The Unencrypted Form of Kerberos 5 KRB-CRED Message
draft-ietf-krb-wg-clear-text-cred-03

Network Working Group                                           R. Yount
Internet-Draft                                Carnegie Mellon University
Intended status: Standards Track                      September 23, 2011
Expires: March 26, 2012

          The Unencrypted Form Of Kerberos 5 KRB-CRED Message
                  draft-ietf-krb-wg-clear-text-cred-03

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.

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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 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

   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].  This
   means that no [RFC3961] 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

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   types as integer values.  These layers are free to use a 0 in an
   encryption type integer as a flag or sentinal 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 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, integrity, and mutual authentication.
   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

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   point to this document.

8.  References

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.

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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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