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.

















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Copyright Notice

  Copyright (c) 2016 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
  (http://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 Simplified BSD License text as described in Section 4.e of
  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



















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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.



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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
     }





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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
     }








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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.













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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>.



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  [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.






















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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
  }



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  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: [email protected]


  Carl Wallace
  Red Hound Software, Inc.

  Email: [email protected]



















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