Internet Engineering Task Force (IETF) A. Jain

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
(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.

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RFC 8129 Authentication Indicator March 2017

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.

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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 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.

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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. 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.

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[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>.

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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: [email protected]

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

Email: [email protected]

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

Email: [email protected]

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