Internet Engineering Task Force A. Popov
Internet-Draft M. Nystroem
Intended status: Standards Track Microsoft Corp.
Expires: April 17, 2016 D. Balfanz, Ed.
A. Langley
Google Inc.
October 15, 2015
Token Binding over HTTP
draft-ietf-tokbind-https-02
Abstract
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 http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on April 17, 2016.
Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. The Token-Binding Header . . . . . . . . . . . . . . . . . . 3
3. Federation Use Cases . . . . . . . . . . . . . . . . . . . . 4
3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
3.3. HTTP Redirects . . . . . . . . . . . . . . . . . . . . . 6
3.4. Negotiated Key Parameters . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
4.1. Security Token Replay . . . . . . . . . . . . . . . . . . 7
4.2. Privacy Considerations . . . . . . . . . . . . . . . . . 7
4.3. Triple Handshake Vulnerability in TLS . . . . . . . . . . 7
4.4. Sensitivity of the Token-Binding Header . . . . . . . . . 8
4.5. Securing Federated Sign-On Protocols . . . . . . . . . . 9
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. Normative References . . . . . . . . . . . . . . . . . . 11
5.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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
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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 Token-Binding header in their HTTP requests.
The ABNF of the Token-Binding header is:
Token-Binding = "Token-Binding" ":" [CFWS] EncodedTokenBindingMessage
The EncodedTokenBindingMessage is a web-safe Base64-encoding of the
TokenBindingMessage as defined in the TokenBindingProtocol [TBPROTO].
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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. Below we
specify one such mechanism, which is suitable for redirect-based
interactions between Token Consumers and Token Providers.
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).
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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:
o A Token Consumer causes the client to issue a token request to the
Token Provider. The goal is for the client to obtain a token and
then use it with the Token Consumer.
o The client delivers the token request to the Token Provider.
o The Token Provider issues the token. The token is issued for the
specific Token Consumer who requested it (thus preventing
malicious Token Consumers from using tokens with other Token
Consumers). The token is, however, typically a bearer token,
meaning that any client can use it with the Token Consumer, not
just the client to which it was issued.
o Therefore, in the previous step, the Token Provider may want to
include in the token the Token-Binding public key that the client
uses when communicating with the Token Consumer, thus _binding_
the token to client's Token-Binding keypair. The client proves
possession of the private key when communicating with the Token
Consumer through the Token Binding Protocol [TBPROTO], and reveals
the corresponding public key of this keypair as part of the Token
Binding ID. Comparing the public key from the token with the
public key from the Token Binding ID allows the Token Consumer to
verify that the token was sent to it by the legitimate client.
o To allow the Token Provider to include the Token-Binding public
key in the token, the Token Binding ID (between client and Token
Consumer) must therefore be communicated to the Token Provider
along with the token request. Communicating a Token Binding ID
involves proving possession of a private key and is described in
the Token Binding Protocol [TBPROTO].
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.
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Below, we specify how Token Consumers can grant this permission.
during redirect-based federation protocols.
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 a Include-
Referer-Token-Binding-ID HTTP response header in its HTTP response.
The ABNF of the Include-Referer-Token-Binding-ID header is:
Include-Referer-Token-Binding-ID = "Include-Referer-Token-Binding-ID" ":"
[CFWS] %x74.72.75.65 ; "true", case-sensitive
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 301, 302,
303, 307 or 308, 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).
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3.4. 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 exported key material (EKM)
value [RFC5705] 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 keying
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manterial 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.
4.4. Sensitivity of the Token-Binding Header
The purpose of the Token Binding protocol is to convince the server
that the client that initiated the TLS connection controls a certain
key pair. For the server to correctly draw this conclusion after
processing the Token-Binding header, certain secrecy and integrity
requirements must be met.
For example, the client's private Token Binding key must be kept
secret by the client. If the private key is not secret, then another
actor in the system could create a valid Token Binding header,
impersonating the client. This can render the main purpose of the
protocol - to bind bearer tokens to certain clients - moot: Consider,
for example, an attacker who obtained (perhaps through a network
intrusion) an authentication cookie that a client uses with a certain
server. Consider further that the server bound that cookie to the
client's Token Binding ID precisely to thwart cookie theft. If the
attacker were to come into possession of the client's private key, he
could then establish a TLS connection with the server and craft a
Token-Binding header that matches the binding present in the cookie,
thus successfully authenticating as the client, and gaining access to
the client's data at the server. The Token Binding protocol, in this
case, didn't successfully bind the cookie to the client.
Likewise, we need integrity protection of the Token-Binding header: A
client shouldn't be tricked into sending a Token-Binding header to a
server that contains Token Binding messages about key pairs that the
client doesn't control. Consider an attacker A that somehow has
knowledge of the exported keying material (EKM) for a TLS connection
between a client C and a server S. (While that is somewhat unlikely,
it's also not entirely out of the question, since the client might
not treat the EKM as a secret - after all, a pre-image-resistant hash
function has been applied to the TLS master secret, making it
impossible for someone knowing the EKM to recover the TLS master
secret. Such considerations might lead some clients to not treat the
EKM as a secret.) Such an attacker A could craft a Token-Binding
header with A's key pair over C's EKM. If the attacker could now
trick C to send such a header to S, it would appear to S as if C
controls a certain key pair when in fact it doesn't (the attacker A
controls the key pair).
