Internet Engineering Task Force P. M. Hallam-Baker
Internet-Draft Comodo Group Inc.
Intended status: Standards Track March 26, 2013
Expires: September 27, 2013
X.509v3 TLS Feature Extension
draft-hallambaker-tlsfeature-00
Abstract
The purpose of the TLS Feature extension is to prevent downgrade
attacks that are not otherwise prevented by the TLS protocol. In
particular, the TLS Feature extension may be used to mandate support
for revocation checking features in the TLS protocol such as OCSP
stapling. Informing clients that an OCSP status response will always
be stapled permits an immediate failure in the case that the response
is not stapled. This in turn prevents a denial of service attack
that might otherwise be possible.
Status of This Memo
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Table of Contents
1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
1.2. TLS Feature . . . . . . . . . . . . . . . . . . . . . . . 2
2. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. TLS Feature . . . . . . . . . . . . . . . . . . . . . . . 4
3.1.1. status_request . . . . . . . . . . . . . . . . . . . 4
3.2. Use . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2.1. Certificate Signing Request . . . . . . . . . . . . . 5
3.2.2. Certificate Signing Certificate . . . . . . . . . . . 5
3.2.3. End Entity Certificate . . . . . . . . . . . . . . . 5
3.3. Processing . . . . . . . . . . . . . . . . . . . . . . . 5
3.3.1. Certification Authority . . . . . . . . . . . . . . . 6
3.3.2. Server . . . . . . . . . . . . . . . . . . . . . . . 6
3.3.3. Client . . . . . . . . . . . . . . . . . . . . . . . 6
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5.1. Alternative Certificates and Certificate Issuers . . . . 6
5.2. Denial of Service . . . . . . . . . . . . . . . . . . . . 7
5.3. Cipher Suite Downgrade Attack . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Normative References . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Definitions
1.1. Requirements Language
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 RFC 2119 [RFC2119].
1.2. TLS Feature
In order to avoid the confusion that would occur in attempting to
describe an X.509 extension describing the use of TLS extensions, in
this document the term 'extension' is reserved to refer to X.509v3
extensions and the term 'feature' is used to refer to a TLS
extension.
2. Purpose
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The purpose of the TLS Feature extension is to prevent downgrade
attacks that are not otherwise prevented by the TLS protocol.
Since the TLS protocol itself provides strong protection against most
forms of downgrade attack, the TLS Feature is only relevant to the
validation of TLS protocol credentials. In particular to the
revocation status of the credentials presented.
At the time of writing, the only TLS feature that is relevant to the
revocation status of credentials is the Certificate Status Request
extension (status_request) used to support in-band exchange of OCSP
tokens, otherwise known as OCSP stapling. This extension is
described in RFC 4366 [RFC4366].
The OCSP stapling mechanism described in RFC 4366 [RFC4366] permits a
TLS server to provide evidence of valid certificate status inband and
thus improve client response. A TLS Feature extension that
advertises the status_request extension informs a client that if the
status_request is specified in a TLS Client Helo, that a server
compliant with the feature declaration MUST respond with a valid OCSP
token for the End Entity Certificate it presents.
Use of the TLS Feature extension in this fashion permits a client to
avoid reliance on certificates that are revoked for the reasons that
occur most frequently. In particular it allows a client to avoid
mis-reliance on certificates that are revoked for cause or at the
request of the subject (e.g. because of a compromised private key).
Advertising the status_request feature permits a client to fail
immediately in the case that the token is not provided by the server
without the need to query the OCSP responder in addition. This
improves client efficiency and more importantly prevents a denial of
service attack against the client by either blocking the OCSP
response or mounting a denial of service attack against the OCSP
responder.
Since the TLS Feature extension is an option, it is not likely that
an attacker attempting to obtain a certificate through fraud will
choose to have a certificate issued with this extension. Such risks
are more approrpriately addressed by mechanisms such as Certificate
Authority Authorization DNS records RFC 6844 [RFC6844] that are
designed to prevent or mitigate mis-issue. Nevertheless a
Certification Authority MAY consider the presence or absence of a
required TLS feature as one factor in determining the level of
additional scruitiny a request should be subject to.
Any TLS feature declaration specified in an End Entity certificate
MUST be offered by the server or clients MAY refuse connection. It
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is important therefore that a Certification Authority only issue
certificates that specify policies that match the configuration of
the server and that the server is capable of verifying that its
configuration is compatible with the feature declaration of the
certificates it offers. Ideally, the TLS feature declaration would
be specified by the certificate request generator as part of the
certificate issue process.
This document describes a mechanism that MAY be used to provide this
communication in-band for the most commonly used certificate
registration protocol.
3. Syntax
The TLS Feature extension has the following format:
cabf-tls-feature OBJECT IDENTIFIER ::= { cabf 1 }
Features ::= SEQUENCE OF INTEGER
The TLS Feature Extension MAY be marked critical. RFC 5280 [RFC5280]
requires that implementations that do not understand the extension
MUST reject the certificate. Marking the TLS Feature Extension
critical breaks backward compatibility and is not recommended unless
this is the desired behavior. Implementations that process the
extension MUST ignore the criticality bit setting.
