Network Working Group                                           R. Yount
Internet-Draft                                Carnegie Mellon University
Intended status: Standards Track                           June 28, 2011
Expires: December 30, 2011

          The Unencrypted Form Of Kerberos 5 KRB-CRED Message


   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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   This Internet-Draft will expire on December 30, 2011.

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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 tranferred 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
   Security Assertion Markup Language (SAML) 2.0
   [OASIS.saml-core-2.0-os] attribute transport.

   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 unencypted 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",
   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.  The cipher text (cipher) is a copy of the EncKrbCredPart
   as defined in Section 5.8.1 [RFC4120] which is in clear text.

4.  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 substitution

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   of a credential by the attacker which can result in the recipients
   use 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 be known to each other and provides 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.

5.  Acknowledgements

   The following individuals have contributed to the development of this

   Thomas HardJono, Massachusetts Institute of Technology

   Josh Howlett, Individual

   Jeffrey Hutzelman, Carnegie Mellon University

6.  IANA Considerations

   This memo includes no request to IANA.

7.  Normative References

              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,

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              July 2005.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

Author's Address

   Russell J. Yount
   Carnegie Mellon University
   5000 Forbes Avenue
   Pittsburgh, Pennsylvania  15213

   Phone: +1 412 268 8391

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