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Internet Engineering Task Force (IETF)                           A. JainRequest for Comments: 8129                                  Georgia TechUpdates:4120                                                  N. KinderCategory: Standards Track                                    N. McCallumISSN: 2070-1721                                            Red Hat, Inc.                                                              March 2017Authentication Indicator in Kerberos TicketsAbstract   This document updatesRFC 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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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.Jain, et al.                 Standards Track                    [Page 1]

RFC 8129                Authentication Indicator              March 2017Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Document Conventions  . . . . . . . . . . . . . . . . . . . .23.  AD Type Specification . . . . . . . . . . . . . . . . . . . .24.  Assigned Numbers  . . . . . . . . . . . . . . . . . . . . . .35.  Security Considerations . . . . . . . . . . . . . . . . . . .36.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .47.  References  . . . . . . . . . . . . . . . . . . . . . . . . .47.1.  Normative References  . . . . . . . . . . . . . . . . . .47.2.  Informative References  . . . . . . . . . . . . . . . . .5Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .6   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .6   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .61.  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 inRFC 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.Jain, et al.                 Standards Track                    [Page 2]

RFC 8129                Authentication Indicator              March 2017   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 NumbersRFC 4120 [RFC4120] is updated in the following way:   o  The ad-type number 97 is assigned for AD-AUTHENTICATION-INDICATOR,      updating the table inSection 7.5.4 of RFC 4120 [RFC4120].   o  The table inSection 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 ofRFC 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.Jain, et al.                 Standards Track                    [Page 3]

RFC 8129                Authentication Indicator              March 2017   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.  References7.1.  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.Jain, et al.                 Standards Track                    [Page 4]

RFC 8129                Authentication Indicator              March 2017   [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>.Jain, et al.                 Standards Track                    [Page 5]

RFC 8129                Authentication Indicator              March 2017Appendix 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   ENDAcknowledgements   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.comJain, et al.                 Standards Track                    [Page 6]