Internet Engineering Task Force A. Jain
Internet-Draft Georgia Tech
Updates: 4120 (if approved) N. Kinder
Intended status: Standards Track N. McCallum
Expires: August 21, 2015 Red Hat, Inc.
February 17, 2015

Authentication Indicator in Kerberos Tickets


This document proposes an extension in the Kerberos protocol [RFC4120]. 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 the service tickets so that the application services can use it as an input into policy decisions.

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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 or Public Key Cryptography. Implementations that have Pre-Authentication mechanisms offering significantly different strengths of client authentication may choose to keep track of the strength of the authentication used as an input into policy decisions. This document proposes a new Authorization Data Type to be used to convey the authentication strength to the application services. The AD type is wrapped in the AD-CAMMAC [I-D.ietf-kitten-cammac] container and contains information about the type of authentication mechanism used by the Kerberos client to authenticate itself to the KDC.

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 KDC MAY include the following Authorization Data element, wrapped in AD-CAMMAC, in the initial credentials and copy it from a ticket-granting ticket into service tickets:


The corresponding ad-data field contains the DER encoding of the ASN.1 type which is defined as


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. These strings MAY be site-defined strings that do not contain a colon such as the name of the Pre-Authentication mechanism used, or alternatively URIs that reference a Level of Assurance Profile [RFC6711].

The AD-AUTHENTICATION-INDICATOR MUST be included in the AD-CAMMAC container so that its contents can be protected. The AD-AUTHENTICATION-INDICATOR MAY safely be ignored by the applications and KDCs that do not implement this element.

4. Security Considerations

AD-AUTHENTICATION-INDICATOR is wrapped in AD-CAMMAC which supersedes AD-KDC-ISSUED container. AD-CAMMAC allows both the application service and the KDC to verify the authenticity of the contained Authorization Data.

A malicious service can replace AD-CAMMAC in a service ticket with a legitimate AD-CAMMAC present in some other ticket that the service has received. The KDC MUST ensure that the service does not tamper with the contents of AD-CAMMAC or the ticket by including a kdc-verifier in the containing CAMMAC. This binding protects AD-AUTHENTICATION-INDICATOR in case of constrained delegation such as S4U2Proxy [MS-SFU] extension.

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.

5. References

5.1. Normative References

[I-D.ietf-kitten-cammac] Sorce, S., Yu, T. and T. Hardjono, "Kerberos Authorization Data Container Authenticated by Multiple MACs", Internet-Draft draft-ietf-kitten-cammac-01, January 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4120] Neuman, C., Yu, T., Hartman, S. and K. Raeburn, "The Kerberos Network Authentication Service (V5)", RFC 4120, July 2005.
[RFC6113] Hartman, S. and L. Zhu, "A Generalized Framework for Kerberos Pre-Authentication", RFC 6113, April 2011.

5.2. Informative References

[MS-SFU] Microsoft, "Kerberos Protocol Extensions: Service for User and Constrained Delegation Protocol", January 2013.
[RFC6711] Johansson, L., "An IANA Registry for Level of Assurance (LoA) Profiles", RFC 6711, August 2012.

Appendix A. Acknowledgements

Dmitri Pal (Red Hat)
Simo Sorce (Red Hat)

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: