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Internet Engineering Task Force (IETF)                          R. YountRequest for Comments: 6448                    Carnegie Mellon UniversityCategory: Standards Track                                  November 2011ISSN: 2070-1721The Unencrypted Form of Kerberos 5 KRB-CRED MessageAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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 20111.  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 inSection 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 inSection 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 inSection 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 encryptionYount                        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 University7.  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 20118.  References8.1.  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.eduYount                        Standards Track                    [Page 4]