Internet Engineering Task Force (IETF)                           A. Jain
Request for Comments: 8129                                  Georgia Tech
Updates: 4120                                                  N. Kinder
Category: Standards Track                                    N. McCallum
ISSN: 2070-1721                                            Red Hat, Inc.
                                                              March 2017


              Authentication Indicator in Kerberos Tickets

Abstract

   This document updates RFC 4120, as it specifies an extension in the
   Kerberos protocol.  It defines a new authorization data type,
   AD-AUTHENTICATION-INDICATOR.  The purpose of introducing this data
   type is to include an indicator of the strength of a client's
   authentication in service tickets so that application services can
   use it as an input into policy decisions.

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/rfc8129.

Copyright Notice

   Copyright (c) 2017 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
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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Document Conventions  . . . . . . . . . . . . . . . . . . . .   2
   3.  AD Type Specification . . . . . . . . . . . . . . . . . . . .   2
   4.  Assigned Numbers  . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .   6
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   Kerberos [RFC4120] allows secure interaction among users and services
   over a network.  It supports a variety of authentication mechanisms
   using its pre-authentication framework [RFC6113].  The Kerberos
   authentication service has been architected to support password-based
   authentication as well as multi-factor authentication using one-time
   password devices, public-key cryptography, and other
   pre-authentication schemes.  Implementations that offer
   pre-authentication mechanisms supporting significantly different
   strengths of client authentication may choose to keep track of the
   strength of the authentication that was used, for use as an input
   into policy decisions.

   This document specifies a new authorization data type to convey
   authentication strength information to application services.
   Elements of this type appear within an AD-CAMMAC (Authorization Data
   type Container Authenticated by Multiple Message Authentication
   Codes) [RFC7751] container.

2.  Document Conventions

   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 RFC 2119 [RFC2119].

3.  AD Type Specification

   The Key Distribution Center (KDC) MAY include authorization data of
   ad-type 97, wrapped in AD-CAMMAC, in initial credentials.  The KDC
   MAY copy it from a ticket-granting ticket into service tickets.

   The corresponding ad-data field contains the DER encoding [X.690] of
   the following ASN.1 [X.680] type:

   AD-AUTHENTICATION-INDICATOR ::= SEQUENCE OF UTF8String

   Each UTF8String value is a short string that indicates that a
   particular set of requirements was met during the initial
   authentication.  These strings are intended to be compared against
   known values.  They are not intended to store structured data.  Each
   string MUST be either:

   o  A URI that references a Level of Assurance Profile [RFC6711], or

   o  A site-defined string, which MUST NOT contain a colon, whose
      meaning is determined by the realm administrator.

   Authorization data elements of type AD-AUTHENTICATION-INDICATOR MUST
   be included in an AD-CAMMAC container so that their contents can be
   verified as originating from the KDC.  Elements of type
   AD-AUTHENTICATION-INDICATOR MAY safely be ignored by applications and
   KDCs that do not implement this element.

4.  Assigned Numbers

   RFC 4120 [RFC4120] is updated in the following way:

   o  The ad-type number 97 is assigned for AD-AUTHENTICATION-INDICATOR,
      updating the table in Section 7.5.4 of RFC 4120 [RFC4120].

   o  The table in Section 5.2.6 of RFC 4120 [RFC4120] is updated to map
      the ad-type 97 to "DER encoding of AD-AUTHENTICATION-INDICATOR".

5.  Security Considerations

   Elements of type AD-AUTHENTICATION-INDICATOR are wrapped in AD-CAMMAC
   containers.  AD-CAMMAC supersedes AD-KDC-ISSUED and allows both
   application services and the KDC to verify the authenticity of the
   contained authorization data.

   KDC implementations MUST use AD-CAMMAC verifiers as described in the
   security considerations of RFC 7751 [RFC7751] to ensure that
   AD-AUTHENTICATION-INDICATOR elements are not modified by an attacker.
   Application servers MUST validate the AD-CAMMAC container before
   making authorization decisions based on AD-AUTHENTICATION-INDICATOR
   elements.  Application servers MUST NOT make authorization decisions
   based on AD-AUTHENTICATION-INDICATOR elements that appear outside of
   AD-CAMMAC containers.

   Using multiple strings in AD-AUTHENTICATION-INDICATOR may lead to
   ambiguity when a service tries to make a decision based on the
   AD-AUTHENTICATION-INDICATOR values.  This ambiguity can be avoided if
   indicator values are always used as a positive indication of certain
   requirements being met during the initial authentication.  For
   example, if a "without-password" indicator is inserted whenever
   authentication occurs without a password, a service might assume this
   is an indication that a higher-strength client authentication
   occurred.  However, this indicator might also be inserted when no
   authentication occurred at all (such as anonymous PKINIT).

   Application service evaluation of site-defined indicators MUST
   consider the realm of original authentication in order to avoid
   cross-realm indicator collisions.  Failure to enforce this property
   can result in invalid authorization decisions.

6.  IANA Considerations

   This document does not require any IANA actions.

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>.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              DOI 10.17487/RFC4120, July 2005,
              <http://www.rfc-editor.org/info/rfc4120>.

   [RFC6113]  Hartman, S. and L. Zhu, "A Generalized Framework for
              Kerberos Pre-Authentication", RFC 6113,
              DOI 10.17487/RFC6113, April 2011,
              <http://www.rfc-editor.org/info/rfc6113>.

   [RFC7751]  Sorce, S. and T. Yu, "Kerberos Authorization Data
              Container Authenticated by Multiple Message Authentication
              Codes (MACs)", RFC 7751, DOI 10.17487/RFC7751, March 2016,
              <http://www.rfc-editor.org/info/rfc7751>.

   [X.680]    ITU-T, "Information technology -- Abstract Syntax Notation
              One (ASN.1): Specification of basic notation", ITU-T
              Recommendation X.680, ISO/IEC International Standard
              8824-1:2008, November 2008.

   [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 International
              Standard 8825-1:2008, November 2008.

7.2.  Informative References

   [RFC6711]  Johansson, L., "An IANA Registry for Level of Assurance
              (LoA) Profiles", RFC 6711, DOI 10.17487/RFC6711, August
              2012, <http://www.rfc-editor.org/info/rfc6711>.

Appendix A.  ASN.1 Module

   KerberosV5AuthenticationIndicators {
           iso(1) identified-organization(3) dod(6) internet(1)
           security(5) kerberosV5(2) modules(4)
           authentication-indicators(9)
   } DEFINITIONS EXPLICIT TAGS ::= BEGIN

   AD-AUTHENTICATION-INDICATOR ::= SEQUENCE OF UTF8String

   END

Acknowledgements

   Dmitri Pal (Red Hat)
   Simo Sorce (Red Hat)
   Greg Hudson (MIT)

Authors' Addresses

   Anupam Jain
   Georgia Tech
   225 North Ave NW
   Atlanta, GA  30332
   United States of America

   Email: ajain323@gatech.edu


   Nathan Kinder
   Red Hat, Inc.
   444 Castro St.
   Suite 500
   Mountain View, CA  94041
   United States of America

   Email: nkinder@redhat.com


   Nathaniel McCallum
   Red Hat, Inc.
   100 East Davie Street
   Raleigh, NC  27601
   United States of America

   Email: npmccallum@redhat.com