rfc9921v1.txt		rfc9921.txt
	Internet Engineering Task Force (IETF) H. Birkholz		Internet Engineering Task Force (IETF) H. Birkholz
	Request for Comments: 9921 Fraunhofer SIT		Request for Comments: 9921 Fraunhofer SIT
	Category: Standards Track T. Fossati		Category: Standards Track T. Fossati
	ISSN: 2070-1721 Linaro		ISSN: 2070-1721 Linaro
	M. Riechert		M. Riechert
	Microsoft		Microsoft
	February 2026		February 2026
	Concise Binary Object Representation (CBOR) Object Signing and		CBOR Object Signing and Encryption (COSE) Header Parameter for Timestamp
	Encryption (COSE) Header Parameter for Timestamp Tokens as Defined in		Tokens as Defined in RFC 3161
	RFC 3161
	Abstract		Abstract
	This document defines two Concise Binary Object Representation (CBOR)		This document defines two CBOR Object Signing and Encryption (COSE)
	Object Signing and Encryption (COSE) header parameters for		header parameters for incorporating timestamping based on RFC 3161
	incorporating timestamping based on RFC 3161 into COSE message		into COSE message structures (COSE_Sign and COSE_Sign1). This
	structures (COSE_Sign and COSE_Sign1). This enables the use of		enables the use of established timestamping infrastructure per RFC
	established timestamping infrastructure per RFC 3161 in COSE-based		3161 in COSE-based protocols.
	protocols.
	Status of This Memo		Status of This Memo
	This is an Internet Standards Track document.		This is an Internet Standards Track document.
	This document is a product of the Internet Engineering Task Force		This document is a product of the Internet Engineering Task Force
	(IETF). It represents the consensus of the IETF community. It has		(IETF). It represents the consensus of the IETF community. It has
	received public review and has been approved for publication by the		received public review and has been approved for publication by the
	Internet Engineering Steering Group (IESG). Further information on		Internet Engineering Steering Group (IESG). Further information on
	Internet Standards is available in Section 2 of RFC 7841.		Internet Standards is available in Section 2 of RFC 7841.
	skipping to change at line 83 ¶		skipping to change at line 81 ¶
	Acknowledgments		Acknowledgments
	Contributors		Contributors
	Authors' Addresses		Authors' Addresses
	1. Introduction		1. Introduction
	RFC 3161 [RFC3161] provides a method for timestamping a message		RFC 3161 [RFC3161] provides a method for timestamping a message
	digest to prove that it was created before a given time.		digest to prove that it was created before a given time.
	This document defines two new CBOR Object Signing and Encryption		This document defines two new CBOR Object Signing and Encryption
	(COSE) [STD96] header parameters that carry the TimeStampToken (TST)		(COSE) [RFC9052] header parameters that carry the TimeStampToken
	output [RFC3161], thus allowing existing and widely deployed trust		(TST) output [RFC3161], thus allowing existing and widely deployed
	infrastructure to be used with COSE structures used for signing		trust infrastructure to be used with COSE structures used for signing
	(COSE_Sign and COSE_Sign1).		(COSE_Sign and COSE_Sign1).
	1.1. Use Cases		1.1. Use Cases
	This section discusses two use cases, each representing one of the		This section discusses two use cases, each representing one of the
	two modes of use defined in Section 2. As the security		two modes of use defined in Section 2. As the security
	characteristics of the two cases differ, care must be taken when		characteristics of the two cases differ, care must be taken when
	choosing the appropriate mode for a given application. See		choosing the appropriate mode for a given application. See
	Section 5.1 for a discussion on the security of the implementations.		Section 5.1 for a discussion on the security of the implementations.
	skipping to change at line 160 ¶		skipping to change at line 158 ¶
	timestamping, motivated by the usage scenarios discussed above.		timestamping, motivated by the usage scenarios discussed above.
	The diagrams in this section illustrate the processing flow of the		The diagrams in this section illustrate the processing flow of the
	specified modes. For simplicity, only the COSE_Sign1 processing is		specified modes. For simplicity, only the COSE_Sign1 processing is
	shown. Similar diagrams for COSE_Sign can be derived by allowing		shown. Similar diagrams for COSE_Sign can be derived by allowing
	multiple private-key parallelogram boxes and replacing the label		multiple private-key parallelogram boxes and replacing the label
	[signature] with [signatures].		[signature] with [signatures].
	2.1. COSE, then Timestamp (CTT)		2.1. COSE, then Timestamp (CTT)
	Figure 1 shows the case where the signature(s) field of the signed		Figure 1 shows the case where the signature(s) field of the COSE
	COSE object is digested and submitted to a TSA to be timestamped.		signed object is digested and submitted to a TSA to be timestamped.
