Internet Engineering Task Force (IETF) Phillip Hallam-Baker
INTERNET-DRAFT Comodo Group Inc.
Intended Status: Standards Track March 23, 2015
Expires: September 24, 2015
X.509v3 TLS Feature Extension
draft-hallambaker-tlsfeature-08
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Hallam-Baker September 24, 2015 [Page 1]
Internet-Draft TLS Feature Extension March 2015
Table of Contents
1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. TLS Feature, X.509 Extension . . . . . . . . . . . . . . 3
2. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. TLS Feature . . . . . . . . . . . . . . . . . . . . . . . 5
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 . . . . . . . . . . . . . . . . . . . . . . . 6
3.3.1. Certification Authority . . . . . . . . . . . . . . 6
3.3.2. Server . . . . . . . . . . . . . . . . . . . . . . . 6
3.3.3. Client . . . . . . . . . . . . . . . . . . . . . . . 6
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5.1. Alternative Certificates and Certificate Issuers . . . . 7
5.2. Denial of Service . . . . . . . . . . . . . . . . . . . . 7
5.3. Cipher Suite Downgrade Attack . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
Hallam-Baker September 24, 2015 [Page 2]
Internet-Draft TLS Feature Extension March 2015
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, X.509 Extension
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
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 including downgrade attacks against cipher
suite choices offered and client credentials, the TLS feature
extension is only relevant to the validation of TLS protocol
credentials.
At the time of writing, the only TLS feature extensions that are
relevant to the revocation status of credentials are the Certificate
Status Request extension (status_request) and the Multiple
Certificate Status Extension (status_request_v2). These extensions
are used to support in-band exchange of OCSP tokens, otherwise known
as OCSP stapling. These extensions are described in [RFC6066] and
[RFC6961].
The OCSP stapling mechanism described in [RFC6066] permits a TLS
server to provide evidence of valid certificate status inband. When
this information is provided inband, the privacy, performance and
reliability concerns arising from the need to make a third party
connection during the TLS handshake are eliminated. A client cannot
however draw any conclusion from the absence of inband status
information unless it knows that the legitimate server would have
provided it. The status information might have been omitted because
the server does not support the extension or because the server is
witholding the information intentionally, knowing the certificate to
be invalid.
The inclusion of a TLS feature extension advertising the
status_request feature in the server end entity certificate permits a
client to fail immediately if the certificate status information is
not provided by the server. The need to query the OCSP responder is
Hallam-Baker September 24, 2015 [Page 3]
Internet-Draft TLS Feature Extension March 2015
eliminated entirely. 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 appropriately addressed by mechanisms such as Certification
Authority Authorization DNS records RFC 6844 [RFC6844] that are
designed to prevent or mitigate mis-issue.
A server offering an end entity certificate with a TLS feature
extension MUST satisfy a client request for the specified feature
unless this would be redundant as described below. Clients MAY
refuse to accept the connection if the server does not accept a
request for a specified feature.
A Certification Authority SHOULD NOT issue certificates that specify
a TLS feature extension advertising features that the server does not
support.
A server MAY advise a Certification Authority that it is capable of
supporting a feature by including the corresponding TLS feature
extension in a Certificate Signing Request [RFC2986]. A server SHOULD
verify that its configuration supports the features advertised in the
credentials presented to a client requesting connection.
This document describes the use of the TLS feature in PKIX end entity
certificates and certificate signing certificates. A mechanism that
MAY be used to describe support for the specified features in-band
for the most commonly used certificate registration protocol is also
provided.
3. Syntax
The TLS feature extension has the following format:
tls-feature OBJECT IDENTIFIER ::= { id-pe 24 }
Features ::= SEQUENCE OF INTEGER
The extnValue of the tls-feature extension is the ASN.1 BER encoding
of the Features structure.
The TLS feature extension SHOULD NOT be marked critical. RFC 5280
[RFC5280] requires that implementations that do not understand
critical extensions MUST reject the certificate. Marking the TLS
feature extension critical breaks backward compatibility and is not
recommended unless this is the desired behavior.
Hallam-Baker September 24, 2015 [Page 4]
Internet-Draft TLS Feature Extension March 2015
3.1. TLS Feature
The object member Features is a sequence of TLS extension identifiers
(features, in this specification's terminology) as specified in the
IANA Transport Layer Security (TLS) Extensions registry. If these
features are requested by the client in its ClientHello message, then
they MUST be present in the server's ServerHello.
