EST CSR attributes: Clarification and Enhancement of the CSR Attributes Definition in RFC 7030

Internet Engineering Task Force (IETF)                M. Richardson, Ed.
Request for Comments: 9908                      Sandelman Software Works
Updates: 7030, 9148                                             O. Friel
Category: Standards Track                                          Cisco
ISSN: 2070-1721                                            D. von Oheimb
                                                                 Siemens
                                                              D. Harkins
                                                   The Industrial Lounge
                                                            January 2026


   Clarification and Enhancement of the CSR Attributes Definition in
                                RFC 7030

Abstract

   This document updates RFC 7030, "Enrollment over Secure Transport"
   (EST), clarifying how the Certificate Signing Request (CSR)
   Attributes Response can be used by an EST server to specify both CSR
   attribute Object Identifiers (OIDs) and CSR attribute values,
   particularly X.509 extension values, that the server expects the
   client to include in a subsequent CSR request.  RFC 9148 is derived
   from RFC 7030 and is also updated.

   RFC 7030 is ambiguous in its specification of the CSR Attributes
   Response.  This has resulted in implementation challenges and
   implementor confusion because there was no universal understanding of
   what was specified.  This document clarifies the encoding rules.

   This document also provides a new straightforward approach: using a
   template for CSR contents that may be partially filled in by the
   server.  This also allows an EST server to specify a subject
   Distinguished Name (DN).

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9908.

Copyright Notice

   Copyright (c) 2026 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.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  CSR Attributes Handling
     3.1.  Extensions to RFC 7030, Section 2.6
     3.2.  Extensions to RFC 7030, Section 4.5.2
     3.3.  Update to RFC 9148
     3.4.  Use of CSR Templates
   4.  Coexistence with Existing Implementations
   5.  Examples Using the Original Approach in RFC 7030
     5.1.  Require an RFC 8994 / ACP subjectAltName with Specific
           otherName
       5.1.1.  Base64-Encoded Example
       5.1.2.  ASN.1 DUMP Output
     5.2.  Original Example in RFC 7030
       5.2.1.  Base64-Encoded Example
       5.2.2.  ASN.1 DUMP Output
     5.3.  Require a Specific subjectAltName Extension
       5.3.1.  Base64-Encoded Example
       5.3.2.  ASN.1 DUMP Output
     5.4.  Require a Public Key of a Specific Size
       5.4.1.  Base64-Encoded Example
       5.4.2.  ASN.1 DUMP Output
     5.5.  Require a Public Key of a Specific Curve
       5.5.1.  Base64-Encoded Example
       5.5.2.  ASN.1 DUMP Output
     5.6.  Require Specific Extensions and Attributes
       5.6.1.  Base64-Encoded Example
       5.6.2.  ASN.1 DUMP Output
   6.  Security Considerations
     6.1.  Identity and Privacy Considerations
   7.  IANA Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Appendix A.  ASN.1 Module
   Acknowledgments
   Authors' Addresses

1.  Introduction

   This document updates RFC 7030 and clarifies how the Certificate
   Signing Request (CSR) Attributes Response can be used by an EST
   server to specify both CSR attribute OIDs and CSR attribute values.
   In particular, the server needs to be able to specify X.509 extension
   values that it expects the client to include in the subsequent CSR.

   "Enrollment over Secure Transport" [RFC7030] 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].

   When bootstrapping the ACP, there is a requirement that each node be
   given a very specific subjectAltName.  In [RFC8994], the ACP
   specification, the EST server is specified to make use of the CSR
   Attributes ("/csrattrs") Request (specified in [RFC7030],
   Section 2.6) to convey the actual subjectAltName to the EST client
   that needs to go into its CSR and thus ultimately into its End Entity
   (EE) 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 [RFC7030],
   Section 2.6.

   [RFC7030], 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 be used.

   After significant discussion, it has been determined that Section 4.5
   of [RFC7030] is sufficiently difficult to read and ambiguous to
   interpret, so clarification is needed.