3.1. TLS Feature
The TLS Feature extension lists a sequence of TLS extension
identifiers that a server compliant with the feature declaration MUST
support and accept on client request.
This specification does not require a TLS client to offer or support
any TLS extension regardless of whether it is specified in the TLS
Feature or not. In particular a client MAY request and a server MAY
support any TLS extension regardless of whether it is specified in a
TLS Feature extension or not.
If a TLS Feature extension specifies a TLS extension, a server
offering the certificate MUST support the extension specified and
MUST comply with any specific requirements specified for that
extension in this document or in the document that specifies the TLS
extension.
3.1.1. status_request
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If the TLS status_request extension is specified in the TLS Feature
extension and a TLS client specifies the status_request extensionin
the Client Hello, a server MUST return a valid OCSP token for the
specified End Entity certificate in the response.
3.2.1. Certificate Signing Request
If the certificate issue mechanism makes use of the PKCS#10
Certificate Signing Request (CSR) RFC 4366 [RFC4366], the CSR MAY
specify a TLS Feature extension as a CSR attribute. A server or
server administration tool should only generate key signing requests
that it knows can be supported by the server for which the
certificate is intended.
3.2.2. Certificate Signing Certificate
When present in a Certificate Signing Certificate, the TLS Feature
extension specifies a constraint on valid certificate chains.
Specifically, a certificate that is signed by a Certificate Signing
Certificate that contains a TLS Feature extension MUST contain a TLS
Feature extension which MUST offer the same set or a superset of the
features advertised in the signing certificate.
While relying clients MAY reject certificates that do not comply with
this requirement, the use of TLS Feature extension in Certificate
Signing Certificates is primarily intended for use by parties seeking
to evaluate the performance of certificate issuers and MAY be ignored
by clients.
3.2.3. End Entity Certificate
When specified in an End Entity Certificate, the TLS Feature
extension specifies criteria that a server MUST meet to be compliant
with the feature declaration.
In the case that a client determines that the server configuration is
inconsistent with the specified feature declaration it MAY reject the
TLS configuration.
In the case that a client determines that the server configuration is
inconsistent with a feature declaration specifying support for the
TLS status_request extension it SHOULD reject the TLS configuration.
3.3. Processing
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3.3.1. Certification Authority
A CA SHOULD NOT issue certs with a Feature extension unless there is
an affirmative statement to the effect that the end entity intends to
support the specified features. For example the use of a Feature
extension in the CSR or through an out of band communication.
The TLS Feature extension MAY be used with any TLS server regardless
of whether the server offers support. Server support for the TLS
Feature extension is nevertheless desirable as it can reduce the risk
of administrative error.
A server SHOULD verify that its configuration is compatible with the
TLS Feature extension expressed in a certificate it presents. A
server MAY override local configuration options if necessary to
ensure consistency but SHOULD inform the administrator whenever such
an inconsitency is discovered.
A server SHOULD support generation of the Feature extension in CSRs
if key generation is supported.
A compliant client MUST process the TLS Feature Extension and MUST
ignore the setting of the X.509 criticality flag.
A compliant client SHOULD reject a TLS connection with security
properties that are inconsistent with the specified TLS Feature
extension. A compliant client MAY accept such a TLS connection
request however if it is determined that doing so is appropriate in
particular circumstances.
4. Acknowledgements
[List of CABForum and PKIX contributors]
5. Security Considerations
5.1. Alternative Certificates and Certificate Issuers
Use of the TLS Feature extension to mandate support for a particular
form of revocation checking is optional. This control can provide
protection in the case that a certificate with a TLS Feature is
compromised after issue but not in the case that the attacker obtains
an unmarked certificate from an issuer through fraud.
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The TLS Feature extension is a post-issue security control. Such
risks can only be addressed by security controls that take effect
before issue.
5.2. Denial of Service
A certificate Issuer could issue a certificate that intentionally
specified a feature statement that they knew the server could not
support.
The risks of such refusal would appear to be negligible since a
Certificate Authority could equally refuse to issue the certificate.
5.3. Cipher Suite Downgrade Attack
The TLS Feature extension does not provide protection against a
cipher suite downgrade attack. This is left to the existing controls
in the TLS protocol itself.
6. IANA Considerations
No action by IANA is required.
7. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, April 2006.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC6844] Hallam-Baker, P. and R. Stradling, "DNS Certification
Authority Authorization (CAA) Resource Record", RFC 6844,
January 2013.
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[X.509] International Telecommunication Union , "ITU-T
Recommendation X.509 (11/2008): Information technology -
Open systems interconnection - The Directory: Public-key
and attribute certificate frameworks ", ITU-T
Recommendation X.509, November 2008.
[X.680] International Telecommunication Union , "ITU-T
Recommendation X.680 (11/2008): Information technology -
Abstract Syntax Notation One (ASN.1): Specification of
basic notation ", ITU-T Recommendation X.680, November
2008.
Author's Address
Phillip Hallam-Baker
Comodo Group Inc.
Email: philliph@comodo.com
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