draft-ietf-krb-wg-clear-text-cred-03.txt   rfc6448.txt 
Network Working Group R. Yount Internet Engineering Task Force (IETF) R. Yount
Internet-Draft Carnegie Mellon University Request for Comments: 6448 Carnegie Mellon University
Intended status: Standards Track September 23, 2011 Category: Standards Track November 2011
Expires: March 26, 2012 ISSN: 2070-1721
The Unencrypted Form Of Kerberos 5 KRB-CRED Message The Unencrypted Form of Kerberos 5 KRB-CRED Message
draft-ietf-krb-wg-clear-text-cred-03
Abstract Abstract
The Kerberos 5 KRB-CRED message is used to transfer Kerberos The Kerberos 5 KRB-CRED message is used to transfer Kerberos
credentials between applications. When used with a secure transport credentials between applications. When used with a secure transport,
the unencrypted form of the KRB-CRED message may be desirable. This the unencrypted form of the KRB-CRED message may be desirable. This
document describes the unencrypted form of the KRB-CRED message. document describes the unencrypted form of the KRB-CRED message.
Status of this Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering This is an Internet Standards Track document.
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
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time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
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Internet Standards is available in Section 2 of RFC 5741.
This Internet-Draft will expire on March 26, 2012. 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 Notice
Copyright (c) 2011 IETF Trust and the persons identified as the Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
1. Introduction 1. Introduction
There are applications which need to transfer Kerberos credentials There are applications that need to transfer Kerberos credentials
between them without having a prior relationship with established between them without having a prior relationship with established
Kerberos keys. When transferred over a transport that provides Kerberos keys. When transferred over a transport that provides
confidentiality and integrity, the unencrypted form of the KRB-CRED confidentiality and integrity, the unencrypted form of the KRB-CRED
message MAY be used. One application employing this method is the message MAY be used. One application employing this method is the
Kerberos attribute transport mechanism described in section 2.8 of Kerberos attribute transport mechanism, described in Section 2.7 of
the SAML V2.0 Kerberos Attribute Profile the Security Assertion Markup Language (SAML) V2.0 Kerberos Attribute
[sstc-saml-attribute-kerberos]. Profile [SAMLv2-KRB-ATTRIB].
In the SAML application, the Identity Provider (IdP) somehow obtains In the SAML application, the Identity Provider (IdP) somehow obtains
a Kerberos service ticket from the Kerberos Key Distribution Center a Kerberos service ticket from the Kerberos Key Distribution Center
(KDC) when required by the SAML system and transfers the credential (KDC) when required by the SAML system and transfers the credential
to a Service Provider (SP) within an attribute statement. The SP can to a Service Provider (SP) within an attribute statement. The SP can
then use the credential to access a Kerberos protected service. then use the credential to access a Kerberos protected service.
The Kerberos 5 specification as described in [RFC4120] mentions the The Kerberos 5 specification as described in [RFC4120] mentions the
non-standard legacy use of unencrypted KRB-CRED with Generic Security non-standard legacy use of unencrypted KRB-CRED messages with the
Services Application Programming Interface (GSS-API) [RFC1964] by the Generic Security Service Application Program Interface (GSS-API)
MIT, Heimdal, and Microsoft Kerberos implementations. This document [RFC1964] by the MIT, Heimdal, and Microsoft Kerberos
provides a formal specification of the unencrypted form of the KRB- implementations. This document provides a formal specification of
CRED message to enable its continued use in new applications. the unencrypted form of the KRB-CRED message to enable its continued
use in new applications.
2. Requirements notation 2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. The Unencrypted Form Of The KRB-CRED 3. The Unencrypted Form of the KRB-CRED Message
The unencrypted form of the KRB-CRED contains EncryptedData as The unencrypted form of the KRB-CRED message contains EncryptedData
defined in Section 5.2.9 [RFC4120]. The encryption type (etype) MUST as defined in Section 5.2.9 of [RFC4120]. The encryption type
be specified as 0. The optional key version number (kvno) SHOULD NOT (etype) MUST be specified as 0. The optional key version number
be present and MUST be ignored by the recipient if present. The (kvno) SHOULD NOT be present and MUST be ignored by the recipient if
cipher text (cipher) is a copy of the EncKrbCredPart as defined in present. The ciphertext (cipher) is a copy of the EncKrbCredPart,
Section 5.8.1 [RFC4120] which is in clear text. which is in cleartext, as defined in Section 5.8.1 of [RFC4120].
