OCSP for PKINIT: Online Certificate Status Protocol (OCSP) Support for Public Key Cryptography for Initial Authentication in Kerberos (PKINIT)
Lets Kerberos clients and servers carry fresh certificate status answers inside PKINIT login messages.
Zhu, L., Jaganathan, K., and N. Williams, "Online Certificate Status Protocol (OCSP) Support for Public Key Cryptography for Initial Authentication in Kerberos (PKINIT)", RFC 4557, DOI 10.17487/RFC4557, June 2006, <https://www.rfc-editor.org/info/rfc4557>.
@misc{rfc4557,
series = {Request for Comments},
number = 4557,
howpublished = {RFC 4557},
publisher = {RFC Editor},
doi = {10.17487/RFC4557},
url = {https://www.rfc-editor.org/info/rfc4557},
author = {L. Zhu and K. Jaganathan and N. Williams},
title = {{Online Certificate Status Protocol (OCSP) Support for Public Key Cryptography for Initial Authentication in Kerberos (PKINIT)}},
pagetotal = 6,
year = 2006,
month = jun,
} RFC text
Contents
Network Working Group L. Zhu
Request for Comments: 4557 K. Jaganathan
Category: Standards Track Microsoft Corporation
N. Williams
Sun Microsystems
June 2006
Online Certificate Status Protocol (OCSP) Support for
Public Key Cryptography for
Initial Authentication in Kerberos (PKINIT)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document defines a mechanism to enable in-band transmission of
Online Certificate Status Protocol (OCSP) responses in the Kerberos
network authentication protocol. These responses are used to verify
the validity of the certificates used in Public Key Cryptography for
Initial Authentication in Kerberos (PKINIT), which is the Kerberos
Version 5 extension that provides for the use of public key
cryptography.
Table of Contents
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................2
3. Message Definition ..............................................2
4. Security Considerations .........................................3
5. Acknowledgements ................................................4
6. References ......................................................4
6.1. Normative References .......................................4
6.2. Informative References .....................................4
1. Introduction
Online Certificate Status Protocol (OCSP) [RFC2560] enables
applications to obtain timely information regarding the revocation
status of a certificate. Because OCSP responses are well bounded and
small in size, constrained clients may wish to use OCSP to check the
validity of the certificates for Kerberos Key Distribution Center
(KDC) in order to avoid transmission of large Certificate Revocation
Lists (CRLs) and therefore save bandwidth on constrained networks
[OCSP-PROFILE].
This document defines a pre-authentication type [RFC4120], where the
client and the KDC MAY piggyback OCSP responses for certificates used
in authentication exchanges, as defined in [RFC4556].
By using this OPTIONAL extension, PKINIT clients and the KDC can
maximize the reuse of cached OCSP responses.
2. Conventions Used in This Document
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in [RFC2119].
3. Message Definition
A pre-authentication type identifier is defined for this mechanism:
PA-PK-OCSP-RESPONSE 18
The corresponding padata-value field [RFC4120] contains the DER [X60]
encoding of the following ASN.1 type:
PKOcspData ::= SEQUENCE OF OcspResponse
-- If more than one OcspResponse is
-- included, the first OcspResponse
-- MUST contain the OCSP response
-- for the signer's certificate.
-- The signer refers to the client for
-- AS-REQ, and the KDC for the AS-REP,
-- respectively.
OcspResponse ::= OCTET STRING
-- Contains a complete OCSP response,
-- as defined in [RFC2560].
The client MAY send OCSP responses for certificates used in PA-PK-
AS-REQ [RFC4556] via a PA-PK-OCSP-RESPONSE.
The KDC that receives a PA-PK-OCSP-RESPONSE SHOULD send a PA-PK-
OCSP-RESPONSE containing OCSP responses for certificates used in the
KDC's PA-PK-AS-REP. The client can request a PA-PK-OCSP-RESPONSE by
using a PKOcspData containing an empty sequence.
The KDC MAY send a PA-PK-OCSP-RESPONSE when it does not receive a
PA-PK-OCSP-RESPONSE from the client.
The PA-PK-OCSP-RESPONSE sent by the KDC contains OCSP responses for
certificates used in PA-PK-AS-REP [RFC4556].
Note the lack of integrity protection for the empty or missing OCSP
response; lack of an expected OCSP response from the KDC for the
KDC's certificates SHOULD be treated as an error by the client,
unless it is configured otherwise.
When using OCSP, the response is signed by the OCSP server, which is
trusted by the receiver. Depending on local policy, further
verification of the validity of the OCSP servers may be needed
The client and the KDC SHOULD ignore invalid OCSP responses received
via this mechanism, and they MAY implement CRL processing logic as a
fall-back position, if the OCSP responses received via this mechanism
alone are not sufficient for the verification of certificate
validity. The client and/or the KDC MAY ignore a valid OCSP response
and perform its own revocation status verification independently.
4. Security Considerations
The pre-authentication data in this document do not actually
authenticate any principals, but are designed to be used in
conjunction with PKINIT.
