draft-ietf-tokbind-https-08.txt   draft-ietf-tokbind-https-09.txt 
Internet Engineering Task Force A. Popov Internet Engineering Task Force A. Popov
Internet-Draft M. Nystroem Internet-Draft M. Nystroem
Intended status: Standards Track Microsoft Corp. Intended status: Standards Track Microsoft Corp.
Expires: August 20, 2017 D. Balfanz, Ed. Expires: October 23, 2017 D. Balfanz, Ed.
A. Langley A. Langley
Google Inc. Google Inc.
J. Hodges J. Hodges
Paypal PayPal
February 16, 2017 April 21, 2017
Token Binding over HTTP Token Binding over HTTP
draft-ietf-tokbind-https-08 draft-ietf-tokbind-https-09
Abstract Abstract
This document describes a collection of mechanisms that allow HTTP This document describes a collection of mechanisms that allow HTTP
servers to cryptographically bind security tokens (such as cookies servers to cryptographically bind security tokens (such as cookies
and OAuth tokens) to TLS [RFC5246] connections. and OAuth tokens) to TLS connections.
We describe both _first-party_ and _federated_ scenarios. In a We describe both first-party and federated scenarios. In a first-
first-party scenario, an HTTP server is able to cryptographically party scenario, an HTTP server is able to cryptographically bind the
bind the security tokens it issues to a client, and which the client security tokens it issues to a client, and which the client
subsequently returns to the server, to the TLS connection between the subsequently returns to the server, to the TLS connection between the
client and server. Such bound security tokens are protected from client and server. Such bound security tokens are protected from
misuse since the server can generally detect if they are replayed misuse since the server can generally detect if they are replayed
inappropriately, e.g., over other TLS connections. inappropriately, e.g., over other TLS connections.
Federated token bindings, on the other hand, allow servers to Federated token bindings, on the other hand, allow servers to
cryptographically bind security tokens to a TLS connection that the cryptographically bind security tokens to a TLS connection that the
client has with a _different_ server than the one issuing the token. client has with a different server than the one issuing the token.
This Internet-Draft is a companion document to The Token Binding This Internet-Draft is a companion document to The Token Binding
Protocol [I-D.ietf-tokbind-protocol] Protocol.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 20, 2017. This Internet-Draft will expire on October 23, 2017.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 31 skipping to change at page 2, line 31
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. The Sec-Token-Binding HTTP Request Header Field . . . . . . . 4 2. The Sec-Token-Binding HTTP Request Header Field . . . . . . . 4
2.1. HTTPS Token Binding Key Pair Scoping . . . . . . . . . . 5 2.1. HTTPS Token Binding Key Pair Scoping . . . . . . . . . . 5
3. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . . . 6 3. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . . . 6
4. First-party Use Cases . . . . . . . . . . . . . . . . . . . . 6 4. First-Party Use Cases . . . . . . . . . . . . . . . . . . . . 6
5. Federation Use Cases . . . . . . . . . . . . . . . . . . . . 6 5. Federation Use Cases . . . . . . . . . . . . . . . . . . . . 6
5.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 6 5.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 6
5.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 7 5.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 7
5.3. HTTP Redirects . . . . . . . . . . . . . . . . . . . . . 8 5.3. HTTP Redirects . . . . . . . . . . . . . . . . . . . . . 8
5.4. Negotiated Key Parameters . . . . . . . . . . . . . . . . 10 5.4. Negotiated Key Parameters . . . . . . . . . . . . . . . . 10
5.5. Federation Example . . . . . . . . . . . . . . . . . . . 11 5.5. Federation Example . . . . . . . . . . . . . . . . . . . 11
6. Implementation Considerations . . . . . . . . . . . . . . . . 13 6. Implementation Considerations . . . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13 7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7.1. Security Token Replay . . . . . . . . . . . . . . . . . . 13 7.1. Security Token Replay . . . . . . . . . . . . . . . . . . 13
7.2. Triple Handshake Vulnerability in TLS 1.2 and Older TLS 7.2. Triple Handshake Vulnerability in TLS 1.2 and Older TLS
Versions . . . . . . . . . . . . . . . . . . . . . . . . 14 Versions . . . . . . . . . . . . . . . . . . . . . . . . 14
7.3. Sensitivity of the Sec-Token-Binding Header . . . . . . . 14 7.3. Sensitivity of the Sec-Token-Binding Header . . . . . . . 14
7.4. Securing Federated Sign-On Protocols . . . . . . . . . . 15 7.4. Securing Federated Sign-On Protocols . . . . . . . . . . 15
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 17 8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 17
8.1. Scoping of Token Binding Key Pairs . . . . . . . . . . . 17 8.1. Scoping of Token Binding Key Pairs . . . . . . . . . . . 17
8.2. Life Time of Token Binding Key Pairs . . . . . . . . . . 18 8.2. Lifetime of Token Binding Key Pairs . . . . . . . . . . . 18
8.3. Correlation . . . . . . . . . . . . . . . . . . . . . . . 18 8.3. Correlation . . . . . . . . . . . . . . . . . . . . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
11.1. Normative References . . . . . . . . . . . . . . . . . . 19 11.1. Normative References . . . . . . . . . . . . . . . . . . 19
11.2. Informative References . . . . . . . . . . . . . . . . . 21 11.2. Informative References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction 1. Introduction
The Token Binding Protocol [I-D.ietf-tokbind-protocol] defines a The Token Binding Protocol [I-D.ietf-tokbind-protocol] defines a
Token Binding ID for a TLS connection between a client and a server. Token Binding ID for a TLS connection between a client and a server.
