| Internet-Draft | CSR Attestation Attributes | July 2023 |
| Ounsworth & Tschofenig | Expires 9 January 2024 | [Page] |
Utilizing information from a device or hardware security module about its posture can help to improve security of the overall system. Information about the manufacturer of the hardware, the version of the firmware running on this hardware and potentially about the layers of software above the firmware, the presence of hardware security functionality to protect keys and many more properties can be made available to remote parties in a cryptographically secured way. This functionality is accomplished with attestation technology.¶
This document describes extensions to encode evidence produced by an attester for inclusion in PKCS10 certificate signing requests. More specifically, two new ASN.1 Attribute definitions, and an ASN.1 CLASS definition to convey attestation information to a Registration Authority or to a Certification Authority are described.¶
This note is to be removed before publishing as an RFC.¶
The latest revision of this draft can be found at https://lamps-wg.github.io/csr-attestation/draft-ounsworth-csr-attestation.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ounsworth-csr-attestation/.¶
Source for this draft and an issue tracker can be found at https://github.com/lamps-wg/csr-attestation.¶
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At the time that it is requesting a certificate from a Certification Authority, a PKI end entity may wish to provide evidence of the security properties of the environment in which the private key is stored to be verified by a relying party such as the Registration Authority or the Certificate Authority. This specification provides a newly defined attestation attribute for carrying remote attestations in PKCS#10 Certification Requests (CSR) [RFC2986].¶
As outlined in the RATS Architecture [RFC9334], an Attester (typically a device) produces a signed collection of Evidence about its running environment, often refered to as an "attestation". A Relying Party may consult that attestation in making policy decisions about the trustworthiness of the entity being attested. Section 3 overviews how the various roles in the RATS Archictecture map to a certificate requester and a CA/RA.¶
At the time of writing, several standardized and proprietary attestation technologies are in use. This specification thereby tries to be technology agnostic with regards to the transport of the produced signed claims.¶
This document is concerned only about the transport of an attesttation inside a CSR and makes minimal assumptions about its content or format. We assume that an attestation can be broken into the following components:¶
This document creates two ATTRIBUTE/Attribute definitions. The first Attribute may be used to carry a set of certificates or public keys that may be necessary to validate evidence. The second Attribute carries a structure that may be used to carry key attestation statements, signatures and related data.¶
A CSR may contain one or more attestations, for example a key attestation asserting the storage properties of the private key as well as a platform attestation asserting the firmware version and other general properties of the device, or multiple key attestations signed by certificate chains on different cryptographic algorithms.¶
With these attributes, an RA or CA has additional information about whether to issue a certificate and what information to populate into the certificate. The scope of this document is, however, limited to the transport of evidence via a CSR. The exact format of the attestation data being carried is defined in various standard and proprietary specifications.¶
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.¶
This document re-uses the terms defined in RFC 9334 related to remote attestation. Readers of this document are assumed to be familiar with the following terms: evidence, claim, attestation result, attester, verifier, and relying party.¶
Figure 1 shows the high-level communication pattern of the RATS passport model where the attester transmits the evidence in the CSR to the RA and the CA. The verifier processes the received evidence and computes an attestation result, which is then processed by the RA/CA prior to the certificate issuance.¶
Note that the verifier is a logical role that may be included in the RA/CA product. In this case the Relying Party and Verifier collapse into a single entity. The verifier functionality can, however, also be kept separate from the RA/CA functionality, such as a utility or library provided by the device manufacturer. For example, security concerns may require parsers of evidence formats to be logically or physically separated from the core CA functionality.¶
.-------------.
| | Compare Evidence
| Verifier | against
| | policy
'--------+----'
^ |
Evidence | | Attestation
| | Result
| v
.------------. .----|----------.