   Also, [RFC7030], Section 4.5.2 is extended to clarify the use of the
   existing ASN.1 syntax [X.680] [X.690].

   This covers all uses and is fully backward compatible with existing
   use, including addressing the needs of [RFC8994] and [RFC8995].

2.  Terminology

   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.

3.  CSR Attributes Handling

3.1.  Extensions to RFC 7030, Section 2.6

   This document replaces the second paragraph in Section 2.6 of
   [RFC7030] with the following text:

   |  These attributes can provide additional 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 ([RFC7030],
   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 id-
   ExtensionReq, which has the value 1.2.840.113549.1.9.14.  Note that
   this is the same as pkcs-9-at-extensionRequest defined in PKCS#9
   [RFC2985].  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 particular
   extnID.

   When not using the template-based approach of Section 3.4, specifying
   the requirement for a public key of a specific type and optionally
   its size and other parameters MUST be done as follows: Include
   exactly one Attribute with the type field being the OID specifying
   the type of the key, such as ecPublicKey or rsaEncryption.  The
   'values' field MAY be empty to indicate no further requirements on
   the key.  Otherwise, it MUST contain suitable parameters for the
   given key type, such as a singleton set containing the OID of an
   elliptic curve (EC) (e.g., secp384r1) or containing an integer value
   for the RSA key size (e.g., 4096).  Many examples for this are given
   in Section 5.

3.3.  Update to RFC 9148

   The updates to EST in this document equally apply when using the
   Constrained Application Protocol (CoAP) as a transport mechanism as
   described in [RFC9148].  This document therefore adds the following
   paragraph after the second paragraph of [RFC9148], Section 1:

   |  EST over CoAP as specified in [RFC9148] applies unchanged to
   |  [RFC7030], which is updated by RFC 9908.  Hence, all references to
   |  [RFC7030] in [RFC9148] are assumed to indicate that [RFC7030] is
   |  updated by RFC 9908.

3.4.  Use of CSR Templates

   As an alternative to the unstructured inclusion of CSR Attributes
   specified in Section 4.5.2 of [RFC7030] with its limitations and
   ambiguities, Appendix B of [RFC8295] describes an approach using a
   CSR template.  An entire CSR object is returned with various fields
   filled out and other fields waiting to be filled in.  In that
   approach, a pKCS7PDU attribute includes a PKIData content type
   [RFC5272] and that, in turn, includes a CSR [RFC2986] or a
   Certificate Request Message Format (CRMF) formatted request (for
   details, see 5 or 9 of [RFC6268], respectively).

   One drawback to that approach, particularly for the CSR, is that some
   unused fields have to be included; specifically, the 'signature'
   field on the CSR is faked with an empty bit string.

   A similar method has been defined in "Internet X.509 Public Key
   Infrastructure -- Certificate Management Protocol (CMP)" [RFC9810]
   and "Lightweight Certificate Management Protocol (CMP) Profile"
   ([RFC9483], Section 4.3.3) using a CSR template as defined for CRMF
   [RFC4211].  Like the approach mentioned before, this method does not
   properly deal with absent Relative Distinguished Name (RDN) values,
   as it would encode them as invalid empty strings.  Also, encoding an
   absent 'subjectPublicKey' value as an empty BIT STRING and an absent
   X.509 extension value as an empty OCTET STRING can cause issues with
   strict ASN.1 parsing and decoding.

   These drawbacks are avoided as follows:

   This specification defines a new Certificate Request Information
   Template attribute for CsrAttrs (as given in Section 3.2) that is
   essentially a partially filled-in PKCS#10 CSR minus the signature
   wrapper:

     CertificationRequestInfoTemplate ::= SEQUENCE {
         version       INTEGER { v1(0) } (v1, ... ),
         subject       NameTemplate OPTIONAL,
         subjectPKInfo [0] SubjectPublicKeyInfoTemplate
                                   {{ PKInfoAlgorithms }} OPTIONAL,
         attributes    [1] Attributes{{ CRIAttributes }}
     }

   Appendix A contains all details.

   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 has been made OPTIONAL.  It MUST be present if
      the server places any requirements on the RDNs of the subject
      name; otherwise, it MUST be absent.

   *  RDNs in the 'subject' fields are allowed to have no value, which
      has been achieved by adding OPTIONAL to the 'value' field of
      SingleAttributeTemplate.  If the client is expected to provide an
      RDN of a certain type such as commonName, the respective RDN MUST
      be present in the 'subject' field; otherwise, it MUST be absent.
      In addition, if the server gives an RDN value, this means that the
      client is expected to use this value for the RDN; otherwise, the
      client is expected to fill in a suitable value.  The example at
      the end of this section has a 'subject' field that contains both
      forms of RDN specifications.

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

   *  The 'subjectPKInfo' field has been made OPTIONAL.  The field MUST
      be absent if the server places no requirements on the key;
      otherwise, it MUST be present, and the 'algorithm' field specifies
      the type of key pair the client is expected to use.

   *  The 'subjectPublicKey' field contained in
      SubjectPublicKeyInfoTemplate has been made OPTIONAL because it is
      usually not needed.  In case the server requires use of an RSA key
      and needs to specify its size, the field MUST be present and
      contain a placeholder public key value of the desired RSA modulus
      length; otherwise, the subjectPublicKey MUST be absent.

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

   *  A new OID id-aa-extensionReqTemplate and the related
      ExtensionTemplate structure is defined where the 'extnValue' field
      has been made OPTIONAL.  This is only needed to enable specifying
      partial extensions with values to be filled in by the client;
      otherwise, the id-ExtensionReq OID and the respective value of
      type ExtensionReq MUST be used for specifying requirements on
      X.509 extensions.

   For each extension of type Extension or ExtensionTemplate provided by
   the server, the client is expected to include an extension of the
   type given by the extnID.  If the 'critical' field is present, the
   client SHOULD use it in the extension as well.  If the 'extnValue' is
   present (which is always the case when type Extension is used), the
   client SHOULD use the given extension value in its CSR.  When the
   type ExtensionTemplate is used, the 'extnValue' can be absent, and
   then the client SHOULD provide an extension value in an Extension
   with the given extnID.  For instance, if the server includes an
   ExtensionTemplate with the extnID 'subjectAltName' but without an
   extnValue, the client SHOULD include a SAN extension with a suitable
   value.

   In case the server includes an ExtensionTemplate with the extnID
   'subjectAltName' and a partially filled-in extnValue, such as a
   'directoryName' choice containing the NULL-DN (i.e., an empty
   sequence of RDNs) or the 'iPAddress' choice with an empty OCTET
   STRING, it means that the client SHOULD fill in the respective
   GeneralName value.

   ExtensionTemplate {EXTENSION:ExtensionSet} ::= SEQUENCE {
      extnID      EXTENSION.&id({ExtensionSet}),
      critical    BOOLEAN 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
     }

   The 'version' field of the CertificationRequestInfoTemplate MUST
   contain v1 (0).

   The 'attributes' field MUST NOT contain multiple id-aa-
   extensionReqTemplate attributes and MUST NOT contain both id-
   ExtensionReq and id-aa-extensionReqTemplate attributes.

   The 'values' field of an id-aa-extensionReqTemplate attribute MUST
   contain a set with exactly one element, and this element MUST be of
   type ExtensionTemplate.

   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 names with given values "myDept" and
      "myGroup",

   *  the 'publicKey' field with an Elliptic Curve Cryptography (ECC)
      public key on curve secp256r1,

   *  the 'subjectAltName' extension with two entries; one DNS entry
      with name "www.myServer.com" and IP entry that is empty for the IP
      address to be filled in,

   *  the 'keyUsage' extension marked critical with the values
      digitalSignature and keyAgreement, and