The Token Binding ID of a TLS connection is constructed using the The Token Binding ID of a TLS connection is constructed using the
public key of a private-public key pair. The client proves public key of a private-public key pair. The client proves
possession of the corresponding private key. This Token Binding key possession of the corresponding private key. This Token Binding key
pair is long-lived (i.e., subsequent TLS connections between the same pair is long-lived. I.e., subsequent TLS connections between the
client and server have the same Token Binding ID, unless specifically same client and server have the same Token Binding ID, unless
reset, e.g., by the user). When issuing a security token (e.g. an specifically reset, e.g., by the user. When issuing a security token
HTTP cookie or an OAuth token) to a client, the server can include (e.g., an HTTP cookie or an OAuth token [RFC6749]) to a client, the
the Token Binding ID in the token, thus cryptographically binding the server can include the Token Binding ID in the token, thus
token to TLS connections between that particular client and server, cryptographically binding the token to TLS connections between that
and inoculating the token against abuse (re-use, attempted particular client and server, and inoculating the token against abuse
impersonation, etc.) by attackers. (re-use, attempted impersonation, etc.) by attackers.
While the Token Binding Protocol [I-D.ietf-tokbind-protocol] defines While the Token Binding Protocol [I-D.ietf-tokbind-protocol] defines
a message format for establishing a Token Binding ID, it does not a message format for establishing a Token Binding ID, it does not
specify how this message is embedded in higher-level protocols. The specify how this message is embedded in higher-level protocols. The
purpose of this specification is to define how TokenBindingMessages purpose of this specification is to define how TokenBindingMessages
are embedded in HTTP (both versions 1.1 [RFC7230] and 2 [RFC7540]). are embedded in HTTP (both versions 1.1 [RFC7230] and 2 [RFC7540]).
Note that TokenBindingMessages are only defined if the underlying Note that TokenBindingMessages are only defined if the underlying
transport uses TLS. This means that Token Binding over HTTP is only transport uses TLS. This means that Token Binding over HTTP is only
defined when the HTTP protocol is layered on top of TLS (commonly defined when the HTTP protocol is layered on top of TLS (commonly
referred to as HTTPS). referred to as HTTPS).
HTTP clients establish a Token Binding ID with a server by including HTTP clients establish a Token Binding ID with a server by including
a special HTTP header field in HTTP requests. The HTTP header field a special HTTP header field in HTTP requests. The HTTP header field
value is a base64url-encoded TokenBindingMessage. value is a base64url-encoded TokenBindingMessage.
TokenBindingMessages allow clients to establish multiple Token TokenBindingMessages allow clients to establish multiple Token
Binding IDs with the server, by including multiple TokenBinding Binding IDs with the server, by including multiple TokenBinding
structures in the TokenBindingMessage. By default, a client will structures in the TokenBindingMessage. By default, a client will
establish a _provided_ Token Binding ID with the server, indicating a establish a provided Token Binding ID with the server, indicating a
Token Binding ID that the client will persistently use with the Token Binding ID that the client will persistently use with the
server. Under certain conditions, the client can also include a server. Under certain conditions, the client can also include a
_referred_ Token Binding ID in the TokenBindingMessage, indicating a referred Token Binding ID in the TokenBindingMessage, indicating a
Token Binding ID that the client is using with a _different_ server Token Binding ID that the client is using with a different server
than the one that the TokenBindingMessage is sent to. This is useful than the one that the TokenBindingMessage is sent to. This is useful
in federation scenarios. in federation scenarios.
1.1. Requirements Language 1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
2. The Sec-Token-Binding HTTP Request Header Field 2. The Sec-Token-Binding HTTP Request Header Field
Once a client and server have negotiated the Token Binding Protocol Once a client and server have negotiated the Token Binding Protocol
with HTTP/1.1 or HTTP/2 (see [I-D.ietf-tokbind-protocol] and with HTTP/1.1 or HTTP/2 (see [I-D.ietf-tokbind-protocol] and
[I-D.ietf-tokbind-negotiation]), clients MUST include a Sec-Token- [I-D.ietf-tokbind-negotiation]), clients MUST include a Sec-Token-
Binding header field in their HTTP requests, and MUST include only Binding header field in their HTTP requests, and MUST include only
one such header field per HTTP request. Also, The Sec-Token-Binding one such header field per HTTP request. Also, The Sec-Token-Binding
header field MUST NOT be included in HTTP responses. The ABNF of the header field MUST NOT be included in HTTP responses. The ABNF of the
Sec-Token-Binding header field is (in [RFC7230] style, see also Sec-Token-Binding header field is (in [RFC7230] style, see also
[RFC7231] Section 8.3): Section 8.3 of [RFC7231]):
Sec-Token-Binding = EncodedTokenBindingMessage Sec-Token-Binding = EncodedTokenBindingMessage
The header field name is "Sec-Token-Binding" and its single value, The header field name is Sec-Token-Binding and its single value,
EncodedTokenBindingMessage, is a base64url encoding of a single EncodedTokenBindingMessage, is a base64url encoding of a single
TokenBindingMessage, as defined in [I-D.ietf-tokbind-protocol], using TokenBindingMessage, as defined in [I-D.ietf-tokbind-protocol], using
the URL- and filename-safe character set described in Section 5 of the URL- and filename-safe character set described in Section 5 of
[RFC4648], with all trailing pad characters '=' omitted and without [RFC4648], with all trailing padding characters '=' omitted and
the inclusion of any line breaks, whitespace, or other additional without the inclusion of any line breaks, whitespace, or other
characters. additional characters.