| +-------------->|----' | Compare Attestation
| Attester | Evidence | Relying | Result against
| | in CSR | Party (RA/CA) | policy
'------------' '---------------'
As discussed in RFC 9334, different security and privacy aspects need to be considered. For example, evidence may need to be protected against replay and Section 10 of RFC 9334 lists approach for offering freshness. There are also concerns about the exposure of persistent identifiers by utilizing attestation technology, which are discussed in Section 11 of RFC 9334. Finally, the keying material used by the attester need to be protected against unauthorized access, and against signing arbitrary content that originated from outside the device. This aspect is described in Section 12 of RFC 9334. Most of these aspects are, however, outside the scope of this specification but relevant for use with a given attestation technology. The focus of this specification is on the transport of evidence from the attester to the relying party via existing certification request messages.¶
-- Root of IETF's PKIX OID tree
id-pkix OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) pkix(7) }
-- S/Mime attributes - can be used here.
id-aa OBJECT IDENTIFIER ::= {iso(1) member-body(2) usa(840)
rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) attributes(2)}
-- Branch for attestation statement types
id-ata OBJECT IDENTIFIER ::= { id-pkix (TBD1) }
¶
By definition, Attributes within a Certification Signing Request are typed as ATTRIBUTE. This attribute definition contains one or more attestation statements of a type "AttestStatement".¶
id-aa-attestStatement OBJECT IDENTIFIER ::= { id-aa (TBDAA2) }
AttestAttribute ATTRIBUTE ::= {
TYPE AttestStatement
IDENTIFIED BY id-aa-attestStatement
}
¶
A CSR MAY contain one or more instance of AttestAttribute to allow,
for example a key attestation
asserting the storage properties of the private key as well as a platform
attestation asserting the firmware version and other general properties
of the device, or multiple key attestations signed by certificate chains
on different cryptographic algorithms.¶
An AttestStatement is a simple type-value pair encoded as a sequence, of which the type of the "value" field is controlled by the value of the "type" field, similar to an Attribute definition.¶
AttestStatement ::= SEQUENCE {
type OBJECT IDENTIFIER,
value ANY
}
¶
The "AttestCertsAttribute" contains a set of certificates that may be needed to validate the contents of an attestation statement contained in an attestAttribute. The set of certificates should contain the object that contains the public key needed to directly validate the AttestAttribute. The remaining elements should chain that data back to an agreed upon root of trust for attestations. No order is implied, it is the Verifier's responsibility to perform the appropriate certificate path building.¶
A CSR MUST contain at most 1 AttestCertsAttribute. In the case where
the CSR contains multiple instances of AttestAttribute representing
multiple attestations, all necessary certificates MUST be contained in
the same instance of AttestCertsAttribute.¶
id-aa-attestChainCerts OBJECT IDENTIFIER ::= { id-aa (TBDAA1) }
AttestCertsAttribute ATTRIBUTE ::= {
TYPE SET OF CertificateChoice
COUNTS MAX 1
IDENTIFIED BY id-aa-attestChainCerts
}
¶
This is an ASN.1 CHOICE construct used to represent an encoding of a broad variety of certificate types.¶
CertificateChoice ::=
CHOICE {
cert Certificate,
