IETF P. Wouters
Internet-Draft No Hats Corporation
Intended status: Standards Track J. Gilmore
Expires: July 10, 2012
S. Weiler
SPARTA, Inc.
T. Kivinen
AuthenTec
H. Tschofenig
Nokia Siemens Networks
January 7, 20, 2012
TLS Out-of-Band Public Key Validation
draft-ietf-tls-oob-pubkey-00.txt
draft-ietf-tls-oob-pubkey-01.txt
Abstract
This document specifies a new TLS certificate type for exchanging raw
public keys in Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS) for use with out-of-band authentication.
Currently, TLS authentication can only occur via PKIX or OpenPGP
certificates. By specifying a minimum resource for raw public key
exchange, implementations can use alternative authentication methods.
One such method is using DANE Resource Records secured by DNSSEC,
Another use case is to provide authentication functionality when used
with devices in a constrained environment that use whitelists and
blacklists, as is the case with sensors and other embedded devices
that are constrained by memory, computational, and communication
limitations where the usage of PKIX is not feasible.
The new certificate type specified can also be used to reduce the
latency of a TLS client that is already in possession of a validated
public key of the TLS server before it starts a (non-resumed) TLS
handshake.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 10, 2012.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Applicability . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Changes to the Handshake Message Contents . . . . . . . . . . 5
2.1. Client Hello . . . . . . . . . . . . . . . . . . . . . . . 6
2.2. Server Hello . . . . . . . . . . . . . . . . . . . . . . . 7
2.3. Certificate Request . . . . . . . . . . . . . . . . . . . 7
2.4. Other Handshake Messages . . . . . . . . . . . . . . . . . 8
3. Security Considerations . . . . . . . . . . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
1.1. Motivation
Traditionally, TLS server public keys are obtained in PKIX containers
in-band using the TLS connection and validated using trust anchors
based on a [PKIX] certification authority (CA). This method can add
a complicated trust relationship that is difficult to validate.
Examples of such complexity can be seen in [Defeating-SSL].
Alternative methods are available that allow a TLS client to obtain
the TLS server public key:
o The TLS server public key is obtained from a DNSSEC secured RRset
using [DANE]
o The TLS server public key is obtained from a [PKIX] certificate
chain from an [LDAP] server
o The TLS server public key is provisioned by the operating system
and updated via software updates
o A TLS client has connected to the TLS server before and has cached
the TLS server certificate chain or TLS server public key for re-
use
[RFC5246] does not provide a mechanism for a TLS client to tell the
TLS server it is already in possession of the authenticated public
key. Therefore, a TLS server must always send a list of trusted CA
keys and its EE certificate containing its public key, even when the
TLS client does not require or desire that data for authentication.
[RFC6066] allows suppression of the certificate trust anchor chain,
but not suppression of the PKIX EE certificate container. These
certificate chains are large opaque blocks of data containing much
more than the public key of the TLS server. Since the TLS client
might only be able to validate the PKIX SubjectPublicKeyInfo via an
out-of-band method such as [DANE], it has to ignore any additional
information received that was sent by the server that it could not
validate. Furthermore, information that comes in via these
certificate chains could contain contradicting or additional
information that the TLS client cannot validate or trust, such as an
expiry date that conflicts with information obtained from DNS or
LDAP. This document specifies a method to suppress sending this
additional information.
Some small embedded devices use the UDP based [CoAP], a specialized
constrained networks and nodes for machine-to-machine applications.
These devices interact with a Web server to upload data such as
temperature sensor readings at a regular intervals. Constrained
Application Protocol (CoAP) [CoAP] can utilize DTLS for its
communication security. As part of the provisioning procedure, the
embeded device is configured with the address and public key of a
dedicated CoAP server to upload sensor data. Receiving PKIX
information [PKIX] from a webserver would be an unneccesarry burden
on a sensor networking deployment environment that requires pre-
configured client-server public keys. These devices often also lack
a real-time clock to perform any PKIX epixry checks.
1.2. Applicability
The Transport Layer Security (TLS) Protocol Version 1.2 is specified
in [RFC5246] and provides a framework for extensions to TLS as well
as considerations for designing such extensions. [RFC6066] defines
several new TLS extensions. This document extends the specifications
of those RFCs with one new TLS Certificate Type to facilitate
suppressing unneeded [PKIX] information from being sent during the
TLS handshake when this information is not required to authenticate
the TLS server.
1.3. Terminology
Most security-related terms in this document are to be understood in
the sense defined in [SECTERMS]; such terms include, but are not
limited to, "attack", "authentication", "authorization",
"certification authority", "certification path", "certificate",
"credential", "identity", "self-signed certificate", "trust", "trust
anchor", "trust chain", "validate", and "verify".
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].
2. Changes to the Handshake Message Contents
This section describes the changes to the TLS handshake message
contents when raw public keys are to be used for authentication.
Figure 1 illustrates the exchange of messages as described in the
sub-sections below. The new "RawPublicKey" value in the cert_type
extension indicates the ability and desire to exchange raw public
keys, which are then exchanged as part of the certificate payloads.
client_hello,
cert_type="RawPublicKey" ->
<- server_hello,
cert_type="RawPublicKey",
certificate,
server_key_exchange,
certificate_request,
server_hello_done
certificate,
client_key_exchange,
certificate_verify,
change_cipher_spec,
finished ->
<- change_cipher_spec,
finished
Application Data <-------> Application Data
Figure 1: Example Message Flow
2.1. Client Hello
In order to indicate the support of out-of-bound out-of-band raw public keys,
clients MUST include an extension of type "cert_type" to the extended
client hello message. The "cert_type" TLS extension, which is
defined in [RFC6091], is assigned the value of 9 from the TLS
ExtensionType registry. This value is used as the extension number
for the extensions in both the client hello message and the server
hello message. The hello extension mechanism is described in
[RFC5246].