For example: For example:
Sec-Token-Binding: <base64url-encoded TokenBindingMessage> Sec-Token-Binding: <base64url-encoded TokenBindingMessage>
If the server receives more than one Sec-Token-Binding header field If the server receives more than one Sec-Token-Binding header field
in an HTTP request, then the server MUST reject the message with a in an HTTP request, then the server MUST reject the message with a
400 (Bad Request) HTTP status code. Additionally, the Sec-Token- 400 (Bad Request) HTTP status code. Additionally, the Sec-Token-
Binding header field: Binding header field:
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MAY be listed by a server in a Vary response header field, and, MAY be listed by a server in a Vary response header field, and,
MUST NOT be used in HTTP trailers. MUST NOT be used in HTTP trailers.
The TokenBindingMessage MUST contain one TokenBinding structure with The TokenBindingMessage MUST contain one TokenBinding structure with
TokenBindingType of provided_token_binding, which MUST be signed with TokenBindingType of provided_token_binding, which MUST be signed with
the Token Binding private key used by the client for connections the Token Binding private key used by the client for connections
between itself and the server that the HTTP request is sent to between itself and the server that the HTTP request is sent to
(clients use different Token Binding key pairs for different servers, (clients use different Token Binding key pairs for different servers,
see Section 2.1 below). The Token Binding ID established by this see Section 2.1 below). The Token Binding ID established by this
TokenBinding is called a _Provided Token Binding ID_. TokenBinding is called a Provided Token Binding ID.
The TokenBindingMessage MAY also contain one TokenBinding structure The TokenBindingMessage MAY also contain one TokenBinding structure
with TokenBindingType of referred_token_binding, as specified in with TokenBindingType of referred_token_binding, as specified in
Section 5.3. In addition to the latter, or rather than the latter, Section 5.3. In addition to the latter, or rather than the latter,
the TokenBindingMessage MAY contain other TokenBinding structures. the TokenBindingMessage MAY contain other TokenBinding structures.
This is use case-specific, and such use cases are outside the scope This is use case-specific, and such use cases are outside the scope
of this specification. of this specification.
A TokenBindingMessage is validated by the server as described in A TokenBindingMessage is validated by the server as described in
Section 4.2. "Server Processing Rules" of Section 4.2. ("Server Processing Rules") of
[I-D.ietf-tokbind-protocol]. If validation fails and a Token Binding [I-D.ietf-tokbind-protocol]. If validation fails and a Token Binding
is rejected, any associated bound tokens MUST also be rejected by the is rejected, any associated bound tokens MUST also be rejected by the
server. HTTP requests containing invalid tokens MUST be rejected. server. HTTP requests containing invalid tokens MUST be rejected.
In this case, the server application MAY return HTTP status code 400 In this case, the server application MAY return HTTP status code 400
(Bad Request) or proceed with an application-specific invalid token (Bad Request) or proceed with an application-specific invalid token
response (e.g. directing the client to re-authenticate and present a response (e.g., directing the client to re-authenticate and present a
different token), or terminate the connection. different token), or terminate the connection.
In HTTP/2, the client SHOULD use Header Compression [RFC7541] to In HTTP/2, the client SHOULD use Header Compression [RFC7541] to
avoid the overhead of repeating the same header field in subsequent avoid the overhead of repeating the same header field in subsequent
HTTP requests. HTTP requests.
2.1. HTTPS Token Binding Key Pair Scoping 2.1. HTTPS Token Binding Key Pair Scoping
HTTPS is used in conjunction with various application protocols, and HTTPS is used in conjunction with various application protocols and
application contexts, in various ways. For example, general purpose application contexts, in various ways. For example, general-purpose
Web browsing is one such HTTP-based application context. Within the Web browsing is one such HTTP-based application context. Within the
latter context, HTTP cookies [RFC6265] are typically utilized for latter context, HTTP cookies [RFC6265] are typically utilized for
state management, including client authentication. A related, though state management, including client authentication. A related, though
distinct, example of other HTTP-based application contexts is where distinct, example of other HTTP-based application contexts is where
OAuth tokens [RFC6749] are utilized to manage authorization for OAuth tokens [RFC6749] are utilized to manage authorization for
third-party application access to resources. The token scoping rules third-party application access to resources. The token scoping rules
of these two examples can differ: the scoping rules for cookies are of these two examples can differ: the scoping rules for cookies are
concisely specified in [RFC6265], whereas OAuth is a framework and concisely specified in [RFC6265], whereas OAuth is a framework and
defines various token types with various scopings, some of which are defines various token types with various scopings, some of which are
determined by the encompassing application. determined by the encompassing application.
The scoping for those Token Binding key pairs generated by Web The scoping of Token Binding key pairs generated by Web browsers for
browsers in the context of the first-party and federation use cases use in first-party and federation use cases defined in this
defined in this specification (below), and to be used for binding specification (Section 5), and intended for binding HTTP cookies,
HTTP cookies MUST be at the granularity of "effective top-level MUST be no wider than the granularity of "effective top-level domain
domain (public suffix) + 1" (eTLD+1), i.e., at the same granularity (public suffix) + 1" (eTLD+1). I.e., the scope of Token Binding key
at which cookies can be set (see [RFC6265]). Key pairs used to bind pairs is no wider than the scope at which cookies can be set (see
other application tokens, such as OAuth tokens or Open ID Connect "ID [RFC6265]), but MAY be more narrow if cookies are scoped more
Tokens", SHOULD generally adhere to the above eTLD+1 scoping narrowly.
requirement for those tokens being employed in first-party or
federation scenarios as described below. Applications other than Web Key pairs used to bind other application tokens, such as OAuth tokens
browsers MAY use different key pair scoping rules. See also or OpenID Connect ID Tokens, SHOULD generally adhere to the above
Section 8.1, below. eTLD+1 scoping requirement for those tokens being employed in first-
party or federation scenarios. Applications other than Web browsers
MAY use different key pair scoping rules. See also Section 8.1,
below.
