LAMPS Working Group                                   M. Richardson, Ed.
Internet-Draft                                  Sandelman Software Works
Intended status: Standards Track                                O. Friel
Expires: 8 September 2022                                          Cisco
                                                           D. von Oheimb
                                                                 Siemens
                                                              D. Harkins
                                                   The Industrial Lounge
                                                            7 March 2022


           Clarification of RFC7030 CSR Attributes definition
               draft-richardson-lamps-rfc7030-csrattrs-02

Abstract

   Enrollment over Secure Transport (EST) is ambiguous in specification
   of the CSR Attributes Response.  This has resulted in implementation
   challenges and implementor confusion.  This document updates 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 that the server expects the client to include in its CSR
   request.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 8 September 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.






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   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  CSR Attributes Handling . . . . . . . . . . . . . . . . . . .   3
     2.1.  Current EST Specification . . . . . . . . . . . . . . . .   3
   3.  Updated CSR Attributes Handling . . . . . . . . . . . . . . .   4
     3.1.  Option two: Extend CSR structure to allow values: . . . .   4
     3.2.  Option three: explicit content for the key
           specification . . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Option four: explicit members for unique attributes . . .   7
     3.4.  Option five: more specific structure, simpler
           extensions  . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Co-existence with existing implementations  . . . . . . . . .   7
     4.1.  Use a new MIME type . . . . . . . . . . . . . . . . . . .   7
     4.2.  Use a new end point of the new format . . . . . . . . . .   8
     4.3.  Insist new format is upwardly compatible with old
           format  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.4.  Return new format to new clients only . . . . . . . . . .   8
   5.  Whether or not to Base64 encoding of results  . . . . . . . .   8
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  RFC8994/ACP subjectAltName with specific otherName
           included  . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.2.  EST server requires public keys of a specific size  . . .   8
     6.3.  EST server requires a public key of a specific algorithm/
           curve . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.4.  EST server requires a specific extension to be present  .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
     7.1.  Identity and Privacy Considerations . . . . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   10. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     11.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10







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1.  Introduction

   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 in fact runs contrary to section 2.6.  Section 2.6
   says that the CSR attributes "provide additional descriptive
   information that the EST server cannot access itself".  This extends
   to specifying that a particular attribute should exist, but not to
   the point of having the EST server actually specify the value.

   The way in which the CSRattributes were understood by [RFC8994] turns
   out to be invalid.  This document, therefore, updates section 2.6 to
   define this behavior.

   This document also updates section 4.5 to include revised ASN.1 that
   covers all uses and is backward compatible with the existing use.

   Additional examples are provided in an appendix.

2.  CSR Attributes Handling

2.1.  Current EST Specification

   The ASN.1 for CSR Attributes as defined in EST section 4.5.2 is:

      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}) }

   That section also states the following:

      the values indicating the particular
      attributes desired to be included in the resulting certificate's
      extensions



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   This has been interpreted by some implementations as meaning that the
   CSR Attributes response can only include values for the attribute
   OIDs that the client should include in its CSR, and cannot include
   the actual values of those attributes.  This is further reinforced by
   the example:

      Attribute:  type = extensionRequest (1.2.840.113549.1.9.14)
                         value = macAddress (1.3.6.1.1.1.1.22)

   This example illustrates that the 'value' specified is an attribute
   OID, for example the macAddress OID, and not the value (such as
   "10-00-00-12-23-45") of the attribute itself.

   There is no clearly documented mechanism with supporting examples
   that specifies how a CSR Attributes response can include a value for
   a given attribute such as SubjectAltName.

   EST section 4.5.2 also states the following:

      The structure of the CSR Attributes Response SHOULD, to the
      greatest extent possible, reflect the structure of the CSR
      it is requesting.

   This statement aligns closely with the goal of this document.
   Additionally, EST Extensions [RFC8295] Appendix A has an informative
   appendix that outlines how a full CSR can be included in the CSR
   Attributes response.

3.  Updated CSR Attributes Handling

   The WG will pick one option as part of the adoption call.

3.1.  Option two: Extend CSR structure to allow values:

   This ASN.1 needs fixing.
















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      CsrAttrs ::= SEQUENCE SIZE (0..MAX) OF AttrOrOID

      AttrOrOID ::= CHOICE (oid OBJECT IDENTIFIER,
                            attribute Attribute,
                            value Value }

      Attribute { ATTRIBUTE:IOSet } ::= SEQUENCE {
           extType  ATTRIBUTE.&id({IOSet}),
           extAttr  SET SIZE(1..MAX) OF ATTRIBUTE.&Type({IOSet}{@type})
      }

      Value { ATTRIBUTE:IOSet } ::= SEQUENCE {
           extType  ATTRIBUTE.&id({IOSet}),
           type     ATTRIBUTE.&Type({IOSet}{@type}),
           value    OCTET STRING
      }

   This would just add a value to the SEQUENCE:

        OBJECT challengePassword
        SEQUENCE
          OBJECT subjectAltName
          SET
            OBJECT someACPgoo
        SEQUENCE
          OBJECT id-ecPublicKey
          SET
            OBJECT secp384r1
            OBJECT ecdsa-with-SHA384

   For example:

     0  30: SEQUENCE {
     2  28:   SEQUENCE {
     4   3:     OBJECT IDENTIFIER subjectAltName (2 5 29 17)
     9  21:     SET {
    11  19:       [1] {
    13  17:         UTF8String 'hello@example.com'
          :         }
          :       }
          :     }
          :   }

3.2.  Option three: explicit content for the key specification

   The following options support complete and unambiguous specification
   of




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   *  CSR ingredients optionally including values to use,

   *  the type of the public key, which is given in the form of a
      public-key algorithm,

   *  and the hash algorithm to use for the self-signature.

