TLS P. Wouters, Ed.
Internet-Draft Red Hat
Intended status: Standards Track H. Tschofenig, Ed.
Expires: April 25, 2013 Nokia Siemens Networks
J. Gilmore
S. Weiler
SPARTA, Inc.
T. Kivinen
AuthenTec
October 22, 2012
Out-of-Band Public Key Validation for Transport Layer Security (TLS)
draft-ietf-tls-oob-pubkey-06.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 April 25, 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 Extension . . . . . . . . . . . . . . . . . . . . . . 4
4. TLS Handshake Extension . . . . . . . . . . . . . . . . . . . 7
4.1. Client Hello . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Server Hello . . . . . . . . . . . . . . . . . . . . . . . 7
4.3. Certificate Request . . . . . . . . . . . . . . . . . . . 8
4.4. Other Handshake Messages . . . . . . . . . . . . . . . . . 8
4.5. Client authentication . . . . . . . . . . . . . . . . . . 8
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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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 [RFC6698].
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 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 Extension
This section describes the changes to the TLS handshake message
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contents when raw public key certificates are to be used. Figure 3
illustrates the exchange of messages as described in the sub-sections
below. The client and the server exchange the newly defined
certificate_type extension to indicate their ability and desire to
exchange raw public keys. These raw public keys, in the form of a
SubjectPublicKeyInfo structure, are then carried inside the
certificate payload. The SubjectPublicKeyInfo structure is defined
in Section 4.1 of RFC 5280. Note that the SubjectPublicKeyInfo block
does not only contain the raw keys, such as the public exponent and
the modulus of an RSA public key, but also an algorithm identifier.
The structure, as shown in Figure 1, is encoded in an ASN.1 format
and therefore contains length information as well.
SubjectPublicKeyInfo ::= SEQUENCE {
algorithm AlgorithmIdentifier,
subjectPublicKey BIT STRING }
Figure 1: SubjectPublicKeyInfo ASN.1 Structure.
The algorithm identifiers are Object Identifiers (OIDs). RFC 3279
[RFC3279], for example, defines the following OIDs shown in Figure 2.
Key Type | Document | OID
-----------------------+----------------------------+-------------------
RSA | Section 2.3.1 of RFC 3279 | 1.2.840.113549.1.1
.......................|............................|...................
Digital Signature | |
Algorithm (DSS) | Section 2.3.2 of RFC 3279 | 1.2.840.10040.4.1
.......................|............................|...................
Elliptic Curve | |
Digital Signature | |
Algorithm (ECDSA) | Section 2.3.5 of RFC 3279 | 1.2.840.10045.2.1
-----------------------+----------------------------+-------------------
Figure 2: Example Algorithm Identifiers.
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client_hello,
certificate_type ->
<- server_hello,
certificate_type,
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 3: Basic Raw Public Key TLS Exchange.
The "certificate_type" TLS extension carries a list of supported
certificate types the client can send and receive, sorted by client
preference. Two values are defined for each certificate types to
differentiate whether a client or a server is able to process a
certificate of a specific type or can also send it. This extension
MUST be omitted if the client only supports X.509 certificates. The
"extension_data" field of this extension contains a CertTypeExtension
structure.
Note that the CertTypeExtension structure is being used both by the
client and the server, even though the structure is only specified
once in this document.
The structure of the CertTypeExtension is defined as follows:
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enum { client, server } ClientOrServerExtension;
enum { X.509-Accept (0),
X.509-Offer (1),
RawPublicKey-Accept (2),
RawPublicKey-Offer (3),
(255)
} CertificateType;
struct {
select(ClientOrServerExtension)
case client:
CertificateType certificate_types<1..2^8-1>;
case server:
CertificateType certificate_type;
}
} CertTypeExtension;
Figure 4: CertTypeExtension Structure.
The '-Offer' postfix indicates that a TLS entity is able to send the
indicated certificate type to the other communication partner. The
'-Accept' postfix indicates that a TLS entity is able to receive the
indicated certificate type.
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.
4. TLS Handshake Extension
4.1. Client Hello
In order to indicate the support of out-of-band raw public keys,
clients MUST include an extension of type "certificate_type" to the
extended client hello message. The "certificate_type" TLS extension
is assigned the value of [TBD] 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 TLS 1.2 [RFC5246].
4.2. Server Hello
If the server receives a client hello that contains the
"certificate_type" extension and chooses a cipher suite then two
outcomes are possible. The server MUST either select a certificate
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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
CertTypeExtension structure, which is included in the extended server
hello message using an extension of type "certificate_type". Servers
that only support X.509 certificates MAY omit including the
"certificate_type" extension in the extended server hello.