Scoping rules for other HTTP-based application contexts are outside Scoping rules for other HTTP-based application contexts are outside
the scope of this specification. the scope of this specification.
3. TLS Renegotiation 3. TLS Renegotiation
Token Binding over HTTP/1.1 [RFC7230] can be performed in combination Token Binding over HTTP/1.1 [RFC7230] can be performed in combination
with TLS renegotiation. In this case, renegotiation MUST only occur with TLS renegotiation. In this case, renegotiation MUST only occur
between a client's HTTP request and the server's response, the client between a client's HTTP request and the server's response, the client
MUST NOT send any pipelined requests, and the client MUST NOT MUST NOT send any pipelined requests, and the client MUST NOT
initiate renegotiation (i.e., the client may only send a initiate renegotiation. (I.e., the client may only send a
renegotiation ClientHello in response to the server's HelloRequest). renegotiation ClientHello in response to the server's HelloRequest.)
These conditions ensure that both the client and the server can These conditions ensure that both the client and the server can
clearly identify which TLS Exported Keying Material value [RFC5705] clearly identify which TLS Exported Keying Material value [RFC5705]
to use when generating or verifying the TokenBindingMessage. This to use when generating or verifying the TokenBindingMessage. This
also prevents a TokenBindingMessage from being split across TLS also prevents a TokenBindingMessage from being split across TLS
renegotiation boundaries (i.e., due to TLS message fragmentation - renegotiation boundaries. (I.e., due to TLS message fragmentation -
see Section 6.2.1 of [RFC5246]). see Section 6.2.1 of [RFC5246].)
4. First-party Use Cases 4. First-Party Use Cases
In a first-party use case (also known as a "same-site" use case), an In a first-party use case (also known as a "same-site" use case), an
HTTP server issues a security token such as a cookie (or similar) to HTTP server issues a security token such as a cookie (or similar) to
a client, and expects the client to return the security token at a a client, and expects the client to return the security token at a
later time, e.g., in order to authenticate. Binding the security later time, e.g., in order to authenticate. Binding the security
token to the TLS connection between client and server protects the token to the TLS connection between client and server protects the
security token from misuse since the server can detect if the security token from misuse, since the server can detect if the
security token is replayed inappropriately, e.g., over other TLS security token is replayed inappropriately, e.g., over other TLS
connections. connections.
See [I-D.ietf-tokbind-protocol] Section 5 for general guidance See Section 5 of [I-D.ietf-tokbind-protocol] for general guidance
regarding binding of security tokens and their subsequent validation. regarding binding of security tokens and their subsequent validation.
5. Federation Use Cases 5. Federation Use Cases
5.1. Introduction 5.1. Introduction
For privacy reasons, clients use different Token Binding key pairs to For privacy reasons, clients use different Token Binding key pairs to
establish Provided Token Binding IDs with different servers. As a establish Provided Token Binding IDs with different servers. As a
result, a server cannot bind a security token (such as an OAuth token 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 or an OpenID Connect ID Token [OpenID.Core]) to a TLS connection that
client has with a different server. This is, however, a common the client has with a different server. This is, however, a common
requirement in federation scenarios: For example, an Identity requirement in federation scenarios: For example, an Identity
Provider may wish to issue an identity token to a client and Provider may wish to issue an identity token to a client and
cryptographically bind that token to the TLS connection between the cryptographically bind that token to the TLS connection between the
client and a Relying Party. client and a Relying Party.
In this section we describe mechanisms to achieve this. The common In this section, we describe mechanisms to achieve this. The common
idea among these mechanisms is that a server (called the _Token idea among these mechanisms is that a server (called the Token
Consumer_ in this document) signals to the client that it should Consumer in this document) signals to the client that it should
reveal the Provided Token Binding ID that is used between the client reveal the Provided Token Binding ID that is used between the client
and itself, to another server (called the _Token Provider_ in this and itself, to another server (called the Token Provider in this
document). Also common across the mechanisms is how the Token document). Also common across the mechanisms is how the Token
Binding ID is revealed to the Token Provider: The client uses the Binding ID is revealed to the Token Provider: The client uses the
Token Binding Protocol [I-D.ietf-tokbind-protocol], and includes a Token Binding Protocol [I-D.ietf-tokbind-protocol], and includes a
TokenBinding structure in the Sec-Token-Binding HTTP header field TokenBinding structure in the Sec-Token-Binding HTTP header field
defined above. What differs between the various mechanisms is _how_ defined above. What differs between the various mechanisms is how
the Token Consumer signals to the client that it should reveal the the Token Consumer signals to the client that it should reveal the
Token Binding ID to the Token Provider. Below we specify one such Token Binding ID to the Token Provider. Below, we specify one such
mechanism, which is suitable for redirect-based interactions between mechanism, which is suitable for redirect-based interactions between
Token Consumers and Token Providers. Token Consumers and Token Providers.
5.2. Overview 5.2. Overview
In a Federated Sign-On protocol, an Identity Provider issues an In a Federated Sign-On protocol, an Identity Provider issues an
identity token to a client, which sends the identity token to a identity token to a client, which sends the identity token to a
Relying Party to authenticate itself. Examples of this include Relying Party to authenticate itself. Examples of this include
OpenID Connect (where the identity token is called "ID Token") and OpenID Connect (in which the identity token is called an "ID Token")
SAML (where the identity token is a SAML assertion). and SAML (in which the identity token is a SAML assertion).
