Token Binding Working Group N. Harper
Internet-Draft Google Inc.
Intended status: Standards Track March 13, 2017
Expires: September 14, 2017

Token Binding for 0-RTT TLS 1.3 Connections


This document describes how Token Binding can be used in the 0-RTT data of a TLS 1.3 connection. This involves updating how Token Binding negotiation works and adding a mechanism for indicating whether a server prevents replay. A TokenBindingMessage sent in 0-RTT data has different security properties than one sent after the TLS handshake has finished, which this document also describes.

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Table of Contents

1. Introduction

Token Binding ([I-D.ietf-tokbind-protocol]) cryptographically binds security tokens (e.g. HTTP cookies, OAuth tokens) to the TLS layer on which they are presented. It does so by signing an [RFC5705] exporter value from the TLS connection. TLS 1.3 introduces a new mode that allows a client to send application data on its first flight. If this 0-RTT data contains a security token, then a client using Token Binding would want to prove possession of its Token Binding private key so that the server can verify the binding. The [RFC5705]-style exporter provided by TLS 1.3 cannot be run until the handshake has finished. TLS 1.3 also provides an exporter that can be used with 0-RTT data, but it requires that the application explicitly specify that use. This document specifies how to use the early_exporter_secret with Token Binding in TLS 1.3 0-RTT data.

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. Proposed Design

A TokenBinding struct as defined in [I-D.ietf-tokbind-protocol] contains a signature of the EKM value from the TLS layer. Under normal circumstances, a TokenBinding on a TLS 1.3 connection would use the exporter_secret to derive the EKM value. When 0-RTT data is assembled to be sent, the exporter_secret is not yet available. This design changes the definition of the TokenBinding.signature field to use the exporter with either early_exporter_secret or exporter_secret. Since no negotiation for the connection can happen before the client sends this TokenBindingMessage in 0-RTT data, this document also describes how a client decides what TokenBindingMessage to send in 0-RTT data and how a server should interpret that message.

If a client does not send any 0-RTT data, or if the server rejects the client’s 0-RTT data, then the client MUST use the 1-RTT exporter, as defined in [I-D.ietf-tokbind-protocol].

2.1. TokenBinding Signature Definition

In [I-D.ietf-tokbind-protocol], the signature field of the TokenBinding struct is defined to be the signature of a concatentation that includes the EKM value. Depending on the circumstances, the exporter value in section 7.3.3 of [I-D.ietf-tls-tls13] is computed using either exporter_secret or early_exporter_secret as the Secret.

When early_exporter_secret is used as the Secret, the client MUST indicate this use so the server knows which secret to use in signature verification. This indication is done through a new Token Binding extension, “early_exporter” (with extension type TBD). This extension always has 0-length data, so the full Extension struct is the bytes {0xTBD, 0x00, 0x00}. The early_exporter extension MUST be present in every TokenBinding struct where the exporter that is signed uses the early_exporter_secret, and it MUST NOT be present in any other TokenBinding structs.

2.1.1. Selecting Which Exporter Secret to Use

A client which is not sending any 0-RTT data on a connection MUST use the exporter defined in [I-D.ietf-tls-tls13] (using exporter_secret as the Secret) for all TokenBindingMessages on that connection so that it is compatible with [I-D.ietf-tokbind-protocol].

When a client sends a TokenBindingMessage in 0-RTT data, it must use the early_exporter_secret. After the client receives an application-layer response from the server, it must use the exporter_secret for all future token bindings on that connection. Requests sent after the client’s TLS Finished message, but before the client processes any application-layer response from the server, may use either exporter secret in their token bindings.

A server may choose to reject an application message containing a Token Binding that uses the early_exporter_secret. If it chooses to do so, it may send an application message indicating that the client should re-send the request (with a new Token Binding). In HTTP, this could be done with a 307 status code.

2.2. Negotiating Token Binding

2.2.1. Negotiation TLS Extension

The behavior of the Token Binding negotiation TLS extension does not change for a 0-RTT connection: the client and server should process this extension the same way regardless of whether the client also sent the EarlyDataIndication extension.

For the sake of choosing a key parameter to use in 0-RTT data, the client MUST use the same key parameter that was used on the connection during which the ticket (now being used for resumption) was established. The server MUST NOT accept early data if the negotiated Token Binding key parameter does not match the parameter from the initial connection. This is the same behavior as ALPN and SNI extensions.

If 0-RTT data is being sent with Token Binding using a PSK obtained out-of-band, then the Token Binding key parameter to use with that PSK must also be provisioned to both parties, and only that key parameter must be used with that PSK.

