Internet DRAFT - draft-ietf-krb-wg-cammac
draft-ietf-krb-wg-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: April 6, 2015 MIT Kerberos Consortium
October 3, 2014
Kerberos Authorization Data Container Authenticated by Multiple MACs
draft-ietf-krb-wg-cammac-11
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. This document updates RFC
4120.
Status of This Memo
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This Internet-Draft will expire on April 6, 2015.
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Copyright (c) 2014 IETF Trust and the persons identified as the
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 2
3. Motivations . . . . . . . . . . . . . . . . . . . . . . . . . 2
4. Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Assigned numbers . . . . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . 6
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
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
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(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 a service ticket
that the KDC previously issued, as evidence that the service is
authorized to impersonate the client principal named in that ticket.
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
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 has the same
functional and security properties as the ad-checksum element of AD-
KDC-ISSUED; the svc-verifier allows the service to verify the
integrity of the AD-CAMMAC 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 for the
issued ticket; this operation 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-
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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.
KerberosV5CAMMAC DEFINITIONS EXPLICIT TAGS ::= BEGIN
AD-CAMMAC ::= SEQUENCE {
elements [0] AuthorizationData,
kdc-verifier [1] Verifier-MAC OPTIONAL,
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:
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Contains an RFC 3961 [RFC3961] Checksum (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. The key usage for computing the MAC (Checksum) is 64.
kdc-verifier:
A Verifier-MAC where the key is a long-term key 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
key. The kvno and enctype SHOULD be specified to disambiguate
which of the long-term keys of the trusted service is used.
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5. Usage
Application servers and KDCs MAY ignore the AD-CAMMAC container and
the authorization data elements it contains. For compatibility with
older Kerberos implementations, a KDC issuing an AD-CAMMAC SHOULD
enclose it in an AD-IF-RELEVANT container unless the KDC knows that
the application server is likely to recognize it.
6. Assigned numbers
The ad-type number for AD-CAMMAC is 96.
The key usage number for the Verifier-MAC checksum is 64.
7. IANA Considerations
[ RFC Editor: please remove this section prior to publication. ]
There are no IANA considerations in this document. Any numbers
assigned in this document are not in IANA-controlled number spaces.
8. 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, AD-SIGNEDPATH,
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attempts to bind to other authorization data in a ticket, and it is
very difficult for two such authorization data types to coexist.
To minimize ticket size when embedding CAMMACs in Kerberos tickets, a
KDC MAY omit the kdc-verifier from the CAMMAC when it is not needed.
In this situation, the KDC cannot always determine whether the CAMMAC
contents are intact. The KDC MUST NOT create a new CAMMAC from an
existing one unless the existing CAMMAC has a valid kdc-verifier,
with two exceptions.
Only KDCs for the local realm have knowledge of the local TGS key, so
the outer encryption of a local TGT is sufficient to protect the
CAMMAC of a local TGT from tampering, assuming that all of the KDCs
in the local realm consistently filter out CAMMAC authorization data
submitted by clients. The KDC MAY create a new CAMMAC from an
existing CAMMAC lacking a kdc-verifier if that CAMMAC is contained
within a local TGT and all of the local realm KDCs are configured to
filter out CAMMAC authorization data submitted by clients.
An application service might not use the S4U2Proxy extension, or the
realm policy might disallow the use of S4U2Proxy by that service. In
such situations where there is no flow of authorization data from the
service to the KDC, the application service could modify the CAMMAC
contents, but such modifications would have no effect on other
services. Because of the lack of security impact, the KDC MAY create
a new CAMMAC from an existing CAMMAC lacking a kdc-verifier if it is
inserting the new CAMMAC into a service ticket for the same service
principal as the ticket that contained the existing CAMMAC, but MUST
NOT place a kdc-verifier in the new CAMMAC.
The kdc-verifier in CAMMAC does not bind the service principal name
to the CAMMAC contents, because the service principal name is not
part of the EncTicketPart. An entity that has access to the keys of
two different service principals can decrypt a ticket for one service
and encrypt it in the key of the other service, altering the svc-
verifier to match. Both the kdc-verifier and the svc-verifier would
still validate, but the KDC never issued this fabricated ticket. The
impact of this manipulation is minor if the CAMMAC contents only
communicate attributes related to the client. If an application
requires an authenticated binding between the service principal name
and the CAMMAC or ticket contents, the KDC MUST include in the CAMMAC
some authorization data element that names the service principal.
9. Acknowledgements
Shawn Emery, Sam Hartman, Greg Hudson, Ben Kaduk, Zhanna Tsitkov, and
Kai Zheng provided helpful technical and editorial feedback on
earlier versions of this document.
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10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, 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
[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>.
Authors' Addresses
Simo Sorce (editor)
Red Hat
Email: ssorce@redhat.com
Tom Yu (editor)
MIT Kerberos Consortium
Email: tlyu@mit.edu
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Thomas Hardjono (editor)
MIT Kerberos Consortium
Email: hardjono@mit.edu
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