If the client supports the receiption of raw public keys and the
server is able to provide such a raw public key then the TLS server
MUST place the SubjectPublicKeyInfo structure into the Certificate
payload. The public key MUST match the selected key exchange
algorithm.
4.3. Certificate Request
The semantics of this message remain the same as in the TLS
specification.
4.4. Other Handshake Messages
All the other handshake messages are identical to the TLS
specification.
4.5. Client authentication
Client authentication by the TLS server is supported only through
authentication of the received client SubjectPublicKeyInfo via an
out-of-band method
5. Examples
Figure 5, Figure 6, and Figure 7 illustrate example exchanges.
The first example shows an exchange where the TLS client indicates
its ability to receive raw public keys. This client is quite
restricted since it is unable to process other certificate types sent
by the server. It also does not have credentials it could send. The
'certificate_type' extension indicates this in [1]. When the TLS
server receives the client hello it processes the certificate_type
extension. 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 [3]. The client uses this raw public key in
the TLS handshake and an out-of-band technique, such as DANE, to
verify its validity.
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client_hello,
certificate_type=(RawPublicKey-Accept) -> // [1]
<- server_hello,
certificate_type=(RawPublicKey-Offer), // [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 5: Example with Raw Public Key provided by the TLS Server
In our second example the TLS client as well as 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
is configured with a raw public key for use with TLS and is also able
to process raw public keys sent by the server. Therefore, it
indicates these capabilities in the 'certificate_type' extension in
[1]. As in the previously shown example the server fulfills the
client's request, indicates this via the 'RawPublicKey-Offer'in the
certificate_type payload, and provides a raw public key into the
Certificate payload back to the client (see [3]). The TLS server,
however, demands client authentication and therefore a
certificate_request is added [4]. The certificate_type payload in
[2] indicates that the TLS server accepts raw public keys. The TLS
client, who has a raw public key pre-provisioned, returns it in the
Certificate payload [5] to the server.
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client_hello,
certificate_type=(RawPublicKey-Offer, RawPublicKey-Accept) -> // [1]
<- server_hello,
certificate_type=(RawPublicKey-Offer,
RawPublicKey-Accept) // [2]
certificate, // [3]
certificate_request, // [4]
server_key_exchange,
server_hello_done
certificate, // [5]
client_key_exchange,
change_cipher_spec,
finished ->
<- change_cipher_spec,
finished
Application Data <-------> Application Data
Figure 6: 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 provided by the server, and the ability to send
raw public keys. The server provides the X.509 certificate in [3]
with the indication present in [2]. For client authentication,
however, the server indicates in [2] that it is able to support raw
public keys and requests a certificate from the client in [4]. The
TLS client provides a raw public key in [5] after receiving and
processing the TLS server hello message.
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client_hello,
certificate_type=(X.509-Accept, RawPublicKey-Offer) -> // [1]
<- server_hello,
certificate_type=(X.509-Offer,
RawPublicKey-Accept), // [2]
certificate, // [3]
certificate_request, // [4]
server_key_exchange,
server_hello_done
certificate, // [5]
client_key_exchange,
change_cipher_spec,
finished ->
<- change_cipher_spec,
finished
Application Data <-------> Application Data
Figure 7: 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 [RFC6698] 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
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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 a new TLS extension, "certificate_type",
assigned a value of [TBD] from the TLS ExtensionType registry defined
in [RFC5246]. This value is used as the extension number for the
extensions in both the client hello message and the server hello
message. The new extension type is used for certificate type
negotiation.
The "certificate_type" extension contains an 8-bit CertificateType
field, for which a new registry, named "TLS Certificate Types", is
established in this document, to be maintained by IANA. The registry
is segmented in the following way:
1. The values 0 - 3 are defined in Figure 4.
2. Values from 3 through 223 decimal inclusive are assigned via IETF
Consensus [RFC5226].
3. Values from 224 decimal through 255 decimal inclusive are
reserved for Private Use [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,
Klaus Hartke, Stefan Jucker, 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-12 (work in progress), October 2012.
[I-D.ietf-tls-cached-info]
Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension",
draft-ietf-tls-cached-info-13 (work in progress),
September 2012.
[LDAP] Sermersheim, J., "Lightweight Directory Access Protocol
(LDAP): The Protocol", RFC 4511, June 2006.
[RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 3279, April 2002.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012.
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Authors' Addresses
Paul Wouters (editor)
Red Hat
Email: paul@nohats.ca
Hannes Tschofenig (editor)
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland
Phone: +358 (50) 4871445
Email: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
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
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