The Unencrypted Form of Kerberos 5 KRB-CRED Message
RFC 6448

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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