Kerberos Auth Indicator: Authentication Indicator in Kerberos Tickets
A Kerberos ticket field that tells services how strongly the client was authenticated.
Jain, A., Kinder, N., and N. McCallum, "Authentication Indicator in Kerberos Tickets", RFC 8129, DOI 10.17487/RFC8129, March 2017, <https://www.rfc-editor.org/info/rfc8129>.
@misc{rfc8129,
series = {Request for Comments},
number = 8129,
howpublished = {RFC 8129},
publisher = {RFC Editor},
doi = {10.17487/RFC8129},
url = {https://www.rfc-editor.org/info/rfc8129},
author = {A. Jain and N. Kinder and N. McCallum},
title = {{Authentication Indicator in Kerberos Tickets}},
pagetotal = 6,
year = 2017,
month = mar,
} RFC text
Contents
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.
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:
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.
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
AI ELI5
On this page
§1Why Tickets Need an Authentication Indicator
Kerberos (RFC 4120) lets users and services interact securely over a network. Through its pre-authentication framework (RFC 6113) it supports password-based authentication, multi-factor authentication with one-time password devices, public-key cryptography and other schemes.
Those schemes can offer very different strengths of client authentication. An implementation may keep track of which strength was used and feed it into policy decisions. This document specifies a new authorization data type that conveys that information to application services. Its elements appear inside an AD-CAMMAC container (RFC 7751).
There are easy ways and hard ways to prove who you are. This gives the ticket a small label saying which way you used.
§2Where the Indicator Fits in a Ticket
The indicator is produced by the KDC, the party that checks the client at initial authentication, and it is read by application services. It travels as authorization data in a ticket: the KDC MAY include it in initial credentials and MAY copy it from a ticket-granting ticket into service tickets, so a service ticket can report how the client first proved its identity.
The element is wrapped in an AD-CAMMAC container, so the contents can be verified as originating from the KDC. Because KDCs and applications that do not implement the element MAY safely ignore it, adding it does not break existing deployments that never look at it.
The security office stamps the label when you first walk in, and the sticker stays on your badge as you move between rooms.
§3The AD-AUTHENTICATION-INDICATOR Type
The KDC MAY include authorization data of ad-type 97, wrapped in AD-CAMMAC, in initial credentials. It MAY copy the element from a ticket-granting ticket into service tickets. The ad-data field holds the DER encoding of this ASN.1 type:
AD-AUTHENTICATION-INDICATOR ::= SEQUENCE OF UTF8String
Each string is a short value showing that a particular set of requirements was met during the initial authentication. Strings are meant to be compared against known values, not to store structured data. Each one MUST be either:
- a URI that references a Level of Assurance Profile (RFC 6711), or
- a site-defined string whose meaning is set by the realm administrator, which MUST NOT contain a colon.
The elements MUST be included in an AD-CAMMAC container so their contents can be verified as coming from the KDC. Applications and KDCs that do not implement the element MAY safely ignore it.
The label is a short list of words, like a sticker that says "showed ID" or "used a key card". Only the building's own security office is allowed to put the sticker on.
AD-AUTHENTICATION-INDICATOR ::= SEQUENCE OF UTF8String§4Assigned Numbers
This RFC updates RFC 4120 in two ways. It assigns ad-type number 97 to AD-AUTHENTICATION-INDICATOR in the table in Section 7.5.4 of that RFC. It also updates the table in Section 5.2.6 to map ad-type 97 to "DER encoding of AD-AUTHENTICATION-INDICATOR". The IANA Considerations section says the document needs no IANA actions.
The number 97 is now officially reserved for this label in the Kerberos rulebook.
§5Security Considerations
Because the elements sit inside AD-CAMMAC, which supersedes AD-KDC-ISSUED, both application services and the KDC can verify them. KDC implementations MUST use AD-CAMMAC verifiers as described in RFC 7751 so an attacker cannot modify the indicators. Application servers MUST validate the container before making authorization decisions, and MUST NOT base decisions on indicators that appear outside an AD-CAMMAC container.
Multiple strings can make a decision ambiguous. The RFC advises always using indicators as a positive sign that requirements were met. Its example is a "without-password" indicator: a service might read it as stronger authentication, but it could also be inserted when no authentication occurred, such as anonymous PKINIT.
Services that evaluate site-defined indicators MUST consider the realm of original authentication, to avoid cross-realm collisions. Failing to do so can lead to invalid authorization decisions.
A label is only trustworthy if it is sealed by the security office, so guards must check the seal. A guard also has to know which building a sticker came from, because two buildings might use the same words differently.