draft-ietf-krb-wg-cammac-08.txt   draft-ietf-krb-wg-cammac-09.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 1, 2015 MIT Kerberos Consortium Expires: March 9, 2015 MIT Kerberos Consortium
June 30, 2014 September 5, 2014
Kerberos Authorization Data Container Authenticated by Multiple MACs Kerberos Authorization Data Container Authenticated by Multiple MACs
draft-ietf-krb-wg-cammac-08 draft-ietf-krb-wg-cammac-09
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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provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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Copyright Notice Copyright Notice
Copyright (c) 2014 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.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . . 3 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 2
3. Motivations . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Motivations . . . . . . . . . . . . . . . . . . . . . . . . . 2
4. Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. AD-CAMMAC . . . . . . . . . . . . . . . . . . . . . . . . . 5 5. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Assigned numbers . . . . . . . . . . . . . . . . . . . . . . . 6 6. Assigned numbers . . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7 8. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . . 7 10.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . . 8 10.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
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. This new container allows both MACs), that supersedes AD-KDC-ISSUED. This new container allows both
the receiving application service and the Key Distribution Center the receiving application service and the Key Distribution Center
(KDC) itself to verify the authenticity of the contained (KDC) itself to verify the authenticity of the contained
authorization data. The AD-CAMMAC container can also include authorization data. The AD-CAMMAC container can also include
additional verifiers that "trusted services" can use to verify the additional verifiers that "trusted services" can use to verify the
skipping to change at page 3, line 45 skipping to change at page 3, line 15
way that the service named in a ticket can verify that the KDC has way that the service named in a ticket can verify that the KDC has
issued the contained authorization data. This capability takes issued the contained authorization data. This capability takes
advantage of a shared symmetric key between the KDC and the service advantage of a shared symmetric key between the KDC and the service
to assure the service that the KDC did not merely copy client- to assure the service that the KDC did not merely copy client-
requested authorization data to the ticket without inspecting them. requested authorization data to the ticket without inspecting them.
The AD-KDC-ISSUED container works well for situations where the flow The AD-KDC-ISSUED container works well for situations where the flow
of authorization data is from the KDC to the service. However, of authorization data is from the KDC to the service. However,
protocol extensions such as Constrained Delegation (S4U2Proxy protocol extensions such as Constrained Delegation (S4U2Proxy
[MS-SFU]) require that a service present to the KDC a service ticket [MS-SFU]) require that a service present to the KDC a service ticket
that the service received from a client, as evidence that the client that the KDC previously issued, as evidence that the service is
authenticated to the service. In the S4U2Proxy extension, the KDC authorized to impersonate the client principal named in that ticket.
uses the evidence ticket as the basis for issuing a derivative ticket In the S4U2Proxy extension, the KDC uses the evidence ticket as the
that the service can then use to impersonate the client. The basis for issuing a derivative ticket that the service can then use
authorization data contained within the evidence ticket constitute a to impersonate the client. The authorization data contained within
flow of authorization data from the application service to the KDC. the evidence ticket constitute a flow of authorization data from the
application service to the KDC. The properties of the AD-KDC-ISSUED
The properties of the AD-KDC-ISSUED container are insufficient for container are insufficient for this use case because the service
this use case because the service knows the symmetric key for the knows the symmetric key for the checksum in the AD-KDC-ISSUED
checksum in the AD-KDC-ISSUED container. Therefore, the KDC has no container. Therefore, the KDC has no way to detect whether the
way to detect whether the service has tampered with the contents of service has tampered with the contents of the AD-KDC-ISSUED container
the AD-KDC-ISSUED container within the evidence ticket. within the evidence ticket.
The new AD-CAMMAC authorization data container specified in this The new AD-CAMMAC authorization data container specified in this
document improves upon AD-KDC-ISSUED by including additional verifier document improves upon AD-KDC-ISSUED by including additional verifier
elements. The svc-verifier element of the CAMMAC is equivalent to elements. The svc-verifier element of the CAMMAC has the same
the ad-checksum element of AD-KDC-ISSUED and allows the service to functional and security properties as the ad-checksum element of AD-
verify the integrity of the contents as it already could with the 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 KDC-ISSUED container. The kdc-verifier and other-verifiers elements
are new to AD-CAMMAC and provide its enhanced capabilities. are new to AD-CAMMAC and provide its enhanced capabilities.