opaqueCert [0] IMPLICIT OCTET STRING,
typedCert [1] IMPLICIT TypedCert,
typedFlatCert [2] IMPLICIT TypedFlatCert
}
¶
"Certificate" is a standard X.509 certificate that MUST be compliant with RFC5280. Enforcement of this constraint is left to the relying parties.¶
"opaqueCert" should be used sparingly as it requires the receiving party to implictly know its format. It is encoded as an OCTET STRING.¶
"TypedCert" is an ASN.1 construct that has the charateristics of a certificate, but is not encoded as an X.509 certificate. The certTypeField indicates how to interpret the certBody field. While it is possible to carry any type of data in this structure, it's intended the content field include data for at least one public key formatted as a SubjectPublicKeyInfo (see [RFC5912]).¶
TYPED-CERT ::= TYPE-IDENTIFIER
CertType ::= TYPED-CERT.&id
TypedCert ::= SEQUENCE {
certType TYPED-CERT.&id({TypedCertSet}),
content TYPED-CERT.&Type ({TypedCertSet}{@certType})
}
TypedCertSet TYPED-CERT ::= {
... -- Empty for now,
}
¶
"TypedFlatCert" is a certificate that does not have a valid ASN.1 encoding. Think compact or implicit certificates as might be used with smart cards. certType indicates the format of the data in the certBody field, and ideally refers to a published specification.¶
TypedFlatCert ::= SEQUENCE {
certType OBJECT IDENTIFIER,
certBody OCTET STRING
}
¶
The IANA is requested to open one new registry, allocate a value from the "SMI Security for PKIX Module Identifier" registry for the included ASN.1 module, and allocate values from "SMI Security for S/MIME Attributes" to identify two Attributes defined within.¶
Please open up a registry for Attestation Statement Formats within the SMI-numbers registry, allocating an assignment from id-pkix ("SMI Security for PKIX" Registry) for the purpose.¶
Columns:¶
The attestation evidence communicated in the attributes and structures defined in this document are meant to be used in a PKCS10 Certification Signing Request (CSR). It is up to the verifier and to the relying party (RA/CA) to place as much or as little trust in this information as dictated by policies.¶
This document defines the transport of evidence of different formats in a CSR. Some of these attestation formats are based on standards while others are proprietary formats. A verifier will need to understand these formats for matching the received values against policies.¶
Policies drive the processing of evidence at the verifier and other policies influence the decision making at the relying party when evaluating the attestation result. The relying party is ultimately responsible for making a decision of what attestation-related information in the CSR it will accept. The presence of the attributes defined in this specification provide the relying party with additional assurance about attester. Policies used at the verifier and the relying party are implementation dependent and out of scope for this document. Whether to require the use of the attestation-related attributes in the CSR is out-of-scope for this document.¶
