| Internet-Draft | CSRAttrs | August 2023 |
| Richardson, et al. | Expires 2 February 2024 | [Page] |
The Enrollment over Secure Transport (EST, RFC7030) is ambiguous in its specification of the CSR Attributes Response. This has resulted in implementation challenges and implementor confusion.¶
This document updates RFC7030 (EST) and clarifies how the CSR Attributes Response can be used by an EST server to specify both CSR attribute OIDs and also CSR attribute values, in particular X.509 extension values, that the server expects the client to include in subsequent CSR request.¶
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This Internet-Draft will expire on 2 February 2024.¶
Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
Enrollment over Secure Transport [RFC7030] (EST) has been used in a wide variety of applications. In particular, [RFC8994] and [RFC8995] describe a way to use it in order to build out an autonomic control plane (ACP) [RFC8368].¶
The ACP requires that each node be given a very specific subjectAltName. In the ACP specification, the solution was for the EST server to use section 2.6 of [RFC7030] to convey to the EST client the actual subjectAltName that will end up in its certificate.¶
As a result of some implementation challenges, it came to light that this particular way of using the CSR attributes was not universally agreed upon, and it was suggested that it went contrary to section 2.6.¶
Section 2.6 says that the CSR attributes "can provide additional descriptive information that the EST server cannot access itself". This is extended to describe how the EST server can provide values that it demands to use.¶
After significant discussion, it has been determined that Section 4.5 of [RFC7030] specification is sufficiently difficult to read and ambiguous to interpret that clarification is needed.¶
This document motivates the different use cases, and provides additional worked out examples.¶
Also, section 4.5.2 is extended to clarify the use of the existing ASN.1 syntax. This covers all uses and is fully backward compatible with existing use.¶
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.¶
Replace the second paragraph with the following text:¶
These attributes can provide additional descriptive information that the EST server cannot access itself, such as the Media Access Control (MAC) address of an interface of the EST client. The EST server can also provide concrete values that it tells the client to include in the CSR, such as a specific X.509 Subject Alternative Name extension. Moreover, these attributes can indicate the type of the included public key or which crypto algorithms to use for the self-signature, such as a specific elliptic curve or a specific hash function that the client is expected to use when generating the CSR.¶
The ASN.1 syntax for CSR Attributes as defined in EST section 4.5.2 is as follows:¶
CsrAttrs ::= SEQUENCE SIZE (0..MAX) OF AttrOrOID
AttrOrOID ::= CHOICE (oid OBJECT IDENTIFIER, attribute Attribute }
Attribute { ATTRIBUTE:IOSet } ::= SEQUENCE {
type ATTRIBUTE.&id({IOSet}),
values SET SIZE(1..MAX) OF ATTRIBUTE.&Type({IOSet}{@type}) }
¶
This remains unchanged, such that bits-on-the-wire compatibility is maintained.¶
Key parts that were unclear were which OID to use in the 'type' field and that the 'values' field can contain an entire sequence of X.509 extensions.¶
The OID to use for such attributes in the 'type' field MUST be extensionRequest, which has the numerical value 1.2.840.113549.1.9.14. There MUST be only one such Attribute.¶
