Internet Engineering Task Force (IETF)                       M. Pritikin
Request for Comments: 7894                           Cisco Systems, Inc.
Category: Standards Track                                     C. Wallace
ISSN: 2070-1721                                 Red Hound Software, Inc.
                                                               June 2016


               Alternative Challenge Password Attributes
                  for Enrollment over Secure Transport

Abstract

   This document defines a set of new Certificate Signing Request
   attributes for use with the Enrollment over Secure Transport (EST)
   protocol.  These attributes provide disambiguation of the existing
   overloaded uses for the challengePassword attribute defined in "PKCS
   #9: Selected Object Classes and Attribute Types Version 2.0" (RFC
   2985).  Uses include the original certificate revocation password,
   common authentication password uses, and EST-defined linking of
   transport security identity.

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
   http://www.rfc-editor.org/info/rfc7894.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................4
   3. Alternative Challenge Password Attributes .......................4
      3.1. OTP Challenge Attribute ....................................4
      3.2. Revocation Challenge Attribute .............................5
      3.3. EST Identity Linking Attribute .............................5
   4. Indicating Support for the Alternative Challenge Attributes .....6
   5. Security Considerations .........................................6
   6. IANA Considerations .............................................7
   7. References ......................................................7
      7.1. Normative References .......................................7
      7.2. Informative References .....................................8
   Appendix A.  ASN.1 Module ..........................................9
   Acknowledgements ..................................................10
   Authors' Addresses ................................................10

1.  Introduction

   "PKCS #9: Selected Object Classes and Attribute Types Version 2.0"
   [RFC2985] defined a challengePassword attribute that has been
   overloaded by modern protocol usage with the appropriate
   interpretation being provided by context rather than OID definition.
   PKCS #9 defines the challengePassword attribute as "a password by
   which an entity may request certificate revocation".  The parsing and
   embedding of this attribute within Certificate Signing Requests is
   well supported by common PKI toolsets, but many workflows leverage
   this supported field as a one-time password for authentication.  For
   example, this is codified in many Simple Certificate Enrollment
   Protocol (SCEP) implementations as indicated by [SCEP].  Continuing
   this trend, Enrollment over Secure Transport (EST) [RFC7030] defines
   an additional semantic for the challengePassword attribute in
   Section 3.5, in order to provide a linking of the Certificate Signing
   Request (CSR) to the secure transport.

   Where the context of the protocol operation fully defined the proper
   semantic, and when only one use was required at a time, the
   overloading of this field did not cause difficulties.  Implementation
   experience with EST has shown this to be a limitation though.  There
   are plausible use cases where it is valuable to use either of the
   existing methods separately or in concert.  For example, an EST
   server might require the client to authenticate itself using the
   existing client X.509 certificate as well as the user's username and
   password, and to include a one-time password within the CSR, all
   while maintaining identity linking to bind the CSR to the secure
   transport.  The overloading of a single attribute type should not be
   the limiting factor for administrators attempting to meet their
   security requirements.

   This document defines the otpChallenge attribute for use when a one-
   time password (OTP) value within the CSR is a requirement.  The
   revocationChallenge attribute is defined to allow disambiguated usage
   of the original challenge password attribute semantics for
   certificate revocation.  The estIdentityLinking attribute is defined
   to reference existing EST challenge password semantics with no
   potential for confusion with legacy challenge password practices.

   The attributes defined in this specification supplement existing EST
   mechanisms and are not intended to displace current usage of any
   existing EST authentication mechanisms.  Conveying the authentication
   value itself as an attribute may be preferable to using an HTTP or
   Transport Layer Security (TLS) password or other TLS authentication
   mechanism in environments where the certificate request processing
   component is removed from the HTTP/TLS termination point, for
   example, when a web application firewall is used.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Alternative Challenge Password Attributes

   The following sections describe three alternative challenge password
   attributes for use with EST [RFC7030].  Appendix A provides an ASN.1
   module containing the new definitions.

   Each attribute described below is defined as a DirectoryString with a
   maximum length of 255, which features several possible encoding
   options.  Attribute values generated in accordance this document
   SHOULD use the PrintableString encoding whenever possible.  If
   internationalization issues make this impossible, the UTF8String
   alternative SHOULD be used.  Attribute processing systems MUST be
   able to recognize and process the PrintableString and UTF8String
   string types in DirectoryString values.  Support for other string
   types is OPTIONAL.