Evidence generated by the attestation generally needs to be fresh to provide value to the verifier since the configuration on the device may change over time. Section 10 of [RFC9334] discusses different approaches for providing freshness, including a nonce-based approach, the use of timestamps and an epoch-based technique. The use of nonces requires an extra message exchange via the relying party and the use of timestamps requires synchronized clocks. Epochs also require communication. The definition of "fresh" is somewhat ambiguous in the context of CSRs, especially considering that non-automated certificate enrollments are often asyncronous, and considering the common practice of re-using the same CSR for multiple certificate renewals across the lifetime of a key. "Freshness" typically implies both asserting that the data was generated at a certain point-in-time, as well as providing non-replayability. Developers, operators, and designers of protocols which embed attestation-carrying-CSRs need to consider what notion of freshness is appropriate and available in-context; thus the issue of freshness is out-of-scope for this document.¶
This section provides two non-normative examples for embedding evidence in in CSRs. The first example conveys Arm Platform Security Architecture tokens, which offers platform attestation, into the CSR. The second example embeds the TPM v2.0 attestation information in the CSR.¶
The following example illustrates a CSR with a signed TPM Quote based on [TPM20]. The Platform Configuration Registers (PCRs) are fixed-size registers in a TPM that record measurements of software and configuration information and are therefore used to capture the system state. The digests stored in these registers are then digitially signed with an attestation key known to the hardware.¶
Note: The information conveyed in the value field of the AttestStatement structure may contain more information than the signed TPM Quote structure defined in the TPM v2.0 specification [TPM20], such as plaintext PCR values, the up-time, the event log, etc. The detailed structure of such payload is, however, not defined in this document and may be subject to future standardization work in supplementary documents.¶
Certification Request:
Data:
Version: 1 (0x0)
Subject: CN = server.example.com
Subject Public Key Info:
Public Key Algorithm: id-ecPublicKey
Public-Key: (256 bit)
pub:
04:b9:7c:02:a1:1f:9c:f3:f4:c4:55:3a:d9:3e:26:
e8:e5:11:63:84:36:5f:93:a6:99:7d:d7:43:23:0a:
4f:c0:a8:40:46:7e:8d:b2:1a:38:19:ff:6a:a7:38:
16:06:1e:12:9f:d1:d5:58:55:e6:be:6d:bb:e1:fb:
f7:70:a7:5c:c9
ASN1 OID: prime256v1
NIST CURVE: P-256
Attributes:
AttestStatement
type: TBD2 (identifying TPM V2.0 attestation)
value:
80:02:00:00:01:99:00:00:00:00:00:00:01:86:00:7e
ff:54:43:47:80:18:00:22:00:0b:76:71:0f:61:80:95
8d:89:32:38:a6:cc:40:43:02:4a:da:26:d5:ea:11:71
99:d7:a5:59:a4:18:54:1e:7b:86:00:0d:30:2e:66:6e
6a:37:66:63:39:31:76:62:74:00:00:00:00:00:00:36
5b:bc:0b:71:4f:d8:84:90:09:01:42:82:48:a6:46:53
98:96:00:00:00:01:00:0b:03:0f:00:00:00:20:49:ce
66:9a:aa:7e:52:ff:93:0e:dd:9f:27:97:88:eb:75:cb
ad:53:22:e5:ad:2c:9d:44:1e:dd:65:48:6b:88:00:14
00:0b:01:00:15:a4:95:8a:0e:af:04:36:be:35:f7:27
85:bd:7f:87:46:74:18:e3:67:2f:32:f2:bf:b2:e7:af
a1:1b:f5:ca:1a:eb:83:8f:2f:36:71:cd:7c:18:ab:50
3d:e6:6e:ab:2e:78:a7:e4:6d:cf:1f:03:e6:46:74:28
a7:6c:d6:1e:44:3f:88:89:36:9a:a3:f0:9a:45:07:7e
01:5e:4c:97:7d:3f:e2:f7:15:59:96:5f:0e:9a:1c:b3
a0:6b:4a:77:a5:c0:e0:93:53:cb:b7:50:59:3d:23:ee
5c:31:00:48:6c:0b:1a:b8:04:a4:14:05:a6:63:bc:36
aa:7f:b9:aa:1f:19:9e:ee:49:48:08:e1:3a:d6:af:5f
d5:eb:96:28:bf:41:3c:89:7a:05:4b:b7:32:a2:fc:e7
f6:ad:c7:98:a6:98:99:f6:e9:a4:30:d4:7f:5e:b3:cb
d7:cc:76:90:ef:2e:cc:4f:7d:94:ab:33:8c:9d:35:5d
d7:57:0b:3c:87:9c:63:89:61:d9:5c:a0:b7:5c:c4:75
21:ae:dc:c9:7c:e3:18:a2:b3:f8:15:27:ff:a9:28:2f
cb:9b:17:fe:96:04:53:c4:19:0e:bf:51:0e:9d:1c:83
49:7e:51:64:03:a1:40:f1:72:8b:74:e3:16:79:af:f1
14:a8:5e:44:00:00:01:00:00
