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: January 15, 2014 MIT Kerberos Consortium
July 14, 2013
Kerberos Authorization Data Container Authenticated by Multiple MACs
draft-ietf-krb-wg-cammac-05
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
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This Internet-Draft will expire on January 15, 2014.
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Assigned numbers . . . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . . 8
Appendix A. Additional Stuff . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
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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
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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.
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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
}
AD-CAMMAC-BINDING ::= OCTET STRING
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.
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AD-CAMMAC-BINDING:
An optional AuthorizationData element that binds the CAMMAC
contents to the enclosing ticket. This AuthorizationData element
has ad-type number TBD, and if it appears in the AD-CAMMAC, it
MUST be the first member of the elements field of the AD-CAMMAC.
The contents of the AD-CAMMAC-BINDING element are a local matter
for the KDC implementation. A KDC can use this element to
checksum portions of the ticket outside of the CAMMAC, to ensure
that a service has not tampered with them. This can be useful if
the KDC implements a capability resembling the Windows Constrained
Delegation (S4U2Proxy) [MS-SFU] extension.
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
6. IANA Considerations
TBD.
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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 necessarily 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,
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-
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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.
[MS-SFU] Microsoft, "[MS-SFU]: Kerberos Protocol Extensions:
Service for User and Constrained Delegation Protocol",
January 2013,
<http://msdn.microsoft.com/en-us/library/cc246071.aspx>.
[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
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Tom Yu (editor)
MIT Kerberos Consortium
Email: tlyu@mit.edu
Thomas Hardjono (editor)
MIT Kerberos Consortium
Email: hardjono@mit.edu
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