Internet Engineering Task Force A. Popov, Ed.
Internet-Draft M. Nystroem
Intended status: Standards Track Microsoft Corp.
Expires: November 30, 2015 D. Balfanz
A. Langley
Google Inc.
May 29, 2015
The Token Binding Protocol Version 1.0
draft-ietf-tokbind-protocol-01
Abstract
This document specifies Version 1.0 of the Token Binding protocol.
The Token Binding protocol allows client/server applications to
create long-lived, uniquely identifiable TLS [RFC5246] bindings
spanning multiple TLS sessions and connections. Applications are
then enabled to cryptographically bind security tokens to the TLS
layer, preventing token export and replay attacks. To protect
privacy, the TLS Token Binding identifiers are only transmitted
encrypted and can be reset by the user at any time.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 30, 2015.
Copyright Notice
Copyright (c) 2015 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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Token Binding Protocol Overview . . . . . . . . . . . . . . . 3
3. Token Binding Protocol Message . . . . . . . . . . . . . . . 4
4. Establishing a TLS Token Binding . . . . . . . . . . . . . . 6
5. TLS Token Binding ID Format . . . . . . . . . . . . . . . . . 7
6. Security Token Validation . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8.1. Security Token Replay . . . . . . . . . . . . . . . . . . 9
8.2. Downgrade Attacks . . . . . . . . . . . . . . . . . . . . 9
8.3. Privacy Considerations . . . . . . . . . . . . . . . . . 9
8.4. Token Binding Key Sharing Between Applications . . . . . 10
8.5. Triple Handshake Vulnerability in TLS . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
Web services generate various security tokens (e.g. HTTP cookies,
OAuth tokens) for web applications to access protected resources.
Any party in possession of such token gains access to the protected
resource. Attackers export bearer tokens from the user's machine,
present them to web services, and impersonate authenticated users.
The idea of Token Binding is to prevent such attacks by
cryptographically binding security tokens to the TLS layer.
A TLS Token Binding is established by the user agent generating a
private-public key pair (possibly within a secure hardware module,
such as TPM) per target server, and proving possession of the private
key on every TLS connection to the target server. The proof of
possession involves signing the tls_unique value [RFC5929] for the
TLS connection with the private key. Such TLS Token Binding is
identified by the corresponding public key. TLS Token Bindings are
long-lived, i.e. they encompass multiple TLS connections and TLS
sessions between a given client and server. To protect privacy, TLS
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Token Binding identifiers are never transmitted in clear text and can
be reset by the user at any time, e.g. when clearing browser cookies.
When issuing a security token to a client that supports TLS Token
Binding, a server includes the client's TLS Token Binding ID in the
token. Later on, when a client presents a security token containing
a TLS Token Binding ID, the server makes sure the ID in the token
matches the ID of the TLS Token Binding established with the client.
In the case of a mismatch, the server discards the token.
In order to successfully export and replay a bound security token,
the attacker needs to also be able to export the client's private
key, which is hard to do in the case of the key generated in a secure
hardware module.
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 Protocol Overview
The client and server use the Token Binding Negotiation TLS Extension
[TBNEGO] to negotiate the Token Binding protocol version and the
parameters (signature and hash algorithm, length) of the Token
Binding key. This negotiation does not require additional round-
trips.
The Token Binding protocol consists of one message sent by the client
to the server, proving possession of one or more client-generated
asymmetric keys. This message is only sent if the client and server
agree on the use of the Token Binding protocol and the key
parameters. The Token Binding message is sent with the application
protocol data in TLS application_data records.
A server receiving the Token Binding message verifies that the key
parameters in the message match the Token Binding parameters
negotiated via [TBNEGO], and then validates the signatures contained
in the Token Binding message. If either of these checks fails, the
server terminates the connection, otherwise the TLS Token Binding is
successfully established with the ID contained in the Token Binding
message.
When a server supporting the Token Binding protocol receives a bound
token, the server compares the TLS Token Binding ID in the security
token with the TLS Token Binding ID established with the client. If
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the bound token came from a TLS connection without a Token Binding,
or if the IDs don't match, the token is discarded.
This document defines the format of the Token Binding protocol
message, the process of establishing a TLS Token Binding, the format
of the Token Binding ID, and the process of validating a security
token. Token Binding Negotiation TLS Extension [TBNEGO] describes
the negotiation of the Token Binding protocol and key parameters.
Token Binding over HTTP [HTTPSTB] explains how the Token Binding
message is encapsulated within HTTP/1.1 [RFC7230] or HTTP/2 [RFC7540]
messages. [HTTPSTB] also describes Token Binding between multiple
communicating parties: User Agent, Identity Provider and Relying
Party.
3. Token Binding Protocol Message
The Token Binding message is sent by the client and proves possession
of one or more private keys held by the client. This message MUST be
sent if the client and server successfully negotiated the use of the
Token Binding protocol via [TBNEGO], and MUST NOT be sent otherwise.
