Internet Engineering Task Force (IETF) R. Yount
Request for Comments: 6448 Carnegie Mellon University
Category: Standards Track November 2011
ISSN: 2070-1721
The Unencrypted Form of Kerberos 5 KRB-CRED Message
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 Section 2 of RFC 5741.
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/rfc6448.
Copyright Notice
Copyright (c) 2011 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.
Yount Standards Track [Page 1]RFC 6448 Kerberos 5 Unencrypted KRB-CRED November 2011
1. Introduction
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 Section 2.7 of
the Security Assertion Markup Language (SAML) V2.0 Kerberos Attribute
Profile [SAMLv2-KRB-ATTRIB].
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 [RFC4120] mentions the
non-standard legacy use of unencrypted KRB-CRED messages with the
Generic Security Service Application Program Interface (GSS-API)
[RFC1964] 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.
2. Requirements Notation
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 [RFC2119].
3. The Unencrypted Form of the KRB-CRED Message
The unencrypted form of the KRB-CRED message contains EncryptedData
as defined in Section 5.2.9 of [RFC4120]. 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 Section 5.8.1 of [RFC4120].
4. Kerberos Encryption Type 0 Is Not an Encryption System
The Kerberos Encryption Type 0 is an invalid value [RFC3961]. 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
Yount Standards Track [Page 2]
RFC 6448 Kerberos 5 Unencrypted KRB-CRED November 2011
type integer as a flag or sentinel value, or for other context-
specific purposes. For example, Section 3 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.
5. Security Considerations
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) [RFC5246], 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 [OASIS-SAMLv2] statement.
6. Acknowledgements
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
7. IANA Considerations
The reference for Kerberos Encryption Type 0 has been updated to
point to this document.
Yount Standards Track [Page 3]RFC 6448 Kerberos 5 Unencrypted KRB-CRED November 2011
8. References
8.1. Normative References
[OASIS-SAMLv2]
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.
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
RFC 1964, June 1996.
[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.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
8.2. Informative References
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005.
[SAMLv2-KRB-ATTRIB]
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: rjy@cmu.edu
Yount Standards Track [Page 4]