   *  the 'extKeyUsage' extension with the value 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"
           }
         }
       }
     [0] {
       SEQUENCE {
         OBJECT IDENTIFIER ecPublicKey (1 2 840 10045 2 1)
         OBJECT IDENTIFIER secp256r1 (1 2 840 10045 3 1 7)
         }
       }
     [1] {
       SEQUENCE {
         OBJECT IDENTIFIER id-aa-extensionReqTemplate
                           (1 2 840 113549 1 9 62)
         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)
               }
             }
           }
         }
       }
     }

4.  Coexistence with Existing Implementations

   EST servers with legacy clients MAY continue to use the unstructured
   list of attribute/value pairs as described in [RFC7030] and MAY also
   include the template style described in Section 3.4 for newer
   clients.  Clients that understand both MUST use the template only,
   and ignore all other CsrAttrs elements.  Older clients will ignore
   the new CertificationRequestInfoTemplate element.

5.  Examples Using the Original Approach in RFC 7030

   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" [dumpasn1] is then provided to
   detail what the contents are.

5.1.  Require an RFC 8994 / ACP subjectAltName with Specific otherName

   A single subjectAltName extension is specified in a single CsrAttrs
   attribute [RFC7030] with an OID 'id-ExtensionReq' indicating type
   Extensions.  This is what might be created by a Registrar [RFC8995]
   that is asking for AcpNodeName [RFC8994] with format 'otherNames'.

5.1.1.  Base64-Encoded Example

   The Base64:

   MGgwZgYJKoZIhvcNAQkOMVkwVzBVBgNVHREBAf8ESzBJoEcG
   CCsGAQUFBwgKoDsWOXJmYzg5OTQrZmQ3MzlmYzIzYzM0NDAx
   MTIyMzM0NDU1MDAwMDAwMDArQGFjcC5leGFtcGxlLmNvbQ==

5.1.2.  ASN.1 DUMP Output

   There is a single subjectAltName Extension with an Attribute with
   Extension type.

       <30 68>
     0 104: SEQUENCE {
       <30 66>
     2 102:   SEQUENCE {
       <06 09>
     4   9:     OBJECT IDENTIFIER
          :       extensionRequest (1 2 840 113549 1 9 14)
          :       (PKCS #9 via CRMF)
       <31 59>
    15  89:     SET {
       <30 57>
    17  87:       SEQUENCE {
       <30 55>
    19  85:         SEQUENCE {
       <06 03>
    21   3:           OBJECT IDENTIFIER
          :             subjectAltName (2 5 29 17)
          :             (X.509 extension)
       <01 01>
    26   1:           BOOLEAN TRUE
       <04 4B>
    29  75:           OCTET STRING, encapsulates {
       <30 49>
    31  73:             SEQUENCE {
       <A0 47>
    33  71:               [0] {
       <06 08>
    35   8:                 OBJECT IDENTIFIER '1 3 6 1 5 5 7 8 10'
       <A0 3B>
    45  59:                 [0] {
       <16 39>
    47  57:                   IA5String
          :                   'rfc8994+fd739fc23c34401122334455'
          :                   '00000000+@acp.example.com'
          :                   }
          :                 }
          :               }
          :             }
          :           }
          :         }
          :       }
          :     }
          :   }

5.2.  Original Example in RFC 7030

   In this example, taken from Section 4.5.2 of [RFC7030], a few
   different attributes are included.  The original encoding of the
   'macAddress' part in the example is NOT CORRECT.  It was not aligned
   with the definition of the Extension Request attribute as specified
   in Section 5.4.2 of [RFC2985].  The revised encoding given here does
   not use an 'id-ExtensionReq' attribute because the MAC Address is not
   an X.509 certificate extension by itself and because the server
   provides its OID without a value, which is not allowed syntactically
   within a structure of type 'Extension'.

5.2.1.  Base64-Encoded Example

   The Base64:

   MDIGCSqGSIb3DQEJBzASBgcqhkjOPQIBMQcGBSuBBAAiBgcr
   BgEBAQEWBggqhkjOPQQDAw==

5.2.2.  ASN.1 DUMP Output

   The CsrAttrs structure contains:

   1.  The challengePassword attribute to indicate that the CSR should
       include this value.

   2.  An ecPublicKey OID with the value secp384r1 to indicate what kind
       of public key should be submitted.

   3.  The macAddress OID 1.3.6.1.1.1.1.22 to indicate that the CSR is
       expected to include (in a subjectDirectoryAttributes extension) a
       MAC address value.