4. Kerberos Encryption Type 0 Is Not An Encryption System 4. Kerberos Encryption Type 0 Is Not an Encryption System
The Kerberos Encryption Type 0 is an invalid value [RFC3961]. This The Kerberos Encryption Type 0 is an invalid value [RFC3961]. This
means that no [RFC3961] encryption type with value 0 will ever be means that no encryption type with value 0 will ever be defined; no
defined; no encryption or key management operations will use this encryption or key management operations will use this value. Layers
value. Layers above the encryption layer often transport encryption above the encryption layer often transport encryption types as
types as integer values. These layers are free to use a 0 in an integer values. These layers are free to use a 0 in an encryption
encryption type integer as a flag or sentinal value or for other type integer as a flag or sentinel value, or for other context-
context-specific purposes. For example, section 3 of this specific purposes. For example, Section 3 of this specification
specification defines the semantics of a 0 carried in the KRB-CRED defines the semantics of a 0 carried in the KRB-CRED message's
message's encryption type field. In the context of the KRB-CRED it encryption type field. In the context of the KRB-CRED message, it is
is a message specific indicator to be interpreted as the message is a message-specific indicator to be interpreted as the message is not
not encrypted. This approach was chosen due to existing Kerberos encrypted. This approach was chosen due to existing Kerberos
implementations which conform to this specification. implementations that conform to this specification.
5. Security Considerations 5. Security Considerations
The KRB-CRED message contains sensitive information related to The KRB-CRED message contains sensitive information related to
Kerberos credentials being transferred, such as their secret session Kerberos credentials being transferred, such as their secret session
keys, client and server principal names, and validity period. keys, client and server principal names, and validity period.
Possession of this information, along with the ticket itself, would Possession of this information, along with the ticket itself, would
allow an attacker to impersonate the client named in the ticket. The allow an attacker to impersonate the client named in the ticket. The
possibility of modification of the KRB-CRED enables the attacker to possibility of modification of the KRB-CRED message enables the
substitute the credentials. This can result in the recipient using attacker to substitute the credentials. This can result in the
the credentials of a client which was not intended. As a result, the recipient using the credentials of a client that was not intended.
KRB-CRED message must be carefully safeguarded. 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 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 used with a transport where sender and recipient identities can be
established to be known to each other. The transport MUST also established to be known to each other. The transport MUST also
provide confidentiality, integrity, and mutual authentication. provide confidentiality, integrity, and mutual authentication.
Examples of transports which MAY be securely used to transport an Examples of transports that MAY be securely used to transport an
unencrypted KRB-CRED message would include Transport Layer Security unencrypted KRB-CRED message would include Transport Layer Security
(TLS) [RFC5246] where mutual authentication has been established and (TLS) [RFC5246], where mutual authentication has been established, or
those encoded within encrypted and signed SAML Security Assertion the use of messages where the KRB-CRED is encoded within an encrypted
Markup Language (SAML) 2.0 [OASIS.saml-core-2.0-os] statement. and signed SAML 2.0 [OASIS-SAMLv2] statement.
6. Acknowledgements 6. Acknowledgements
The following individuals have contributed to the development of this The following individuals have contributed to the development of this
specification. specification.
Thomas Hardjono, Massachusetts Institute of Technology Thomas Hardjono, Massachusetts Institute of Technology
Josh Howlett, Individual Josh Howlett, Individual
Jeffrey Hutzelman, Carnegie Mellon University Jeffrey Hutzelman, Carnegie Mellon University
7. IANA Considerations 7. IANA Considerations
The reference for Kerberos encryption type 0 should be updated to The reference for Kerberos Encryption Type 0 has been updated to
point to this document. point to this document.
8. References 8. References
8.1. Normative References 8.1. Normative References
[OASIS.saml-core-2.0-os] [OASIS-SAMLv2]
Cantor, S., Kemp, J., Philpott, R., and E. Maler, Cantor, S., Ed., Kemp, J., Ed., Philpott, R., Ed., and E.
"Assertions and Protocol for the OASIS Security Assertion Maler, Ed., "Assertions and Protocol for the OASIS
Markup Language (SAML) V2.0", OASIS Standard saml-core- Security Assertion Markup Language (SAML) V2.0", OASIS
2.0-os, March 2005. Standard saml-core-2.0-os, March 2005.
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", [RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
RFC 1964, June 1996. RFC 1964, June 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120, Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005. July 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008. (TLS) Protocol Version 1.2", RFC 5246, August 2008.
8.2. Informative References 8.2. Informative References
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005. Kerberos 5", RFC 3961, February 2005.
[sstc-saml-attribute-kerberos] [SAMLv2-KRB-ATTRIB]
Howlett, J. and T. Hardjono, "SAML V2.0 Kerberos Attribute Howlett, J., Ed., and T. Hardjono, Ed., "SAML V2.0
Profile Version 1.0, OASIS Security Services Draft, sstc- Kerberos Attribute Profile Version 1.0",
saml-attribute-kerberos.odt (work in progress)", sstc-saml-attribute-kerberos.odt, August 2011.
December 2010.
Author's Address Author's Address
Russell J. Yount Russell J. Yount
Carnegie Mellon University Carnegie Mellon University
5000 Forbes Avenue 5000 Forbes Avenue
Pittsburgh, Pennsylvania 15213 Pittsburgh, Pennsylvania 15213
US US
Phone: +1 412 268 8391 Phone: +1 412 268 8391
Email: rjy@cmu.edu EMail: rjy@cmu.edu
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