Internet Engineering Task Force (IETF) R. Yount
Request for Comments: 6448 Carnegie Mellon University
Category: Standards Track November 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.
Status of This Memo
This is an Internet Standards Track document.
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(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.
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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 in Section 2.7 of
the Security Assertion Markup Language (SAML) V2.0 Kerberos Attribute
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-