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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]