This message MUST be sent in the client's first application protocol
message. This message MAY also be sent in subsequent application
protocol messages, proving possession of other keys by the same
client, to facilitate token binding between more than two
communicating parties. Token Binding over HTTP [HTTPSTB] specifies
the encapsulation of the Token Binding message in the application
protocol messages, and the scenarios involving more than two
communicating parties. The Token Binding message format is defined
using TLS specification language, and reuses existing TLS structures
and IANA registrations where possible:
enum {
sha256(4), (255)
} HashAlgorithm;
enum {
rsa(1), ecdsap256(3), (255)
} SignatureAlgorithm;
struct {
HashAlgorithm hash;
SignatureAlgorithm signature;
} SignatureAndHashAlgorithm;
struct {
opaque modulus<1..2^16-1>;
opaque publicexponent<1..2^8-1>;
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} RSAPublicKey;
enum {
secp256r1 (23), (0xFFFF)
} NamedCurve;
struct {
opaque point <1..2^8-1>;
} ECPoint;
struct {
NamedCurve namedcurve;
ECPoint point; // Uncompressed format
} ECDSAParams;
enum {
provided_token_binding(0), referred_token_binding(1), (255)
} TokenBindingType;
struct {
TokenBindingType tokenbinding_type;
SignatureAndHashAlgorithm algorithm;
select (algorithm.signature) {
case rsa: RSAPublicKey rsapubkey;
case ecdsa: ECDSAParams ecdsaparams;
}
} TokenBindingID;
enum {
(255) // No initial ExtensionType registrations
} ExtensionType;
struct {
ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} Extension;
struct {
TokenBindingID tokenbindingid;
opaque signature<0..2^16-1>;// Signature over hashed ("token binding", tls_unique)
Extension extensions<0..2^16-1>;
} TokenBinding;
struct {
TokenBinding tokenbindings<0..2^16-1>;
} TokenBindingMessage;
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The Token Binding message consists of a series of TokenBinding
structures containing the TokenBindingID, a signature over the hash
of the NUL-terminated, ASCII label ("token binding") and the
tls_unique, optionally followed by Extension structures. An
implementation MUST ignore any unknown extensions. Initially, no
extension types are defined. At least one TokenBinding MUST be
included in the Token Binding message. The signature and hash
algorithm and key length used in the TokenBinding MUST match the
parameters negotiated via [TBNEGO]. The client SHOULD generate and
store Token Binding keys in a secure manner that prevents key export.
In order to prevent cooperating servers from linking user identities,
different keys SHOULD be used by the client for connections to
different servers, according to the token scoping rules of the
application protocol.
4. Establishing a TLS Token Binding
The triple handshake vulnerability in the TLS protocol affects the
security of the Token Binding protocol, as described in the "Security
Considerations" section below. Therefore, the server MUST NOT
negotiate the use of the Token Binding protocol unless the server
also negotiates Extended Master Secret TLS extension
[I-D.ietf-tls-session-hash].
The server MUST terminate the connection if the use of the Token
Binding protocol has been successfully negotiated via [TBNEGO] within
the TLS handshake, but the client's first application message does
not contain the Token Binding message. The server MUST terminate the
connection if the use of the Token Binding protocol was not
negotiated, but the client sends the Token Binding message.
If the Token Binding type is "provided_token_binding", the server
MUST verify that the signature algorithm (including elliptic curve in
the case of ECDSA) and key length in the Token Binding message match
those negotiated via [TBNEGO]. In the case of a mismatch, the server
MUST terminate the connection. As described in [HTTPSTB], Token
Bindings of type "referred_token_binding" may have different key
parameters than those negotiated via [TBNEGO].
If the Token Binding message does not contain at least one
TokenBinding structure, or the signature contained in a TokenBinding
structure is invalid, the server MUST terminate the connection.
Otherwise, the TLS Token Binding is successfully established and its
ID can be provided to the application for security token validation.
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5. TLS Token Binding ID Format
The ID of the TLS Token Binding established as a result of Token
Binding message processing is a binary representation of the
following structure:
struct {
TokenBindingType tokenbinding_type;
SignatureAndHashAlgorithm algorithm;
select (algorithm.signature) {
case rsa: RSAPublicKey rsapubkey;
case ecdsa: ECDSAParams ecdsaparams;
}
} TokenBindingID;
TokenBindingID includes the type of the token binding and the key
parameters negotiated via [TBNEGO]. This document defines two token
binding types: provided_token_binding used to establish a Token
Binding when connecting to a server, and referred_token_binding used
when requesting tokens to be presented to a different server. Token
Binding over HTTP [HTTPSTB] describes Token Binding between multiple
communicating parties: User Agent, Identity Provider and Relying
Party. TLS Token Binding ID can be obtained from the TokenBinding
structure described in the "Token Binding Protocol Message" section
of this document by discarding the signature and extensions. TLS
Token Binding ID will be available at the application layer and used
by the server to generate and verify bound tokens.
