Out-of-Band Public Key Validation for Transport Layer Security
draft-ietf-tls-oob-pubkey-04
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| Document | Type | Active Internet-Draft (tls WG) | |
|---|---|---|---|
| Authors | Paul Wouters , John IETF Gilmore , Samuel Weiler , Tero Kivinen , Hannes Tschofenig | ||
| Last updated | 2012-07-16 | ||
| Replaces | draft-wouters-tls-oob-pubkey | ||
| Stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-tls-oob-pubkey-04
TLS P. Wouters
Internet-Draft Red Hat
Intended status: Standards Track J. Gilmore
Expires: January 17, 2013
S. Weiler
SPARTA, Inc.
T. Kivinen
AuthenTec
H. Tschofenig
Nokia Siemens Networks
July 16, 2012
Out-of-Band Public Key Validation for Transport Layer Security
draft-ietf-tls-oob-pubkey-04.txt
Abstract
This document specifies a new 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.
This document introduces the support for raw public keys in TLS.
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 17, 2013.
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. New TLS Extensions . . . . . . . . . . . . . . . . . . . . . . 4
4. TLS Handshake Extension . . . . . . . . . . . . . . . . . . . 5
4.1. Client Hello . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Server Hello . . . . . . . . . . . . . . . . . . . . . . . 6
4.3. Certificate Request . . . . . . . . . . . . . . . . . . . 6
4.4. Certificate Payload . . . . . . . . . . . . . . . . . . . 6
4.5. Other TLS Messages . . . . . . . . . . . . . . . . . . . . 6
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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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] does 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 defines an extension to
indicate the support for 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].
3. New TLS Extensions
In order to indicate the support for multiple certificate types two
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new extensions are defined by this specification with the following
semantic:
cert-send: The certificate payload in this message contains a
certificate of the type indicated by this extension.
cert-receive: By including this extension an entity indicates that
it is able to recieve and process the indicated certificate types.
This list is sorted by preference.
enum { X.509(0), RawPublicKey(1), (255) } CertType;
CertType cert-receive <1..2^8-1>;
CertType cert-send;
Figure 1: New TLS Extension Structures
No new cipher suites are required for use with raw public keys. All
existing cipher suites that support a key exchange method compatible
with the key in the certificate can be used in combination with raw
public key certificate types.
4. TLS Handshake Extension
This section describes the semantic of the 'cert-send' and the 'cert-
receive' extensions for the different handshake messages.
4.1. Client Hello
To allow a TLS client to indicate that it is able to receive a
certificate of a specific type it MAY include the 'cert-receive'
extension in the client hello message. To indicate the ability to
process a raw public key by the server the TLS client MUST include
the 'cert-receive' with the value one (1) (indicating "RawPublicKey")
in the list of supported certificate types. If a TLS client only
supports X.509 certificates it MAY include this extension to indicate
support for it.
Future documents may define additional certificate types that require
addition values to be registered.
Note: No new cipher suites are required to use raw public keys. All
existing cipher suites that support a key exchange method compatible
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with the defined extension can be used.
4.2. Server Hello
If the server receives a client hello that contains the 'cert-
receive' extension then two outcomes are possible. The server MUST
either select a certificate type from client-provided list or
terminate the session with a fatal alert of type
"unsupported_certificate". In the former case the procedure in
Section 4.4 MUST be followed.
4.3. Certificate Request
The Certificate Request payload sent by the TLS server to the TLS
client MUST be accompanied by a 'cert-receive' extension, which
indicates to the TLS client the certificate type the server supports.
4.4. Certificate Payload
Certificate payloads MUST be accompanied by a 'cert-send' extension,
which indicates the certificate format found in the Certificate
payload itself.
The list of supported certificate types to choose from MUST have been
obtained via the 'cert-receive' extension. This ensures that a
Certificate payload only contains a certificate type that is also
supported by the recipient.
When the 'RawPublicKey' certificate type is selected then the
SubjectPublicKeyInfo structure MUST be placed into the Certificate
payload. The type of the asymmetric key MUST match the selected key
exchange algorithm.
4.5. Other TLS Messages
All the other handshake messages are identical to the TLS
specification.
5. Examples
Figure 2, Figure 3, and Figure 4 illustrate example message
exchanges.
The first example shows an exchange where the TLS client indicates
its ability to process two certificate types, namely raw public keys
and X.509 certificates via the 'cert-receive' extension (see [1]).