The "cert_type" TLS extension carries a list of supported certificate
types the client can use, sorted by client preference. This
extension MUST be omitted if the client only supports X.509
certificates. The "extension_data" field of this extension contains
a CertificateTypeExtension structure. Note that the
CertificateTypeExtension structure is being used both by the client
and the server, even though the structure is only specified once in
this document.
The [RFC6091] defined CertificateTypeExtension is extended as
follows:
enum { client, server } ClientOrServerExtension;
enum { X.509(0), OpenPGP(1),
RawPublicKey([TBD]),
(255) } CertificateType;
struct {
select(ClientOrServerExtension)
case client:
CertificateType certificate_types<1..2^8-1>;
case server:
CertificateType certificate_type;
}
} CertificateTypeExtension;
No new cipher suites are required to use raw public keys. All
existing cipher suites that support a key exchange method compatible
with the defined extension can be used.
2.2. Server Hello
If the server receives a client hello that contains the "cert_type"
extension and chooses a cipher suite then two outcomes are possible.
The server MUST either select a certificate type from the
certificate_types field in the extended client hello or terminate the
session with a fatal alert of type "unsupported_certificate".
The certificate type selected by the server is encoded in a
CertificateTypeExtension structure, which is included in the extended
server hello message using an extension of type "cert_type". Servers
that only support X.509 certificates MAY omit including the
"cert_type" extension in the extended server hello.
If the negotiated certificate type is RawPublicKey the TLS server
MUST send a CertificateTypeExtension structure with a PKIX [PKIX]
certificate containing ONLY the SubjectPublicKeyInfo. The public key
MUST match the selected key exchange algorithm.
2.3. Certificate Request
The semantics of this message remain the same as in the TLS
specification. However, if this message is sent, and the negotiated
certificate type is RawPublicKey, the "certificate_authorities" list
MUST be empty.
2.4. Other Handshake Messages
All the other handshake messages are identical to the TLS
specification.
3. Security Considerations
The TLS cert_type extension defined here lets a TLS client attempt to
supress the sending of server certificate as well as the
certification chain for that certificate.
A client using this cert_type needs to be confident in the
authenticity of the public key it is using. Since those public keys
were obtained out-of-band extension), the authentication must also be
out-of-band.
Depending on exactly how the public keys were obtained, it may be
appropriate to use authentication mechanisms tied to the public key
transport. For example, if public keys were obtained using [DANE] it
is appropriate to use DNSSEC to authenticate the public keys.
4. IANA Considerations
We request that IANA assign a TLS cert_type value for RawPublicKey.
5. Contributors
The following individuals made important contributions to this
document: Paul Hoffman.
6. Acknowledgements
This document is based on material from RFC 6066 for which the author
is Donald Eastlake 3rd. Contributions to that document also include
Joseph Salowey, Alexey Melnikov, Peter Saint-Andre, and Adrian
Farrel.
The feedback from the TLS working group meeting at IETF#81 has
substantially shaped the document and we would like to thank the
meeting participants for their input. The support for hashes of
public keys has been removed after the discussions at the IETF#82
meeting and the feedback from Eric Rescorla.
7. References
7.1. Normative References
[PKIX] 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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[SECTERMS]
Shirey, R., "Internet Security Glossary, Version 2",
RFC 4949, August 2007.
7.2. Informative References
[CoAP] Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
"Constrained Application Protocol",
draft-ietf-core-coap-07 (work in progress), July 2011.
[DANE] Hoffman, P. and J. Schlyter, "Using Secure DNS to
Associate Certificates with Domain Names For TLS",
draft-ietf-dane-protocol-12
draft-ietf-dane-protocol-14 (work in progress),
September 2011.
[Defeating-SSL]
Marlinspike, M., "New Tricks for Defeating SSL in
Practice", February 2009, <http://www.blackhat.com/
presentations/bh-dc-09/Marlinspike/
BlackHat-DC-09-Marlinspike-Defeating-SSL.pdf>.
[LDAP] Sermersheim, J., "Lightweight Directory Access Protocol
(LDAP): The Protocol", RFC 4511, June 2006.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
[RFC6091] Mavrogiannopoulos, N. and D. Gillmor, "Using OpenPGP Keys
for Transport Layer Security (TLS) Authentication",
RFC 6091, February 2011.
Authors' Addresses
Paul Wouters
No Hats Corporation
Email: paul@nohats.ca
John Gilmore
PO Box 170608
San Francisco, California 94117
USA
Phone: +1 415 221 6524
Email: gnu@toad.com
URI: https://www.toad.com/
Samuel Weiler
SPARTA, Inc.
7110 Samuel Morse Drive
Columbia, Maryland 21046
US
Email: weiler@tislabs.com
Tero Kivinen
AuthenTec
Eerikinkatu 28
HELSINKI FI-00180
FI
Email: kivinen@iki.fi
Hannes Tschofenig
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland
Phone: +358 (50) 4871445
Email: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at