Internet Engineering Task Force S. Sorce, Ed.
Internet-Draft Red Hat
Updates: 4120 (if approved) T. Yu, Ed.
Intended status: Standards Track T. Hardjono, Ed.
Expires: April 4, 2013 MIT Kerberos Consortium
Oct 2012
Kerberos Authorization Data Container Authenticated by Multiple MACs
draft-ietf-krb-wg-cammac-03
Abstract
Abstract: This document specifies a Kerberos Authorization Data
container that supersedes AD-KDC-ISSUED. It allows for multiple
Message Authentication Codes (MACs) or signatures to authenticate the
contained Authorization Data elements.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 4, 2013.
Copyright Notice
Copyright (c) 2012 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
Sorce, et al. Expires April 4, 2013 [Page 1]
Internet-Draft Container Authenticated by Multiple MACs Oct 2012
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . . 3
3. Validation . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. AD-CAMMAC . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Assigned numbers . . . . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . . 7
Appendix A. Additional Stuff . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
Sorce, et al. Expires April 4, 2013 [Page 2]
Internet-Draft Container Authenticated by Multiple MACs Oct 2012
1. Introduction
This document specifies a new Authorization Data container for
Kerberos, called AD-CAMMAC (Container Authenticated by Multiple
MACs), that supersedes AD-KDC-ISSUED. The container allows both the
receiving application service and the Key Distribution Center (KDC)
itself to verify the authenticity of the contained authorization
data. The AD-CAMMAC container can also include additional verifiers
that "trusted services" can use to verify the contained authorization
data.
2. Requirements Language
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 RFC 2119 [RFC2119].
3. Validation
Kerberos ticket authorization data are highly sensitive and must be
validated to insure that no tampering has occurred. Although
authorization data are in the encrypted part of a Kerberos ticket and
therefore have their integrity protected by the ticket encryption,
clients can request that KDCs insert potentially arbitrary
authorization data into tickets on their behalf. The Kerberos
protocol specifications allow this client behavior because the
originally envisioned usage of authorization data was to serve as
restrictions on the client's privileges. Services that need to
interpret specific authorization data as granting increased
privileges need some way to ensure that the KDC originated those
authorization data.
In order to validate any information, the receiving application
service needs to be able to cryptographically verify the data. This
is done by introducing a new AuthorizationData element called AD-
CAMMAC that contains enough information to bind the contents to a
principal in a way that a receiving application service can verify
autonomously without further contact with the KDC.
The following information is needed:
o The KDC MAC
o The Service MAC
Sorce, et al. Expires April 4, 2013 [Page 3]
Internet-Draft Container Authenticated by Multiple MACs Oct 2012
o Optional Trusted Service MAC
The KDC MAC is required to allow the KDC to validate the data without
needing to recompute the contents at every Ticket Granting Service
(TGS) request.
The Service MAC is required so that the Service can verify that the
authorization data has been validated by the KDC.
The Trusted Service MAC is useful to verify the authenticity of the
contents on the same host, when the data is received by a less
trusted service and passed to a more trusted service on the same host
without the need for additional round trips to the KDC.
The ad-type for AD-CAMMAC is (TBD).
4. Encoding
The Kerberos protocol is defined in [RFC4120] using Abstract Syntax
Notation One (ASN.1) [X.680][X.690]. As such, this specification
also uses the ASN.1 syntax for specifying both the abstract layout of
the AD-CAMMAC attributes, as well as its encoding.
4.1. AD-CAMMAC
KerberosV5CAMMAC DEFINITIONS EXPLICIT TAGS ::= BEGIN
AD-CAMMAC ::= SEQUENCE {
elements [0] AuthorizationData,
kdc-verifier [1] Verifier-MAC,
svc-verifier [2] Verifier-MAC OPTIONAL,
other-verifiers [3] SEQUENCE OF Verifier
}
Verifier ::= CHOICE {
mac Verifier-MAC,
...
}
Verifier-MAC ::= SEQUENCE {
identifier [0] PrincipalName OPTIONAL,
kvno [1] UInt32,
enctype [2] Int32,
mac [3] Checksum
}
Sorce, et al. Expires April 4, 2013 [Page 4]
Internet-Draft Container Authenticated by Multiple MACs Oct 2012
END
elements
A sequence of authorization data elements issued by the
KDC. These elements are the authorization data that the verifier
fields authenticate.