The 'values' field of this attribute MUST contain a set with exactly one element, and this element MUST by of type Extensions, as per Section 4.1 of [RFC5280]:¶
Extensions ::= SEQUENCE SIZE (1..MAX) OF Extension
Extension ::= SEQUENCE {
extnID OBJECT IDENTIFIER,
critical BOOLEAN DEFAULT FALSE,
extnValue OCTET STRING
-- contains the DER encoding of an ASN.1 value
-- corresponding to the extension type identified
-- by extnID
}
¶
An Extension comprises the OID of the specific X.509 extension (extnID), optionally the 'critical' bit, and the extension value (extnValue).¶
An Extensions structure, which is a sequence of elements of type Extension, MUST NOT include more than one element with a particiular extnID.¶
With this understanding, the needs of [RFC8994] and [RFC8995] are satisfied with no change to the bits on the wire.¶
[RFC8295], Appendix B suggests an alternative for the piecemeal specification of attributes that [RFC7030] documented. Instead, a CSR object is returned with various fields filled out, and other fields waiting to be filled in. This is also the method defined CMP Update [I-D.ietf-lamps-cmp-updates] and the Lightweight CMP profile [I-D.ietf-lamps-lightweight-cmp-profile], Section 4.3.3, which uses a CSR template in CRMF.¶
CertificationRequestInfo ::= SEQUENCE {
version INTEGER { v1(0) } (v1,...),
subject Name,
subjectPKInfo SubjectPublicKeyInfo{{ PKInfoAlgorithms }},
attributes [0] Attributes{{ CRIAttributes }}
}
¶
The CertificationRequestInfo is included as a new Attribute in the CsrAttrs elements sequence. It has the type OID TBD. Legacy servers MAY continue to use the [RFC7030] style piecemeal attribute/value pairs, and MAY also include the template style described here. Clients which understand both MUST use the template only, and ignore all other CSRattrs elements. Older clients will ignore this new element.¶
The use of the CertificationRequestInfo avoids a useless fake signature in the CSR object. The version code is always v1 (0). As shown in the example below, any empty values in the subject DN, and in any included X509v3 extensions are expected to be filled in by the client.¶
The SubjectPublicKeyInfo field must be present, but it MUST have an empty bit string for the key, as the server does not know what key will be used. The server MAY specify (in the OID), the type of the key to use, but otherwise the OID type MUST be NULL.¶
Each example has a high-level (English) explanation of what is expected. Some mapping back to the Attribute and Extension definitions above are included. The base64 DER encoding is then shown. The output of "dumpasn1" is then provided to detail what the contents are.¶
A single subjectAltName extension is specified in a single Extension attribute. This is what might be created by an [RFC8995] Registrar that is asking for [RFC8994] AcpNodeName format otherNames.¶
<CODE BEGINS> MGQwYgYJKoZIhvcNAQkOMVUwUwYDVR0RAQH/BEmgRzBFBggr BgEFBQcICgw5cmZjODk5NCtmZDczOWZjMjNjMzQ0MDExMjIz MzQ0NTUwMDAwMDAwMCtAYWNwLmV4YW1wbGUuY29t <CODE ENDS>¶
There is a single subjectAltName Extension with an Attribute with Extension type.¶
<30 64>
0 100: SEQUENCE {
<30 62>
2 98: SEQUENCE {
<06 09>
4 9: OBJECT IDENTIFIER extensionRequest (1 2 840 113549 1 9 14)
: (PKCS #9 via CRMF)
<31 55>
15 85: SET {
<30 53>
17 83: SEQUENCE {
<06 03>
19 3: OBJECT IDENTIFIER subjectAltName (2 5 29 17)
: (X.509 extension)
<01 01>
24 1: BOOLEAN TRUE
<04 49>
27 73: OCTET STRING
: A0 47 30 45 06 08 2B 06 .G0E..+.
: 01 05 05 07 08 0A 0C 39 .......9
: 72 66 63 38 39 39 34 2B rfc8994+
: 66 64 37 33 39 66 63 32 fd739fc2
: 33 63 33 34 34 30 31 31 3c344011
: 32 32 33 33 34 34 35 35 22334455
: 30 30 30 30 30 30 30 30 00000000
: 2B 40 61 63 70 2E 65 78 [email protected]
: 61 6D 70 6C 65 2E 63 6F ample.co
: 6D m
: }
: }
: }
: }
¶
In this example, taken from [RFC7030], a few different attributes are included.¶
<CODE BEGINS> MEEGCSqGSIb3DQEJBzASBgcqhkjOPQIBMQcGBSuBBAAiMBYG CSqGSIb3DQEJDjEJBgcrBgEBAQEWBggqhkjOPQQDAw== <CODE ENDS>¶
<30 41>
0 65: SEQUENCE {
<06 09>
2 9: OBJECT IDENTIFIER challengePassword (1 2 840 113549 1 9 7)
: (PKCS #9)
<30 12>
13 18: SEQUENCE {
<06 07>
15 7: OBJECT IDENTIFIER ecPublicKey (1 2 840 10045 2 1)
: (ANSI X9.62 public key type)
<31 07>
24 7: SET {
<06 05>
26 5: OBJECT IDENTIFIER secp384r1 (1 3 132 0 34)
: (SECG (Certicom) named elliptic curve)
: }
: }
<30 16>
33 22: SEQUENCE {
<06 09>
35 9: OBJECT IDENTIFIER extensionRequest (1 2 840 113549 1 9 14)
: (PKCS #9 via CRMF)
<31 09>
46 9: SET {
<06 07>
48 7: OBJECT IDENTIFIER '1 3 6 1 1 1 1 22'
: }
: }
<06 08>
57 8: OBJECT IDENTIFIER ecdsaWithSHA384 (1 2 840 10045 4 3 3)
: (ANSI X9.62 ECDSA algorithm with SHA384)
: }
¶
This example is the same as the previous one except that instead of the OID for a macAddress, a subjectAltName is specified as the only Extension element.¶
<CODE BEGINS> MGYGCSqGSIb3DQEJBzASBgcqhkjOPQIBMQcGBSuBBAAiMDsG CSqGSIb3DQEJDjEuMCwGA1UdEQEB/wQioCAwHgYIKwYBBQUH CAoMEnBvdGF0b0BleGFtcGxlLmNvbQYIKoZIzj0EAwM= <CODE ENDS>¶
<30 66>
0 102: SEQUENCE {
<06 09>
2 9: OBJECT IDENTIFIER challengePassword (1 2 840 113549 1 9 7)
: (PKCS #9)
<30 12>
13 18: SEQUENCE {
<06 07>
15 7: OBJECT IDENTIFIER ecPublicKey (1 2 840 10045 2 1)
: (ANSI X9.62 public key type)
<31 07>
24 7: SET {
<06 05>
26 5: OBJECT IDENTIFIER secp384r1 (1 3 132 0 34)
: (SECG (Certicom) named elliptic curve)
: }
: }
<30 3B>
33 59: SEQUENCE {
<06 09>
35 9: OBJECT IDENTIFIER extensionRequest (1 2 840 113549 1 9 14)
: (PKCS #9 via CRMF)
<31 2E>
46 46: SET {
<30 2C>
48 44: SEQUENCE {
<06 03>
50 3: OBJECT IDENTIFIER subjectAltName (2 5 29 17)
: (X.509 extension)
<01 01>
55 1: BOOLEAN TRUE
<04 22>
58 34: OCTET STRING
: A0 20 30 1E 06 08 2B 06 . 0...+.
: 01 05 05 07 08 0A 0C 12 ........
: 70 6F 74 61 74 6F 40 65 potato@e
: 78 61 6D 70 6C 65 2E 63 xample.c
: 6F 6D om
: }
: }
: }
<06 08>
94 8: OBJECT IDENTIFIER ecdsaWithSHA384 (1 2 840 10045 4 3 3)
: (ANSI X9.62 ECDSA algorithm with SHA384)
: }
¶
The CSR requires a public key of a specific size¶
<CODE BEGINS> MCkGCSqGSIb3DQEJBzARBgkqhkiG9w0BAQExBAICEAAGCSqG SIb3DQEBCw== <CODE ENDS>¶
<30 29>
0 41: SEQUENCE {
<06 09>
2 9: OBJECT IDENTIFIER challengePassword (1 2 840 113549 1 9 7)
: (PKCS #9)
<30 11>
13 17: SEQUENCE {
<06 09>
15 9: OBJECT IDENTIFIER rsaEncryption (1 2 840 113549 1 1 1)
: (PKCS #1)
<31 04>
26 4: SET {
<02 02>
28 2: INTEGER 4096
: }
: }
<06 09>
32 9: OBJECT IDENTIFIER sha256WithRSAEncryption (1 2 840 113549 1 1 11)
: (PKCS #1)
: }
¶
The CSR requires a public key with a specific curve¶