3.1.  OTP Challenge Attribute

   The otpChallenge attribute is defined as a DirectoryString with a
   maximum length of 255.  This is consistent with the challengePassword
   attribute as originally defined in PKCS #9 [RFC2985].  The
   otpChallenge attribute is identified by the id-aa-otpChallenge object
   identifier.  This facilitates reuse of the existing challengePassword
   code by associating the new object identifiers with the existing
   parsing and generation code.  This attribute provides a means of
   conveying a one-time password value as part of a CSR request.
   Generation, verification, storage, etc., of the value is not
   addressed by this specification.  [RFC4226] and [RFC6238] define one-
   time password mechanisms that MAY be used with this attribute.

      ub-aa-otpChallenge INTEGER ::= 255
      id-aa-otpChallenge OBJECT IDENTIFIER ::= {
          id-smime 56
      }
      otpChallenge ATTRIBUTE ::= {
          WITH SYNTAX DirectoryString {ub-aa-otpChallenge}
          EQUALITY MATCHING RULE caseExactMatch
          SINGLE VALUE TRUE
          ID id-aa-otpChallenge
      }

3.2.  Revocation Challenge Attribute

   The original PKCS #9 challengePassword field has been overloaded, and
   the common use is unclear.  The revocationChallenge attribute defined
   here provides an unambiguous method of indicating the original PKCS
   #9 intent for this attribute type.  The revocationChallenge attribute
   is identified by the id-aa-revocationChallenge object identifier.
   [RFC2985] discusses the original semantics for the PKCS #9 challenge
   password attribute.

      ub-aa-revocationChallenge INTEGER ::= 255
      id-aa-revocationChallenge OBJECT IDENTIFIER ::= {
          id-smime 57
      }
      revocationChallenge ATTRIBUTE ::= {
          WITH SYNTAX DirectoryString {ub-aa-revocationChallenge}
          EQUALITY MATCHING RULE caseExactMatch
          SINGLE VALUE TRUE
          ID id-aa-revocationChallenge
      }

3.3.  EST Identity Linking Attribute

   EST defines a mechanism for associating identity information from an
   authenticated TLS session with proof-of-possession information in a
   certificate request.  The mechanism was labeled using the pkcs-9-at-
   challengePassword identifier from [RFC2985].  To avoid any confusion
   with the semantics described in [RFC2985] or any other specifications
   that similarly defined use of the PKCS #9 challenge password
   attribute for their own purposes, a new object identifier is defined
   here and associated with the semantics described in Section 3.5 of
   [RFC7030].

      ub-aa-est-identity-linking INTEGER ::= 255
      id-aa-estIdentityLinking OBJECT IDENTIFIER ::= {
          id-smime 58
      }
      estIdentityLinking ATTRIBUTE ::= {
          WITH SYNTAX DirectoryString {ub-aa-est-identity-linking}
          EQUALITY MATCHING RULE caseExactMatch
          SINGLE VALUE TRUE
          ID id-aa-estIdentityLinking
      }

4.  Indicating Support for the Alternative Challenge Attributes

   The EST server MUST indicate these attributes, as the particular use
   case requires, in every CSR Attributes Response.  An EST server MAY
   send both the estIdentityLinking attribute and the challengePassword
   attribute [RFC7030] in a CSR Attributes Response to ensure support
   for legacy clients.

   The client MUST include every indicated attribute for which it has
   values in the subsequent CSR.  If a client sees an estIdentityLinking
   attribute in a CSR Attributes Response, it SHOULD prefer that and not
   include a challengePassword attribute [RFC7030] in the resulting CSR.
   EST clients that include an unsolicited estIdentityLinking attribute
   MAY also include the challengePassword attribute [RFC7030] to ensure
   support for legacy servers.

   EST servers MUST evaluate each challenge attribute independently.
   All challenge attributes included by an EST client MUST be
   successfully processed by an EST server for a request to be
   considered valid.  The EST server MAY ignore challenge attributes
   according to local policy, for example, if the EST client is an
   authenticated Registration Authority, the EST server may ignore the
   estIdentityLinking attribute within a CSR (see Section 3.7 of
   [RFC7030]).  The EST server MAY refuse enrollment requests that are
   not encoded according to the policy of the Certification Authority
   (CA).

5.  Security Considerations

   In addition to the security considerations expressed in the EST
   specification [RFC7030], additional security considerations may be
   associated with the mechanism used to generate and verify the
   otpChallenge value.  Where a one-time password is used, the security
   considerations expressed in "HOTP: An HMAC-Based One-Time Password
   Algorithm" [RFC4226] or "TOTP: Time-Based One-Time Password
   Algorithm" [RFC6238] may be relevant.  Similarly, the security
   considerations from [RFC2985] that apply to the challenge attribute
   are relevant as well.

