]> 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 in COSE-based protocols. 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 . As the security characteristics of the two cases differ, care must be taken when choosing the appropriate mode for a given application. See for a discussion on the security of the implementations. The primary use case is that of "long-term signatures", i.e., signatures that can still be verified even after the signing certificate has expired. This can address situations where it is important to prevent subsequent denial by the signer or to verify signatures made using (very) short-term certificates. To achieve this, the document signer acquires a fresh TST for the document's signature from a trusted TSA and concatenates it with the document. Later, when a relying party verifies the signed document and its associated TST, they can be certain that the document was signed at least at the time specified by the TSA, and that the signing certificate was valid at the time the signature was made. This primary usage scenario motivates the "COSE then Timestamp" mode described in . The second use case is new. It is the notarization of a signed document by registering it with a transparency service. This is common practice for ensuring the accountability and auditability of issued documents, which are typically referred to as "statements" in this context. It is also common practice to only register the signed parts of a statement (the "signed statement" portion) with a transparency service, in order to reduce the complexity of consistency checks at a later stage, as well as avoiding the need to retrieve or reconstruct unsigned parts. Once the signed parts of a document have been registered in the append-only log at a transparency service, the log entry cannot be changed. In order to avoid losing the TST during the registration process, the TST must be included in the signed statement. To achieve this, the issuer acquires a TST from a TSA, includes it in the to-be-signed part of the statement so that the resulting signed statement includes the TST, and then registers the signed parts (rendering it a "transparent statement"). Later on, a relying party consuming the transparent statement including the TST can be certain that the statement was signed by the issuer at least at the time specified by the TSA. If the issuer's signing key has expired (or been compromised), the authenticity of the statement can be ascertained by ensuring that no revocation information was made public before the time asserted by the issuer and registered at the transparency service. This new usage scenario motivates the "Timestamp then COSE" mode defined 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 parallelogram boxes and replacing the label [signature] with [signatures].

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.

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 +<-----' '--------------------------' ]]>

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-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 The following illustrates how MessageImprint is computed using a sample COSE_Sign1 message. Given the COSE_Sign1 message >, / unprotected / { / kid / 4:'11' }, / payload / 'This is the content.', / signature / h'8eb33e4ca31d1c465ab05aac34cc6b23d58fef5c083106c4 d25a91aef0b0117e2af9a291aa32e14ab834dc56ed2a223444547e01f11d3b0916e5 a4c345cacb36' ] ) ]]> 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 The following illustrates how MessageImprint is computed using a sample COSE_Sign message. Given the COSE_Sign message >, / unprotected / { / kid / 4:'11' }, / signature / h'e2aeafd40d69d19dfe6e52077c5d7ff4e408282cbefb 5d06cbf414af2e19d982ac45ac98b8544c908b4507de1e90b717c3d34816fe926a2b 98f53afd2fa0f30a' ] ] ] ) ]]> the signatures array is used as input for computing the MessageImprint. When using SHA-256, the resulting MessageImprint is

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. (For an example, see .) 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.

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, particularly with regard to the need for implementations to protect private key material. Additionally, solutions based on the COSE header parameters defined in this document must be able to report compromised keys promptly. 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. The resolution, accuracy, and precision of the TSA clock, as well as the expected latency introduced by round trips to and from the TSA must be taken into account when implementing solutions based on the COSE header parameters defined in this document.

Avoiding Semantic Confusion 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. Implementers MUST clearly differentiate between RFC 3161 TSA timestamps proving the existence of payload data at an earlier point in time (TTC) and timestamps explicitly providing evidence of the existence of the cryptographic signature (CTT). Failure to clearly distinguish between these timestamp semantics can result in vulnerabilities, such as incorrectly accepting signatures created after key revocation based on older payload-only timestamps. Validators must not interpret protected-header payload timestamps as proof of signature creation time and should rely exclusively on RFC 3161 TSA timestamps explicitly covering signature data for determining signature validity timing.

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 RFC Editor, please note that the following examples use fictitious numbers for TBD1 and TBD2. Once the assignments have been made by the IANA, they will need to be recomputed.

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'8eb33e4ca31d1c465ab05aac34cc6b23d58fef5c083106c4d 25a91aef0b0117e2af9a291aa32e14ab834dc56ed2a223444547e01f11d3b0916e5a4 c345cacb36' ] ) ]]> 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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636573311830160603550403130f7777772e667265657473612e6f72673122302006\ 092a864886f70d0109011613627573696c657a617340676d61696c2e636f6d311230\ 1006035504071309577565727a62757267310b3009060355040613024445310f300d\ 0603550408130642617965726e30820222300d06092a864886f70d01010105000382\ 020f003082020a0282020100b591048c4e486f34e9dc08627fc2375162236984b82c\ b130beff517cfc38f84bce5c65a874dab2621ae0bce7e33563e0ede934fd5f882315\ 9f07848808227460c1ed88261706f4281334359dfbb81bd1353fc179610af1a8c8c8\ 65dc00ea23b3a89be6bd03ba85a9ec827d60565905e22d6a584ed1380ae150280cee\ 397e98a012f380464007862443bc077cb95f421af31712d9683cdb6dffbaf3c8ba5b\ a566ae523d459d6177346d4d840e27886b7c01c5b890d78a2e27bba8dd2f9a2812e1\ 57d62f921c65962548069dcdb7d06de181de0e9570d66f87220ce28b628ab55906f3\ ee0c210f7051e8f4858af8b9a92d09e46af2d9cba5bfcfad168cdf604491a4b06603\ b114caf7031f065e7eeefa53c575f3490c059d2e32ddc76ac4d4c4c710683b97fd1b\ e591bc61055186d88f9a0391b307b6f91ed954daa36f9acd6a1e14aa2e4adf17464b\ 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5d1260928eba009e803429ae324cf96f042354f37bca5afddc79f79346ab388bfc79\ f01dc9861254ea6cc129941076b83d20556f3be51326837f2876f7833b370e7c3d41\ 0523827d4f53400c72218d75229ff10c6f8893a9a3a1c0c42bb4c898c13df41c7f65\ 73b4fc56515971a610a7b0d2857c8225a9fb204eaceca2e8971aa1af87886a2ae3c7\ 2fe0a0aae842980a77bef16b92115458090d982b5946603764e75a0ad3d11454b998\ 6f678b9ab6afe8497033ae3abfd4eb43b7bc9dee68815949e6481582a82e785277f2\ 282107efe390200e0508acb8ea82ea2505276f3c9da2a3d3b4ad38bbf8842bda36fc\ 2448291f558dc02dd1e0308207ff308205e7a003020102020900c1e986160da8e980\ 300d06092a864886f70d01010d05003081953111300f060355040a13084672656520\ 5453413110300e060355040b1307526f6f74204341311830160603550403130f7777\ 772e667265657473612e6f72673122302006092a864886f70d010901161362757369\ 6c657a617340676d61696c2e636f6d3112301006035504071309577565727a627572\ 67310f300d0603550408130642617965726e310b3009060355040613024445301e17\ 0d3136303331333031353231335a170d3431303330373031353231335a3081953111\ 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Acknowledgments The editors would like to thank Alexey Melnikov, Carl Wallace, Carsten Bormann, Deb Cooley, Éric Vyncke, Francesca Palombini, Leonard Rosenthol, Linda Dunbar, Michael B. Jones, Michael Prorock, Mike Bishop, Mohamed Boucadair, Orie Steele, Roman Danyliw, 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.