To better protect the security of the identity token, the Identity To better protect the security of the identity token, the Identity
Provider may wish to bind the identity token to the TLS connection Provider may wish to bind the identity token to the TLS connection
between the client and the Relying Party, thus ensuring that only between the client and the Relying Party, thus ensuring that only
said client can use the identity token: The Relying Party will said client can use the identity token. The Relying Party will
compare the Token Binding ID in the identity token with the Token compare the Token Binding ID (or a cryptographic hash of it) in the
Binding ID of the TLS connection between it and the client. identity token with the Token Binding ID (or a hash thereof) of the
TLS connection between this Relying Party and the client.
This is an example of a federation scenario, which more generally can This is an example of a federation scenario, which more generally can
be described as follows: be described as follows:
o A Token Consumer causes the client to issue a token request to the 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 Token Provider. The goal is for the client to obtain a token and
then use it with the Token Consumer. then use it with the Token Consumer.
o The client delivers the token request to the Token Provider. o The client delivers the token request to the Token Provider.
o The Token Provider issues the token. The token is issued for the o The Token Provider issues the token. The token is issued for the
specific Token Consumer who requested it (thus preventing specific Token Consumer who requested it (thus preventing
malicious Token Consumers from using tokens with other Token malicious Token Consumers from using tokens with other Token
Consumers). The token is, however, typically a bearer token, Consumers). The token is, however, typically a bearer token,
meaning that any client can use it with the Token Consumer, not meaning that any client can use it with the Token Consumer, not
just the client to which it was issued. just the client to which it was issued.
o Therefore, in the previous step, the Token Provider may want to o Therefore, in the previous step, the Token Provider may want to
include in the token the Token Binding ID that the client uses include in the token the Token Binding ID (or a cryptographic hash
when communicating with the Token Consumer, thus _binding_ the of it) that the client uses when communicating with the Token
token to the client's Token Binding key pair. The client proves Consumer, thus binding the token to the client's Token Binding key
possession of the private key when communicating with the Token pair. The client proves possession of the private key when
Consumer through the Token Binding Protocol communicating with the Token Consumer through the Token Binding
[I-D.ietf-tokbind-protocol], and reveals the corresponding public Protocol [I-D.ietf-tokbind-protocol], and uses the corresponding
key of this key pair as part of the Token Binding ID. Comparing public key of this key pair as a component of the Token Binding
the Token Binding ID from the token to the Token Binding ID ID. Comparing the Token Binding ID from the token to the Token
established with the client allows the Token Consumer to verify Binding ID established with the client allows the Token Consumer
that the token was sent to it by the legitimate client. to verify that the token was sent to it by the legitimate client.
o To allow the Token Provider to include the Token Binding ID in the o To allow the Token Provider to include the Token Binding ID in the
token, the Token Binding ID (between client and Token Consumer) token, the Token Binding ID (between client and Token Consumer)
must therefore be communicated to the Token Provider along with must therefore be communicated to the Token Provider along with
the token request. Communicating a Token Binding ID involves the token request. Communicating a Token Binding ID involves
proving possession of a private key and is described in the Token proving possession of a private key and is described in the Token
Binding Protocol [I-D.ietf-tokbind-protocol]. Binding Protocol [I-D.ietf-tokbind-protocol].
The client will perform this last operation (proving possession of a The client will perform this last operation (proving possession of a
private key that corresponds to a Token Binding ID between the client private key that corresponds to a Token Binding ID between the client
and the Token Consumer while delivering the token request to the and the Token Consumer while delivering the token request to the
Token Provider) only if the Token Consumer requests the client to do Token Provider) only if the Token Consumer requests the client to do
so. so.
Below, we specify how Token Consumers can signal this request in Below, we specify how Token Consumers can signal this request in
redirect-based federation protocols. Note that this assumes that the redirect-based federation protocols. Note that this assumes that the
federated sign-on flow starts at the Token Consumer, or at the very federated sign-on flow starts at the Token Consumer, or at the very
least includes a redirect from Token Consumer to Token Provider. It least, includes a redirect from the Token Consumer to the Token
is outside the scope of this document to specify similar mechanisms Provider. It is outside the scope of this document to specify
for flows that do not include such redirects. similar mechanisms for flows that do not include such redirects.
5.3. HTTP Redirects 5.3. HTTP Redirects
When a Token Consumer redirects the client to a Token Provider as a When a Token Consumer redirects the client to a Token Provider as a
means to deliver the token request, it SHOULD include a Include- means to deliver the token request, it SHOULD include an Include-
Referred-Token-Binding-ID HTTP response header field in its HTTP Referred-Token-Binding-ID HTTP response header field in its HTTP
response. The ABNF of the Include-Referred-Token-Binding-ID header response. The ABNF of the Include-Referred-Token-Binding-ID header
is (in [RFC7230] style, see also [RFC7231] Section 8.3): is (in [RFC7230] style, see also Section 8.3 of [RFC7231]):
Include-Referred-Token-Binding-ID = "true" Include-Referred-Token-Binding-ID = "true"
Where the header field name is "Include-Referred-Token-Binding-ID", Where the header field name is "Include-Referred-Token-Binding-ID",
and the field-value of "true" is case-insensitive. For example: and the field-value of "true" is case-insensitive. For example:
Include-Referred-Token-Binding-ID: true Include-Referred-Token-Binding-ID: true
Including this response header field signals to the client that it Including this response header field signals to the client that it
should reveal, to the Token Provider, the Token Binding ID used should reveal, to the Token Provider, the Token Binding ID used
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If the Include-Referred-Token-Binding-ID header field is received in If the Include-Referred-Token-Binding-ID header field is received in
response to a request that did not include the Token-Binding header response to a request that did not include the Token-Binding header
field, the client MUST ignore the Include-Referred-Token-Binding-ID field, the client MUST ignore the Include-Referred-Token-Binding-ID
header field. header field.
