Internet Engineering Task Force A. Popov
Internet-Draft M. Nyström
Intended status: Standards Track Microsoft Corp.
Expires: September 28, 2015 D. Balfanz, Ed.
A. Langley
Google Inc.
March 27, 2015

Token Binding over HTTP


This document describes a collection of mechanisms that allow HTTP servers to cryptographically bind authentication tokens (such as cookies and OAuth tokens) to a TLS [RFC5246] connection.

We describe both first-party as well as federated scenarios. In a first-party scenario, an HTTP server issues a security token (such as a cookie) to a client, and expects the client to send the security token back to the server at a later time in order to authenticate. Binding the token to the TLS connection between client and server protects the security token from theft, and ensures that the security token can only be used by the client that it was issued to.

Federated token bindings, on the other hand, allow servers to cryptographically bind security tokens to a TLS [RFC5246] connection that the client has with a different server than the one issuing the token.

This Internet-Draft is a companion document to The Token Binding Protocol [TBPROTO]

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

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This Internet-Draft will expire on September 28, 2015.

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Table of Contents

1. Introduction

The Token Binding Protocol [TBPROTO] defines a Token Binding ID for a TLS connection between a client and a server. The Token Binding ID of a TLS connection is related to a private key that the client proves possession of to the server, and is long-lived (i.e., subsequent TLS connections between the same client and server have the same Token Binding ID). When issuing a security token (e.g. an HTTP cookie or an OAuth token) to a client, the server can include the Token Binding ID in the token, thus cryptographically binding the token to TLS connections between that particular client and server, and inoculating the token against theft by attackers.

While the Token Binding Protocol [TBPROTO] defines a message format for establishing a Token Binding ID, it doesn't specify how this message is embedded in higher-level protocols. The purpose of this specification is to define how TokenBindingMessages are embedded in HTTP (both versions 1.1 [RFC2616] and 2 [I-D.ietf-httpbis-http2]). Note that TokenBindingMessages are only defined if the underlying transport uses TLS. This means that Token Binding over HTTP is only defined when the HTTP protocol is layered on top of TLS (commonly referred to as HTTPS).

HTTP clients establish a Token Binding ID with a server by including a special HTTP header in HTTP requests. The HTTP header value is a TokenBindingMessage.

TokenBindingMessages allow clients to establish multiple Token Binding IDs with the server, by including multiple TokenBinding structures in the TokenBindingMessage. By default, a client will establish a provided Token Binding ID with the server, indicating a Token Binding ID that the client will persistently use with the server. Under certain conditions, the client can also include a referred Token Binding ID in the TokenBindingMessage, indicating a Token Binding ID that the client is using with a different server than the one that the TokenBindingMessage is sent to. This is useful in federation scenarios.

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. The Token-Binding Header

Once a client and server have negotiated the Token Binding Protocol with HTTP/1.1 or HTTP/2 (see The Token Binding Protocol [TBPROTO]), clients MUST include the following header in their HTTP requests:

 Token-Binding: EncodedTokenBindingMessage

The EncodedTokenBindingMessage is a web-safe Base64-encoding of the TokenBindingMessage as defined in the TokenBindingProtocol [TBPROTO].

The TokenBindingMessage MUST contain a TokenBinding with TokenBindingType provided_token_binding, which MUST be signed with the Token Binding key used by the client for connections between itself and the server that the HTTP request is sent to (clients use different Token Binding keys for different servers). The Token Binding ID established by this TokenBinding is called a Provided Token Binding ID

In HTTP/2, the client SHOULD use Header Compression [I-D.ietf-httpbis-header-compression] to avoid the overhead of repeating the same header in subsequent HTTP requests.

3. Federation Use Cases

3.1. Introduction

For privacy reasons, clients use different private keys to establish Provided Token Binding IDs with different servers. As a result, a server cannot bind a security token (such as an OAuth token or an OpenID Connect identity token) to a TLS connection that the client has with a different server. This is, however, a common requirement in federation scenarios: For example, an Identity Provider may wish to issue an identity token to a client and cryptographically bind that token to the TLS connection between the client and a Relying Party.

In this section we describe mechanisms to achieve this. The common idea among these mechanisms is that a server (called the Token Consumer in this document) gives the client permission to reveal the Provided Token Binding ID that is used between the client and itself, to another server (called the Token Provider in this document). Also common across the mechanisms is how the Token Binding ID is revealed to the Token Provider: The client uses the Token Binding Protocol [TBPROTO], and includes a TokenBinding structure in the Token-Binding HTTP header defined above. What differs between the various mechanisms is how the Token Consumer grants the permission to reveal the Token Binding ID to the Token Provider.

3.2. Overview

In a Federated Sign-On protocol, an Identity Provider issues an identity token to a client, which sends the identity token to a Relying Party to authenticate itself. Examples of this include OpenID Connect (where the identity token is called "ID Token") and SAML (where the identity token is a SAML assertion).

To better protect the security of the identity token, the Identity Provider may wish to bind the identity token to the TLS connection between the client and the Relying Party, thus ensuring that only said client can use the identity token: The Relying Party will compare the Token Binding ID in the identity token with the Token Binding ID of the TLS connection between it an the client.

This is an example of a federation scenario, which more generally can be described as follows:

The client will perform this last operation (proving possession of a private key that corresponds to a Token Binding ID between the client and the Token Consumer while delivering the token request to the Token Provider) only if the Token Consumer permits the client to do so.

Below, we will enumerate a number of mechanisms available to Token Consumers to grant this permission.

3.3. HTTP Redirects

When a Token Consumer redirects the client to a Token Provider as a means to deliver the token request, it SHOULD include the following HTTP response header in its HTTP response:

 Include-Referer-Token-Binding-ID: true

Including this response header signals to the client that it should reveal the Token Binding ID used between the client and the Token Consumer to the Token Provider. In the absence of this response header, the client will not disclose any information about the Token Binding used between the client and the Token Consumer to the Token Provider.

This header has only meaning if the HTTP status code is 302 or 301, and MUST be ignored by the client for any other status codes. If the client supports the Token Binding Protocol, and has negotiated the Token Binding Protocol with both the Token Consumer and the Token Provider, it already sends the following header to the Token Provider with each HTTP request (see above):

 Token-Binding: EncodedTokenBindingMessage

The TokenBindingMessage SHOULD contain a TokenBinding with TokenBindingType referred_token_binding. If included, this TokenBinding MUST be signed with the Token Binding key used by the client for connections between itself and the Token Consumer (more specifically, the web origin that issued the Include-Referer-Token-Binding-ID response header). The Token Binding ID established by this TokenBinding is called a Referred Token Binding ID.

As described above, the TokenBindingMessage MUST additionally contain a Provided Token Binding ID, i.e., a TokenBinding structure with TokenBindingType provided_token_binding, which MUST be signed with the Token Binding key used by the client for connections between itself and the Token Privider (more specifically, the web origin that the token request sent to).

3.4. Cross-Origin Resource Sharing

When issuing an XML HTTP request across origins to a Token Provider, a Token Consumer can reveal its Token Binding ID through the withRefererTokenBindingID property of the XmlHttpRequest object. Example:

 var xhr = new XMLHttpRequest();
 xhr.withCredentials = true; // send cookies
 xhr.withRefererTokenBindingID = true;, url, true);

The client SHOULD include the Token-Binding: header to the outgoing request as described above if:

3.5. Negotiated Key Parameters

The Token Binding Protocol [TBPROTO] allows the server and client to negotiate a signature algorithm used in the TokenBindingMessage. It is possible that the Token Binding ID used between the client and the Token Consumer, and the Token Binding ID used between the client and Token Provider, use different signature algorithms. The client MUST use the signature algorithm negotiated with the Token Consumer in the referred_token_binding TokenBinding of the TokenBindingMessage, even if that signature algorithm is different from the one negotiated with the origin that the header is sent to.

Token Providers SHOULD support all the SignatureAndHashAlgorithms specified in the Token Binding Protocol [TBPROTO]. If a token provider does not support the SignatureAndHashAlgorithm specified in the referred_token_binding TokenBinding in the TokenBindingMessage, it MUST issue an unbound token.

4. Security Considerations

4.1. Security Token Replay

The goal of the Federated Token Binding mechanisms is to prevent attackers from exporting and replaying tokens used in protocols between the client and Token Consumer, thereby impersonating legitimate users and gaining access to protected resources. Bound tokens can still be replayed by malware present in the client. In order to export the token to another machine and successfully replay it, the attacker also needs to export the corresponding private key. The Token Binding private key is therefore a high-value asset and MUST be strongly protected, ideally by generating it in a hardware security module that prevents key export.

4.2. Privacy Considerations

The Token Binding protocol uses persistent, long-lived TLS Token Binding IDs. To protect privacy, TLS Token Binding IDs are never transmitted in clear text and can be reset by the user at any time, e.g. when clearing browser cookies. Unique Token Binding IDs MUST be generated for connections to different origins, so they cannot be used by cooperating servers to link user identities.

4.3. Triple Handshake Vulnerability in TLS

The Token Binding protocol relies on the tls_unique value to associate a TLS connection with a TLS Token Binding. The triple handshake attack [TRIPLE-HS] is a known TLS protocol vulnerability allowing the attacker to synchronize tls_unique values 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.

5. References

5.1. Normative References

[I-D.ietf-httpbis-header-compression] Peon, R. and H. Ruellan, "HPACK - Header Compression for HTTP/2", Internet-Draft draft-ietf-httpbis-header-compression-12, February 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C. and B. Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)", RFC 4492, May 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5929] Altman, J., Williams, N. and L. Zhu, "Channel Bindings for TLS", RFC 5929, July 2010.
[RFC7301] Friedl, S., Popov, A., Langley, A. and E. Stephan, "Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension", RFC 7301, July 2014.
[TBPROTO] Popov, A., "The Token Binding Protocol Version 1.0", 2014.

5.2. Informative References

[I-D.ietf-httpbis-http2] Belshe, M., Peon, R. and M. Thomson, "Hypertext Transfer Protocol version 2", Internet-Draft draft-ietf-httpbis-http2-17, February 2015.
[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", Internet-Draft draft-ietf-tls-session-hash-04, March 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 Microsoft Corp. USA EMail:
Magnus Nyström Microsoft Corp. USA EMail:
Dirk Balfanz (editor) Google Inc. USA EMail:
Adam Langley Google Inc. USA EMail: