Internet DRAFT - draft-nharper-tokbind-tls13
draft-nharper-tokbind-tls13
Network Working Group N. Harper
Internet-Draft Google Inc.
Updates: TBNEGO (if approved) July 27, 2017
Intended status: Standards Track
Expires: January 28, 2018
Token Binding for Transport Layer Security (TLS) Version 1.3 Connections
draft-nharper-tokbind-tls13-00
Abstract
Negotiation of the Token Binding protocol is only defined for
Transport Layer Security (TLS) versions 1.2 and earlier. Token
Binding users may wish to use it with TLS 1.3; this document defines
a backwards compatible way to negotiate Token Binding on TLS 1.3
connections.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 28, 2018.
Copyright Notice
Copyright (c) 2017 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
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
1. Introduction
Negotiating Token Binding using a TLS [I-D.ietf-tls-tls13] extension
as described in [I-D.ietf-tokbind-negotiation] is fairly
straightforward, but is restricted to TLS 1.2 and earlier. Only one
minor change is needed to use this extension to negotiate Token
Binding on connections using TLS 1.3 and later. Instead of the
server putting the "token_binding" extension in the ServerHello like
in TLS 1.2, in TLS 1.3 the server puts it in EncryptedExtensions
instead.
This document also non-normatively provides a clarification for the
definition of the TokenBinding.signature field from
[I-D.ietf-tokbind-protocol], since TLS 1.3 defines an alternate (but
API-compatible) exporter mechanism to the one in [RFC5705] used in
[I-D.ietf-tokbind-protocol].
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 TLS Extension
In TLS 1.3, the "token_binding" TLS extension may be present only in
ClientHello and EncryptedExtensions handshake messages. The format
of the "token_binding" TLS extension remains the same as defined in
[I-D.ietf-tokbind-negotiation].
A client puts the "token_binding" TLS extension in its ClientHello to
indicate its support for the Token Binding protocol. The client
should follow the same rules for when to send this extension and the
contents of its data as in section 2 of
[I-D.ietf-tokbind-negotiation]. Since the "token_binding" extension
remains unchanged from TLS 1.2 to TLS 1.3 in the ClientHello, a
client sending the "token_binding" extension in a TLS 1.3 ClientHello
is backwards compatible with a server that only supports TLS 1.2.
A server puts the "token_binding" TLS extension in the
EncryptedExtensions message following its ServerHello to indicate
support for the Token Binding protocol and to select protocol version
and key parameters. The server includes the extension following the
same rules as section 3 of [I-D.ietf-tokbind-negotiation], with the
following changes:
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o The "token_binding" TLS extension is in EncryptedExtensions
instead of ServerHello.
o The server MUST NOT include both the "token_binding" extension and
the "early_data" extension on the same connection.
3. Interaction with 0-RTT Data
[I-D.ietf-tls-tls13] requires that extensions define their
interaction with 0-RTT. The "token_binding" extension MUST NOT be
used with 0-RTT unless otherwise specified in another draft. A
client MAY include both "early_data" and "token_binding" extensions
in its ClientHello - this indicates that the client is willing to
resume a connection and send early data (without Token Binding), or
negotiate Token Binding on the connection and have early data
rejected.
4. Clarification of TokenBinding.signature
This non-normative section provides a clarification on the definition
of the TokenBinding.signature field when used on a TLS 1.3
connection.
[I-D.ietf-tokbind-protocol] defines the TokenBinding.signature field
in terms of an exported keying material (EKM) value as defined in
[RFC5705]. [I-D.ietf-tls-tls13] provides an equivalent interface in
section 7.5. For clarity, using the terminology from
[I-D.ietf-tls-tls13], the EKM used in section 3.3 of
[I-D.ietf-tokbind-protocol] in TLS 1.3 is the exporter value (section
7.5 of [I-D.ietf-tls-tls13]) computed with the following parameters:
o Secret: exporter_master_secret.
o label: The ASCII string "EXPORTER-Token-Binding" with no
terminating NUL.
o context_value: No context value is supplied.
o key_length: 32 bytes.
These are the same input values as specified in section 3.3 of
[I-D.ietf-tokbind-protocol].
5. Security Considerations
The consideration regarding downgrade attacks in
[I-D.ietf-tokbind-negotiation] still apply here: The parameters
negotiated in the "token_binding" extension are protected by the TLS
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handshake. An active network attacker cannot modify or remove the
"token_binding" extension without also breaking the TLS connection.
This extension cannot be used with 0-RTT data, so the concerns in
[I-D.ietf-tls-tls13] about replay do not apply here.
6. References
6.1. Normative References
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-21 (work in progress),
July 2017.
[I-D.ietf-tokbind-negotiation]
Popov, A., Nystrom, M., Balfanz, D., and A. Langley,
"Transport Layer Security (TLS) Extension for Token
Binding Protocol Negotiation", draft-ietf-tokbind-
negotiation-09 (work in progress), July 2017.
[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>.
6.2. Informative References
[I-D.ietf-tokbind-protocol]
Popov, A., Nystrom, M., Balfanz, D., Langley, A., and J.
Hodges, "The Token Binding Protocol Version 1.0", draft-
ietf-tokbind-protocol-15 (work in progress), July 2017.
[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>.
Author's Address
Nick Harper
Google Inc.
Email: nharper@google.com
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