The Unencrypted Form Of Kerberos 5 KRB-CRED Message
draft-ietf-krb-wg-clear-text-cred-01

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

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This Internet-Draft will expire on January 27, 2012.

Copyright Notice

Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved.

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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.

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]. Layers above the encryption layer are left to interpret its use in their own context specific manner. The use of encryption type 0 in the unencrypted form of the KRB-CRED is not to specify an encryption type. 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 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.

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 point to this document.

8. References

8.1. Normative References

8.2. Informative References

Author's Address

Table of Contents

• 1. Introduction
• 2. Requirements notation
• 3. The Unencrypted Form Of The KRB-CRED
• 4. Kerberos Encryption Type 0 Is Not An Encryption System
• 5. Security Considerations
• 6. Acknowledgements
• 7. IANA Considerations
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Author's Address