Internet Engineering Task Force A. Popov, Ed.
Internet-Draft M. Nystroem
Intended status: Standards Track Microsoft Corp.
Expires: March 13, 2016 D. Balfanz
A. Langley
Google Inc.
September 10, 2015
Transport Layer Security (TLS) Extension for Token Binding Protocol
Negotiation
draft-ietf-tokbind-negotiation-00
Abstract
This document specifies a Transport Layer Security (TLS) [RFC5246]
extension for the negotiation of Token Binding protocol
[I-D.ietf-tokbind-protocol] version and key parameters.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. Token Binding Negotiation Client Hello Extension . . . . . . 2
3. Token Binding Negotiation Server Hello Extension . . . . . . 3
4. Negotiating Token Binding Protocol Version and Key Parameters 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6.1. Downgrade Attacks . . . . . . . . . . . . . . . . . . . . 6
6.2. Triple Handshake Vulnerability in TLS . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
In order to use the Token Binding protocol
[I-D.ietf-tokbind-protocol], the client and server need to agree on
the Token Binding protocol version and the parameters (signature and
hash algorithm, length) of the Token Binding key. This document
specifies a new TLS extension to accomplish this negotiation without
introducing additional network round-trips.
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 [RFC2119].
2. Token Binding Negotiation Client Hello Extension
The client uses the "token_binding" TLS extension to indicate the
highest supported Token Binding protocol version and key parameters.
enum {
token_binding(TBD), (65535)
} ExtensionType;
The "extension_data" field of this extension contains a
"TokenBindingParameters" value.
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struct {
uint8 major;
uint8 minor;
} ProtocolVersion;
enum {
rsa2048_pkcs1.5_sha256(0), rsa2048_pss_sha256(1), ecdsap256_sha256(2), (255)
} TokenBindingKeyParameters;
struct {
ProtocolVersion token_binding_version;
TokenBindingKeyParameters key_parameters_list<1..2^8-1>
} TokenBindingParameters;
"token_binding_version" indicates the version of the Token Binding
protocol the client wishes to use during this connection. This
SHOULD be the latest (highest valued) version supported by the
client. [I-D.ietf-tokbind-protocol] describes version {1, 0} of the
protocol. Prototype implementations of Token Binding drafts can
indicate support of a specific draft version, e.g. {0, 1} or {0, 2}.
"key_parameters_list" contains the list of identifiers of the Token
Binding key parameters supported by the client, in descending order
of preference.
3. Token Binding Negotiation Server Hello Extension
The server uses the "token_binding" TLS extension to indicate support
for the Token Binding protocol and to select the protocol version and
key parameters.
The server that supports Token Binding and receives a client hello
message containing the "token_binding" extension, will include the
"token_binding" extension in the server hello if all of the following
conditions are satisfied:
1. The server supports the Token Binding protocol version offered by
the client or a lower version.
2. The server finds acceptable Token Binding key parameters on the
client's list.
3. The server is also negotiating Extended Master Secret TLS
extension [I-D.ietf-tls-session-hash] (see security
considerations section below for more details).
The server will ignore any key parameters that it does not recognize.
The "extension_data" field of the "token_binding" extension is
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structured the same as described above for the client
"extension_data".
"token_binding_version" contains the lower of the Token Binding
protocol version offered by the client in the "token_binding"
extension and the highest version supported by the server.
"key_parameters_list" contains exactly one Token Binding key
parameters identifier selected by the server from the client's list.
4. Negotiating Token Binding Protocol Version and Key Parameters
It is expected that a server will have a list of Token Binding key
parameters identifiers that it supports, in preference order. The
server MUST only select an identifier that the client offered. The
server SHOULD select the most highly preferred key parameters
identifier it supports which is also advertised by the client. In
the event that the server supports none of the key parameters that
the client advertises, then the server MUST NOT include
"token_binding" extension in the server hello.
The client receiving the "token_binding" extension MUST terminate the
handshake with a fatal "unsupported_extension" alert if any of the
following conditions are true:
1. The client did not include the "token_binding" extension in the
client hello.
2. "token_binding_version" is higher than the Token Binding protocol
version advertised by the client.
3. "key_parameters_list" includes more than one Token Binding key
parameters identifier.
4. "key_parameters_list" includes an identifier that was not
advertised by the client.