There is no binding between PA-PK-OCSP-RESPONSE pre-authentication
data and PKINIT pre-authentication data other than a given OCSP
response corresponding to a certificate used in a PKINIT pre-
authentication data element. Attacks involving removal or
replacement of PA-PK-OCSP-RESPONSE pre-authentication data elements
are, at worst, downgrade attacks, where a PKINIT client or KDC would
proceed without use of CRLs or OCSP for certificate validation, or
denial-of-service attacks, where a PKINIT client or KDC that cannot
validate the other's certificate without an accompanying OCSP
response might reject the AS exchange or might have to download very
large CRLs in order to continue. Kerberos V does not protect against
denial-of-service attacks; therefore, the denial-of-service aspect of
these attacks is acceptable.
If a PKINIT client or KDC cannot validate certificates without the
aid of a valid PA-PK-OCSP-RESPONSE, then it SHOULD fail the AS
exchange, possibly according to local configuration.
5. Acknowledgements
This document was based on conversations among the authors, Jeffrey
Altman, Sam Hartman, Martin Rex, and other members of the Kerberos
working group.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and
C. Adams, "X.509 Internet Public Key Infrastructure
Online Certificate Status Protocol - OCSP", RFC 2560,
June 1999.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC
4120, July 2005.
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for
Initial Authentication in Kerberos (PKINIT)", RFC
4556, June 2006.
[X690] ASN.1 encoding rules: Specification of Basic Encoding
Rules (BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER), ITU-T
Recommendation X.690 (1997) | ISO/IEC International
Standard 8825-1:1998.
6.2. Informative References
[OCSP-PROFILE] Deacon, A. and R. Hurst, "Lightweight OCSP Profile for
High Volume Environments", Work in Progress, May 2006.
Authors' Addresses
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: lzhu@microsoft.com
Karthik Jaganathan
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: karthikj@microsoft.com
Nicolas Williams
Sun Microsystems
5300 Riata Trace Ct
Austin, TX 78727
US
EMail: Nicolas.Williams@sun.com
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AI ELI5
On this page
§1Introduction
The Online Certificate Status Protocol (OCSP, RFC 2560) lets applications get timely information on whether a certificate has been revoked. OCSP responses are well bounded and small, so constrained clients may prefer them over downloading large Certificate Revocation Lists (CRLs) when checking the certificates of a Kerberos Key Distribution Center (KDC). That saves bandwidth on constrained networks.
RFC 4557 defines a pre-authentication type, as in RFC 4120, in which the client and the KDC MAY piggyback OCSP responses for certificates used in the authentication exchanges of PKINIT (RFC 4556). The extension is OPTIONAL, and it lets both sides reuse cached OCSP responses.
Instead of fetching a huge list of cancelled ID cards, you get a short note that says whether this one card is still good.
§2The PA-PK-OCSP-RESPONSE Message
The new pre-authentication type is PA-PK-OCSP-RESPONSE, with identifier 18. Its padata-value holds the DER encoding of a PKOcspData, which is a sequence of OcspResponse. Each OcspResponse is an octet string holding one complete OCSP response as defined in RFC 2560.
If more than one response is included, the first MUST be the response for the signer's certificate. The signer is the client in the AS-REQ and the KDC in the AS-REP.
The message is a small envelope holding one or more certificate-check answers. The first answer must be about the sender's own card.
§3How Clients and KDCs Use It
The client MAY send OCSP responses for the certificates used in PA-PK-AS-REQ. A KDC that receives a PA-PK-OCSP-RESPONSE SHOULD send back one with OCSP responses for the certificates used in its PA-PK-AS-REP. A client can ask for one by sending a PKOcspData containing an empty sequence. The KDC MAY also send one even if the client sent none.
The OCSP response is signed by the OCSP server, which the receiver trusts. Depending on local policy, further checking of the OCSP servers may be needed.
Both sides SHOULD ignore invalid OCSP responses received this way. They MAY fall back to CRL processing if the responses alone do not settle certificate validity, and they MAY ignore a valid response and do their own revocation checking.
You can hand over a revocation note, or hand over an empty envelope to ask the other side for theirs. If a note looks bad, you may throw it away and check another way.
- PA-PK-AS-REQ + PA-PK-OCSP-RESPONSEOptional; an empty sequence asks the KDC for its responses.
- PA-PK-AS-REP + PA-PK-OCSP-RESPONSECovers the certificates used in the PA-PK-AS-REP.
§4Missing or Empty Responses
The empty or missing OCSP response has no integrity protection. The RFC therefore says that a client SHOULD treat a missing expected OCSP response for the KDC's certificates as an error, unless it is configured otherwise.
More generally, if a PKINIT client or KDC cannot validate certificates without a valid PA-PK-OCSP-RESPONSE, it SHOULD fail the AS exchange, possibly according to local configuration.
If you expected a note saying the card is good and no note comes, you should be suspicious and may refuse to go on.
§5Security Considerations
The pre-authentication data defined here does not authenticate any principal. It is meant to be used together with PKINIT. The only binding between it and the PKINIT data is that an OCSP response corresponds to a certificate used in a PKINIT element.
Because of this, removing or replacing PA-PK-OCSP-RESPONSE elements is at worst a downgrade attack, where a client or KDC proceeds without CRLs or OCSP, or a denial-of-service attack, where a party that cannot validate the other's certificate without an OCSP response rejects the exchange or must download very large CRLs. Kerberos V does not protect against denial of service, so the RFC calls that aspect acceptable.
Someone could remove the note so you skip the check, or so you have to do extra work. The RFC accepts that risk and says to stop the login if you cannot check the card.