The kdc-verifier element of 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 authorization data that it previously verify the integrity of authorization data that it previously
inserted into a ticket, by using a key that only the KDC knows. The inserted into a ticket, by using a key that only the KDC knows. The
KDC thus avoids recomputing all of the authorization data, an KDC thus avoids recomputing all of the authorization data for the
operation that might not always be possible when that data includes issued ticket; this operation might not always be possible when that
ephemeral information such as the strength or type of authentication data includes ephemeral information such as the strength or type of
method used to obtain the original ticket. authentication method used to obtain the original ticket.
The verifiers in the other-verifiers element of the AD-CAMMAC The verifiers in the other-verifiers element of the AD-CAMMAC
container are not required, but can be useful when a lesser- container are not required, but can be useful when a lesser-
privileged service receives a ticket from a client and needs to privileged service receives a ticket from a client and needs to
extract the CAMMAC to demonstrate to a higher-privileged "trusted extract the CAMMAC to demonstrate to a higher-privileged "trusted
service" on the same host that it is legitimately acting on behalf of 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- that client. The trusted service can use a verifier in the other-
verifiers element to validate the contents of the CAMMAC without verifiers element to validate the contents of the CAMMAC without
further communication with the KDC. further communication with the KDC.
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] and using the ASN.1 Distinguished Notation One (ASN.1) [X.680] and using the ASN.1 Distinguished
Encoding Rules (DER) [X.690]. For consistency, this specification Encoding Rules (DER) [X.690]. For consistency, this specification
also uses ASN.1 for specifying the layout of AD-CAMMAC. The ad-data 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 authorization data element is the ASN.1 DER encoding
of the AD-CAMMAC ASN.1 type specified below. of the AD-CAMMAC ASN.1 type specified below.
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 OPTIONAL,
svc-verifier [2] Verifier-MAC OPTIONAL, svc-verifier [2] Verifier-MAC OPTIONAL,
other-verifiers [3] SEQUENCE (SIZE (1..MAX)) other-verifiers [3] SEQUENCE (SIZE (1..MAX))
OF Verifier OPTIONAL OF Verifier OPTIONAL
} }
Verifier ::= CHOICE { Verifier ::= CHOICE {
mac Verifier-MAC, mac Verifier-MAC,
... ...
} }
skipping to change at page 6, line 6 skipping to change at page 5, line 16
AuthorizationData value in the elements field of the AD-CAMMAC. AuthorizationData value in the elements field of the AD-CAMMAC.
The identifier, kvno, and enctype fields help the recipient locate The identifier, kvno, and enctype fields help the recipient locate
the key required for verifying the MAC. For the kdc-verifier and the key required for verifying the MAC. For the kdc-verifier and
the svc-verifier, the identifier, kvno and enctype fields are the svc-verifier, the identifier, kvno and enctype fields are
often obvious from context and MAY be omitted. For the kdc- often obvious from context and MAY be omitted. For the kdc-
verifier, the MAC is computed differently than for the svc- verifier, the MAC is computed differently than for the svc-
verifier and the other-verifiers, as described later. The key verifier and the other-verifiers, as described later. The key
usage for computing the MAC (Checksum) is 64. usage for computing the MAC (Checksum) is 64.
kdc-verifier: kdc-verifier:
A Verifier-MAC where the key is that of the local Ticket-Granting A Verifier-MAC where the key is a long-term key of the local
Service (TGS). The checksum type is the required checksum type Ticket-Granting Service (TGS). The checksum type is the required
for the enctype of the TGS key. In contrast to the other checksum type for the enctype of the TGS key. In contrast to the
Verifier-MAC elements, the KDC computes the MAC in the kdc- other Verifier-MAC elements, the KDC computes the MAC in the kdc-
verifier over the ASN.1 DER encoding of the EncTicketPart of the verifier over the ASN.1 DER encoding of the EncTicketPart of the
surrounding ticket, but where the AuthorizationData value in the surrounding ticket, but where the AuthorizationData value in the
EncTicketPart contains the AuthorizationData value contained in EncTicketPart contains the AuthorizationData value contained in
the CAMMAC instead of the AuthorizationData value that would the CAMMAC instead of the AuthorizationData value that would
otherwise be present in the ticket. This altered Verifier-MAC otherwise be present in the ticket. This altered Verifier-MAC
computation binds the kdc-verifier to the other contents of the computation binds the kdc-verifier to the other contents of the
ticket, assuring the KDC that a malicious service has not ticket, assuring the KDC that a malicious service has not
substituted a mismatched CAMMAC received from another ticket. substituted a mismatched CAMMAC received from another ticket.