5. Extended Master Secret [I-D.ietf-tls-session-hash] is not
negotiated (see security considerations section below for more
details).
If the "token_binding" extension is included in the server hello and
the client supports the Token Binding protocol version selected by
the server, it means that the version and key parameters have been
negotiated between the client and the server and SHALL be definitive
for the TLS connection. In this case, the client MUST use the
negotiated key parameters in the "provided_token_binding" as
described in [I-D.ietf-tokbind-protocol].
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If the client does not support the Token Binding protocol version
selected by the server, then the connection proceeds without Token
Binding.
Please note that the Token Binding protocol version and key
parameters are negotiated for each TLS connection, which means that
the client and server include their "token_binding" extensions both
in the full TLS handshake that establishes a new TLS session and in
the subsequent abbreviated TLS handshakes that resume the TLS
session.
5. IANA Considerations
This document defines a new TLS extension "token_binding", which
needs to be added to the IANA "Transport Layer Security (TLS)
Extensions" registry.
This document establishes a registry for identifiers of Token Binding
key parameters entitled "Token Binding Key Parameters" under the
"Token Binding Protocol" heading.
Entries in this registry require the following fields:
o Value: The octet value that identifies a set of Token Binding key
parameters (0-255).
o Description: The description of the Token Binding key parameters.
o Specification: A reference to a specification that defines the
Token Binding key parameters.
This registry operates under the "Expert Review" policy as defined in
[RFC5226]. The designated expert is advised to encourage the
inclusion of a reference to a permanent and readily available
specification that enables the creation of interoperable
implementations using the identified set of Token Binding key
parameters.
An initial set of registrations for this registry follows:
Value: 0
Description: rsa2048_pkcs1.5_sha256
Specification: this document
Value: 1
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Description: rsa2048_pss_sha256
Specification: this document
Value: 2
Description: ecdsap256_sha256
Specification: this document
6. Security Considerations
6.1. Downgrade Attacks
The Token Binding protocol version and key parameters are negotiated
via "token_binding" extension within the TLS handshake. TLS prevents
active attackers from modifying the messages of the TLS handshake,
therefore it is not possible for the attacker to remove or modify the
"token_binding" extension. The signature and hash algorithms and key
length used in the TokenBinding of type "provided_token_binding" MUST
match the parameters negotiated via "token_binding" extension.
6.2. Triple Handshake Vulnerability in TLS
The Token Binding protocol relies on the TLS Exporters [RFC5705] to
associate a TLS connection with a Token Binding. The triple
handshake attack [TRIPLE-HS] is a known TLS protocol vulnerability
allowing the attacker to synchronize exported keying material between
TLS connections. The attacker can then successfully replay bound
tokens. For this reason, the Token Binding protocol MUST NOT be
negotiated unless the Extended Master Secret TLS extension
[I-D.ietf-tls-session-hash] has also been negotiated.
7. Acknowledgements
This document incorporates comments and suggestions offered by Eric
Rescorla, Gabriel Montenegro, Martin Thomson, Vinod Anupam, Bill Cox,
Nick Harper and others.
8. References
8.1. Normative References
[I-D.ietf-tokbind-protocol]
Popov, A., Nystrom, M., Balfanz, D., and A. Langley, "The
Token Binding Protocol Version 1.0", draft-ietf-tokbind-
protocol-01 (work in progress), May 2015.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <http://www.rfc-editor.org/info/rfc5705>.
8.2. Informative References
[I-D.ietf-tls-session-hash]
Bhargavan, K., Delignat-Lavaud, A., Pironti, A., Langley,
A., and M. Ray, "Transport Layer Security (TLS) Session
Hash and Extended Master Secret Extension", draft-ietf-
tls-session-hash-06 (work in progress), July 2015.
[TRIPLE-HS]
Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti,
A., and P. Strub, "Triple Handshakes and Cookie Cutters:
Breaking and Fixing Authentication over TLS. IEEE
Symposium on Security and Privacy", 2014.
Authors' Addresses
Andrei Popov (editor)
Microsoft Corp.
USA
Email: andreipo@microsoft.com
Magnus Nystroem
Microsoft Corp.
USA
Email: mnystrom@microsoft.com
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Dirk Balfanz
Google Inc.
USA
Email: balfanz@google.com
Adam Langley
Google Inc.
USA
Email: agl@google.com
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