Signature Algorithm: ecdsa-with-SHA256
Signature Value:
30:45:02:21:00:93:fd:81:03:75:d1:7d:ab:53:6c:a5:19:a7:
68:3d:d6:e2:39:14:d6:9e:47:24:38:b5:76:db:18:a6:ca:c4:
8a:02:20:36:be:3d:71:93:5d:05:c3:ac:fa:a8:f3:e5:46:db:
57:f9:23:ee:93:47:6d:d6:d3:4f:c2:b7:cc:0d:89:71:fe
The example shown in Figure 3 illustrates how platform attestation is conveyed in a CSR. The content of the evidence in this example is re-used from [I-D.tschofenig-rats-psa-token] and contains a digitally signed Entity Attestation Token (EAT).¶
While the PSA token is digitally signed with an attestation private key, it does not offer key attestation.¶
Certification Request:
Data:
Version: 1 (0x0)
Subject: CN = server.example.com
Subject Public Key Info:
Public Key Algorithm: id-ecPublicKey
Public-Key: (256 bit)
pub:
04:b9:7c:02:a1:1f:9c:f3:f4:c4:55:3a:d9:3e:26:
e8:e5:11:63:84:36:5f:93:a6:99:7d:d7:43:23:0a:
4f:c0:a8:40:46:7e:8d:b2:1a:38:19:ff:6a:a7:38:
16:06:1e:12:9f:d1:d5:58:55:e6:be:6d:bb:e1:fb:
f7:70:a7:5c:c9
ASN1 OID: prime256v1
NIST CURVE: P-256
Attributes:
AttestStatement
type: TBD1 (referring to PSA attestation)
value: d2:84:43:a1:01:26:a0:59:01:3b:aa:19:01:09:78:
18:68:74:74:70:3a:2f:2f:61:72:6d:2e:63:6f:6d:
2f:70:73:61:2f:32:2e:30:2e:30:19:09:5a:1a:7f:
ff:ff:ff:19:09:5b:19:30:00:19:09:5c:58:20:00:
00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:
00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:
00:19:09:5d:48:00:00:00:00:00:00:00:00:19:09:
5e:73:31:32:33:34:35:36:37:38:39:30:31:32:33:
2d:31:32:33:34:35:19:09:5f:81:a2:02:58:20:03:
03:03:03:03:03:03:03:03:03:03:03:03:03:03:03:
03:03:03:03:03:03:03:03:03:03:03:03:03:03:03:
03:05:58:20:04:04:04:04:04:04:04:04:04:04:04:
04:04:04:04:04:04:04:04:04:04:04:04:04:04:04:
04:04:04:04:04:04:0a:58:20:01:01:01:01:01:01:
01:01:01:01:01:01:01:01:01:01:01:01:01:01:01:
01:01:01:01:01:01:01:01:01:01:01:19:01:00:58:
21:01:02:02:02:02:02:02:02:02:02:02:02:02:02:
02:02:02:02:02:02:02:02:02:02:02:02:02:02:02:
02:02:02:02:19:09:60:78:2e:68:74:74:70:73:3a:
2f:2f:76:65:72:61:69:73:6f:6e:2e:65:78:61:6d:
70:6c:65:2f:76:31:2f:63:68:61:6c:6c:65:6e:67:
65:2d:72:65:73:70:6f:6e:73:65:58:40:56:f5:0d:
13:1f:a8:39:79:ae:06:4e:76:e7:0d:c7:5c:07:0b:
6d:99:1a:ec:08:ad:f9:f4:1c:ab:7f:1b:7e:2c:47:
f6:7d:ac:a8:bb:49:e3:11:9b:7b:ae:77:ae:c6:c8:
91:62:71:3e:0c:c6:d0:e7:32:78:31:e6:7f:32:84:
1a
Signature Algorithm: ecdsa-with-SHA256
Signature Value:
30:45:02:21:00:93:fd:81:03:75:d1:7d:ab:53:6c:a5:19:a7:
68:3d:d6:e2:39:14:d6:9e:47:24:38:b5:76:db:18:a6:ca:c4:
8a:02:20:36:be:3d:71:93:5d:05:c3:ac:fa:a8:f3:e5:46:db:
57:f9:23:ee:93:47:6d:d6:d3:4f:c2:b7:cc:0d:89:71:fe
The decoded evidence is shown in Appendix A of [I-D.tschofenig-rats-psa-token], the shown attestation information, provides the following information to an RA/CA:¶
{::include CSR-ATTESTATION-2023.asn}
¶
This specification is the work of a design team created by the chairs of the LAMPS working group. The following persons, in no specific order, contributed to the work: Richard Kettlewell, Chris Trufan, Bruno Couillard, Jean-Pierre Fiset, Sander Temme, Jethro Beekman, Zsolt Rózsahegyi, Ferenc Pető, Mike Agrenius Kushner, Tomas Gustavsson, Dieter Bong, Christomer Meyer, Michael StJohns, Carl Wallace, Michael Ricardson, Tomofumi Okubo, Olivier Couillard, John Gray, Eric Amador, Johnson Darren, Herman Slatman, Tiru Reddy, Thomas Fossati, Corey Bonnel, Argenius Kushner, James Hagborg.¶
We would like to specifically thank Mike StJohns for his work on an earlier version of this draft.¶