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.

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   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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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

   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.

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