   CSR ingredients may be the subject DN, any X.509 extensions, and
   special attributes like a challenge password.

   For specifying the type of keys allowed in CSRs, they use a to-the-
   point KeySpec type.  It can be defined for instance as

      KeySpec ::= CHOICE {
                      keyAlg AlgorithmIdentifier,
                      rsaKeyLen INTEGER
      }

   The keyAlg type use used to specify public-key alorithms and can
   include parameters, such as the name of an elliptic curve.  The
   rsaKeyLen choice allows specifying the size of RSA keys, which it is
   not possible using values of type AlgorithmIdentifier.

   The keySpec could also be sequence of such specs, such that the
   server can give several key types from which the client can choose,
   e.g., EC keys on certain curves and/or RSA keys of certain sizes.

   Stick for syntactic backward compatibility with

       CsrAttrs ::= SEQUENCE SIZE (0..MAX) OF AttrOrOID

   Each OID given in AttrOrOID must occur only once.

   Plain OIDs are used mostly for challengePassword.

   Attributes are used mostly for any X.509 extensions, subject DN, key
   spec, and hash alg, while defining new generally usable OIDs for

   *  a subject DN of type Name

   *  a key spec of type KeySpec

   *  a hash alg spec of type AlgorithmIdentifier

   to be given on demand as attribute IDs of type
   ATTRIBUTE.&id({IOSet}).





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3.3.  Option four: explicit members for unique attributes

   Define a new and more to-the-point type, which does not require new
   OIDs:

      CsrAttrs ::= SEQUENCE {
           oids      SEQUENCE OF OBJECT IDENTIFIER,
           attrs     SEQUENCE OF Attribute,
           subject   [0] Name OPTIONAL,
           keySpec   [1] KeySpec OPTIONAL,
           hashAlg   [2] AlgorithmIdentifier OPTIONAL

      }

   Each OID given in oids or attrs must occur only once.

   The oids are used mostly for requiring a challenge password.

   The atttrs are used mostly for requiring certain X.509 extensions.

   This is, typically just challengePassword and extensionRequest are
   used.

3.4.  Option five: more specific structure, simpler extensions

   Define a new fully to-the-point type, which does not require any
   (direct) OIDs:

      CsrAttrs ::= SEQUENCE {
           subject               Name OPTIONAL,
           extensions            SEQUENCE OF Extension,
           challengePassword     BOOLEAN,
           keySpec           [0] KeySpec OPTIONAL,
           hashAlg           [1] AlgorithmIdentifier OPTIONAL

      }

4.  Co-existence with existing implementations

   There are some ways in which the new CSRattributes could co-exist
   with RFC7030.

4.1.  Use a new MIME type

   The client can signal that it supports the new attribute format by
   using an Accept: header in the transaction.  This acts as a signal to
   a server that it can/should return the attributes in the new format.




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4.2.  Use a new end point of the new format

   Clients that want to use the new format would use a new end point,
   such as "csrvalues" which would only support the new format.  A
   client which supported both would have to try both "csrvalues" and
   then fall back "csrattrs" if the EST server did not support the new
   format.  Some uses (such as [RFC8994]) require the new format, so if
   it was not suppored, that would be a protocol error.

4.3.  Insist new format is upwardly compatible with old format

   ASN.1 encoding is self-describing, and some formats proposed above
   could possibly be parsed by legacy clients without a problem.

4.4.  Return new format to new clients only

   The Registrar may know which clients are which by the kind of
   authentication that they do.  An [RFC8994] client which has just
   performed a [RFC8995] enrollment would be assumed to require the new
   format only.  A client which authenticates with an LDevID for a
   renewal would be strongly identified, and the Registrar could be
   programmed whether to return new format, or legacy CSR attributes.

5.  Whether or not to Base64 encoding of results

   [RFC8951] clarified that the csrattrs end point was to be Base64
   encoded even though the HTTP transport was 8-bit clean.

   If this document establishes a new end point, then the new end point
   will not be base64 encoded according to current HTTP usage.

6.  Examples

6.1.  RFC8994/ACP subjectAltName with specific otherName included

   TBD

6.2.  EST server requires public keys of a specific size

   TBD

6.3.  EST server requires a public key of a specific algorithm/curve

   TBD







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6.4.  EST server requires a specific extension to be present

   TBD

7.  Security Considerations

   All security considertions from EST [RFC7030] section 6 are
   applicable.

7.1.  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 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.

8.  IANA Considerations

   None.

9.  Acknowledgements

   TODO

10.  Changelog

11.  References

11.1.  Normative References

   [BCP14]    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>.

   [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>.






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   [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>.

11.2.  Informative References

   [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>.

   [RFC8951]  Richardson, M., Werner, T., and W. Pan, "Clarification of
              Enrollment over Secure Transport (EST): Transfer Encodings
              and ASN.1", RFC 8951, DOI 10.17487/RFC8951, November 2020,
              <https://www.rfc-editor.org/info/rfc8951>.

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




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