6448
PROPOSED STANDARD
The Unencrypted Form of Kerberos 5 KRB-CRED Message
Authors: R. Yount
Date: November 2011
Area: sec
Working Group: krb-wg
Stream: IETF
Abstract
The Kerberos 5 KRB-CRED message is used to transfer Kerberos credentials between applications. When used with a secure transport, the unencrypted form of the KRB-CRED message may be desirable. This document describes the unencrypted form of the KRB-CRED message. [STANDARDS-TRACK]
RFC 6448
PROPOSED STANDARD
Internet Engineering Task Force (IETF) R. Yount
Request for Comments: 6448 Carnegie Mellon University
Category: Standards Track November 2011
ISSN: 2070-1721
<span class="h1">The Unencrypted Form of Kerberos 5 KRB-CRED Message</span>
Abstract
The Kerberos 5 KRB-CRED message is used to transfer Kerberos
credentials between applications. When used with a secure transport,
the unencrypted form of the KRB-CRED message may be desirable. This
document describes the unencrypted form of the KRB-CRED message.
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 <a href="./rfc5741#section-2">Section 2 of RFC 5741</a>.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
<a href="https://www.rfc-editor.org/info/rfc6448">http://www.rfc-editor.org/info/rfc6448</a>.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
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described in the Simplified BSD License.
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<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
There are applications that need to transfer Kerberos credentials
between them without having a prior relationship with established
Kerberos keys. When transferred over a transport that provides
confidentiality and integrity, the unencrypted form of the KRB-CRED
message MAY be used. One application employing this method is the
Kerberos attribute transport mechanism, described in <a href="#section-2.7">Section 2.7</a> of
the Security Assertion Markup Language (SAML) V2.0 Kerberos Attribute
Profile [<a href="#ref-SAMLv2-KRB-ATTRIB">SAMLv2-KRB-ATTRIB</a>].
In the SAML application, the Identity Provider (IdP) somehow obtains
a Kerberos service ticket from the Kerberos Key Distribution Center
(KDC) when required by the SAML system and transfers the credential
to a Service Provider (SP) within an attribute statement. The SP can
then use the credential to access a Kerberos protected service.
The Kerberos 5 specification as described in [<a href="./rfc4120" title=""The Kerberos Network Authentication Service (V5)"">RFC4120</a>] mentions the
non-standard legacy use of unencrypted KRB-CRED messages with the
Generic Security Service Application Program Interface (GSS-API)
[<a href="./rfc1964" title=""The Kerberos Version 5 GSS-API Mechanism"">RFC1964</a>] by the MIT, Heimdal, and Microsoft Kerberos
implementations. This document provides a formal specification of
the unencrypted form of the KRB-CRED message to enable its continued
use in new applications.
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Requirements Notation</span>
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 [<a href="./rfc2119" title=""Key words for use in RFCs to Indicate Requirement Levels"">RFC2119</a>].
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. The Unencrypted Form of the KRB-CRED Message</span>
The unencrypted form of the KRB-CRED message contains EncryptedData
as defined in <a href="./rfc4120#section-5.2.9">Section 5.2.9 of [RFC4120]</a>. The encryption type
(etype) MUST be specified as 0. The optional key version number
(kvno) SHOULD NOT be present and MUST be ignored by the recipient if
present. The ciphertext (cipher) is a copy of the EncKrbCredPart,
which is in cleartext, as defined in <a href="./rfc4120#section-5.8.1">Section 5.8.1 of [RFC4120]</a>.
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. Kerberos Encryption Type 0 Is Not an Encryption System</span>
The Kerberos Encryption Type 0 is an invalid value [<a href="./rfc3961" title=""Encryption and Checksum Specifications for Kerberos 5"">RFC3961</a>]. This
means that no encryption type with value 0 will ever be defined; no
encryption or key management operations will use this value. Layers
above the encryption layer often transport encryption types as
integer values. These layers are free to use a 0 in an encryption
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type integer as a flag or sentinel value, or for other context-
specific purposes. For example, <a href="#section-3">Section 3</a> of this specification
defines the semantics of a 0 carried in the KRB-CRED message's
encryption type field. In the context of the KRB-CRED message, it is
a message-specific indicator to be interpreted as the message is not
encrypted. This approach was chosen due to existing Kerberos
implementations that conform to this specification.
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Security Considerations</span>
The KRB-CRED message contains sensitive information related to
Kerberos credentials being transferred, such as their secret session
keys, client and server principal names, and validity period.
Possession of this information, along with the ticket itself, would
allow an attacker to impersonate the client named in the ticket. The
possibility of modification of the KRB-CRED message enables the
attacker to substitute the credentials. This can result in the
recipient using the credentials of a client that was not intended.
As a result, the KRB-CRED message must be carefully safeguarded.
The use of an unencrypted form of the KRB-CRED message MUST only be
used with a transport where sender and recipient identities can be
established to be known to each other. The transport MUST also
provide confidentiality, integrity, and mutual authentication.
Examples of transports that MAY be securely used to transport an
unencrypted KRB-CRED message would include Transport Layer Security
(TLS) [<a href="./rfc5246" title=""The Transport Layer Security (TLS) Protocol Version 1.2"">RFC5246</a>], where mutual authentication has been established, or
the use of messages where the KRB-CRED is encoded within an encrypted
and signed SAML 2.0 [<a href="#ref-OASIS-SAMLv2">OASIS-SAMLv2</a>] statement.
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Acknowledgements</span>
The following individuals have contributed to the development of this
specification.
Thomas Hardjono, Massachusetts Institute of Technology
Josh Howlett, Individual
Jeffrey Hutzelman, Carnegie Mellon University
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. IANA Considerations</span>
The reference for Kerberos Encryption Type 0 has been updated to
point to this document.
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<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. References</span>
<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>. Normative References</span>
[<a id="ref-OASIS-SAMLv2">OASIS-SAMLv2</a>]
Cantor, S., Ed., Kemp, J., Ed., Philpott, R., Ed., and E.
Maler, Ed., "Assertions and Protocol for the OASIS
Security Assertion Markup Language (SAML) V2.0", OASIS
Standard saml-core-2.0-os, March 2005.
[<a id="ref-RFC1964">RFC1964</a>] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
<a href="./rfc1964">RFC 1964</a>, June 1996.
[<a id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.
[<a id="ref-RFC4120">RFC4120</a>] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", <a href="./rfc4120">RFC 4120</a>,
July 2005.
[<a id="ref-RFC5246">RFC5246</a>] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", <a href="./rfc5246">RFC 5246</a>, August 2008.
<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>. Informative References</span>
[<a id="ref-RFC3961">RFC3961</a>] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", <a href="./rfc3961">RFC 3961</a>, February 2005.
[<a id="ref-SAMLv2-KRB-ATTRIB">SAMLv2-KRB-ATTRIB</a>]
Howlett, J., Ed., and T. Hardjono, Ed., "SAML V2.0
Kerberos Attribute Profile Version 1.0",
sstc-saml-attribute-kerberos.odt, August 2011.
Author's Address
Russell J. Yount
Carnegie Mellon University
5000 Forbes Avenue
Pittsburgh, Pennsylvania 15213
US
Phone: +1 412 268 8391
EMail: [email protected]
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