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Versions: (draft-wouters-tls-oob-pubkey) 00
01 02 03 04 05 06 07 08 09 10 11 RFC 7250
TLS P. Wouters
Internet-Draft No Hats Corporation
Intended status: Standards Track J. Gilmore
Expires: October 27, 2012
S. Weiler
SPARTA, Inc.
T. Kivinen
AuthenTec
H. Tschofenig
Nokia Siemens Networks
April 25, 2012
TLS Out-of-Band Public Key Validation
draft-ietf-tls-oob-pubkey-03.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 public key validation.
Currently, TLS authentication can only occur via X.509-based Public
Key Infrastructure (PKI) or OpenPGP certificates. By specifying a
minimum resource for raw public key exchange, implementations can use
alternative public key validation methods.
One such alternative public key valiation method is offered by the
DNS-Based Authentication of Named Entities (DANE) together with DNS
Security. Another alternative is to utilize pre-configured keys, as
is the case with sensors and other embedded devices. The usage of
raw public keys, instead of X.509-based certificates, leads to a
smaller code footprint.
The support for raw public keys is introduced into TLS via a new non-
PKIX certificate type.
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
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 27, 2012.
Copyright Notice
Copyright (c) 2012 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. TLS Handshake Extension . . . . . . . . . . . . . . . . . . . . 5
3.1. Client Hello . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Server Hello . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Certificate Request . . . . . . . . . . . . . . . . . . . . 7
3.4. Other Handshake Messages . . . . . . . . . . . . . . . . . 7
3.5. Client authentication . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
Traditionally, TLS server public keys are obtained in PKIX containers
in-band using the TLS handshake 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
resource records using DANE [I-D.ietf-dane-protocol].
o The TLS server public key is obtained from a [PKIX] certificate
chain from an Lightweight Directory Access Protocol (LDAP) [LDAP]
server.
o The TLS client and server public key is provisioned into the
operating system firmware image, and updated via software updates.
Some smart objects use the UDP-based Constrained Application Protocol
(CoAP) [I-D.ietf-core-coap] to interact with a Web server to upload
sensor data at a regular intervals, such as temperature readings.
CoAP [I-D.ietf-core-coap] can utilize DTLS for securing the client-
to-server communication. As part of the manufacturing process, the
embeded device may be configured with the address and the public key
of a dedicated CoAP server, as well as a public key for the client
itself. The usage of X.509-based PKIX certificates [PKIX] may not
suit all smart object deployments and would therefore be an
unneccesarry burden.
The Transport Layer Security (TLS) Protocol Version 1.2 [RFC5246]
provides a framework for extensions to TLS as well as guidelines for
designing such extensions. This document uses the TLS Certificate
Type extension point to define a new non-X.509 certificate type for
carrying raw public keys.
2. Terminology
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].
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3. TLS Handshake Extension
This section describes the changes to the TLS handshake message
contents when raw public key certificates are to be used. 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. Note that
the certificate payloads only contain the SubjectPublicKeyInfo
structure instead of the entire certificate.
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
3.1. Client Hello
In order to indicate the support of 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].
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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.
3.2. 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
CertificateType 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
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MUST place the SubjectPublicKeyInfo structure into the Certificate
payload. The public key MUST match the selected key exchange
algorithm.
3.3. Certificate Request
The semantics of this message remain the same as in the TLS
specification.
3.4. Other Handshake Messages
All the other handshake messages are identical to the TLS
specification.
3.5. Client authentication
Client authentication by the TLS server is supported only through
authentication of the received client SubjectPublicKeyInfo via an
out-of-band method
4. Security Considerations
The transmission of raw public keys, as described in this document,
provides benefits by lowering the over-the-air transmission overhead
since raw public keys are quite naturally smaller than an entire
certificate. There are also advantages from a codesize point of view
for parsing and processing these keys. The crytographic procedures
for assocating the public key with the possession of a private key
also follows standard procedures.
The main security challenge is, however, how to associate the public
key with a specific entity. This information will be needed to make
authorization decisions. Without a secure binding, man-in-the-middle
attacks may be the consequence. This document assumes that such
binding can be made out-of-band and we list a few examples in
Section 1. DANE [I-D.ietf-dane-protocol] offers one such approach.
If public keys are obtained using DANE, these public keys are
authenticated via DNSSEC. Pre-configured keys is another out of band
method for authenticating raw public keys. While pre-configured keys
are not suitable for a generic Web-based e-commerce environment such
keys are a reasonable approach for many smart object deployments
where there is a close relationship between the software running on
the device and the server-side communication endpoint. Regardless of
the chosen mechanism for out-of-band public key validation an
assessment of the most suitable approach has to be made prior to the
start of a deployment to ensure the security of the system.
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5. IANA Considerations
This document requests IANA to assign a TLS cert_type value for
RawPublicKey. The cert_type registry is established with [RFC6091].
6. Contributors
The following individuals made important contributions to this
document: Paul Hoffman.
7. Acknowledgements
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 moved to [I-D.ietf-tls-cached-info] after the
discussions at the IETF#82 meeting and the feedback from Eric
Rescorla.
We would like to thank Martin Rex, Bill Frantz, Zach Shelby, Carsten
Bormann, Cullen Jennings, Rene Struik, Alper Yegin, and Jim Schaad.
8. References
8.1. 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.
8.2. Informative References
[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>.
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[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
"Constrained Application Protocol (CoAP)",
draft-ietf-core-coap-09 (work in progress), March 2012.
[I-D.ietf-dane-protocol]
Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Protocol for Transport Layer
Security (TLS)", draft-ietf-dane-protocol-19 (work in
progress), April 2012.
[I-D.ietf-tls-cached-info]
Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension",
draft-ietf-tls-cached-info-11 (work in progress),
December 2011.
[LDAP] Sermersheim, J., "Lightweight Directory Access Protocol
(LDAP): The Protocol", RFC 4511, June 2006.
[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/
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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
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