<CODE BEGINS> MD0GCSqGSIb3DQEJBzASBgcqhkjOPQIBMQcGBSuBBAAiMBIGCSqGSIb3DQEJDjEFBgNVBAUGCCqGSM49BAMD <CODE ENDS>¶
Provide a CSR with an ECC key from p384, include your serial number, and sign it with sha384.¶
<30 3D>
0 61: SEQUENCE {
<06 09>
2 9: OBJECT IDENTIFIER challengePassword (1 2 840 113549 1 9 7)
: (PKCS #9)
<30 12>
13 18: SEQUENCE {
<06 07>
15 7: OBJECT IDENTIFIER ecPublicKey (1 2 840 10045 2 1)
: (ANSI X9.62 public key type)
<31 07>
24 7: SET {
<06 05>
26 5: OBJECT IDENTIFIER secp384r1 (1 3 132 0 34)
: (SECG (Certicom) named elliptic curve)
: }
: }
<30 12>
33 18: SEQUENCE {
<06 09>
35 9: OBJECT IDENTIFIER extensionRequest (1 2 840 113549 1 9 14)
: (PKCS #9 via CRMF)
<31 05>
46 5: SET {
<06 03>
48 3: OBJECT IDENTIFIER serialNumber (2 5 4 5)
: (X.520 DN component)
: }
: }
<06 08>
53 8: OBJECT IDENTIFIER ecdsaWithSHA384 (1 2 840 10045 4 3 3)
: (ANSI X9.62 ECDSA algorithm with SHA384)
: }
¶
The CSR is required to have an EC key, to include a serial number, a friendly name, favorite drink, and be signed with SHA512.¶
<CODE BEGINS> MFQGCSqGSIb3DQEJBzASBgcqhkjOPQIBMQcGBSuBBAAjMCkG CSqGSIb3DQEJDjEcBgNVBAUGCSqGSIb3DQEJFAYKCZImiZPy LGQBBQYIKoZIzj0EAwQ= <CODE ENDS>¶
Provide a CSR with an EC key from sha521, include your serial number, friendly name, and favorite drink, and sign it with sha512¶
<30 54>
0 84: SEQUENCE {
<06 09>
2 9: OBJECT IDENTIFIER challengePassword (1 2 840 113549 1 9 7)
: (PKCS #9)
<30 12>
13 18: SEQUENCE {
<06 07>
15 7: OBJECT IDENTIFIER ecPublicKey (1 2 840 10045 2 1)
: (ANSI X9.62 public key type)
<31 07>
24 7: SET {
<06 05>
26 5: OBJECT IDENTIFIER secp521r1 (1 3 132 0 35)
: (SECG (Certicom) named elliptic curve)
: }
: }
<30 29>
33 41: SEQUENCE {
<06 09>
35 9: OBJECT IDENTIFIER extensionRequest (1 2 840 113549 1 9 14)
: (PKCS #9 via CRMF)
<31 1C>
46 28: SET {
<06 03>
48 3: OBJECT IDENTIFIER serialNumber (2 5 4 5)
: (X.520 DN component)
<06 09>
53 9: OBJECT IDENTIFIER
: friendlyName (for PKCS #12) (1 2 840 113549 1 9 20)
: (PKCS #9 via PKCS #12)
<06 0A>
64 10: OBJECT IDENTIFIER '0 9 2342 19200300 100 1 5'
: }
: }
<06 08>
76 8: OBJECT IDENTIFIER ecdsaWithSHA512 (1 2 840 10045 4 3 4)
: (ANSI X9.62 ECDSA algorithm with SHA512)
: }
¶
The security considerations from EST [RFC7030] section 6 are unchanged.¶
An EST server may use this mechanism to instruct the EST client about the identities it should include in the CSR it sends as part of enrollment. The client may only be aware of its IDevID Subject, which includes a manufacturer serial number. The EST server can use this mechanism to tell the client to include a specific fully qualified domain name in the CSR in order to complete domain ownership proofs required by the CA. Additionally, the EST server may deem the manufacturer serial number in an IDevID as personally identifiable information, and may want to specify a new random opaque identifier that the pledge should use in its CSR. This may be desirable if the CA and EST server have different operators.¶
No requests are made to IANA.¶
Corey Bonnell crafted example02 using a different tool, and this helped debug other running code.¶