draft-ietf-krb-wg-cammac-05.txt   draft-ietf-krb-wg-cammac-06.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: January 15, 2014 MIT Kerberos Consortium Expires: April 24, 2014 MIT Kerberos Consortium
July 14, 2013 October 21, 2013
Kerberos Authorization Data Container Authenticated by Multiple MACs Kerberos Authorization Data Container Authenticated by Multiple MACs
draft-ietf-krb-wg-cammac-05 draft-ietf-krb-wg-cammac-06
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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Copyright Notice Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . . 3 2. Requirements Language . . . . . . . . . . . . . . . . . . . . . 3
3. Validation . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Motivations . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. AD-CAMMAC . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.1. AD-CAMMAC . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Assigned numbers . . . . . . . . . . . . . . . . . . . . . . . 6 5. Assigned numbers . . . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 8. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . . 7 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . . 8 10.1. Normative References . . . . . . . . . . . . . . . . . . . 8
Appendix A. Additional Stuff . . . . . . . . . . . . . . . . . . . 8 10.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8 Appendix A. Additional Stuff . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction 1. Introduction
This document specifies a new Authorization Data container for This document specifies a new Authorization Data container for
Kerberos, called AD-CAMMAC (Container Authenticated by Multiple Kerberos, called AD-CAMMAC (Container Authenticated by Multiple
MACs), that supersedes AD-KDC-ISSUED. The container allows both the MACs), that supersedes AD-KDC-ISSUED. This new container allows both
receiving application service and the Key Distribution Center (KDC) the receiving application service and the Key Distribution Center
itself to verify the authenticity of the contained authorization (KDC) itself to verify the authenticity of the contained
data. The AD-CAMMAC container can also include additional verifiers authorization data. The AD-CAMMAC container can also include
that "trusted services" can use to verify the contained authorization additional verifiers that "trusted services" can use to verify the
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. Validation 3. Motivations
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 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.
o The Service MAC The existing AD-KDC-ISSUED authorization data container allows a
o Optional Trusted Service MAC 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.
The KDC MAC is required to allow the KDC to validate the data without For example, the new AD-CAMMAC container can protect authorization
needing to recompute the contents at every Ticket Granting Service data when using the Constrained Delegation (S4U2Proxy [MS-SFU])
(TGS) request. 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.
The Service MAC is required so that the Service can verify that the If the KDC had issued a AD-KDC-ISSUED container in the S4U2Proxy
authorization data has been validated by the KDC. evidence ticket instead of AD-CAMMAC, it would have no way to
subsequently verify whether the service had tampered with the
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 Trusted Service MAC is useful to verify the authenticity of the The kdc-verifier MAC in the AD-CAMMAC container allows a KDC to
contents on the same host, when the data is received by a less verify the integrity of the contained authorization data without
trusted service and passed to a more trusted service on the same host having to compute all of the authorization data, an operation that
without the need for additional round trips to the KDC. 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.
The ad-type for AD-CAMMAC is (TBD). 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 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]. As such, this specification Notation One (ASN.1) [X.680][X.690]. For consistency, this
also uses the ASN.1 syntax for specifying both the abstract layout of specification also uses the ASN.1 syntax for specifying the layout of
the AD-CAMMAC attributes, as well as its encoding. 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.
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 OF Verifier
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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 methods on the ticket, some authorization data requires
the additional protection provided by the CAMMAC. 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 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.
A KDC that needs to verify the contents of a CAMMAC in a non-TGS 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 ticket MUST ensure that the CAMMAC in the ticket is the same one that
one that it inserted into the ticket. A malicious service could it inserted into the ticket. A malicious service could substitute
substitute legitimate CAMMACs from other tickets that it has received legitimate CAMMACs from other tickets that it has received (but not
(but not fabricate completely new CAMMACs) into a service ticket. A fabricate completely new CAMMACs) into a service ticket. A CAMMAC by
CAMMAC by itself does not contain sufficient information to itself does not contain sufficient information to accomplish this,
accomplish this. but including an AD-CAMMAC-BINDING element could be sufficient.
8. Acknowledgements 8. Open Issues
TBD. Consider making other-verifiers "[3] SEQUENCE (SIZE (1..MAX)) OF
VERIFIER OPTIONAL" to make the common case encoding smaller.
9. References Enclose in AD-IF-RELEVANT?
9.1. Normative References 9. Acknowledgements
TBD.
10. 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.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120, Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005. July 2005.
skipping to change at page 8, line 12 skipping to change at page 8, line 27
One (ASN.1): Specification of basic notation -- ITU-T One (ASN.1): Specification of basic notation -- ITU-T
Recommendation X.680 (ISO/IEC International Standard 8824- Recommendation X.680 (ISO/IEC International Standard 8824-
1:2008)", 2008. 1:2008)", 2008.
[X.690] ISO, "Information technology -- ASN.1 encoding rules: [X.690] ISO, "Information technology -- ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules Encoding Rules (CER) and Distinguished Encoding Rules
(DER) -- ITU-T Recommendation X.690 (ISO/IEC International (DER) -- ITU-T Recommendation X.690 (ISO/IEC International
Standard 8825-1:2008)", 1997. Standard 8825-1:2008)", 1997.
9.2. Informative References 10.2. Informative References
[MIT-Athena] [MIT-Athena]
Steiner, J., Neuman, B., and J. Schiller, "Kerberos: An Steiner, J., Neuman, B., and J. Schiller, "Kerberos: An
Authentication Service for Open Network Systems. In Authentication Service for Open Network Systems. In
Proceedings of the Winter 1988 Usenix Conference. Proceedings of the Winter 1988 Usenix Conference.
February.", 1988. February.", 1988.
[MS-SFU] Microsoft, "[MS-SFU]: Kerberos Protocol Extensions: [MS-SFU] Microsoft, "[MS-SFU]: Kerberos Protocol Extensions:
Service for User and Constrained Delegation Protocol", Service for User and Constrained Delegation Protocol",
January 2013, January 2013,
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