One document matched: draft-ietf-krb-wg-cammac-06.txt
Differences from draft-ietf-krb-wg-cammac-05.txt
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 24, 2014 MIT Kerberos Consortium
October 21, 2013
Kerberos Authorization Data Container Authenticated by Multiple MACs
draft-ietf-krb-wg-cammac-06
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 24, 2014.
Copyright Notice
Copyright (c) 2013 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
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . . 3
3. Motivations . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. AD-CAMMAC . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Assigned numbers . . . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
8. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . . 8
Appendix A. Additional Stuff . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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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. This new 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. Motivations
The new AD-CAMMAC authorization data container specified in this
document is an improvement upon AD-KDC-ISSUED because it provides
assurance to the KDC that the service named in the ticket did not
tamper with the contained authorization data. By adding MACs
verifiable by the KDC and trusted services, AD-CAMMAC enables several
new use cases for the Kerberos protocol that AD-KDC-ISSUED does not
accommodate.
The existing AD-KDC-ISSUED authorization data container allows a
service to verify that the KDC has issued the contained authorization
data. However, because the symmetric key for the MAC is known to
both the KDC and the service, the KDC cannot generally detect whether
the service has forged the contents of an AD-KDC-ISSUED container in
an existing ticket. The new kdc-verifier MAC in the AD-CAMMAC
container, because it uses a key known only to the KDC, allows the
KDC to verify the integrity of the contents of that container.
For example, the new AD-CAMMAC container can protect authorization
data when using the Constrained Delegation (S4U2Proxy [MS-SFU])
protocol extension. This extension allows a service to use a ticket
to itself as evidence that it received a user request and
consequently ask the KDC to issue a new ticket on behalf of the user
to perform operations against another service.
If the KDC had issued a AD-KDC-ISSUED container in the S4U2Proxy
evidence ticket instead of AD-CAMMAC, it would have no way to
subsequently verify whether the service had tampered with the
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contents of that container. The service would know the key for the
MAC for the AD-KDC-ISSUED container in the evidence ticket, and could
therefore forge its contents.
The kdc-verifier MAC in the AD-CAMMAC container allows a KDC to
verify the integrity of the contained authorization data without
having to compute all of the authorization data, an operation that
might not always be possible when the data contains ephemeral
information such as the strength or type of authentication method
used to obtain the original ticket.
A lesser-privileged service on a host may receive an authentication
from a client, and might then ask a higher-privileged service
("trusted service") on the same host to act on behalf of the client.
To demonstrate that the client has authenticated to it, the lesser-
privileged service can extract the AD-CAMMAC container from the
ticket and submit it to the trusted service. The trusted service can
either ask a specialized service (not yet specified) on the KDC to
validate the AD-CAMMAC container, or use verify the optional
additional verifiers (the other-verifiers field) that are part of the
AD-CAMMAC.
4. Encoding
The Kerberos protocol is defined in [RFC4120] using Abstract Syntax
Notation One (ASN.1) [X.680][X.690]. For consistency, this
specification also uses the ASN.1 syntax for specifying the layout of
AD-CAMMAC. The ad-data of the AD-CAMMAC authorization data element
is the ASN.1 DER encoding of the AD-CAMMAC ASN.1 type specified
below.
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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.
Some protocol extensions such as S4U2Proxy allow the KDC to issue a
new ticket based on an evidence ticket provided by the service. If
the evidence ticket contains authorization data that needs to be
preserved in the new ticket, then the KDC MUST revalidate it.
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
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,
but including an AD-CAMMAC-BINDING element could be sufficient.
8. Open Issues
Consider making other-verifiers "[3] SEQUENCE (SIZE (1..MAX)) OF
VERIFIER OPTIONAL" to make the common case encoding smaller.
Enclose in AD-IF-RELEVANT?
9. Acknowledgements
TBD.
10. References
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10.1. 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-
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.
10.2. 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.
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Appendix A. Additional Stuff
This becomes an Appendix.
Authors' Addresses
Simo Sorce (editor)
Red Hat
Email: ssorce@redhat.com
Tom Yu (editor)
MIT Kerberos Consortium
Email: tlyu@mit.edu
Thomas Hardjono (editor)
MIT Kerberos Consortium
Email: hardjono@mit.edu
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