Network Working Group R. Yount
Internet-Draft Carnegie Mellon University
Intended status: Standards Track July 26, 2011
Expires: January 27, 2012
The Unencrypted Form Of Kerberos 5 KRB-CRED Message
draft-ietf-krb-wg-clear-text-cred-01
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
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This Internet-Draft will expire on January 27, 2012.
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1. Introduction
There are applications which 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 2.8 of
the SAML V2.0 Kerberos Attribute Profile
[sstc-saml-attribute-kerberos].
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 with Generic Security
Services Application Programming 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.
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
The unencrypted form of the KRB-CRED contains EncryptedData as
defined in Section 5.2.9 [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
cipher text (cipher) is a copy of the EncKrbCredPart as defined in
Section 5.8.1 [RFC4120] which is in clear text.
4. Kerberos Encryption Type 0 Is Not An Encryption System
The Kerberos Encryption Type 0 is an invalid value [RFC3961]. Layers
above the encryption layer are left to interpret its use in their own
context specific manner. The use of encryption type 0 in the
unencrypted form of the KRB-CRED is not to specify an encryption
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type. In the context of the KRB-CRED it is a message specific
indicator to be interpreted as the message is not encrypted. This
approach was chosen due to existing Kerberos implementations which
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 enables the attacker to
substitute the credentials. This can result in the recipient using
the credentials of a client which 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 been
established to be known to each other. The transport MUST also
provide confidentiality and integrity. Examples of transports which
MAY be securely used to transport an unencrypted KRB-CRED message
would include Transport Layer Security (TLS) [RFC5246] where mutual
authentication has been established and those encoded within
encrypted and signed SAML Security Assertion Markup Language (SAML)
2.0 [OASIS.saml-core-2.0-os] 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 should be updated to
point to this document.
8. References
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8.1. Normative References
[OASIS.saml-core-2.0-os]
Cantor, S., Kemp, J., Philpott, R., and E. Maler,
"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.
[sstc-saml-attribute-kerberos]
Howlett, J. and T. Hardjono, "SAML V2.0 Kerberos Attribute
Profile Version 1.0, OASIS Security Services Draft, sstc-
saml-attribute-kerberos.odt (work in progress)",
December 2010.
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
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