	The obtained timestamp token is then added back as an unprotected		The obtained timestamp token is then added back as an unprotected
	header into the same COSE object.		header into the same COSE object.
	This mode is utilized when a record of the timing of the signature		This mode is utilized when a record of the timing of the signature
	operation is desired.		operation is desired.
	.--------. .-----.		.--------. .-----.
	| Signer | | TSA |		| Signer | | TSA |
	+--------+----------------------------------. +-----+-------------.		+--------+----------------------------------. +-----+-------------.
	| .-------------. .-----------. .-------. | | .-------------. |		| .-------------. .-----------. .-------. | | .-------------. |
	skipping to change at line 257 ¶		skipping to change at line 255 ¶
	into the timestamping machinery and consequently create different		into the timestamping machinery and consequently create different
	kinds of bindings between COSE and TST. To clearly separate their		kinds of bindings between COSE and TST. To clearly separate their
	semantics, two different COSE header parameters are defined as		semantics, two different COSE header parameters are defined as
	described in the following subsections.		described in the following subsections.
	3.1. 3161-ctt		3.1. 3161-ctt
	The 3161-ctt COSE _unprotected_ header parameter MUST be used for the		The 3161-ctt COSE _unprotected_ header parameter MUST be used for the
	mode described in Section 2.1.		mode described in Section 2.1.
	The 3161-ctt unprotected header parameter contains a DER-encoded		The 3161-ctt unprotected header parameter contains a DER-encoded TST
	TimeStampToken [RFC3161] wrapped in a CBOR byte string (Major type		[RFC3161] wrapped in a CBOR byte string (Major type 2).
	2).
	The MessageImprint sent in the request to the TSA MUST be		The MessageImprint sent in the request to the TSA MUST be
	* the hash of the CBOR-encoded signature field of the COSE_Sign1		* the hash of the CBOR-encoded signature field of the COSE_Sign1
	message, or		message, or
	* the hash of the CBOR-encoded signatures field of the COSE_Sign		* the hash of the CBOR-encoded signatures field of the COSE_Sign
	message.		message.
	In either case, to minimize dependencies, the hash algorithm SHOULD		In either case, to minimize dependencies, the hash algorithm SHOULD
	skipping to change at line 385 ¶		skipping to change at line 382 ¶
	OCTET STRING		OCTET STRING
	80 3F AD A2 91 2D 6B 7A 83 3A 27 BD 96 1C C0 5B		80 3F AD A2 91 2D 6B 7A 83 3A 27 BD 96 1C C0 5B
	C1 CC 16 47 59 B1 C5 6F 7A A7 71 E4 E2 15 26 F7		C1 CC 16 47 59 B1 C5 6F 7A A7 71 E4 E2 15 26 F7
	}		}
	3.2. 3161-ttc		3.2. 3161-ttc
	The 3161-ttc COSE _protected_ header parameter MUST be used for the		The 3161-ttc COSE _protected_ header parameter MUST be used for the
	mode described in Section 2.2.		mode described in Section 2.2.
	The 3161-ttc protected header parameter contains a DER-encoded		The 3161-ttc protected header parameter contains a DER-encoded TST
	TimeStampToken [RFC3161] wrapped in a CBOR byte string (Major type		[RFC3161] wrapped in a CBOR byte string (Major type 2).
	2).
	The MessageImprint sent to the TSA (Section 2.4 of [RFC3161]) MUST be		The MessageImprint sent to the TSA (Section 2.4 of [RFC3161]) MUST be
	the hash of the payload of the COSE signed object. This does not		the hash of the payload of the COSE signed object. This does not
	include the bstr wrapping -- only the payload bytes. (For an		include the bstr wrapping -- only the payload bytes. (For an
	example, see Appendix A.1.)		example, see Appendix A.1.)
	To minimize dependencies, the hash algorithm used for signing the		To minimize dependencies, the hash algorithm used for signing the
	COSE message SHOULD be the same as the algorithm used in the		COSE message SHOULD be the same as the algorithm used in the
	MessageImprint [RFC3161]. However, this may not be possible if the		MessageImprint [RFC3161]. However, this may not be possible if the
	timestamp requester and the COSE message signer are different		timestamp requester and the COSE message signer are different
	entities.		entities.
	4. Timestamp Processing		4. Timestamp Processing
	Timestamp tokens [RFC3161] use Cryptographic Message Syntax (CMS) as		Timestamp tokens [RFC3161] use Cryptographic Message Syntax (CMS) as
	the signature envelope format. [STD70] provides details about		the signature envelope format. [RFC5652] provides details about
	signature verification, and [RFC3161] provides details specific to		signature verification, and [RFC3161] provides details specific to
	timestamp token validation. The payload of the signed timestamp		timestamp token validation. The payload of the signed timestamp
	token is the TSTInfo structure defined in [RFC3161], which contains		token is the TSTInfo structure defined in [RFC3161], which contains
	the MessageImprint that was sent to the TSA. The hash algorithm is		the MessageImprint that was sent to the TSA. The hash algorithm is
	contained in the MessageImprint structure, together with the hash		contained in the MessageImprint structure, together with the hash
	itself.		itself.
	As part of the signature verification, the receiver MUST make sure		As part of the signature verification, the receiver MUST make sure
	that the MessageImprint in the embedded timestamp token matches a		that the MessageImprint in the embedded timestamp token matches a
	hash of either the payload, signature, or signature fields, depending		hash of either the payload, signature, or signature fields, depending
	skipping to change at line 425 ¶		skipping to change at line 421 ¶
	Appendix B of [RFC3161] provides an example that illustrates how		Appendix B of [RFC3161] provides an example that illustrates how
	timestamp tokens can be used to verify signatures of a timestamped		timestamp tokens can be used to verify signatures of a timestamped
	message when utilizing X.509 certificates.		message when utilizing X.509 certificates.
	5. Security Considerations		5. Security Considerations
	Please review the Security Considerations section in [RFC3161]; these		Please review the Security Considerations section in [RFC3161]; these
	considerations apply to this document as well.		considerations apply to this document as well.
	Also review the Security Considerations section in [STD96]. These		Also review the Security Considerations section in [RFC9052]. These
	considerations apply to this document as well, particularly with		considerations apply to this document as well, particularly with
	regard to the need for implementations to protect private key		regard to the need for implementations to protect private key
	material. Additionally, solutions based on the COSE header		material. Additionally, solutions based on the COSE header
	parameters defined in this document must be able to report		parameters defined in this document must be able to report
	compromised keys promptly.		compromised keys promptly.
	The following scenario assumes that an attacker can manipulate the		The following scenario assumes that an attacker can manipulate the
	clocks on the COSE signer and its relying parties, but not the TSA.		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		It is also assumed that the TSA is a trusted third party, so the
	attacker cannot impersonate the TSA and create valid timestamp		attacker cannot impersonate the TSA and create valid timestamp
	tokens. In such a setting, any tampering with the COSE signer's		tokens. In such a setting, any tampering with the COSE signer's
	clock does not have an impact, because once the timestamp is obtained		clock does not have an impact, because once the timestamp is obtained
	from the TSA, it becomes the only reliable source of time. However,		from the TSA, it becomes the only reliable source of time. However,
	in both CTT mode and TTC mode, a denial of service can occur if the		in both CTT mode and TTC mode, a denial of service can occur if the
	attacker can adjust the relying party's clock so that the CMS		attacker can adjust the relying party's clock so that the CMS
	validation fails. This could disrupt the timestamp validation.		validation fails. This could disrupt the timestamp validation.
	In CTT mode, an attacker could manipulate the unprotected header by		In CTT mode, an attacker could manipulate the unprotected header by
	removing or replacing the timestamp. To avoid that, the signed COSE		removing or replacing the timestamp. To avoid that, the COSE signed
	object should be integrity protected during transit and at rest.		object should be integrity protected during transit and at rest.
	In TTC mode, the TSA is given an opaque identifier (a cryptographic		In TTC mode, the TSA is given an opaque identifier (a cryptographic
	hash value) for the payload. While this means that the content of		hash value) for the payload. While this means that the content of
	the payload is not directly revealed, to prevent comparison with		the payload is not directly revealed, to prevent comparison with
	known payloads or disclosure of identical payloads being used over		known payloads or disclosure of identical payloads being used over
	time, the payload would need to be armored, e.g., with a nonce that		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.		is shared with the recipient of the header parameter but not the TSA.
	Such a mechanism can be employed inside the parameters described in		Such a mechanism is out of scope for this document.
	this specification but is out of scope for this document.
	The resolution, accuracy, and precision of the TSA clock, as well as		The resolution, accuracy, and precision of the TSA clock, as well as
	the expected latency introduced by round trips to and from the TSA,		the expected latency introduced by round trips to and from the TSA,
	must be taken into account when implementing solutions based on the		must be taken into account when implementing solutions based on the
	COSE header parameters defined in this document.		COSE header parameters defined in this document.
	5.1. Avoiding Semantic Confusion		5.1. Avoiding Semantic Confusion
	CTT mode and TTC mode have different semantic meanings. An		CTT mode and TTC mode have different semantic meanings. An
	implementation must ensure that the contents of the CTT and TTC		implementation must ensure that the contents of the CTT and TTC
	skipping to change at line 522 ¶		skipping to change at line 517 ¶
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,		DOI 10.17487/RFC2119, March 1997,
	<https://www.rfc-editor.org/info/rfc2119>.		<https://www.rfc-editor.org/info/rfc2119>.
	[RFC3161] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,		[RFC3161] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
	"Internet X.509 Public Key Infrastructure Time-Stamp		"Internet X.509 Public Key Infrastructure Time-Stamp
	Protocol (TSP)", RFC 3161, DOI 10.17487/RFC3161, August		Protocol (TSP)", RFC 3161, DOI 10.17487/RFC3161, August
	2001, <https://www.rfc-editor.org/info/rfc3161>.		2001, <https://www.rfc-editor.org/info/rfc3161>.
			[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
			RFC 5652, DOI 10.17487/RFC5652, September 2009,
			<https://www.rfc-editor.org/info/rfc5652>.
	[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC		[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
	2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,		2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
	May 2017, <https://www.rfc-editor.org/info/rfc8174>.		May 2017, <https://www.rfc-editor.org/info/rfc8174>.
	[STD70] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,		[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
	RFC 5652, DOI 10.17487/RFC5652, September 2009,
	<https://www.rfc-editor.org/info/rfc5652>.
	[STD96] Schaad, J., "CBOR Object Signing and Encryption (COSE):
	Structures and Process", STD 96, RFC 9052,		Structures and Process", STD 96, RFC 9052,
	DOI 10.17487/RFC9052, August 2022,		DOI 10.17487/RFC9052, August 2022,
	<https://www.rfc-editor.org/info/rfc9052>.		<https://www.rfc-editor.org/info/rfc9052>.
	Appendix A. Examples		Appendix A. Examples
	A.1. TTC		A.1. TTC
	The payload		The payload
	skipping to change at line 561 ¶		skipping to change at line 556 ¶
	}		}
	OCTET STRING		OCTET STRING
	09 E6 38 D4 AA 95 FD 72 71 86 62 03 59 53 03 BC		09 E6 38 D4 AA 95 FD 72 71 86 62 03 59 53 03 BC
	E2 32 F4 62 A9 4D 38 E3 93 77 3C D3 AA E3 F6 B0		E2 32 F4 62 A9 4D 38 E3 93 77 3C D3 AA E3 F6 B0
	}		}
	BOOLEAN TRUE		BOOLEAN TRUE
	}		}
	which is sent to the TSA.		which is sent to the TSA.
	A TimeStampResp containing the following TimeStampToken is returned:		A TimeStampResp containing the following TST is returned:
	SEQUENCE {		SEQUENCE {
	OBJECT IDENTIFIER signedData (1 2 840 113549 1 7 2)		OBJECT IDENTIFIER signedData (1 2 840 113549 1 7 2)
	[0] {		[0] {
	SEQUENCE {		SEQUENCE {
	INTEGER 3		INTEGER 3
	SET {		SET {
	SEQUENCE {		SEQUENCE {
	OBJECT IDENTIFIER sha-512 (2 16 840 1 101 3 4 2 3)		OBJECT IDENTIFIER sha-512 (2 16 840 1 101 3 4 2 3)
	NULL		NULL
	skipping to change at line 595 ¶		skipping to change at line 590 ¶
	}		}
	OCTET STRING		OCTET STRING
	09 E6 38 D4 AA 95 FD 72 71 86 62 03 59 53 03 BC		09 E6 38 D4 AA 95 FD 72 71 86 62 03 59 53 03 BC
	E2 32 F4 62 A9 4D 38 E3 93 77 3C D3 AA E3 F6 B0		E2 32 F4 62 A9 4D 38 E3 93 77 3C D3 AA E3 F6 B0
	}		}
	INTEGER 12096870		INTEGER 12096870
	GeneralizedTime 29/08/2025 07:45:46 GMT		GeneralizedTime 29/08/2025 07:45:46 GMT
	BOOLEAN TRUE		BOOLEAN TRUE
	[...]		[...]
	The contents of the TimeStampToken are bstr-wrapped and added to the		The contents of the TST are bstr-wrapped and added to the protected
	protected headers bucket, which is then signed alongside the original		headers bucket, which is then signed alongside the original payload
	payload to obtain the COSE_Sign1 object.		to obtain the COSE_Sign1 object.
