Network Working Group L. Hornquist Astrand
Internet-Draft Apple, Inc
Intended status: Standards Track November 19, 2008
Expires: May 23, 2009
Kerberos ticket extensions
draft-ietf-krb-wg-ticket-extensions-00
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Abstract
The Kerberos protocol does not allow ticket extensions. This make it
harder to deploy features like PKCROSS.
Since the Kerberos protocol did not specified extensibility for the
Ticket structure and the current implementations are aware of the
contents of tickets, the extension protocol cannot simply extend the
Ticket ASN.1 structure. Instead, the extension data needs to be
hidden inside the ticket.
This protocol defines two methods to add extend the tickets. The
first method requires updated clients and is more in line with the
future development of Kerberos. The second way does not require
update client. To take advantage of this protocol the server (KDC or
application server) need to update a well. The two methods are
equivalent and there is a 1-1 mapping between them.
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Table of Contents
1. Requirements Notation . . . . . . . . . . . . . . . . . . . . 4
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Extending the ticket . . . . . . . . . . . . . . . . . . . . . 6
3.1. Update Kerberos 5 ticket . . . . . . . . . . . . . . . . . 6
3.2. Backward compatible format . . . . . . . . . . . . . . . . 6
4. Ticket extentions . . . . . . . . . . . . . . . . . . . . . . 8
5. How to request a new assignment for a ticket extension . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
9. Normative References . . . . . . . . . . . . . . . . . . . . . 13
Appendix A. Ticket-extensions ASN.1 Module . . . . . . . . . . . 14
Appendix B. Changes . . . . . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . . . 17
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1. Requirements Notation
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 [RFC2119].
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2. Background
The ticket and enc-part as defined by [RFC4120] is defined as follow:
Ticket ::= [APPLICATION 1] SEQUENCE {
tkt-vno [0] INTEGER (5),
realm [1] Realm,
sname [2] PrincipalName,
enc-part [3] EncryptedData -- EncTicketPart
}
EncryptedData ::= SEQUENCE {
etype [0] Int32 -- EncryptionType --,
kvno [1] UInt32 OPTIONAL,
cipher [2] OCTET STRING -- ciphertext
}
The reason that the ticket can't be extended is that Kerberos clients
parses the returned ticket and any additions field will not be
preserved.
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3. Extending the ticket
This document describe two methods to extend tickets in Section 3.1
and Section 3.2. The two methods are equivalent and there is a 1-1
mapping between them, copy the fields into the respetive fields.
Anyone that creates protocols that uses ticket extentions MUST
support the Section 3.2 and SHOULD support both, ie, not depend on
the encoding of the Ticket structure itself.
3.1. Update Kerberos 5 ticket
The first method to extend the ticket is add a new field, ext-data
that extends the ticket with an array of type-value ticket
extensions.
Ticket ::= [APPLICATION 1] SEQUENCE {
tkt-vno[0] Int32,
realm[1] Realm,
sname[2] PrincipalName,
enc-part[3] EncryptedData
ext-data[4] SEQUENCE OF TicketExtension OPTIONAL
}
The client signals support by sending the PA-DATA type pa-data-
Client-Extensions setting the bit Client-Extensions-support-et-ticket
(the zero bit) to 1.
XXX write IANA registration for pa-data-Client-Extensions. Require
standard action, private/experimental gets to use their define their
own pa data.
If the KDC implement any protocols that uses Ticket extentions, it
MUST implement this method. Clients MAY support it. Servers MUST
support if they told the KDC they support the extended keys via an
administative command.
3.2. Backward compatible format
The second method is the backward compatible ticket that doesn't
change the format of the Ticket structure uses hides the extension
data inside the enc-part of the ticket.
It does this by using a special encryption type etype-TBETicket to
signal that enc-part.cipher contains the DER-encoded TBETicket
structure, instead of an encrypted EncTicketPart.
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etype-TBETicket INTEGER ::= 4711 -- TBA XXX --
krb5int32 ::= INTEGER (-2147483648..2147483647)
TBETicket ::= SEQUENCE {
etype [0] krb5int32 -- EncryptionType --,
cipher [1] OCTET STRING
extensions [2] SEQUENCE OF TicketExtension OPTIONAL
}
The content of cipher data and encryption type fields is moved inside
TBETicket. The kvno field is not moved and have the same mening as
before.
If the KDC implement this protocol, it MUST support this method,
Clients MAY support it. Servers MUST support if they told the KDC
they support the extended keys via an administative command.
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4. Ticket extentions
Ticket extentions are for communicating between the KDC and the
service/KDC the ticket is for. Clients and 4th parties can read the
data, but should do no attempt to modify, remove or add extentions.
The ticket extentions them self is defined as follows:
TicketExtension ::= SEQUENCE {
te-type [0] krb5int32,
te-data [1] OCTET STRING
te-csum [2] Checksum OPTIONAL,
te-kvno [3] krb5int32 OPTIONAL
}
Negative ticket extension types (te-type) is private extensions and
MUST only be used for experimentation or private use.
The te-type field specifies the type of the content in te-data.
Unknown te-types MUST be ignored both by the client and the server.
The te-csum field is optional for the type, specified by each ticket
extension type. The ticket extension type have to be specified and
the key usage number to use for the check sum. The key is usually
the session key of the ticket, but doesn't have to be, an extension
could specify an new session key used for the ticket. The data that
is signed is also specifed specific type.
The (te-kvno) field is to allow changing keys if they keys is some
unrelated key.
The KDC MUST NOT use extended ticket in an AS or TGS reply unless it
is known that all instances of the service in question support it.
In particular, a (local or cross-realm) TGT MUST NOT use extended
tickets unless all of the KDCs to which it may be sent are known to
support it.
The KDC MAY return extended tickets to servers supporting ticket
extensions even if the extended ticket does not contain any
extensions.
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5. How to request a new assignment for a ticket extension
When anyone is writing a Internet-draft for which a new assignment
for te-type is needed/wanted under the ticket extension, then the
proper way to do so is as follows:
EXAMPLE-MODULE DEFINITIONS ::= BEGIN
krb5-ticket-extension-Name ::= INTEGER nnn
-- IANA: please assign nnn
-- RFC-Editor: replace nnn with IANA-assigned
-- number and remove this note
END
IANA: Don't do note above, its an example, remove this note RFC-
Editor: Don't do note above, its an example, remove this note IANA
will assign the number as part of the RFC publication process.
When reviewing the document, the reviewer should take sure to check
that if te-csum is used, the signing key and key usage is specified.
The data that is signed also needs to be specified.
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6. Security Considerations
This document describes how to extend Kerberos tickets to include
additional data in the ticket. This does have a security
implications since the extension data in the TBETicket is only
optionally signed, not encrypted and is not replay protected. It is
up to the consumers of this interface to make sure its used safely.
Some of the issues that the extensions need to protect them self from
are: MITM downgrade to normal ticket, add or remove extensions, cut
and paste extensions between requests, retransmission of requests to
a different KDC. The data is sent in clear text, so can should be
taken to not send private data.
The ticket extension is mainly to communicate information from the
KDC to the server. The information can either be protected by the
session key, or the key of the server. If its protected by the
session key they both the client and the server can modify the data,
and if its protected the servers key is can modified by the server.
Any extension using Kerberos extension needs to define what the data
is needs protection from.
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7. Acknowledgements
Thanks to Leif Johansson, Kamada Ken'ichi, and Ken Raeburn for
reviewing the document and provided suggestions for improvements.
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8. IANA Considerations
There are currently no ticket extensions. Future ticket extensions
will be published at:
http://www.iana.org/assignments/NNNNNNNN
-- IANA: please name registry, proposal: krb5-ticket-extensions
IANA is requested to maintain this registry for future assignments.
New assignments can only be made via Specification Required as
described in [RFC2434].
IANA will assign the number as part of the RFC publication process.
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9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
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Appendix A. Ticket-extensions ASN.1 Module
KerberosV5-TicketExtensions {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) kerberosV5(2) modules(4) ticket-extensions(TBA)
--- XXX who is the registerar for this number ?
} DEFINITIONS EXPLICIT TAGS ::= BEGIN
IMPORTS
-- as defined in RFC 4120
Int32, Checksum
FROM KerberosV5Spec2 { iso(1) identified-organization(3)
dod(6) internet(1) security(5) kerberosV5(2)
modules(4) krb5spec2(2) }
pa-data-Client-Extensions INTEGER ::= 4710 -- XXX TBA --
PA-DATA-CLIENT-EXTENSIONS ::= BIT STRING
Client-Extensions-support-et-ticket INTEGER ::= 1
Ticket ::= [APPLICATION 1] SEQUENCE {
tkt-vno[0] Int32,
realm[1] Realm,
sname[2] PrincipalName,
enc-part[3] EncryptedData
ext-data[4] SEQUENCE OF TicketExtension OPTIONAL
}
etype-TBETicket INTEGER ::= 4711 -- XXX TBA --
TBETicket ::= SEQUENCE {
etype [0] Int32 -- EncryptionType --,
cipher [1] OCTET STRING
extensions [2] SEQUENCE OF TicketExtension OPTIONAL
}
TicketExtension ::= SEQUENCE {
te-type [0] Int32,
te-data [1] OCTET STRING
te-csum [2] Checksum
}
END
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Appendix B. Changes
RFC-EDITOR: please remove this section.
o Version lha-krb-wg-ticket-extensions-00 - initial version, after
review of Leif Johansson, Kamada Ken'ichi
o Version lha-krb-wg-ticket-extensions-01 - comments from Ken
Raeburn: experimentation or private use, attack types, asn1. nits.
o Version lha-krb-wg-ticket-extensions-02 - comments from Ken
Raeburn: new format for the Ticket PDU message. protocol neg from
the client via pa-data. kvno is Ticket.enc-data, added kvno for
te-csum field. Clearifed between what parties the messages are
for.
o Version ietf-krb-wg-ticket-extensions-00 - make the Backward
compatible format default and MUST support. KDC must always
support both. Comment from Sam Hartman.
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Author's Address
Love Hornquist Astrand
Apple, Inc
Cupertino
USA
Email: lha@apple.com
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