Internet Engineering Task Force A. Jain
Internet-Draft Georgia Tech
Updates: 4120 (if approved) N. Kinder
Intended status: Standards Track N. McCallum
Expires: August 13, 2017 Red Hat, Inc.
February 9, 2017

Authentication Indicator in Kerberos Tickets


This document updates RFC 4120 in order to specify 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

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Table of Contents

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:


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 which references a Level of Assurance Profile [RFC6711]

* 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:

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 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 which 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).

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.

6. IANA Considerations

This document has no actions for IANA.

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.
[RFC4120] Neuman, C., Yu, T., Hartman, S. and K. Raeburn, "The Kerberos Network Authentication Service (V5)", RFC 4120, DOI 10.17487/RFC4120, July 2005.
[RFC6113] Hartman, S. and L. Zhu, "A Generalized Framework for Kerberos Pre-Authentication", RFC 6113, DOI 10.17487/RFC6113, April 2011.
[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.
[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)", 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)", 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.

Appendix A. ASN.1 Module

KerberosV5AuthenticationIndicators {
        iso(1) identified-organization(3) dod(6) internet(1)
        security(5) kerberosV5(2) modules(4)



Appendix B. 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 USA EMail:
Nathan Kinder Red Hat, Inc. 444 Castro St. Suite 500 Mountain View, CA 94041 USA EMail:
Nathaniel McCallum Red Hat, Inc. 100 East Davie Street Raleigh, NC 27601 USA EMail: