draft-ietf-krb-wg-cammac-06.txt   draft-ietf-krb-wg-cammac-07.txt 
Internet Engineering Task Force S. Sorce, Ed. Internet Engineering Task Force S. Sorce, Ed.
Internet-Draft Red Hat Internet-Draft Red Hat
Updates: 4120 (if approved) T. Yu, Ed. Updates: 4120 (if approved) T. Yu, Ed.
Intended status: Standards Track T. Hardjono, Ed. Intended status: Standards Track T. Hardjono, Ed.
Expires: April 24, 2014 MIT Kerberos Consortium Expires: November 8, 2014 MIT Kerberos Consortium
October 21, 2013 May 7, 2014
Kerberos Authorization Data Container Authenticated by Multiple MACs Kerberos Authorization Data Container Authenticated by Multiple MACs
draft-ietf-krb-wg-cammac-06 draft-ietf-krb-wg-cammac-07
Abstract Abstract
Abstract: This document specifies a Kerberos Authorization Data Abstract: This document specifies a Kerberos Authorization Data
container that supersedes AD-KDC-ISSUED. It allows for multiple container that supersedes AD-KDC-ISSUED. It allows for multiple
Message Authentication Codes (MACs) or signatures to authenticate the Message Authentication Codes (MACs) or signatures to authenticate the
contained Authorization Data elements. contained Authorization Data elements.
Status of this Memo Status of this Memo
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 24, 2014. This Internet-Draft will expire on November 8, 2014.
Copyright Notice Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
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contained authorization data. contained authorization data.
2. Requirements Language 2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
3. Motivations 3. Motivations
The new AD-CAMMAC authorization data container specified in this The Kerberos protocol allows clients to submit arbitrary
document is an improvement upon AD-KDC-ISSUED because it provides authorization data for a KDC to insert into a Kerberos ticket. These
assurance to the KDC that the service named in the ticket did not client-requested authorization data allow the client to express
tamper with the contained authorization data. By adding MACs authorization restrictions that the application service will
verifiable by the KDC and trusted services, AD-CAMMAC enables several interpret. With few exceptions, the KDC can safely copy these
new use cases for the Kerberos protocol that AD-KDC-ISSUED does not client-requested authorization data to the issued ticket without
accommodate. necessarily inspecting, interpreting, or filtering their contents.
The existing AD-KDC-ISSUED authorization data container allows a The AD-KDC-ISSUED authorization data container specified in RFC 4120
service to verify that the KDC has issued the contained authorization [RFC4120] is a means for KDCs to include positive or permissive
data. However, because the symmetric key for the MAC is known to (rather than restrictive) authorization data in service tickets in a
both the KDC and the service, the KDC cannot generally detect whether way that the service named in a ticket can verify that the KDC has
the service has forged the contents of an AD-KDC-ISSUED container in issued the contained authorization data. This capability takes
an existing ticket. The new kdc-verifier MAC in the AD-CAMMAC advantage of a shared symmetric key between the KDC and the service
container, because it uses a key known only to the KDC, allows the to assure the service that the KDC did not merely copy client-
KDC to verify the integrity of the contents of that container. requested authorization data to the ticket without inspecting them.
For example, the new AD-CAMMAC container can protect authorization The AD-KDC-ISSUED container works well for situations where the flow
data when using the Constrained Delegation (S4U2Proxy [MS-SFU]) of authorization data is from the KDC to the service. However,
protocol extension. This extension allows a service to use a ticket protocol extensions such as Constrained Delegation (S4U2Proxy
to itself as evidence that it received a user request and [MS-SFU]) require that a service present to the KDC an "evidence"
consequently ask the KDC to issue a new ticket on behalf of the user service ticket that the service received from a client. In the
to perform operations against another service. 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.
If the KDC had issued a AD-KDC-ISSUED container in the S4U2Proxy The new AD-CAMMAC authorization data container specified in this
evidence ticket instead of AD-CAMMAC, it would have no way to document improves upon AD-KDC-ISSUED by including additional verifier
subsequently verify whether the service had tampered with the elements. The svc-verifier element of the CAMMAC is equivalent to
contents of that container. The service would know the key for the the ad-checksum element of AD-KDC-ISSUED and allows the service to
MAC for the AD-KDC-ISSUED container in the evidence ticket, and could verify the integrity of the contents as it already could with the AD-
therefore forge its contents. KDC-ISSUED container. The kdc-verifier and other-verifiers elements
are new to AD-CAMMAC and provide its enhanced capabilities.