This header field has only meaning if the HTTP status code is 301, This header field 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 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 status codes. If the client supports the Token Binding Protocol, and
has negotiated the Token Binding Protocol with both the Token has negotiated the Token Binding Protocol with both the Token
Consumer and the Token Provider, it already sends the Sec-Token- Consumer and the Token Provider, it already sends the Sec-Token-
Binding header field to the Token Provider with each HTTP request Binding header field to the Token Provider with each HTTP request (as
(see above). described in Section 2 above).
The TokenBindingMessage SHOULD contain a TokenBinding with The TokenBindingMessage SHOULD contain a TokenBinding with
TokenBindingType referred_token_binding. If included, this TokenBindingType referred_token_binding. If included, this
TokenBinding MUST be signed with the Token Binding private key used TokenBinding MUST be signed with the Token Binding private key used
by the client for connections between itself and the Token Consumer by the client for connections between itself and the Token Consumer
(more specifically, the server that issued the Include-Referred- (more specifically, the server that issued the Include-Referred-
Token-Binding-ID response header field). The Token Binding ID Token-Binding-ID response header field). The Token Binding ID
established by this TokenBinding is called a _Referred Token Binding established by this TokenBinding is called a Referred Token Binding
ID_. ID.
As described above, the TokenBindingMessage MUST additionally contain As described above, the TokenBindingMessage MUST additionally contain
a Provided Token Binding ID, i.e., a TokenBinding structure with a Provided Token Binding ID, i.e., a TokenBinding structure with
TokenBindingType of provided_token_binding, which MUST be signed with TokenBindingType of provided_token_binding, which MUST be signed with
the Token Binding private key used by the client for connections the Token Binding private key used by the client for connections
between itself and the Token Provider (more specifically, the server between itself and the Token Provider (more specifically, the server
that the token request is being sent to). that the token request is being sent to).
If for some deployment-specific reason the initial Token Provider If, for some deployment-specific reason, the initial Token Provider
("TP1") needs to redirect the client to another Token Provider ("TP1") needs to redirect the client to another Token Provider
("TP2"), rather than directly back to the Token Consumer, it can be ("TP2"), rather than directly back to the Token Consumer, it can be
accommodated using the header fields defined in this specification in accommodated using the header fields defined in this specification in
the following fashion ("the redirect-chain approach"): the following fashion ("the redirect-chain approach"):
Initially, the client is redirected to TP1 by the Token Consumer Initially, the client is redirected to TP1 by the Token Consumer
("TC"), as described above. Upon receiving the client's request, ("TC"), as described above. Upon receiving the client's request,
containing a TokenBindingMessage which contains both provided and containing a TokenBindingMessage which contains both provided and
referred TokenBindings (for TP1 and TC, respectively), TP1 referred TokenBindings (for TP1 and TC, respectively), TP1
responds to the client with a redirect response containing the responds to the client with a redirect response containing the
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Token Providers SHOULD support all the Token Binding key parameters Token Providers SHOULD support all the Token Binding key parameters
specified in [I-D.ietf-tokbind-protocol]. If a token provider does specified in [I-D.ietf-tokbind-protocol]. If a token provider does
not support the key parameters specified in the not support the key parameters specified in the
referred_token_binding TokenBinding in the TokenBindingMessage, it referred_token_binding TokenBinding in the TokenBindingMessage, it
MUST NOT issue a bound token. MUST NOT issue a bound token.
5.5. Federation Example 5.5. Federation Example
The diagram below shows a typical HTTP Redirect-based Web Browser SSO The diagram below shows a typical HTTP Redirect-based Web Browser SSO
Profile (no artifact, no callbacks), featuring binding of, e.g., a Profile (no artifact, no callbacks), featuring binding of, e.g., a
TLS Token Binding ID into an OpenID Connect "ID Token". TLS Token Binding ID into an OpenID Connect ID Token.
Legend: Legend:
+------------+------------------------------------------------------+ +------------+------------------------------------------------------+
| EKM: | TLS Exported Keying Material [RFC5705] | | EKM: | TLS Exported Keying Material [RFC5705] |
| {EKMn}Ksm: | EKM for server "n", signed by private key of TBID | | {EKMn}Ksm: | EKM for server "n", signed by private key of TBID |
| | "m", where "n" must represent server receiving the | | | "m", where "n" must represent server receiving the |
| | ETBMSG, if a conveyed TB's type is | | | ETBMSG, if a conveyed TB's type is |
| | provided_token_binding, then m = n, else if TB's | | | provided_token_binding, then m = n, else if TB's |
| | type is referred_token_binding, then m != n. E.g., | | | type is referred_token_binding, then m != n. E.g., |
| | see step 1b in diagram below. | | | see step 1b in diagram below. |
| ETBMSG: | "Sec-Token-Binding" HTTP header field conveying an | | ETBMSG: | "Sec-Token-Binding" HTTP header field conveying an |
| | EncodedTokenBindingMessage, in turn conveying | | | EncodedTokenBindingMessage, in turn conveying |
| | TokenBinding (TB)struct(s), e.g.: ETBMSG[[TB]] or | | | TokenBinding (TB)struct(s), e.g.: ETBMSG[[TB]] or |
| | ETBMSG[[TB1],[TB2]] | | | ETBMSG[[TB1],[TB2]] |
| ID Token: | the "ID Token" in OIDC, it is the semantic | | ID Token: | the ID Token in OpenID Connect, it is the semantic |
| | equivalent of a SAML "authentication assertion". "ID | | | equivalent of a SAML "authentication assertion". "ID |
| | Token w/TBIDn" denotes a "token bound" ID Token | | | Token w/TBIDn" denotes a "token bound" ID Token |
| | containing TBIDn. | | | containing TBIDn. |
| Ks & Kp: | private (aka secret) key, and public key, | | Ks & Kp: | private (aka secret) key, and public key, |
| | respectively, of client-side Token Binding key pair | | | respectively, of client-side Token Binding key pair |
| OIDC: | Open ID Connect | | OIDC: | OpenID Connect |
| TB: | TokenBinding struct containing signed EKM, TBID, and | | TB: | TokenBinding struct containing signed EKM, TBID, and |
| | TB type, e.g.: | | | TB type, e.g.: |
| | [{EKM1}Ks1,TBID1,provided_token_binding] | | | [{EKM1}Ks1,TBID1,provided_token_binding] |
| TBIDn: | Token Binding ID for client and server n's token- | | TBIDn: | Token Binding ID for client and server n's token- |
| | bound TLS association. TBIDn contains Kpn. | | | bound TLS association. TBIDn contains Kpn. |
+------------+------------------------------------------------------+ +------------+------------------------------------------------------+
Client, Token Consumer, Token Provider, Client, Token Consumer, Token Provider,
aka: aka: aka: aka: aka: aka:
User Agent OpenID Client, OpenID Provider, User Agent OpenID Client, OpenID Provider,
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However, such implementations MUST only convey Token Binding IDs to However, such implementations MUST only convey Token Binding IDs to
servers if signaled to do so by an application. For example, a servers if signaled to do so by an application. For example, a
server can return an Include-Referred-Token-Binding-ID HTTP response server can return an Include-Referred-Token-Binding-ID HTTP response
header field to a Web browser, thus signaling to the Token Binding header field to a Web browser, thus signaling to the Token Binding
implementation in the Web browser that the server intends to convey implementation in the Web browser that the server intends to convey
the Web browser's Token Binding ID to another server. Other the Web browser's Token Binding ID to another server. Other
signaling mechanisms are possible, and are specific to the signaling mechanisms are possible, and are specific to the
application layer protocol, but are outside the scope of this application layer protocol, but are outside the scope of this
specification. specification.
NOTE: See Section 8 "Privacy Considerations", for privacy guidance NOTE: See Section 8 ("Privacy Considerations"), for privacy guidance
regarding the use of this functionality. regarding the use of this functionality.
7. Security Considerations 7. Security Considerations
7.1. Security Token Replay 7.1. Security Token Replay
The goal of the Federated Token Binding mechanisms is to prevent The goal of the Federated Token Binding mechanisms is to prevent
attackers from exporting and replaying tokens used in protocols attackers from exporting and replaying tokens used in protocols
between the client and Token Consumer, thereby impersonating between the client and Token Consumer, thereby impersonating
legitimate users and gaining access to protected resources. Although legitimate users and gaining access to protected resources. Although
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The purpose of the Token Binding protocol is to convince the server The purpose of the Token Binding protocol is to convince the server
that the client that initiated the TLS connection controls a certain that the client that initiated the TLS connection controls a certain
key pair. For the server to correctly draw this conclusion after key pair. For the server to correctly draw this conclusion after
processing the Sec-Token-Binding header field, certain secrecy and processing the Sec-Token-Binding header field, certain secrecy and
integrity requirements must be met. integrity requirements must be met.
For example, the client's Token Binding private key must be kept For example, the client's Token Binding private key must be kept
secret by the client. If the private key is not secret, then another secret by the client. If the private key is not secret, then another
actor in the system could create a valid Token Binding header field, actor in the system could create a valid Token Binding header field,
impersonating the client. This can render the main purpose of the impersonating the client. This can render the main purpose of the
protocol - to bind bearer tokens to certain clients - moot: Consider, protocol - to bind bearer tokens to certain clients - moot.
for example, an attacker who obtained (perhaps through a network Consider, for example, an attacker who obtained (perhaps through a
intrusion) an authentication cookie that a client uses with a certain network intrusion) an authentication cookie that a client uses with a
server. Consider further that the server bound that cookie to the certain server. Consider further that the server bound that cookie
client's Token Binding ID precisely to thwart misuse of the cookie. to the client's Token Binding ID precisely to thwart misuse of the
If the attacker were to come into possession of the client's private cookie. If the attacker were to come into possession of the client's
key, he could then establish a TLS connection with the server and private key, he could then establish a TLS connection with the server
craft a Sec-Token-Binding header field that matches the binding and craft a Sec-Token-Binding header field that matches the binding
present in the cookie, thus successfully authenticating as the present in the cookie, thus successfully authenticating as the
client, and gaining access to the client's data at the server. The client, and gaining access to the client's data at the server. The
Token Binding protocol, in this case, did not successfully bind the Token Binding protocol, in this case, did not successfully bind the
cookie to the client. cookie to the client.
Likewise, we need integrity protection of the Sec-Token-Binding Likewise, we need integrity protection of the Sec-Token-Binding
header field: A client should not be tricked into sending a Sec- header field. A client should not be tricked into sending a Sec-
Token-Binding header field to a server that contains Token Binding Token-Binding header field to a server that contains Token Binding
messages about key pairs that the client does not control. Consider messages about key pairs that the client does not control. Consider
an attacker A that somehow has knowledge of the exported keying an attacker A that somehow has knowledge of the exported keying
material (EKM) for a TLS connection between a client C and a server material (EKM) for a TLS connection between a client C and a server
S. (While that is somewhat unlikely, it is also not entirely out of S. (While that is somewhat unlikely, it is also not entirely out of
the question, since the client might not treat the EKM as a secret - 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 after all, a pre-image-resistant hash function has been applied to
the TLS master secret, making it impossible for someone knowing the the TLS master secret, making it impossible for someone knowing the
EKM to recover the TLS master secret. Such considerations might lead 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 some clients to not treat the EKM as a secret.) Such an attacker A
could craft a Sec-Token-Binding header field with A's key pair over could craft a Sec-Token-Binding header field with A's key pair over
C's EKM. If the attacker could now trick C to send such a header C's EKM. If the attacker could now trick C into sending such a
field to S, it would appear to S as if C controls a certain key pair header field to S, it would appear to S as if C controls a certain
when in fact it does not (the attacker A controls the key pair). key pair, when in fact it does not (the attacker A controls the key
pair).
