Clarification and enhancement of RFC7030 CSR Attributes definition

Internet-Draft	CSRAttrs	September 2024
Richardson, et al.	Expires 7 March 2025	[Page]

Abstract

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.¶

Moreover, it provides new convenient and straightforward approach: using a template for CSR contents that may be partially filled in by the server. This also allows specifying a subject Distinguished Name (DN).¶

3. CSR Attributes Handling

3.1. Extensions to RFC 7030 section 2.6.

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.

3.2. Extensions to RFC 7030 section 4.5.2.

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 be 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.¶

3.3. Use of CSR templates

[RFC8295], Appendix B describes a mechanism that avoids the piecemeal inclusion of attributes that [RFC7030] documented. Instead, an entire CSR object is returned with various fields filled out, and other fields waiting to be filled in, in a pKCS7PDU attribute. In that approach, the pKCS7PDU attribute includes a Full PKI Data content type [RFC5272] and that in turn includes a CSR or CRMF formatted request; see [RFC6268] Sections 5 and 9, respectively.¶

The drawback to that approach, particularly for the CSR, is that some useless fields have to be included; specifically, the signature field on the CSR is faked with an empty bit string. We avoid this drawback as follows.¶

This specification defines the Certificate Request Information Template attribute for CsrAttrs, see Section 3.2, that is essentially a partially filled in PKCS#10 CSR minus the signature wrapper as follows:¶

  aa-certificationRequestInfoTemplate ATTRIBUTE ::=
    { TYPE CertificationRequestInfoTemplate IDENTIFIED BY
      id-aa-certificationRequestInfoTemplate }

  id-aa-certificationRequestInfoTemplate OBJECT IDENTIFIER ::=
    { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
      smime(16) aa(2) csrinfo(TBD2) }

  CertificationRequestInfoTemplate ::= SEQUENCE {
      version           INTEGER { v1(0) } (v1, ... ),
      subject           NameTemplate OPTIONAL,
      subjectPKInfo     SubjectPublicKeyInfoTemplate {{
                            PKInfoAlgorithms }} OPTIONAL,
      attributes    [0] Attributes{{ CRIAttributes }}
  }

  NameTemplate ::= CHOICE { -- only one possibility for now --
      rdnSequence  RDNSequenceTemplate }

  RDNSequenceTemplate ::= SEQUENCE OF RelativeDistinguishedNameTemplate

  RelativeDistinguishedNameTemplate  ::=
      SET SIZE (1 .. MAX) OF AttributeTypeAndValueTemplate {
                             {SupportedAttributes} }

  AttributeTypeAndValueTemplate {ATTRIBUTE:AttrSet} ::= SEQUENCE {
      type      ATTRIBUTE.&id({AttrSet}),
      value     ATTRIBUTE.&Type({AttrSet}{@type}) OPTIONAL
  }

  SubjectPublicKeyInfoTemplate {PUBLIC-KEY: IOSet} ::= SEQUENCE {
      algorithm        AlgorithmIdentifier {PUBLIC-KEY, {IOSet}},
      subjectPublicKey BIT STRING OPTIONAL
  }

  pkcs-9-at-extensionRequestTemplate OBJECT IDENTIFIER ::= {pkcs-9 TBD3}

  extensionRequestTemplate ATTRIBUTE ::= {
          WITH SYNTAX ExtensionRequestTemplate
          SINGLE VALUE TRUE
          ID pkcs-9-at-extensionRequestTemplate
  }

  ExtensionRequestTemplate ::= ExtensionTemplates

  ExtensionTemplates  ::=  SEQUENCE SIZE (1..MAX) OF ExtensionTemplate

  ExtensionTemplate {EXTENSION:ExtensionSet} ::= SEQUENCE {
     extnID      EXTENSION.&id({ExtensionSet}),
     critical    BOOLEAN -- (
                   EXTENSION.&Critical({ExtensionSet}{@extnID}))
                   DEFAULT FALSE,
     extnValue   OCTET STRING (CONTAINING
                 EXTENSION.&ExtnType({ExtensionSet}{@extnID})) OPTIONAL
                 --  contains the DER encoding of the ASN.1 value
                 --  corresponding to the extension type identified
                 --  by extnID when present
  }

