Transport Layer Security (TLS) Extension for Token Binding Protocol Negotiation

In order to use the Token Binding protocol , the client and server need to agree on the Token Binding protocol version and the parameters (signature algorithm, length) of the Token Binding key. This document specifies a new TLS extension to accomplish this negotiation without introducing additional network round-trips.

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 .

The client uses the "token_binding" TLS extension to indicate the highest supported Token Binding protocol version and key parameters.

The "extension_data" field of this extension contains a "TokenBindingParameters" value.

} 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. 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. defines an initial set of identifiers for Token Binding key parameters.

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: The server supports the Token Binding protocol version offered by the client or a lower version. The server finds acceptable Token Binding key parameters on the client's list. The server is also negotiating the Extended Master Secret TLS extension. This requirement only applies when TLS 1.2 or an older TLS version is used (see security considerations section below for more details). The server is also negotiating the Renegotiation Indication TLS extension. This requirement only applies if the TLS server is configured to initiate or allow renegotiation. The server will ignore any key parameters that it does not recognize. The "extension_data" field of the "token_binding" extension is 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.

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: The client did not include the "token_binding" extension in the client hello. "token_binding_version" is higher than the Token Binding protocol version advertised by the client. "key_parameters_list" includes more than one Token Binding key parameters identifier. "key_parameters_list" includes an identifier that was not advertised by the client. TLS 1.2 or an older TLS version is used, but the Extended Master Secret TLS extension is not negotiated (see security considerations section below for more details). The client is configured to initiate or allow renegotiation, but the Renegotiation Indication TLS extension is not negotiated. 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 . 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.

This document updates the TLS "ExtensionType Values" registry originally created in . IANA has provided the following temporary registration for the "token_binding" TLS extension: Value: 24 Extension name: token_binding Reference: this document IANA is requested to make this registration permanent, keeping the value of 24, which has been used by the prototype implementations of the Token Binding protocol. This document uses "Token Binding Key Parameters" registry originally created in . This document creates no new registrations in this registry.

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 algorithm and key length used in the TokenBinding of type "provided_token_binding" MUST match the parameters negotiated via "token_binding" extension.

The Token Binding protocol relies on the TLS Exporters to associate a TLS connection with a Token Binding. The triple handshake attack 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 with these TLS versions, unless the Extended Master Secret TLS extension has also been negotiated. In addition, TLS renegotiation MUST NOT be initiated or allowed, unless the Renegotiation Indication TLS extension has been negotiated.

This document incorporates comments and suggestions offered by Eric Rescorla, Gabriel Montenegro, Martin Thomson, Vinod Anupam, Anthony Nadalin, Michael B. Jones, Bill Cox, Nick Harper, Brian Campbell and others.