Attested TLS Token Binding
draft-mandyam-tokbind-attest-01

Abstract

Token binding allows HTTP servers to bind bearer tokens to TLS connections. In order to do this, clients or user agents must prove possession of a private key. However, proof-of-possession of a private key becomes truly meaningful to a server when accompanied by an attestation statement. This specification describes extensions to the existing token binding protocol to allow for attestation statements to be sent along with the related token binding messages.

Status of This Memo

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This Internet-Draft will expire on September 8, 2017.

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Table of Contents

• 1. Introduction
• 2. Attestation Enhancement to TLS Token Binding Message
• 3. Example - Platform Attestation for Anomaly Detection
• 4. IANA Considerations
• 5. References
• 5.1. Normative References
• 5.2. Informative References
• Authors' Addresses

1. Introduction

[I-D.ietf-tokbind-protocol] and [I-D.ietf-tokbind-negotiation] describe a framework whereby servers can leverage cryptographically-bound authentication tokens to verify TLS connections. This is useful for prevention of man-in-the-middle attacks on TLS sessions, and provides a mechanism by which identity federation systems can be leveraged by a relying party to verify a client based on proof-of-possession of a private key.

Once the use of token binding is negotiated as part of the TLS handshake, an application layer message (the Token Binding message) may be sent from the client to the relying party whose primary purpose is to encapsulate a signature over a value associated with the current TLS session (Exported Key Material, i.e. EKM - see [I-D.ietf-tokbind-protocol]).

Proof-of-possession of a private key is useful to a relying party, but the associated signature in the Token Binding message does not provide an indication as to how the private key is stored and in what kind of environment the associated cryptographic operation takes place. This information may be required by a relying party in order to satisfy requirements regarding client platform integrity. Therefore, attestations are sometimes required by relying parties in order for them to accept signatures from clients. As per the definition in [I-D.birkholz-tuda], "remote attestation describes the attempt to determine the integrity and trustworthiness of an endpoint -- the attestee -- over a network to another endpoint -- the verifier -- without direct access." Attestation statements are therefore widely used in any server verification operation that leverages client cryptography.

TLS token binding can therefore be enhanced with remote attestation statements. The attestation statement can be used to augment Token Binding message. This could be used by a relying party for several different purpose, including (1) to determine whether to accept token binding messages from the associated client, or (2) require an additional mechanism for binding the TLS connection to an authentication operation by the client.

2. Attestation Enhancement to TLS Token Binding Message

The attestation statement can be processed 'in-band' as part of the Token Binding Message itself. This document leverages the TokenBinding.extensions field of the Token Binding Message as described in Section 3.4 of [I-D.ietf-tokbind-protocol], where the extension data conforms to the guidelines of Section 6.3 of the same document. The extension data takes the form of a CBOR (compact binary object representation) Data Definition Language construct, i.e. CDDL.

          
          extension_data = {attestation}
          attestation = (
            attestation_type:  tstr,
            attestation_data:  bstr,
            )
        
        

The attestation data is determined according to the attestation type. In this document, the following types are defined: "packed" (where the corresponding attestation data defined in [Webauthn]) and "TPM" (where the corresponding attestation data defined in [TPM]). Additional attestation types may be accepted by the token binding implementation.

3. Example - Platform Attestation for Anomaly Detection

An example of where a platform-based attestation is useful can be for remote attestation based on client traffic anomaly detection. Many network infrastructure deployments employ network traffic monitors for anomalous pattern detection. Examples of anomalous patterns detectable in the TLS handshake could be unexpected cipher suite negotiation for a given source/destination pairing. In this case, it may be desirable for a client-enhanced attestation reflecting for instance that an expected offered cipher suite in the client hello message is present or the originating browser integrity is intact (e.g. through a hash over the browser application package). If the network traffic monitor can interpret the atttestation included in the token binding message, then it can verify the attestation and potentially emit alerts based on an unexpected attestation.

4. IANA Considerations

This memo includes no request to IANA.

5. References

5.1. Normative References

5.2. Informative References

Authors' Addresses