Note that the CertificationRequestInfoTemplate closely resembles the CertificationRequestInfo from [RFC5912], Section 5:¶

  CertificationRequestInfo ::= SEQUENCE {
    version       INTEGER { v1(0) } (v1,...),
    subject       Name,
    subjectPKInfo SubjectPublicKeyInfo{{ PKInfoAlgorithms }},
    attributes    [0] Attributes{{ CRIAttributes }}
  }

with the following differences.¶

The subject field is made OPTIONAL to cover the case the server does not provide any requrements on the subject name and its RDNs.¶
RelativeDistinguishedNames (RDNs) in the subject fields are allowed to be empty, which is achieved by adding OPTIONAL to the value field of AttributeTypeAndValueTemplate. This means that the client is required to provide an RDN of the given type and fill in its value.¶
The subjectPKInfo field is made OPTIONAL, such that the server can leave it out in case there are no requirements on the key.\ Otherwise the server specifies the algorithm of the key, i.e., the key type, in the algorithm field. The subjectPublicKey field contained in SubjectPublicKeyInfoTemplate is made OPTIONAL because usually it is left out, but in case the server needs to specify the size of an RSA key, the field is used to provide a dummy public key value of the desired RSA modulus length.¶
A new OID pkcs-9-at-extensionRequestTemplate and the related ExtensionTemplate structure is defind where the extnValue field is optional. If the field is absent this means that the client is required to provide an X.509 extension with the given extnID and potentially the critical flag and fill in its value.¶

A similar method has been defined in CMP Updates [RFC9480] and the Lightweight CMP profile [RFC9483], Section 4.3.3, using a CSR template as defined for CRMF [RFC4211]. That method does not properly deal with empty RDNs (encoding them as empty strings), absent subjectPublicKey fields (encoding them as empty BIT STRING), and empty X.509v3 extension values (encoding them as empty OCTET STRING), which may cause issues with strict ASN.1 parsing and decoding.¶

The version code MUST be v1 (0).¶

If the subjectPKInfo field is present, it indicates the type of the key pair the client is expected to use, where the subjectPublicKey MUST NOT be given except if the algorithm indicates RSA.¶

The attributes field MUST NOT contain multiple extensionRequestTemplate attributes and MUST NOT contain both extensionRequest and extensionRequestTemplate attributes.¶

Suppose the server requires that the CSR will contain:¶

the subject field with a common name to be filled in by the EE and two organizational unit fields with given values "myDept" and "myGroup",¶
the publicKey field contains an Elliptic Curve Cryptography (ECC) key on curve secp256r1,¶
the subjectAltName X.509 extension with DNS name "www.myServer.com" and an IP address to be filled in,¶
the keyUsage extension marked critical with the value digitalSignature and keyAgreement, and¶
the extKeyUsage extension with values to be filled in by the EE.¶

Then the CertificationRequestInfo structure constructed by the server will be as follows:¶

 SEQUENCE {
   INTEGER 0
   SEQUENCE {
     SET {
       SEQUENCE {
         OBJECT IDENTIFIER commonName (2 5 4 3)
         }
       }
     SET {
       SEQUENCE {
         OBJECT IDENTIFIER organizationalUnitName (2 5 4 11)
        UTF8String "myDept"
        }
      }
     SET {
       SEQUENCE {
         OBJECT IDENTIFIER organizationalUnitName (2 5 4 11)
         UTF8String "myGroup"
         }
       }
     }
  SEQUENCE {
    SEQUENCE {
      OBJECT IDENTIFIER ecPublicKey (1 2 840 10045 2 1)
      OBJECT IDENTIFIER secp256r1 (1 2 840 10045 3 1 7)
      }
    }
  [0] {
    SEQUENCE {
      OBJECT IDENTIFIER extensionRequest (1 2 840 113549 1 9 TBD3)
      SET {
        SEQUENCE {
          SEQUENCE {
            OBJECT IDENTIFIER subjectAltName (2 5 29 17)
            OCTET STRING, encapsulates {
              SEQUENCE {
                [2] "www.myServer.com"
                [7] ""
                }
              }
            }
          SEQUENCE {
            OBJECT IDENTIFIER keyUsage (2 5 29 15)
            BOOLEAN TRUE
            OCTET STRING, encapsulates {
              BIT STRING 3 unused bits
                "10001"B
              }
            }
          SEQUENCE {
            OBJECT IDENTIFIER extKeyUsage (2 5 29 37)
            }
          }
        }
      }
    }
  }

5. Examples

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.¶

5.1. RFC8994/ACP subjectAltName with specific otherName

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.¶

5.1.1. Base64 encoded example

The Base64:¶

MGQwYgYJKoZIhvcNAQkOMVUwUwYDVR0RAQH/BEmgRzBFBggr
BgEFBQcICgw5cmZjODk5NCtmZDczOWZjMjNjMzQ0MDExMjIz
MzQ0NTUwMDAwMDAwMCtAYWNwLmV4YW1wbGUuY29t

5.1.2. ASN.1 DUMP output

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    +@acp.ex
       :           61 6D 70 6C 65 2E 63 6F    ample.co
       :           6D                         m
       :         }
       :       }
       :     }
       :   }

5.2. RFC7030 original example

In this example, taken from [RFC7030], a few different attributes are included.¶

5.2.1. Base64 encoded example

The Base64:¶

MEEGCSqGSIb3DQEJBzASBgcqhkjOPQIBMQcGBSuBBAAiMBYG
CSqGSIb3DQEJDjEJBgcrBgEBAQEWBggqhkjOPQQDAw==

5.2.2. ASN.1 DUMP output

The challengePassword attribute is included to indicate that the CSR should include this value.¶
An ecPublicKey attribute is provided with the value secp384r1 to indicate what kind of key should be submitted.¶
An extensionRequest container with an OID 1.3.6.1.1.1.1.22 (macAddress), but without a value, to indicate that the CSR should include an X.509v3 extension with this value.¶
The ecdsaWithSHA384 OID is included to indicate what kind of hash is expected to be used for the self-signature of the PCKS#10 CSR structure.¶

    <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)
       :   }

5.3. EST server requires a specific subjectAltName extension

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.¶

5.3.1. Base64 encoded example

The Base64:¶

MGYGCSqGSIb3DQEJBzASBgcqhkjOPQIBMQcGBSuBBAAiMDsG
CSqGSIb3DQEJDjEuMCwGA1UdEQEB/wQioCAwHgYIKwYBBQUH
CAoMEnBvdGF0b0BleGFtcGxlLmNvbQYIKoZIzj0EAwM=

5.3.2. ASN.1 DUMP output

The challengePassword attribute is included to indicate that the CSR should include this value.¶
An ecPublicKey attribute is provided with the value secp384r1 to indicate what kind of key should be submitted.¶
An extensionRequest container with a subjectAltName value containing the name potato@example.com¶
The ecdsaWithSHA384 OID is included to indicate what kind of hash is expected to be used for the self-signature of the PCKS#10 CSR structure.¶

    <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)
       :   }

5.4. Require a public key of a specific size

The CSR requires a public key of a specific size¶

5.4.1. Base64 encoded example

The Base64:¶

MCkGCSqGSIb3DQEJBzARBgkqhkiG9w0BAQExBAICEAAGCSqG
SIb3DQEBCw==

5.4.2. ASN.1 DUMP output

Provide a CSR with an RSA key that's 4096 bits and sign it with sha256¶

    <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)
       :   }

5.5. Require a public key of a specific curve

The CSR requires a public key with a specific curve¶

5.5.1. Base64 encoded example

The Base64:¶

MD0GCSqGSIb3DQEJBzASBgcqhkjOPQIBMQcGBSuBBAAiMBIGCSqGSIb3DQEJDjEF
BgNVBAUGCCqGSM49BAMD

5.5.2. ASN.1 DUMP output

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)
       :   }

5.6. Require a specific extension

The CSR is required to have an EC key, to include a serial number, a friendly name, favorite drink, and be signed with SHA512.¶