svc-verifier: svc-verifier:
skipping to change at page 6, line 41 skipping to change at page 6, line 5
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 KDC SHOULD determine the "trusted service" principal name by the KDC SHOULD determine the "trusted service" principal name by
replacing the service identifier component of the sname of the replacing the service identifier component of the sname of the
surrounding ticket with "host". The checksum is computed using a surrounding ticket with "host". The checksum is computed using a
long-term key of the identified principal, and the checksum type long-term key of the identified principal, and the checksum type
is the required checksum type for the enctype of that long-term is the required checksum type for the enctype of that long-term
key. The kvno and enctype SHOULD be specified to disambiguate key. The kvno and enctype SHOULD be specified to disambiguate
which of the long-term keys of the trusted service is used. which of the long-term keys of the trusted service is used.
5. Assigned numbers 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 ad-type number for AD-CAMMAC is 96.
The key usage number for the Verifier-MAC checksum is 64. The key usage number for the Verifier-MAC checksum is 64.
6. IANA Considerations 7. IANA Considerations
[ RFC Editor: please remove this section prior to publication. ] [ RFC Editor: please remove this section prior to publication. ]
There are no IANA considerations in this document. Any numbers There are no IANA considerations in this document. Any numbers
assigned in this document are not in IANA-controlled number spaces. assigned in this document are not in IANA-controlled number spaces.
7. Security Considerations 8. 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 scheme used for the surrounding ticket, some authorization encryption scheme used for the surrounding ticket, some authorization
data requires 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.
skipping to change at page 7, line 32 skipping to change at page 6, line 51
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. The extent of this duplication would depend on within the CAMMAC. The extent of this duplication would depend on
the security properties required by the application protocol. the security properties required by the application protocol.
The method for computing the kdc-verifier does not bind it to any The method for computing the kdc-verifier does not bind it to any
authorization data within the ticket but outside of the CAMMAC. At authorization data within the ticket but outside of the CAMMAC. At
least one (non-standard) authorization data type attempts to bind to least one (non-standard) authorization data type, AD-SIGNEDPATH,
other authorization data in a ticket, and it is very difficult to attempts to bind to other authorization data in a ticket, and it is
have two such authorization data types coexist. very difficult for two such authorization data types to coexist.
8. Acknowledgements 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 CAMAMC
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.
Shawn Emery, Ben Kaduk, and Zhanna Tsitkov provided helpful technical Only KDCs for the local realm have knowledge of the local TGS key, so
and editorial feedback on earlier versions of this document. 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.
9. References An application service might be use the S4U2Proxy extension, or the
realm policy might disallow the use of S4U2Proxy by that service. In
this situation, the application service could modify the CAMMAC
contents, but such modifications would have no effect on other
services. 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, and if all of the realm's KDCs are
configured to reject S4U2Proxy requests made by that service
principal.
9.1. Normative References 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.
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 [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)
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.
[X.680] ISO, "Information technology -- Abstract Syntax Notation [X.680] ISO, , "Information technology -- Abstract Syntax Notation
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
1:2008)", 2008. 8824-1:2008)", 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,
<http://msdn.microsoft.com/en-us/library/cc246071.aspx>. <http://msdn.microsoft.com/en-us/library/cc246071.aspx>.
[RFC1510] Kohl, J. and B. Neuman, "The Kerberos Network [RFC1510] Kohl, J. and B. Neuman, "The Kerberos Network
Authentication Service (V5)", RFC 1510, September 1993. 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.
Authors' Addresses Authors' Addresses
Simo Sorce (editor) Simo Sorce (editor)
Red Hat Red Hat
Email: ssorce@redhat.com Email: ssorce@redhat.com
Tom Yu (editor) Tom Yu (editor)
MIT Kerberos Consortium MIT Kerberos Consortium
Email: tlyu@mit.edu Email: tlyu@mit.edu
Thomas Hardjono (editor) Thomas Hardjono (editor)
MIT Kerberos Consortium MIT Kerberos Consortium
Email: hardjono@mit.edu Email: hardjono@mit.edu
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