The kdc-verifier MAC in the AD-CAMMAC container allows a KDC to The kdc-verifier element of the AD-CAMMAC container allows a KDC to
verify the integrity of the contained authorization data without verify the integrity of authorization data that it previously
having to compute all of the authorization data, an operation that inserted into a ticket, by using a key that only the KDC knows. The
might not always be possible when the data contains ephemeral KDC thus avoids recomputing all of the authorization data, an
information such as the strength or type of authentication method operation that might not always be possible when that data includes
used to obtain the original ticket. 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 The verifiers in the other-verifiers element of the AD-CAMMAC
from a client, and might then ask a higher-privileged service container are not required, but can be useful when a lesser-
("trusted service") on the same host to act on behalf of the client. privileged service receives a ticket from a client and needs to
To demonstrate that the client has authenticated to it, the lesser- extract the CAMMAC to demonstrate to a higher-privileged "trusted
privileged service can extract the AD-CAMMAC container from the service" on the same host that it is legitimately acting on behalf of
ticket and submit it to the trusted service. The trusted service can that client. The trusted service can use a verifier in the other-
either ask a specialized service (not yet specified) on the KDC to verifiers element to validate the contents of the CAMMAC without
validate the AD-CAMMAC container, or use verify the optional further communication with the KDC.
additional verifiers (the other-verifiers field) that are part of the
AD-CAMMAC.
4. Encoding 4. Encoding
The Kerberos protocol is defined in [RFC4120] using Abstract Syntax The Kerberos protocol is defined in [RFC4120] using Abstract Syntax
Notation One (ASN.1) [X.680][X.690]. For consistency, this Notation One (ASN.1) [X.680] and using the ASN.1 Distinguished
specification also uses the ASN.1 syntax for specifying the layout of Encoding Rules (DER) [X.690]. For consistency, this specification
AD-CAMMAC. The ad-data of the AD-CAMMAC authorization data element also uses ASN.1 for specifying the layout of AD-CAMMAC. The ad-data
is the ASN.1 DER encoding of the AD-CAMMAC ASN.1 type specified of the AD-CAMMAC authorization data element is the ASN.1 DER encoding
below. of the AD-CAMMAC ASN.1 type specified below.
4.1. AD-CAMMAC 4.1. AD-CAMMAC
KerberosV5CAMMAC DEFINITIONS EXPLICIT TAGS ::= BEGIN KerberosV5CAMMAC DEFINITIONS EXPLICIT TAGS ::= BEGIN
AD-CAMMAC ::= SEQUENCE { AD-CAMMAC ::= SEQUENCE {
elements [0] AuthorizationData, elements [0] AuthorizationData,
kdc-verifier [1] Verifier-MAC, kdc-verifier [1] Verifier-MAC,
svc-verifier [2] Verifier-MAC OPTIONAL, svc-verifier [2] Verifier-MAC OPTIONAL,
other-verifiers [3] SEQUENCE OF Verifier other-verifiers [3] SEQUENCE (SIZE (1..MAX))
OF Verifier OPTIONAL
} }
Verifier ::= CHOICE { Verifier ::= CHOICE {
mac Verifier-MAC, mac Verifier-MAC,
... ...
} }
Verifier-MAC ::= SEQUENCE { Verifier-MAC ::= SEQUENCE {
identifier [0] PrincipalName OPTIONAL, identifier [0] PrincipalName OPTIONAL,
kvno [1] UInt32, kvno [1] UInt32 OPTIONAL,
enctype [2] Int32, enctype [2] Int32 OPTIONAL,
mac [3] Checksum mac [3] Checksum
} }
AD-CAMMAC-BINDING ::= OCTET STRING
END END
elements: elements:
A sequence of authorization data elements issued by the KDC. A sequence of authorization data elements issued by the KDC.
These elements are the authorization data that the verifier fields These elements are the authorization data that the verifier fields
authenticate. authenticate.
Verifier: Verifier:
A CHOICE type that currently contains only one alternative: A CHOICE type that currently contains only one alternative:
Verifier-MAC. Future extensions might add support for public-key Verifier-MAC. Future extensions might add support for public-key
signatures. signatures.
Verifier-MAC: Verifier-MAC:
Contains a MAC computed over the encoding of the AuthorizationData Contains a MAC computed over the ASN.1 DER encoding of the
value in the elements field of the AD-CAMMAC. The identifier, AuthorizationData value in the elements field of the AD-CAMMAC.
kvno, and enctype fields help the recipient locate the key The identifier, kvno, and enctype fields help the recipient locate
required for verifying the MAC. the key required for verifying the MAC. For the kdc-verifier and
the svc-verifier, the identifier, kvno and enctype fields are
AD-CAMMAC-BINDING: often obvious from context and MAY be omitted. For the kdc-
An optional AuthorizationData element that binds the CAMMAC verifier, the MAC is computed differently than for the svc-
contents to the enclosing ticket. This AuthorizationData element verifier and the other-verifiers, as described later.
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: kdc-verifier:
A Verifier-MAC where the key is the TGS key. The checksum type is A Verifier-MAC where the key is that of the local Ticket-Granting
the mandatory checksum type for the TGS key. 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: svc-verifier:
A Verifier-MAC where the key is the long-term key of the service A Verifier-MAC where the key is the same long-term service key
for which the ticket is issued. The checksum type is the that the KDC uses to encrypt the surrounding ticket. The checksum
mandatory checksum type for the long-term key of the service. type is the required checksum type for the enctype of the service
This field MUST be present if the service principal of the ticket key used to encrypt the ticket. This field MUST be present if the
is not the local TGS, including when the ticket is a cross-realm service principal of the ticket is not the local TGS, including
TGT. when the ticket is a cross-realm TGT.
other-verifiers: other-verifiers:
A sequence of additional verifiers. In each additional Verifier- A sequence of additional verifiers. In each additional Verifier-
MAC, the key is the long-term key of the principal name specified MAC, the key is a long-term key of the principal name specified in
in the identifier field. The PrincipalName MUST be present and be the identifier field. The PrincipalName MUST be present and be a
a valid principal in the realm. KDCs MAY add one or more 'trusted valid principal in the realm. KDCs MAY add one or more "trusted
service' verifiers. Unless otherwise administratively configured, service" verifiers. Unless otherwise administratively configured,
the 'trusted service' SHOULD be found by replacing the service the KDC SHOULD determine the "trusted service" principal name by
identifier component of the principal name of the svc-verifier replacing the service identifier component of the sname of the
with 'host'. The checksum type is the mandatory checksum type for surrounding ticket with "host". The checksum is computed using a
the long-term key (which one?) of the principal. The key usage is long-term key of the identified principal, and the checksum type
TBD. 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 5. Assigned numbers
TBD TBD
6. IANA Considerations 6. IANA Considerations
TBD. TBD.
7. Security Considerations 7. Security Considerations
Although authorization data are generally conveyed within the Although authorization data are generally conveyed within the
encrypted part of a ticket and are thereby protected by the existing encrypted part of a ticket and are thereby protected by the existing
encryption methods on the ticket, some authorization data requires encryption scheme used for the surrounding ticket, some authorization
the additional protection provided by the CAMMAC. data requires the additional protection provided by the CAMMAC.
Some protocol extensions such as S4U2Proxy allow the KDC to issue a Some protocol extensions such as S4U2Proxy allow the KDC to issue a
new ticket based on an evidence ticket provided by the service. If new ticket based on an evidence ticket provided by the service. If
the evidence ticket contains authorization data that needs to be the evidence ticket contains authorization data that needs to be
preserved in the new ticket, then the KDC MUST revalidate it. preserved in the new ticket, then the KDC MUST revalidate it.
Extracting a CAMMAC from a ticket for use as a credential removes 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 from the context of the ticket. In the general case, this could turn
it into a bearer token, with all of the associated security it into a bearer token, with all of the associated security
implications. Also, the CAMMAC does not itself necessarily contain implications. Also, the CAMMAC does not itself necessarily contain
sufficient information to identify the client principal. Therefore, sufficient information to identify the client principal. Therefore,
application protocols that rely on extracted CAMMACs might need to application protocols that rely on extracted CAMMACs might need to
duplicate a substantial portion of the ticket contents and include duplicate a substantial portion of the ticket contents and include
that duplicated information in the authorization data contained that duplicated information in the authorization data contained
within the CAMMAC. within the CAMMAC. The extent of this duplication would depend on
the security properties required by the application protocol.
A KDC that needs to verify the contents of a CAMMAC in a non-TGS The method for computing the kdc-verifier does not bind it to any
ticket MUST ensure that the CAMMAC in the ticket is the same one that authorization data within the ticket but outside of the CAMMAC. At
it inserted into the ticket. A malicious service could substitute least one (non-standard) authorization data type attempts to bind to
legitimate CAMMACs from other tickets that it has received (but not other authorization data in a ticket, and it is very difficult to
fabricate completely new CAMMACs) into a service ticket. A CAMMAC by have two such authorization data types coexist.
itself does not contain sufficient information to accomplish this,
but including an AD-CAMMAC-BINDING element could be sufficient.
8. Open Issues 8. Open Issues
Consider making other-verifiers "[3] SEQUENCE (SIZE (1..MAX)) OF Allow an optional KDC-verified element, kdc-verifier-unbound, that is
VERIFIER OPTIONAL" to make the common case encoding smaller. not bound to the ticket contents? This would allow a KDC to provide
the service of verifying an extracted CAMMAC without needing a copy
Enclose in AD-IF-RELEVANT? of the ticket ciphertext.
9. Acknowledgements 9. Acknowledgements
TBD. Ben Kaduk and Zhanna Tsitkov provided helpful technical and editorial
feedback on earlier versions of this document.
10. References 10. References
10.1. Normative References 10.1. Normative References
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005. Kerberos 5", RFC 3961, February 2005.
[RFC3962] Raeburn, K., "Advanced Encryption Standard (AES) [RFC3962] Raeburn, K., "Advanced Encryption Standard (AES)
Encryption for Kerberos 5", RFC 3962, February 2005. Encryption for Kerberos 5", RFC 3962, February 2005.
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