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draft-ietf-kitten-cammac
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: November 8, 2014 MIT Kerberos Consortium
May 7, 2014
Kerberos Authorization Data Container Authenticated by Multiple MACs
draft-ietf-krb-wg-cammac-07
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 November 8, 2014.
Copyright Notice
Copyright (c) 2014 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
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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
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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 Kerberos protocol allows clients to submit arbitrary
authorization data for a KDC to insert into a Kerberos ticket. These
client-requested authorization data allow the client to express
authorization restrictions that the application service will
interpret. With few exceptions, the KDC can safely copy these
client-requested authorization data to the issued ticket without
necessarily inspecting, interpreting, or filtering their contents.
The AD-KDC-ISSUED authorization data container specified in RFC 4120
[RFC4120] is a means for KDCs to include positive or permissive
(rather than restrictive) authorization data in service tickets in a
way that the service named in a ticket can verify that the KDC has
issued the contained authorization data. This capability takes
advantage of a shared symmetric key between the KDC and the service
to assure the service that the KDC did not merely copy client-
requested authorization data to the ticket without inspecting them.
The AD-KDC-ISSUED container works well for situations where the flow
of authorization data is from the KDC to the service. However,
protocol extensions such as Constrained Delegation (S4U2Proxy
[MS-SFU]) require that a service present to the KDC an "evidence"
service ticket that the service received from a client. In the
S4U2Proxy extension, the KDC uses the evidence ticket as the basis
for issuing a derivative ticket that the service can then use to
impersonate the client. The authorization data contained within the
evidence ticket constitute a flow of authorization data from the
application service to the KDC. The properties of the AD-KDC-ISSUED
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container are insufficient for this use case because the service
knows the symmetric key for the checksum in the AD-KDC-ISSUED
container. Therefore, the KDC has no way to detect whether the
service has tampered with the contents of the AD-KDC-ISSUED container
within the evidence ticket.
The new AD-CAMMAC authorization data container specified in this
document improves upon AD-KDC-ISSUED by including additional verifier
elements. The svc-verifier element of the CAMMAC is equivalent to
the ad-checksum element of AD-KDC-ISSUED and allows the service to
verify the integrity of the contents as it already could with the AD-
KDC-ISSUED container. The kdc-verifier and other-verifiers elements
are new to AD-CAMMAC and provide its enhanced capabilities.
The kdc-verifier element of the AD-CAMMAC container allows a KDC to
verify the integrity of authorization data that it previously
inserted into a ticket, by using a key that only the KDC knows. The
KDC thus avoids recomputing all of the authorization data, an
operation that might not always be possible when that data includes
ephemeral information such as the strength or type of authentication
method used to obtain the original ticket.
The verifiers in the other-verifiers element of the AD-CAMMAC
container are not required, but can be useful when a lesser-
privileged service receives a ticket from a client and needs to
extract the CAMMAC to demonstrate to a higher-privileged "trusted
service" on the same host that it is legitimately acting on behalf of
that client. The trusted service can use a verifier in the other-
verifiers element to validate the contents of the CAMMAC without
further communication with the KDC.
4. Encoding
The Kerberos protocol is defined in [RFC4120] using Abstract Syntax
Notation One (ASN.1) [X.680] and using the ASN.1 Distinguished
Encoding Rules (DER) [X.690]. For consistency, this specification
also uses ASN.1 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 (SIZE (1..MAX))
OF Verifier OPTIONAL
}
Verifier ::= CHOICE {
mac Verifier-MAC,
...
}
Verifier-MAC ::= SEQUENCE {
identifier [0] PrincipalName OPTIONAL,
kvno [1] UInt32 OPTIONAL,
enctype [2] Int32 OPTIONAL,
mac [3] Checksum
}
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 ASN.1 DER 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. For the kdc-verifier and
the svc-verifier, the identifier, kvno and enctype fields are
often obvious from context and MAY be omitted. For the kdc-
verifier, the MAC is computed differently than for the svc-
verifier and the other-verifiers, as described later.
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kdc-verifier:
A Verifier-MAC where the key is that of the local Ticket-Granting
Service (TGS). The checksum type is the required checksum type
for the enctype of the TGS key. In contrast to the other
Verifier-MAC elements, the KDC computes the MAC in the kdc-
verifier over the ASN.1 DER encoding of the EncTicketPart of the
surrounding ticket, but where the AuthorizationData value in the
EncTicketPart contains the AuthorizationData value contained in
the CAMMAC instead of the AuthorizationData value that would
otherwise be present in the ticket. This altered Verifier-MAC
computation binds the kdc-verifier to the other contents of the
ticket, assuring the KDC that a malicious service has not
substituted a mismatched CAMMAC received from another ticket.
svc-verifier:
A Verifier-MAC where the key is the same long-term service key
that the KDC uses to encrypt the surrounding ticket. The checksum
type is the required checksum type for the enctype of the service
key used to encrypt the ticket. 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 a 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 KDC SHOULD determine the "trusted service" principal name by
replacing the service identifier component of the sname of the
surrounding ticket with "host". The checksum is computed using a
long-term key of the identified principal, and the checksum type
is the required checksum type for the enctype of that long-term
the key. The kvno and enctype SHOULD be specified to disambiguate
which of the long-term keys of the trusted service is used. 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 scheme used for the surrounding 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. The extent of this duplication would depend on
the security properties required by the application protocol.
The method for computing the kdc-verifier does not bind it to any
authorization data within the ticket but outside of the CAMMAC. At
least one (non-standard) authorization data type attempts to bind to
other authorization data in a ticket, and it is very difficult to
have two such authorization data types coexist.
8. Open Issues
Allow an optional KDC-verified element, kdc-verifier-unbound, that is
not bound to the ticket contents? This would allow a KDC to provide
the service of verifying an extracted CAMMAC without needing a copy
of the ticket ciphertext.
9. Acknowledgements
Ben Kaduk and Zhanna Tsitkov provided helpful technical and editorial
feedback on earlier versions of this document.
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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