If A has a pre-existing relationship with S (perhaps has an account
on S), it now appears to the server S as if A is connecting to it,
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even though it is really C. (If the server S doesn't simply use
Token Binding keys to identify clients, but also uses bound
authentication cookies, then A would also have to trick C into
sending one of A's cookies to S, which it can do through a variety of
means - inserting cookies through Javascript APIs, setting cookies
through related-domain attacks, etc.) In other words, A tricked C
into logging into A's account on S. This could lead to a loss of
privacy for C, since A presumably has some other way to also access
the account, and can thus indirectly observe A's behavior (for
example, if S has a feature that lets account holders see their
activity history on S).
Therefore, we need to protect the integrity of the Token-Binding
header. One origin should not be able to set the Token-Binding
header (through a DOM API or otherwise) that the User Agent uses with
another origin.
4.5. Securing Federated Sign-On Protocols
As explained above, in a federated sign-in scenario a client will
prove possession of two different key pairs to a Token Provider: One
key pair is the "provided" Token Binding key pair (which the client
normally uses with the Token Provider), and the other is the
"referred" Token Binding key pair (which the client normally uses
with the Token Consumer). The Token Provider is expected to issue a
token that is bound to the referred Token Binding key.
Both proofs (that of the provided Token Binding key and that of the
referred Token Binding key) are necessary. To show this, consider
the following scenario:
o The client has an authentication token with the Token Provider
that is bound to the client's Token Binding key.
o The client wants to establish a secure (i.e., free of men-in-the-
middle) authenticated session with the Token Consumer, but hasn't
done so yet (in other words, we're about to run the federated
sign-on protocol).
o A man-in-the-middle is allowed to intercept the connection between
client and Token Consumer or between Client and Token Provider (or
both).
The goal is to detect the presence of the man-in-the-middle in these
scenarios.
First, consider a man-in-the-middle between the client and the Token
Provider. Recall that we assume that the client possesses a bound
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authentication token (e.g., cookie) for the Token Provider. The man-
in-the-middle can intercept and modify any message sent by the client
to the Token Provider, and any message sent by the Token Provider to
the client. (This means, among other things, that the man-in-the-
middle controls the Javascript running at the client in the origin of
the Token Provider.) It is not, however, in possession of the
client's Token Binding key. Therefore, it can either choose to
replace the Token Binding key in requests from the client to the
Token Provider, and create a Token-Binding header that matches the
TLS connection between the man-in-the-middle and the Token Provider;
or it can choose to leave the Token-Binding header unchanged. If it
chooses the latter, the signature in the Token Binding message
(created by the original client on the exported keying material (EKM)
for the connection between client and man-in-the-middle) will not
match the EKM between man-in-the-middle and the Token Provider. If
it chooses the former (and creates its own signature, with its own
Token Binding key, over the EKM for the connection between man-in-
the-middle and Token Provider), then the Token Binding message will
match the connection between man-in-the-middle and Token Provider,
but the Token Binding key in the message will not match the Token
Binding key that the client's authentication token is bound to.
Either way, the man-in-the-middle is detected by the Token Provider,
but only if the proof of key possession of the provided Token Binding
key is required in the protocol (as we do above).
Next, consider the presence of a man-in-the-middle between client and
Token Consumer. That man-in-the-middle can intercept and modify any
message sent by the client to the Token Consumer, and any message
sent by the Token Consumer to the client. The Token Consumer is the
party that redirects the client to the Token Provider. In this case,
the man-in-the-middle controls the redirect URL, and can tamper with
any redirect URL issued by the Token Consumer (as well as with any
Javascript running in the origin of the Token Consumer). The goal of
the man-in-the-middle is to trick the Token Issuer to issue a token
bound to _its_ Token Binding key, not to the Token Binding key of the
legitimate client. To thwart this goal of the man-in-the-middle, the
client's referred Token Binding key must be communicated to the Token
Producer in a manner that can not be affected by the man-in-the-
middle (who, as we recall, can modify redirect URLs and Javascript at
the client). Including the referred Token Binding message in the
Token-Binding header (as opposed to, say, including the referred
Token Binding key in an application-level message as part of the
redirect URL) is one way to assure that the man-in-the-middle between
client and Token Consumer cannot affect the communication of the
referred Token Binding key to the Token Provider.
Therefore, the Token-Binding header in the federated sign-on use case
contains both, a proof of possession of the provided Token Binding
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key, as well as a proof of possession of the referred Token Binding
key.
5. References
5.1. Normative References
[]
Peon, R. and H. Ruellan, "HPACK - Header Compression for
HTTP/2", draft-ietf-httpbis-header-compression-12 (work in
progress), February 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[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, DOI 10.17487/
RFC2616, June 1999,
<http://www.rfc-editor.org/info/rfc2616>.
[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, DOI
10.17487/RFC4492, May 2006,
<http://www.rfc-editor.org/info/rfc4492>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <http://www.rfc-editor.org/info/rfc5705>.
[RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
<http://www.rfc-editor.org/info/rfc5929>.
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[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <http://www.rfc-editor.org/info/rfc7301>.
[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", draft-ietf-httpbis-http2-17 (work in
progress), 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", draft-ietf-
tls-session-hash-06 (work in progress), July 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: andreipo@microsoft.com
Magnus Nystroem
Microsoft Corp.
USA
Email: mnystrom@microsoft.com
Dirk Balfanz (editor)
Google Inc.
USA
Email: balfanz@google.com
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Adam Langley
Google Inc.
USA
Email: agl@google.com
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