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heimdal/doc/standardisation/draft-ietf-krb-wg-pkinit-alg-agility-00.txt
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Network Working Group L. Hornquist Astrand
Internet-Draft Stockholm University
Expires: September 2, 2006 L. Zhu
Microsoft Corporation
March 2006
PK-INIT algorithm agility
draft-ietf-krb-wg-pkinit-alg-agility-00
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on September 2, 2006.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
The PK-INIT protocol have in several places hard coded crypto
algorithms. The protocol specification needs to be updated so it can
support negotiation to upgrading to newer versions of crypto
algorithms. This document addresses this issue.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4
3. paChecksum agility . . . . . . . . . . . . . . . . . . . . . . 5
4. CMS Digest Algorithm agility . . . . . . . . . . . . . . . . . 6
5. Certificate Signer Algorithm Identifier agility . . . . . . . 7
6. octetstring2key function agility . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Normative References . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 12
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1. Introduction
The Kerberos PK-INIT document contains several hardcoded algorithms
that was know designed at design time that they had to be replaced by
something else at a later time, this document described how to use
other algorithms other then those that are hard-coded.
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2. 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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3. paChecksum agility
The paChecksum binds the PK-INIT part of the request to main body of
the Kerberos request (KDC-REQ-BODY). This is to makes sure an
attacker can not change the request from the client to the server.
The problem is that paChecksum is hardcoded to use SHA1-1, however,
there is a mechaism to provide algorithm agility for the paChecksum
within the PK-INIT prototcol. Newer clients can choose not send the
paChecksum field, but rather add some new fields after the existing
fields, older KDC will send back know failure-code so that newer
clients can fall back to the old protocol if local policy allows
that.
If the attacker can preserve the checksum in paChecksum, an attacker
can, for example, change the KDC-REQ-BODY is to downgrade the
encryption types used, expend the expiration time, etc, and then try
to brute-force the request.
In the Public Key Encryption case of PK-INIT the reply contains a
checksum over the whole request in the asChecksum field, in this case
the client will detect any modifications to the request. Since the
asChecksum is using the associated checksum of the session key
encryption type, asChecksum field is algorithm agile.
One way to solve this problem is to add the asChecksum to the Diffie-
Hellman case reply too, and just ignore the paCheckSum field. The
KDC should still not issue tickets that are too weak, since that
exposes the problem. This is regardless of the using PK-INIT or not.
Questions for wg: Wait for Kerberos Extensions that will solve this
problem (ignore the problem for how), or use add asChecksum to DH
case.
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4. CMS Digest Algorithm agility
The client can tell KDC what the supported CMS types are in the
requset packet, but there are no equivalent for KDC to the the client
what the digest algorithm are support in an reply.
Have KDC send the CMS list of supported encryption types in the
e-data field of KRB-ERROR when returning the
KDC_ERR_DIGEST_IN_SIGNED_DATA_NOT_ACCEPTED error.
DER encoded TS-SD-PARAMETERS specifies supported digest algorithms.
The list is in decreasing preference order.
TD-SD-PARAMETERS ::= SEQUENCE OF AlgorithmIdentifier
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5. Certificate Signer Algorithm Identifier agility
The KDC can reject a certificate based on the signers hash algorithm
with the error KDC_ERR_DIGEST_IN_CERT_NOT_ACCEPTED, but doesn't tell
the client what algorithm are supported.
DER encoded TS-DC-PARAMETERS specifies supported certificate digest
algorithms. The AllowedAlgorithms is in decreasing preference order.
RejectedAlgorithm may be include my the KDC to tell what algorithm
was rejected in case the rejected certificate was part of a computed
chain.
TD-DC-PARAMETERS ::= SEQUENCE {
AllowedAlgorithms [0] SEQUENCE OF AlgorithmIdentifier,
RejectedAlgorithm [1] AlgorithmIdentifier OPTIONAL
}
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6. octetstring2key function agility
The PK-INIT standard uses a home-grown string 2 key function in the
DH case. The function uses SHA-1 to mix and stretch the DH shared
key.
Describe how the client announces that is supports the new String to
key function. Probably by stuffing it into the supportCMSTypes field
in the request.
Use NIST SP 800 56B when its published.
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7. Security Considerations
This document describes negotiation of checksum types and other
cryptographic functions. Most of this negotiation is done
unauthenticated with no way to very
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8. IANA Considerations
No IANA considerations.
9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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Authors' Addresses
Love Hornquist Astrand
Stockholm University
SE-106 91 STOCKHOLM
SWEDEN
Email: lha@it.su.se
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: lzhu@microsoft.com
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Intellectual Property Statement
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Copyright (C) The Internet Society (2006). This document is subject
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except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Hornquist Astrand & Zhu Expires September 2, 2006 [Page 12]
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