Verifier
A CHOICE type that currently contains only one alternative:
Verifier-MAC. Future extensions might add support for public-key
signatures.
Verifier-MAC
Contains a MAC computed over the encoding of the
AuthorizationData value in the elements field of the
AD-CAMMAC. The identifier, kvno, and enctype fields help the
recipient locate the key required for verifying the MAC.
kdc-verifier
A Verifier-MAC where the key is the TGS key. The checksum type
is the mandatory checksum type for the TGS key.
svc-verifier
A Verifier-MAC where the key is the long-term key of the service
for which the ticket is issued. The checksum type is the
mandatory checksum type for the long-term key of the
service. This field MUST be present if the service principal of
the ticket is not the local TGS, including when the ticket is a
cross-realm TGT.
other-verifiers
A sequence of additional verifiers. In each additional
Verifier-MAC, the key is the long-term key of the principal name
specified in the identifier field. The PrincipalName MUST be
present and be a valid principal in the realm. KDCs MAY add one
or more 'trusted service' verifiers. Unless otherwise
administratively configured, the 'trusted service' SHOULD be
found by replacing the service identifier component of the
principal name of the svc-verifier with 'host'. The checksum
type is the mandatory checksum type for the long-term key (which
one?) of the principal. The key usage is TBD.
5. Assigned numbers
TBD
Sorce, et al. Expires April 4, 2013 [Page 5]
Internet-Draft Container Authenticated by Multiple MACs Oct 2012
6. IANA Considerations
TBD.
7. Security Considerations
Although authorization data are generally conveyed within the
encrypted part of a ticket and are thereby protected by the existing
encryption methods on the ticket, some authorization data requires
the additional protection provided by the CAMMAC.
Extracting a CAMMAC from a ticket for use as a credential removes it
from the context of the ticket. In the general case, this could turn
it into a bearer token, with all of the associated security
implications. Also, the CAMMAC does not itself contain sufficient
information to identify the client principal. Therefore, application
protocols that rely on extracted CAMMACs might need to duplicate a
substantial portion of the ticket contents and include that
duplicated information in the authorization data contained within the
CAMMAC.
A KDC that needs to verify the contents of a CAMMAC in a non-TGS
service ticket MUST ensure that the CAMMAC in the ticket is the same
one that it inserted into the ticket. A malicious service could
substitute legitimate CAMMACs from other tickets that it has received
(but not fabricate completely new CAMMACs) into a service ticket. A
CAMMAC by itself does not contain sufficient information to
accomplish this.
8. Acknowledgements
TBD.
9. References
9.1. Normative References
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005.
[RFC3962] Raeburn, K., "Advanced Encryption Standard (AES)
Encryption for Kerberos 5", RFC 3962, February 2005.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
Sorce, et al. Expires April 4, 2013 [Page 6]
Internet-Draft Container Authenticated by Multiple MACs Oct 2012
July 2005.
[X.680] ISO, "Information technology -- Abstract Syntax Notation
One (ASN.1): Specification of basic notation -- ITU-T
Recommendation X.680 (ISO/IEC International Standard 8824-
1:2008)", 2008.
[X.690] ISO, "Information technology -- ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER) -- ITU-T Recommendation X.690 (ISO/IEC International
Standard 8825-1:2008)", 1997.
9.2. Informative References
[MIT-Athena]
Steiner, J., Neuman, B., and J. Schiller, "Kerberos: An
Authentication Service for Open Network Systems. In
Proceedings of the Winter 1988 Usenix Conference.
February.", 1988.
[RFC1510] Kohl, J. and B. Neuman, "The Kerberos Network
Authentication Service (V5)", RFC 1510, September 1993.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
July 2003.
Appendix A. Additional Stuff
This becomes an Appendix.
Authors' Addresses
Simo Sorce (editor)
Red Hat
Email: ssorce@redhat.com
Sorce, et al. Expires April 4, 2013 [Page 7]
Internet-Draft Container Authenticated by Multiple MACs Oct 2012
Tom Yu (editor)
MIT Kerberos Consortium
Email: tlyu@mit.edu
Thomas Hardjono (editor)
MIT Kerberos Consortium
Email: hardjono@mit.edu
Sorce, et al. Expires April 4, 2013 [Page 8]