6. Security Token Validation
Security tokens can be bound to the TLS layer either by embedding the
Token Binding ID in the token, or by maintaining a database mapping
tokens to Token Binding IDs. The specific method of generating bound
security tokens is application-defined and beyond the scope of this
document.
Upon receipt of a security token, the server attempts to retrieve TLS
Token Binding ID information from the token and from the TLS
connection with the client. Application-provided policy determines
whether to honor non-bound (bearer) tokens. If the token is bound
and a TLS Token Binding has not been established for the client
connection, the server MUST discard the token. If the TLS Token
Binding ID for the token does not match the TLS Token Binding ID
established for the client connection, the server MUST discard the
token.
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7. IANA Considerations
This document establishes a registry for Token Binding type
identifiers entitled "Token Binding Types" under the "Token Binding
Protocol" heading.
Entries in this registry require the following fields:
o Value: The octet value that identifies the Token Binding type
(0-255).
o Description: The description of the Token Binding type.
o Specification: A reference to a specification that defines the
Token Binding type.
This registry operates under the "Expert Review" policy as defined in
[RFC5226]. The designated expert is advised to encourage the
inclusion of a reference to a permanent and readily available
specification that enables the creation of interoperable
implementations using the identified Token Binding type.
An initial set of registrations for this registry follows:
Value: 0
Description: provided_token_binding
Specification: this document
Value: 1
Description: referred_token_binding
Specification: this document
This document establishes a registry for Token Binding extensions
entitled "Token Binding Extensions" under the "Token Binding
Protocol" heading.
Entries in this registry require the following fields:
o Value: The octet value that identifies the Token Binding extension
(0-255).
o Description: The description of the Token Binding extension.
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o Specification: A reference to a specification that defines the
Token Binding extension.
This registry operates under the "Expert Review" policy as defined in
[RFC5226]. The designated expert is advised to encourage the
inclusion of a reference to a permanent and readily available
specification that enables the creation of interoperable
implementations using the identified Token Binding extension. This
document creates no initial registrations in the "Token Binding
Extensions" registry.
This document uses "TLS SignatureAlgorithm" and "TLS HashAlgorithm"
registries originally created in [RFC5246], and "TLS NamedCurve"
registry originally created in [RFC4492]. This document creates no
new registrations in these registries.
8. Security Considerations
8.1. Security Token Replay
The goal of the Token Binding protocol is to prevent attackers from
exporting and replaying security tokens, thereby impersonating
legitimate users and gaining access to protected resources. Bound
tokens can still be replayed by the malware present in the User
Agent. In order to export the token to another machine and
successfully replay it, the attacker also needs to export the
corresponding private key. Token Binding private keys are therefore
high-value assets and SHOULD be strongly protected, ideally by
generating them in a hardware security module that prevents key
export.
8.2. Downgrade Attacks
The Token Binding protocol is only used when negotiated via [TBNEGO]
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
Negotiation TLS Extension used to negotiate the Token Binding
protocol and key parameters. The signature algorithm and key length
used in the TokenBinding of type "provided_token_binding" MUST match
the parameters negotiated via [TBNEGO].
8.3. 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. In order to prevent cooperating
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servers from linking user identities, different keys SHOULD be used
by the client for connections to different servers, according to the
token scoping rules of the application protocol.
8.4. Token Binding Key Sharing Between Applications
Existing systems provide a variety of platform-specific mechanisms
for certain applications to share tokens, e.g. to enable single sign-
on scenarios. For these scenarios to keep working with bound tokens,
the applications that are allowed to share tokens will need to also
share Token Binding keys. Care must be taken to restrict the sharing
of Token Binding keys to the same group(s) of applications that share
the same tokens.
8.5. 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.
9. References
9.1. Normative References
[HTTPSTB] Balfanz, D., Langley, A., Popov, A., and M. Nystroem,
"Token Binding over HTTP", 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
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[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Message Syntax and Routing", RFC 7230, June
2014.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, "Hypertext Transfer
Protocol Version 2 (HTTP/2)", RFC 7540, May 2015.
[TBNEGO] Popov, A., Nystroem, M., Balfanz, D., and A. Langley,
"Transport Layer Security (TLS) Extension for Token
Binding Protocol Negotiation", 2015.
9.2. Informative References
[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", draft-ietf-
tls-session-hash-05 (work in progress), April 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 (editor)
Microsoft Corp.
USA
Email: andreipo@microsoft.com
Magnus Nystroem
Microsoft Corp.
USA
Email: mnystrom@microsoft.com
Dirk Balfanz
Google Inc.
USA
Email: balfanz@google.com
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Adam Langley
Google Inc.
USA
Email: agl@google.com
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