	18([		18([
	<<{1: -7, 269: h'3082154906092a864886f70d010702a082153a308215		<<{1: -7, 269: h'3082154906092a864886f70d010702a082153a308215
	36020103310f300d0609608648016503040203050030820184060b2a864886f70d010		36020103310f300d0609608648016503040203050030820184060b2a864886f70d010
	9100104a08201730482016f3082016b02010106042a0304013031300d060960864801		9100104a08201730482016f3082016b02010106042a0304013031300d060960864801
	65030402010500042009e638d4aa95fd7271866203595303bce232f462a94d38e3937		65030402010500042009e638d4aa95fd7271866203595303bce232f462a94d38e3937
	73cd3aae3f6b0020400b89566180f32303235303832393037343534365a0101ffa082		73cd3aae3f6b0020400b89566180f32303235303832393037343534365a0101ffa082
	0111a482010d308201093111300f060355040a13084672656520545341310c300a060		0111a482010d308201093111300f060355040a13084672656520545341310c300a060
	355040b130354534131763074060355040d136d546869732063657274696669636174		355040b130354534131763074060355040d136d546869732063657274696669636174
	65206469676974616c6c79207369676e7320646f63756d656e747320616e642074696		65206469676974616c6c79207369676e7320646f63756d656e747320616e642074696
	skipping to change at line 804 ¶		skipping to change at line 799 ¶
	}		}
	OCTET STRING		OCTET STRING
	DD 94 71 EF E7 43 C4 05 13 35 DF 8F 6D 28 82 F3		DD 94 71 EF E7 43 C4 05 13 35 DF 8F 6D 28 82 F3
	BA DC 38 77 00 F7 ED 3F 70 91 67 2A 3E EA F7 C8		BA DC 38 77 00 F7 ED 3F 70 91 67 2A 3E EA F7 C8
	}		}
	BOOLEAN TRUE		BOOLEAN TRUE
	}		}
	which is sent to the TSA.		which is sent to the TSA.
	A TimeStampResp containing the following TimeStampToken is returned:		A TimeStampResp containing the following TST is returned:
	SEQUENCE {		SEQUENCE {
	OBJECT IDENTIFIER signedData (1 2 840 113549 1 7 2)		OBJECT IDENTIFIER signedData (1 2 840 113549 1 7 2)
	[0] {		[0] {
	SEQUENCE {		SEQUENCE {
	INTEGER 3		INTEGER 3
	SET {		SET {
	SEQUENCE {		SEQUENCE {
	OBJECT IDENTIFIER sha-512 (2 16 840 1 101 3 4 2 3)		OBJECT IDENTIFIER sha-512 (2 16 840 1 101 3 4 2 3)
	NULL		NULL
	skipping to change at line 838 ¶		skipping to change at line 833 ¶
	}		}
	OCTET STRING		OCTET STRING
	DD 94 71 EF E7 43 C4 05 13 35 DF 8F 6D 28 82 F3		DD 94 71 EF E7 43 C4 05 13 35 DF 8F 6D 28 82 F3
	BA DC 38 77 00 F7 ED 3F 70 91 67 2A 3E EA F7 C8		BA DC 38 77 00 F7 ED 3F 70 91 67 2A 3E EA F7 C8
	}		}
	INTEGER 12100074		INTEGER 12100074
	GeneralizedTime 29/08/2025 07:53:00 GMT		GeneralizedTime 29/08/2025 07:53:00 GMT
	BOOLEAN TRUE		BOOLEAN TRUE
	[...]		[...]
	The contents of the TimeStampToken are bstr-wrapped and added to the		The contents of the TST are bstr-wrapped and added to the unprotected
	unprotected headers bucket in the original COSE_Sign1 object to		headers bucket in the original COSE_Sign1 object to obtain the
	obtain the following:		following:
	18(		18(
	[		[
	/ protected h'a10126' / << {		/ protected h'a10126' / << {
	/ alg / 1:-7 / ECDSA 256 /		/ alg / 1:-7 / ECDSA 256 /
	} >>,		} >>,
	/ unprotected / {		/ unprotected / {
	/ 3161-ctt / 270 : h'3082154906092a864886f70d010702a082153a3082		/ 3161-ctt / 270 : h'3082154906092a864886f70d010702a082153a3082
	1536020103310f300d0609608648016503040203050030820184060b2a864886f70d0		1536020103310f300d0609608648016503040203050030820184060b2a864886f70d0
	109100104a08201730482016f3082016b02010106042a0304013031300d0609608648		109100104a08201730482016f3082016b02010106042a0304013031300d0609608648
End of changes. 16 change blocks.
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