2.2.2. Replay Protection Indication Extension

The signed exporter value used in a 0-RTT connection is not guaranteed to be unique to the connection, so an attacker may be able to replay the signature without having possession of the private key. To combat this attack, a server may implement some sort of replay prevention, and indicate this to the client. A new TLS extension “token_binding_replay_indication” is defined for the client to query and server to indicate whether it has implemented a mechanism to prevent replay.

enum {
    token_binding_replay_indication(TBD), (65535)
} ExtensionType;

When sent, this extension always has zero length. If a client wishes to know whether its peer is preventing replay of TokenBinding structs across multiple connections, the client can include this extension in its ClientHello. Upon receiving this extension, the server must echo it back if it is using such a mechanism (like those described in Section 5.4.1) to prevent replay. A client that only wishes to send 0-RTT Token Binding if the server implements replay protection can send this extension on first connection establishment, and if the server doesn’t send it back (but does support Token Binding) the client can choose to not send 0-RTT messages to that server.

A client that wishes to use this extension should send it every time it sends a “token_binding” [I-D.ietf-tokbind-negotiation] extension.

3. Implementation Challenges

The client has to be able to modify the message it sends in 0-RTT data if the 0-RTT data gets rejected and needs to be retransmitted in 1-RTT data. Even if the Token Binding integration with 0-RTT were modified so that Token Binding never caused a 0-RTT reject that required rewriting a request, the client still has to handle the server rejecting the 0-RTT data for other reasons.

HTTP2 allows for requests to different domains to share the same TLS connection if the SAN of the cert covers those domains. If supports 0-RTT and Token Binding, but only supports Token Binding as defined in [I-D.ietf-tokbind-protocol], those servers cannot share a cert and use HTTP2.

4. IANA Considerations

This document defines a new TLS extension “token_binding_replay_indication”, which needs to be added to the IANA “Transport Layer Security (TLS) Extensions” registry.

This document defines a new Token Binding extension “early_exporter”, which needs to be added to the IANA “Token Binding Extensions” registry.

5. Security Considerations

Token Binding messages that use the 0-RTT exporter have weaker security properties than with the [RFC5705] exporter. If either party of a connection using Token Binding does not wish to use 0-RTT token bindings, they can do so: a client can choose to never send 0-RTT data on a connection where it uses token binding, and a server can choose to reject any 0-RTT data sent on a connection that negotiated token binding.

0-RTT data in TLS 1.3 has weaker security properties than other kinds of TLS data. Specifically, TLS 1.3 does not guarantee non-replayability of data between connections. Token Binding has similar replayability issues when in 0-RTT data, but preventing replay of Token Binding and preventing replay of 0-RTT data are two separate problems. Token Binding is not designed to prevent replay of 0-RTT data, although solutions for preventing the replay of Token Binding might also be applicable to 0-RTT data.

5.1. Proof of Possession of Token Binding Key

When a Token Binding signature is generated using the exporter with early_exporter_secret, the value being signed is under the client’s control. An attacker with temporary access to the Token Binding private key can generate Token Binding signatures for as many future connections as it has NewSessionTickets for. An attacker can construct these to be usable at any time in the future up until the NewSessionTicket’s expiration. Section 4.6.1 of [I-D.ietf-tls-tls13] requires that a NewSessionTicket be valid for a maximum of 7 days.

Unlike in [I-D.ietf-tokbind-protocol], where the proof of possession of the Token Binding key proves that the client had possession at the time the TLS handshake finished, 0-RTT Token Binding only proves that the client had possession of the Token Binding key at some point after receiving the NewSessionTicket used for that connection.

5.2. Attacks on PSK-only Key Exchange and Token Binding

An attacker who possesses the PSK can eavesdrop on an existing connection that uses that PSK to obtain a TokenBindingMessage that is valid on the connection and then hijack the connection to send whatever attacker-controlled data it wishes. Because the regular exporter closes over the server random, this TokenBindingMessage is valid only for that connection.

If the attacker does the same thing with a pure-PSK connection and 0-RTT Token Binding, the attacker can replay the original ClientHello and the exporter will stay the same, allowing the attacker to obtain a TokenBindingMessage from one connection and replay it on future connections. The only way for a server to prevent this replay is to prevent the client from ever repeating a client random in the handshake.

If a server accepting connections with PSK-only key establishment is concerned about the threat of PSK theft and also implements Token Binding, then that server must either reject all 0-RTT token bindings, or implement some form of preventing reuse of a client random.