   4.  The ecdsaWithSHA384 OID to indicate what kind of hash is expected
       to be used for the self-signature in the PKCS#10 CSR.

       <30 32>
     0  50: 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)
          :       }
          :     }
       <06 07>
    33   7:   OBJECT IDENTIFIER '1 3 6 1 1 1 1 22'
       <06 08>
    42   8:   OBJECT IDENTIFIER ecdsaWithSHA384 (1 2 840 10045 4 3 3)
          :     (ANSI X9.62 ECDSA algorithm with SHA384)
          :   }

5.3.  Require 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:

   MEUGCSqGSIb3DQEJBzASBgcqhkjOPQIBMQcGBSuBBAAjBgkq
   hkiG9w0BCRQGCgmSJomT8ixkAQUGA1UEBQYIKoZIzj0EAwQ=

5.3.2.  ASN.1 DUMP Output

   The CsrAttrs structure contains:

   1.  The challengePassword attribute to indicate that the CSR should
       include this value.

   2.  An ecPublicKey OID with the value secp521r1 to indicate what kind
       of public key should be submitted.

   3.  An extensionRequest container with a subjectAltName value
       containing the name potato@example.com.

   4.  The ecdsaWithSHA512 OID to indicate the SHA-512 hash is expected
       to be used for the self-signature in the PKCS#10 CSR.

       <30 45>
     0  69: 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)
          :       }
          :     }
       <06 09>
    33   9:   OBJECT IDENTIFIER friendlyName (for PKCS #12)
                                (1 2 840 113549 1 9 20)
          :     (PKCS #9 via PKCS #12)
       <06 0A>
    44  10:   OBJECT IDENTIFIER '0 9 2342 19200300 100 1 5'
       <06 03>
    56   3:   OBJECT IDENTIFIER serialNumber (2 5 4 5)
          :     (X.520 DN component)
       <06 08>
    61   8:   OBJECT IDENTIFIER ecdsaWithSHA512 (1 2 840 10045 4 3 4)
          :     (ANSI X9.62 ECDSA algorithm with SHA512)
          :   }

5.4.  Require a Public Key of a Specific Size

   The CSR requires an RSA 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 use SHA256 as the
   hash algorithm within the signature.

       <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 an ECC public key with a specific curve.

5.5.1.  Base64-Encoded Example

   The Base64:

   MC4GCSqGSIb3DQEJBzASBgcqhkjOPQIBMQcGBSuBBAAiBgNV
   BAUGCCqGSM49BAMD

5.5.2.  ASN.1 DUMP Output

   Provide a CSR with an ECC public key from p384, include your serial
   number, and use SHA384 as the hash algorithm within the signature.

       <30 2E>
     0  46: 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)
          :       }
          :     }
       <06 03>
    33   3:   OBJECT IDENTIFIER serialNumber (2 5 4 5)
          :     (X.520 DN component)
       <06 08>
    38   8:   OBJECT IDENTIFIER ecdsaWithSHA384 (1 2 840 10045 4 3 3)
          :     (ANSI X9.62 ECDSA algorithm with SHA384)
          :   }

5.6.  Require Specific Extensions and Attributes

   The CSR is required to have an ECC public key, include a serial
   number, include a friendly name, include a favorite drink
   [favoritedrink] [OID 0.9.2342.19200300.100.1.5], and use SHA512 as
   the hash algorithm within the signature.

5.6.1.  Base64-Encoded Example

   The Base64:

   MEUGCSqGSIb3DQEJBzASBgcqhkjOPQIBMQcGBSuBBAAjBgkq
   hkiG9w0BCRQGCgmSJomT8ixkAQUGA1UEBQYIKoZIzj0EAwQ=

5.6.2.  ASN.1 DUMP Output

   Provide a CSR with an ECC public key from sha521 and include your
   serial number, friendly name, and favorite drink, and hash it with
   SHA512.

       <30 45>
     0  69: 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)
          :       }
          :     }
       <06 09>
    33   9:   OBJECT IDENTIFIER friendlyName (for PKCS #12)
                                (1 2 840 113549 1 9 20)
          :     (PKCS #9 via PKCS #12)
       <06 0A>
    44  10:   OBJECT IDENTIFIER '0 9 2342 19200300 100 1 5'
       <06 03>
    56   3:   OBJECT IDENTIFIER serialNumber (2 5 4 5)
          :     (X.520 DN component)
       <06 08>
    61   8:   OBJECT IDENTIFIER ecdsaWithSHA512 (1 2 840 10045 4 3 4)
          :     (ANSI X9.62 ECDSA algorithm with SHA512)
          :   }

6.  Security Considerations

   The security considerations from [RFC7030], Section 6 are unchanged.

6.1.  Identity and Privacy Considerations

   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 Initial Device
   Identifier (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.

7.  IANA Considerations

   For the ASN.1 module in Appendix A, IANA has assigned the following
   OID in the "SMI Security for S/MIME Module Identifier
   (1.2.840.113549.1.9.16.0)" registry:

               +=========+====================+===========+
               | Decimal | Description        | Reference |
               +=========+====================+===========+
               | 82      | id-mod-critemplate | RFC 9908  |
               +---------+--------------------+-----------+

                                 Table 1

   For the Certification Request Information Template and Extension
   Request Template attributes in Appendix A, IANA has assigned the
   following OIDs in the "SMI Security for S/MIME Attributes
   (1.2.840.113549.1.9.16.2)" registry:

     +=========+========================================+===========+
     | Decimal | Description                            | Reference |
     +=========+========================================+===========+
     | 61      | id-aa-certificationRequestInfoTemplate | RFC 9908  |
     +---------+----------------------------------------+-----------+
     | 62      | id-aa-extensionReqTemplate             | RFC 9908  |
     +---------+----------------------------------------+-----------+

                                 Table 2

8.  References

8.1.  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>.

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              DOI 10.17487/RFC2986, November 2000,
              <https://www.rfc-editor.org/info/rfc2986>.

   [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>.

   [RFC9148]  van der Stok, P., Kampanakis, P., Richardson, M., and S.
              Raza, "EST-coaps: Enrollment over Secure Transport with
              the Secure Constrained Application Protocol", RFC 9148,
              DOI 10.17487/RFC9148, April 2022,
              <https://www.rfc-editor.org/info/rfc9148>.

   [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>.

8.2.  Informative References

   [dumpasn1] Gutmann, P., "Dump ASN", 22 April 2021,
              <https://www.cs.auckland.ac.nz/~pgut001/dumpasn1.c>.

   [favoritedrink]
              OID Repository, "drink(5) [other identifier:
              favouriteDrink]", OID 0.9.2342.19200300.100.1.5, 4 July
              2019,
              <https://oid-base.com/get/0.9.2342.19200300.100.1.5>.

   [RFC2985]  Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
              Classes and Attribute Types Version 2.0", RFC 2985,
              DOI 10.17487/RFC2985, November 2000,
              <https://www.rfc-editor.org/info/rfc2985>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [RFC9810]  Brockhaus, H., von Oheimb, D., Ounsworth, M., and J. Gray,
              "Internet X.509 Public Key Infrastructure -- Certificate
              Management Protocol (CMP)", RFC 9810,
              DOI 10.17487/RFC9810, July 2025,
              <https://www.rfc-editor.org/info/rfc9810>.

Appendix A.  ASN.1 Module

   This appendix provides an ASN.1 module [X.680] for the Certification
   Request Information Template attribute and its sub-template
   structures.  It follows the conventions established in [RFC5911],
   [RFC5912], and [RFC6268].