5.6.1. Base64 encoded example

The Base64:¶

MFQGCSqGSIb3DQEJBzASBgcqhkjOPQIBMQcGBSuBBAAjMCkG
CSqGSIb3DQEJDjEcBgNVBAUGCSqGSIb3DQEJFAYKCZImiZPy
LGQBBQYIKoZIzj0EAwQ=

5.6.2. ASN.1 DUMP output

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)
       :   }

10. References

10.1. Normative References

[RFC2119]: Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC5272]: Schaad, J. and M. Myers, "Certificate Management over CMS (CMC)", RFC 5272, DOI 10.17487/RFC5272, June 2008, <https://www.rfc-editor.org/info/rfc5272>.
[RFC5280]: Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, <https://www.rfc-editor.org/info/rfc5280>.
[RFC5911]: Hoffman, P. and J. Schaad, "New ASN.1 Modules for Cryptographic Message Syntax (CMS) and S/MIME", RFC 5911, DOI 10.17487/RFC5911, June 2010, <https://www.rfc-editor.org/info/rfc5911>.
[RFC5912]: Hoffman, P. and J. Schaad, "New ASN.1 Modules for the Public Key Infrastructure Using X.509 (PKIX)", RFC 5912, DOI 10.17487/RFC5912, June 2010, <https://www.rfc-editor.org/info/rfc5912>.
[RFC6268]: Schaad, J. and S. Turner, "Additional New ASN.1 Modules for the Cryptographic Message Syntax (CMS) and the Public Key Infrastructure Using X.509 (PKIX)", RFC 6268, DOI 10.17487/RFC6268, July 2011, <https://www.rfc-editor.org/info/rfc6268>.
[RFC7030]: Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed., "Enrollment over Secure Transport", RFC 7030, DOI 10.17487/RFC7030, October 2013, <https://www.rfc-editor.org/info/rfc7030>.
[RFC8174]: Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8994]: Eckert, T., Ed., Behringer, M., Ed., and S. Bjarnason, "An Autonomic Control Plane (ACP)", RFC 8994, DOI 10.17487/RFC8994, May 2021, <https://www.rfc-editor.org/info/rfc8994>.
[RFC8995]: Pritikin, M., Richardson, M., Eckert, T., Behringer, M., and K. Watsen, "Bootstrapping Remote Secure Key Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995, May 2021, <https://www.rfc-editor.org/info/rfc8995>.
[X.680]: ITU-T, "Information technology -- Abstract Syntax Notation One (ASN.1): Specification of basic notation", ITU-T Recommendation X.680, ISO/IEC 8824-1:2021, February 2021, <https://www.itu.int/rec/T-REC-X.680>.
[X.690]: ITU-T, "Information technology -- ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", ITU-T Recommendation X.690, ISO/IEC 8825-1:2021, February 2021, <https://www.itu.int/rec/T-REC-X.690>.

10.2. Informative References

[RFC4211]: Schaad, J., "Internet X.509 Public Key Infrastructure Certificate Request Message Format (CRMF)", RFC 4211, DOI 10.17487/RFC4211, September 2005, <https://www.rfc-editor.org/info/rfc4211>.
[RFC8295]: Turner, S., "EST (Enrollment over Secure Transport) Extensions", RFC 8295, DOI 10.17487/RFC8295, January 2018, <https://www.rfc-editor.org/info/rfc8295>.
[RFC8368]: Eckert, T., Ed. and M. Behringer, "Using an Autonomic Control Plane for Stable Connectivity of Network Operations, Administration, and Maintenance (OAM)", RFC 8368, DOI 10.17487/RFC8368, May 2018, <https://www.rfc-editor.org/info/rfc8368>.
[RFC9480]: Brockhaus, H., von Oheimb, D., and J. Gray, "Certificate Management Protocol (CMP) Updates", RFC 9480, DOI 10.17487/RFC9480, November 2023, <https://www.rfc-editor.org/info/rfc9480>.
[RFC9483]: Brockhaus, H., von Oheimb, D., and S. Fries, "Lightweight Certificate Management Protocol (CMP) Profile", RFC 9483, DOI 10.17487/RFC9483, November 2023, <https://www.rfc-editor.org/info/rfc9483>.