5.3. Exporter Replayability

The exporter specified in [I-D.ietf-tokbind-protocol] is chosen so that a client and server have the same exporter value only if they are on the same TLS connection. This prevents an attacker who can read the plaintext of a TokenBindingMessage sent on that connection from replaying that message on another connection (without also having the token binding private key). The 0-RTT exporter only covers the ClientHello and the PSK of the connection, so it does not provide this guarantee.

An attacker with possession of the PSK secret and a transcript of the ClientHello and early data sent by a client under that PSK can extract the TokenBindingMessage, create a new connection to the server (using the same ClientHello and PSK), and send different application data with the same TokenBindingMessage. Note that the ClientHello contains public values for the (EC)DHE key agreement that is used as part of deriving the traffic keys for the TLS connection, so if the attacker does not also have the corresponding private values, they will not be able to read the server’s response or send a valid Finished message in the handshake for this TLS connection. Nevertheless, by that point the server has already processed the attacker’s message with the replayed TokenBindingMessage.

This sort of replayability of a TokenBindingMessage is different than the replayability caveat of 0-RTT application data in TLS 1.3. A network observer can replay 0-RTT data from TLS 1.3 without knowing any secrets of the client or server, but the application data that is replayed is untouched. This replay is done by a more powerful attacker who is able to view the plaintext and then spoof a connection with the same parameters so that the replayed TokenBindingMessage still validates when sent with different application data.

5.4. Replay Mitigations

This section presents multiple ways that a client or server can prevent the replay of a TokenBinding while still using Token Binding with 0-RTT data.

If a client or server implements a measure that prevents all replays, then its peer does not also need to implement such a mitigation. A client that is concerned about replay SHOULD implement a replay mitigation instead of relying solely on a signal from the server through the replay indication extension. Note that even with replay mitigations, 0-RTT Token Binding is vulnerable to other attacks.

5.4.1. Server Mitigations

If a server uses a session cache instead of stateless tickets, it can enforce that a PSK generated for resumption can only be used once. If an attacker tries to replay 0-RTT data (with a TokenBindingMessage), the server will reject it because the PSK was already used.

Preventing all replay of 0-RTT data is not necessary to prevent replay of a TokenBinding. A server could implement a mechanism to prevent a particular TokenBinding from being presented on more than one connection. In cases where a server’s TLS termination and application layer processing happen in different locations, this option might be easier to implement, especially when not all requests have bound tokens. This processing can also take advantage of the structure of the bound token, e.g. a token that identifies which user is making a request could shard its store of which TokenBindings have been seen based on the user ID.

A server can prevent some, but not all, 0-RTT data replay with a tight time window for the ticket age that it will accept. See Section 5.5 for more details.

5.4.2. Client Mitigations

A client cannot prevent a sufficiently motivated attacker from replaying a TokenBinding, but it can make it so difficult to replay the TokenBinding that it is easier for the attacker to steal the Token Binding key directly. If the client secures the resumption secret with the same level of protection as the Token Binding key, then the client has made it not worth the effort of the attacker to attempt to replay a TokenBinding. Ideally the resumption secret (and Token Binding key) are protected strongly and virtually non-exportable.

5.5. Early Data Ticket Age Window

When an attacker with control of the PSK secret replays a TokenBindingMessage, it has to use the same ClientHello that the client used. The ClientHello includes an “obfuscated_ticket_age” in its EarlyDataIndication extension, which the server can use to narrow the window in which that ClientHello will be accepted. Even if a PSK is valid for a week, the server will only accept that particular ClientHello for a smaller time window based on the ticket age. A server should make their acceptance window for this value as small as practical to limit an attacker’s ability to replay a ClientHello and send new application data with the stolen TokenBindingMessage.

6. Acknowledgements

The author would like to thank David Benjamin, Steven Valdez, Bill Cox, and Andrei Popov for their feedback and suggestions.

7. Normative References

[I-D.ietf-tls-tls13] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", Internet-Draft draft-ietf-tls-tls13-19, March 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", Internet-Draft draft-ietf-tokbind-negotiation-07, February 2017.
[I-D.ietf-tokbind-protocol] Popov, A., Nystrom, M., Balfanz, D., Langley, A. and J. Hodges, "The Token Binding Protocol Version 1.0", Internet-Draft draft-ietf-tokbind-protocol-13, February 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705, March 2010.

Author's Address

Nick Harper Google Inc. EMail: