]>

CTT Starting with the following COSE_Sign1 object >, / unprotected / { / kid / 4:'11' }, / payload / 'This is the content.', / signature / h'\ 8eb33e4ca31d1c465ab05aac34cc6b23d58fef5c083106c4d25a91aef0b0117e2af9\ a291aa32e14ab834dc56ed2a223444547e01f11d3b0916e5a4c345cacb36' ] ) ]]> The CBOR-encoded signature field is hashed using SHA-256 to create the following TimeStampReq object which is sent to the Time Stamping Authority. A TimeStampResp is returned which contains the following TimeStampToken The contents of the TimeStampToken are bstr-wrapped and added to the unprotected headers bucket in the original COSE_Sign1 object to obtain the following >, / unprotected / { / 3161-ctt / 259 : h'\ 3082154906092a864886f70d010702a082153a30821536020103310f300d06096086\ 48016503040203050030820184060b2a864886f70d0109100104a08201730482016f\ 3082016b02010106042a0304013031300d06096086480165030402010500042044c2\ 419d131d53d55584b5dd33b788c24e551c6d44b1afc8b2b85e6954763b4e0204050f\ 65b3180f32303235303131373138323931335a0101ffa0820111a482010d30820109\ 3111300f060355040a13084672656520545341310c300a060355040b130354534131\ 763074060355040d136d54686973206365727469666963617465206469676974616c\ 6c79207369676e7320646f63756d656e747320616e642074696d65207374616d7020\ 7265717565737473206d616465207573696e672074686520667265657473612e6f72\ 67206f6e6c696e65207365727669636573311830160603550403130f7777772e6672\ 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Acknowledgments The editors would like to thank Carl Wallace, Carsten Bormann, Francesca Palombini, Leonard Rosenthol, Linda Dunbar, Michael B. Jones, Michael Prorock, Orie Steele, Shuping Peng, Stefan Santesson, Steve Lasker, and Yingzhen Qu 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.