This specification does not require a TLS client to offer or support
any TLS feature regardless of whether it is specified in the server
certificate's TLS feature extension 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.
A server that offers a certificate that contains a TLS feature
extension MUST support the features specified and comply with the
corresponding requirements.
3.2. Use
3.2.1. Certificate Signing Request
If the certificate issue mechanism makes use of the PKCS#10
Certificate Signing Request (CSR) [RFC2986], the CSR MAY specify a
TLS feature extension as a CSR Attribute as defined in [RFC2986]
section 4.1. 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 (i.e., CA
certificate with the key usage extension value set to keyCertSign),
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.
This behavior provides a means of requiring support for a particular
set of features for certificates issued under a particular
Certificate Signing Certificate without requiring TLS clients to
verify compliance with TLS feature extensions in multiple
certificates.
3.2.3. End Entity Certificate
When specified in a server end entity Certificate (i.e. a certificate
that specifies the id-kp-server EKU), the TLS feature extension
specifies criteria that a server MUST meet to be compliant with the
feature declaration.
Hallam-Baker September 24, 2015 [Page 5]
Internet-Draft TLS Feature Extension March 2015
In the case that a client determines that the server configuration is
inconsistent with the specified feature declaration it MAY reject the
TLS configuration.
3.2.3.1. TLS status_request
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.
A client MAY accept a TLS configuration despite it being inconsistent
with the TLS feature declaration if the validity of the certificate
chain presented can be established through other means. For example,
by successfully obtaining the OCSP data from another source.
There are certain situations in which the alternative to establishing
a connection with imperfect TLS security is to transmit the same
information with no security controls whatsoever. Accordingly, a
client MAY accept a TLS configuration despite it being inconsistent
with the TLS feature declaration but MUST NOT distinguish that
connection as secure.
3.3. Processing
3.3.1. Certification Authority
A CA SHOULD NOT issue certs with a TLS 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.
3.3.2. Server
A TLS server certificate containing a TLS feature extension MAY be
used with any TLS server that supports the specified features. It is
not necessary for the server to provide support for the TLS feature
extension itself. Such support 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 inconsistency is discovered.
A server SHOULD support generation of the feature extension in CSRs
if key generation is supported.
Hallam-Baker September 24, 2015 [Page 6]
Internet-Draft TLS Feature Extension March 2015
3.3.3. Client
A client MUST treat a certificate with a TLS feature extension as an
invalid certificate if the features offered by the server do not
contain all features present in both the client's ClientHello message
and the TLS feature extension
In the case that use of TLS with a valid certificate is mandated by
explicit security policy, application protocol specification or other
means, the client MUST refuse the connection. If the use of TLS with
a valid certificate is optional, a client MAY accept the connection
but MUST NOT treat the certificate as valid.
4. Acknowledgements
This proposal incorporates text and other contributions from
participants in the IETF and CA-Browser forum. In particular, Robin
Alden, Richard Barnes, Viktor Dukhovni, Stephen Farrell, Gervase
Markham, Yoav Nir, Tom Ritter, Jeremy Rowley, Stefan Santesson, Ryan
Sleevi, Brian Smith, Rob Stradling and Sean Turner
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.
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
Certification 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.
Hallam-Baker September 24, 2015 [Page 7]
Internet-Draft TLS Feature Extension March 2015
6. IANA Considerations
On approval, IANA shall add in the SMI Security for PKIX Certificate
Extension (1.3.6.1.5.5.7.1) registry the following entry:
Decimal Description References
------- ------------------------------ ---------------------
24 id-pe-tlsfeature {this RFC}
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
[RFC6961] Pettersen, Y., "The Transport Layer Security (TLS)
Multiple Certificate Status Request Extension", RFC 6961,
June 2013.
[RFC6844] Hallam-Baker, P.,Stradling, R., "DNS Certification
Authority Authorization (CAA) Resource Record", RFC 6844,
January 2013.
[RFC2986] Nystrom, M.,Kaliski, B., "PKCS #10: Certification Request
Syntax Specification Version 1.7", RFC 2986, November
2000.
[RFC5280] Cooper, D.,Santesson, S.,Farrell, S.,Boeyen, S.,Housley,
R.,Polk, W., "Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation List (CRL)
Profile", RFC 5280, May 2008.
Author's Address
Phillip Hallam-Baker
Comodo Group Inc.
philliph@comodo.com
Hallam-Baker September 24, 2015 [Page 8]