   CRITemplateModule
     { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
       pkcs-9(9) smime(16) modules(0) id-mod-critemplate(82) }

   DEFINITIONS IMPLICIT TAGS ::=

   BEGIN

   IMPORTS -- from [RFC5912]

   SupportedAttributes
    FROM PKIX1Explicit-2009
     { iso(1) identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-explicit-02(51)}

   ATTRIBUTE, EXTENSION
    FROM PKIX-CommonTypes-2009
     { iso(1) identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) pkix(7) id-mod(0) id-mod-pkixCommon-02(57) }

   PUBLIC-KEY, AlgorithmIdentifier{}
    FROM AlgorithmInformation-2009
     { iso(1) identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) pkix(7) id-mod(0)
       id-mod-algorithmInformation-02(58)}

   CertExtensions
    FROM PKIX1Implicit-2009
     { iso(1) identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-implicit-02(59)}

   Attributes{}, CRIAttributes, PKInfoAlgorithms
    FROM PKCS-10
     { iso(1) identified-organization(3) dod(6) internet(1)
       security(5) mechanisms(5) pkix(7)
       id-mod(0) id-mod-pkcs10-2009(69) }
   ;

   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) id-aa-certificationRequestInfoTemplate(61) }

   --  like CertificationRequestInfo but OPTIONAL subject, subjectPKInfo
   CertificationRequestInfoTemplate ::= SEQUENCE {
       version       INTEGER { v1(0) } (v1, ... ),
       subject       NameTemplate OPTIONAL,
       subjectPKInfo [0] SubjectPublicKeyInfoTemplate
                                 {{ PKInfoAlgorithms }} OPTIONAL,
       attributes    [1] Attributes{{ CRIAttributes }}
   }


   --  like Name, but with OPTIONAL RDN values
   NameTemplate ::= CHOICE { -- only one possibility for now --
       rdnSequence  RDNSequenceTemplate }

   RDNSequenceTemplate ::= SEQUENCE OF RelativeDistinguishedNameTemplate

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

   --  like Attributes, but with OPTIONAL value
   SingleAttributeTemplates{ATTRIBUTE:AttrSet} ::= SEQUENCE OF
       SingleAttributeTemplates{ {AttrSet} }

   --  like SingleAttribute, but with OPTIONAL value
   SingleAttributeTemplate{ATTRIBUTE:AttrSet} ::= SEQUENCE {
       type      ATTRIBUTE.&id({AttrSet}),
       value     ATTRIBUTE.&Type({AttrSet}{@type}) OPTIONAL
   }

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

   id-aa-extensionReqTemplate OBJECT IDENTIFIER ::=
   { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
     smime(16) aa(2) id-aa-extensionReqTemplate(62) }

   --  like extensionRequest, but with OPTIONAL Extension extnValues
   --  original definition was in PKCS#9 RFC 2985, Section 5.4.2
   at-extensionReqTemplate ATTRIBUTE ::= {
       TYPE ExtensionReqTemplate IDENTIFIED BY
                    id-aa-extensionReqTemplate }

   ExtensionReqTemplate ::= ExtensionTemplates{{CertExtensions}}

   --  like Extensions, but with OPTIONAL extnValue
   ExtensionTemplates{EXTENSION:ExtensionSet} ::=
       SEQUENCE SIZE (1..MAX) OF ExtensionTemplate{{ExtensionSet}}

   --  like Extension, but with OPTIONAL extnValue
   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
   }

   END

Acknowledgments

   Corey Bonnell crafted Example 2 using a different tool, and this
   helped debug other running code.

   Carl Wallace provided major parts of the
   CertificationRequestInfoTemplate syntax declaration.

   Russ Housley conducted many reviews of the ASN.1 module and suggested
   many fixes.

   Deb Cooley conducted the usual Area Director Review.

Authors' Addresses

   Michael Richardson (editor)
   Sandelman Software Works
   Email: mcr+ietf@sandelman.ca


   Owen Friel
   Cisco
   Email: ofriel@cisco.com


   Dr. David von Oheimb
   Siemens
   Email: dev@ddvo.net


   Dan Harkins
   The Industrial Lounge
   Email: dharkins@lounge.org