When the TLS server receives the client hello it processes the cert-
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receive extension and since it also has a raw public key it indicates
in [2] that it had choosen to place the SubjectPublicKeyInfo
structure into the Certificate payload (see [3]). The client uses
this raw public key in the TLS handshake and an out-of-band
technique, such as DANE, to verify its validatity.
client_hello,
cert-receive=(RawPublicKey, X.509) -> // [1]
<- server_hello,
cert-send=RawPublicKey, // [2]
certificate, // [3]
server_key_exchange,
server_hello_done
client_key_exchange,
change_cipher_spec,
finished ->
<- change_cipher_spec,
finished
Application Data <-------> Application Data
Figure 2: Example with Raw Public Key provided by the TLS Server
In our second example the TLS client and the TLS server use raw
public keys. This is a use case envisioned for smart object
networking. The TLS client in this case is an embedded device that
only supports raw public keys and therefore it indicates this
capability via the 'cert-receive' extension in [1]. As in the
previously shown example the server fulfills the client's request and
provides a raw public key into the Certificate payload back to the
client (see [2] and [3]). The TLS server, however, demands client
authentication and for this reason a Certificate_Request payload is
added [4], which comes with an indication of the supported
certificate types by the server, see [5]. The TLS client, who has a
raw public key pre-provisioned, returns it in the Certificate payload
[7] to the server with the indication about its content [6].
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client_hello,
cert-receive=(RawPublicKey) -> // [1]
<- server_hello,
cert-send=RawPublicKey,// [2]
certificate, // [3]
certificate_request, // [4]
cert-receive=(RawPublicKey, X.509) // [5]
server_key_exchange,
server_hello_done
cert-send=RawPublicKey, // [6]
certificate, // [7]
client_key_exchange,
change_cipher_spec,
finished ->
<- change_cipher_spec,
finished
Application Data <-------> Application Data
Figure 3: Example with Raw Public Key provided by the TLS Server and
the Client
In our last example we illustrate a combination of raw public key and
X.509 usage. The client uses a raw public key for client
authentication but the server provides an X.509 certificate. This
exchange starts with the client indicating its ability to process
X.509 certificates. The server provides the X.509 certificate using
that format in [3] with the indication present in [2]. For client
authentication, however, the server indicates in [5] that it is able
to support raw public keys as well as X.509 certificates. The TLS
client provides a raw public key in [7] and the indication in [6].
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client_hello,
cert-receive=(X.509) -> // [1]
<- server_hello,
cert-send=X.509,// [2]
certificate, // [3]
certificate_request, // [4]
cert-receive=(RawPublicKey, X.509) // [5]
server_key_exchange,
server_hello_done
cert-send=RawPublicKey, // [6]
certificate, // [7]
client_key_exchange,
change_cipher_spec,
finished ->
<- change_cipher_spec,
finished
Application Data <-------> Application Data
Figure 4: Hybrid Certificate Example
6. 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
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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.
7. IANA Considerations
This document defines two new TLS extension, 'cert-send' and 'cert-
receive', and their values need to be added to the TLS ExtensionType
registry created by RFC 5246 [RFC5246].
The values in these new extensions contains an 8-bit CertificateType
field, for which a new registry, named "Certificate Types", is
established in this document, to be maintained by IANA. The registry
is segmented in the following way:
1. The value (0) is defined in this document.
2. Values from 2 through 223 decimal inclusive are assigned using
the 'Specification Required' policy defined in RFC 5226
[RFC5226].
3. Values from 224 decimal through 255 decimal inclusive are
reserved for 'Private Use', see [RFC5226].
8. 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 the following persons for their review
comments: Martin Rex, Bill Frantz, Zach Shelby, Carsten Bormann,
Cullen Jennings, Rene Struik, Alper Yegin, Jim Schaad, Paul Hoffman,
Robert Cragie, Nikos Mavrogiannopoulos, Phil Hunt, John Bradley, and
James Manger.
9. References
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9.1. 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.
9.2. 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>.
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
"Constrained Application Protocol (CoAP)",
draft-ietf-core-coap-10 (work in progress), June 2012.
[I-D.ietf-dane-protocol]
Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", draft-ietf-dane-protocol-23 (work in
progress), June 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.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC6091] Mavrogiannopoulos, N. and D. Gillmor, "Using OpenPGP Keys
for Transport Layer Security (TLS) Authentication",
RFC 6091, February 2011.
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Authors' Addresses
Paul Wouters
Red Hat
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
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