6.  IANA Considerations

   Section 3 defines three attributes that have been assigned object
   identifiers in the "SMI Security for S/MIME Attributes
   (1.2.840.113549.1.9.16.2)" registry [RFC7107]:

        Value     Description                        Reference
        --------  ---------------------------------  ----------
        56        id-aa-otpChallenge                 RFC 7894
        57        id-aa-revocationChallenge          RFC 7894
        58        id-aa-estIdentityLinking           RFC 7894

   Appendix A contains an ASN.1 module.  A module identifier has been
   assigned in the "SMI Security for PKIX Module Identifier" registry
   [RFC7299].

        Value     Description                        Reference
        --------  ---------------------------------  ----------
        87        id-mod-EST-Alt-Challenge           RFC 7894

7.  References

7.1.  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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [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,
              <http://www.rfc-editor.org/info/rfc2985>.

   [RFC5272]  Schaad, J. and M. Myers, "Certificate Management over CMS
              (CMC)", RFC 5272, DOI 10.17487/RFC5272, June 2008,
              <http://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,
              <http://www.rfc-editor.org/info/rfc5280>.

   [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,
              <http://www.rfc-editor.org/info/rfc5912>.

   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,
              <http://www.rfc-editor.org/info/rfc7030>.

7.2.  Informative References

   [RFC4226]  M'Raihi, D., Bellare, M., Hoornaert, F., Naccache, D., and
              O. Ranen, "HOTP: An HMAC-Based One-Time Password
              Algorithm", RFC 4226, DOI 10.17487/RFC4226, December 2005,
              <http://www.rfc-editor.org/info/rfc4226>.

   [RFC6238]  M'Raihi, D., Machani, S., Pei, M., and J. Rydell, "TOTP:
              Time-Based One-Time Password Algorithm", RFC 6238,
              DOI 10.17487/RFC6238, May 2011,
              <http://www.rfc-editor.org/info/rfc6238>.

   [RFC7107]  Housley, R., "Object Identifier Registry for the S/MIME
              Mail Security Working Group", RFC 7107,
              DOI 10.17487/RFC7107, January 2014,
              <http://www.rfc-editor.org/info/rfc7107>.

   [RFC7299]  Housley, R., "Object Identifier Registry for the PKIX
              Working Group", RFC 7299, DOI 10.17487/RFC7299, July 2014,
              <http://www.rfc-editor.org/info/rfc7299>.

   [SCEP]     Gutmann, P. and M. Pritikin, "Simple Certificate Enrolment
              Protocol", Work in Progress, draft-gutmann-scep-02, March
              2016.

Appendix A.  ASN.1 Module

   The following ASN.1 module includes the definitions to support usage
   of the attributes defined in this specification.  Modules from
   [RFC5912] are imported (the original Standards Track source for the
   imported structures is [RFC5280] and [RFC5272]).

   Mod-EST-Alt-Challenge {
      iso(1) identified-organization(3) dod(6) internet(1) security(5)
         mechanisms(5) pkix(7) id-mod(0) 87
   }

   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN
   IMPORTS

   DirectoryString{}
   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
   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)
   };

   ub-aa-otpChallenge INTEGER ::= 255
   id-aa-otpChallenge OBJECT IDENTIFIER ::= {
      iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
      smime(16) aa(2) 56
   }
   otpChallenge ATTRIBUTE ::= {
      TYPE DirectoryString {ub-aa-otpChallenge}
      COUNTS MIN 1 MAX 1
      IDENTIFIED BY id-aa-otpChallenge
   }
   ub-aa-revocationChallenge INTEGER ::= 255
   id-aa-revocationChallenge OBJECT IDENTIFIER ::= {
      iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
      smime(16) aa(2) 57
   }
   revocationChallenge ATTRIBUTE ::= {
      TYPE DirectoryString {ub-aa-revocationChallenge}
      COUNTS MIN 1 MAX 1
      IDENTIFIED BY id-aa-revocationChallenge
   }

   ub-aa-est-identity-linking INTEGER ::= 255
   id-aa-estIdentityLinking OBJECT IDENTIFIER ::= {
      iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
      smime(16) aa(2) 58
   }
   estIdentityLinking ATTRIBUTE ::= {
      TYPE DirectoryString {ub-aa-est-identity-linking}
      COUNTS MIN 1 MAX 1
      IDENTIFIED BY id-aa-estIdentityLinking
   }
   END

Acknowledgements

   Thanks to Jim Schaad, Dan Harkins, Phil Scheffler, Geoff Beier, Mike
   Jenkins, and Deb Cooley for their feedback.

Authors' Addresses

   Max Pritikin
   Cisco Systems, Inc.
   510 McCarthy Drive
   Milpitas, CA  95035
   United States

   Email: pritikin@cisco.com


   Carl Wallace
   Red Hound Software, Inc.

   Email: carl@redhoundsoftware.com