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

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

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