If A has a pre-existing relationship with S (perhaps has an account 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, on S), it now appears to the server S as if A is connecting to it,
even though it is really C. (If the server S does not simply use even though it is really C. (If the server S does not simply use
Token Binding IDs to identify clients, but also uses bound Token Binding IDs to identify clients, but also uses bound
authentication cookies, then A would also have to trick C into 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 sending one of A's cookies to S, which it can do through a variety of
means - inserting cookies through Javascript APIs, setting cookies means - inserting cookies through Javascript APIs, setting cookies
through related-domain attacks, etc.) In other words, A tricked C 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 into logging into A's account on S. This could lead to a loss of
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Therefore, we need to protect the integrity of the Sec-Token-Binding Therefore, we need to protect the integrity of the Sec-Token-Binding
header field. One eTLD+1 should not be able to set the Sec-Token- header field. One eTLD+1 should not be able to set the Sec-Token-
Binding header field (through a DOM API or otherwise) that the User Binding header field (through a DOM API or otherwise) that the User
Agent uses with another eTLD+1. Employing the "Sec-" header field Agent uses with another eTLD+1. Employing the "Sec-" header field
prefix helps to meet this requirement by denoting the header field prefix helps to meet this requirement by denoting the header field
name to be a "forbidden header name", see [fetch-spec]. name to be a "forbidden header name", see [fetch-spec].
7.4. Securing Federated Sign-On Protocols 7.4. Securing Federated Sign-On Protocols
As explained above, in a federated sign-in scenario a client will As explained above, in a federated sign-in scenario, a client will
prove possession of two different Token Binding private keys to a prove possession of two different Token Binding private keys to a
Token Provider: One private key corresponds to the "provided" Token Token Provider: One private key corresponds to the "provided" Token
Binding ID (which the client normally uses with the Token Provider), Binding ID (which the client normally uses with the Token Provider),
and the other is the Token Binding private key corresponding to the and the other is the Token Binding private key corresponding to the
"referred" Token Binding ID (which the client normally uses with the "referred" Token Binding ID (which the client normally uses with the
Token Consumer). The Token Provider is expected to issue a token Token Consumer). The Token Provider is expected to issue a token
that is bound to the referred Token Binding ID. that is bound to the referred Token Binding ID.
Both proofs (that of the provided Token Binding private key and that Both proofs (that of the provided Token Binding private key and that
of the referred Token Binding private key) are necessary. To show of the referred Token Binding private key) are necessary. To show
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authentication token (e.g., cookie) for the Token Provider. The man- authentication token (e.g., cookie) for the Token Provider. The man-
in-the-middle can intercept and modify any message sent by the client 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 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- the client. (This means, among other things, that the man-in-the-
middle controls the Javascript running at the client in the origin of middle controls the Javascript running at the client in the origin of
the Token Provider.) It is not, however, in possession of the the Token Provider.) It is not, however, in possession of the
client's Token Binding private key. Therefore, it can either choose client's Token Binding private key. Therefore, it can either choose
to replace the Token Binding ID in requests from the client to the to replace the Token Binding ID in requests from the client to the
Token Provider, and create a Sec-Token-Binding header field that Token Provider, and create a Sec-Token-Binding header field that
matches the TLS connection between the man-in-the-middle and the matches the TLS connection between the man-in-the-middle and the
Token Provider; or it can choose to leave the Sec-Token-Binding Token Provider, or it can choose to leave the Sec-Token-Binding
header field unchanged. If it chooses the latter, the signature in header field unchanged. If it chooses the latter, the signature in
the Token Binding message (created by the original client on the the Token Binding message (created by the original client on the
exported keying material (EKM) for the connection between client and exported keying material (EKM) for the connection between client and
man-in-the-middle) will not match a signature on the EKM between man- man-in-the-middle) will not match a signature on the EKM between man-
in-the-middle and the Token Provider. If it chooses the former (and in-the-middle and the Token Provider. If it chooses the former (and
creates its own signature, using its own Token Binding private key, creates its own signature, using its own Token Binding private key,
over the EKM for the connection between itself, the man-in-the- over the EKM for the connection between itself, the man-in-the-
middle, and Token Provider), then the Token Binding message will middle, and Token Provider), then the Token Binding message will
match the connection between man-in-the-middle and Token Provider, match the connection between man-in-the-middle and Token Provider,
but the Token Binding ID in the message will not match the Token but the Token Binding ID in the message will not match the Token
Binding ID that the client's authentication token is bound to. Binding ID that the client's authentication token is bound to.
Either way, the man-in-the-middle is detected by the Token Provider, Either way, the man-in-the-middle is detected by the Token Provider,
but only if the proof of possession of the provided Token Binding but only if the proof of possession of the provided Token Binding
private key is required in the protocol (as we do above). private key is required in the protocol (as is done above).
Next, consider the presence of a man-in-the-middle between client and 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 Token Consumer. That man-in-the-middle can intercept and modify any
message sent by the client to the Token Consumer, and any message message sent by the client to the Token Consumer and any message sent
sent by the Token Consumer to the client. The Token Consumer is the by the Token Consumer to the client. The Token Consumer is the party
party that redirects the client to the Token Provider. In this case, that redirects the client to the Token Provider. In this case, the
the man-in-the-middle controls the redirect URL, and can tamper with man-in-the-middle controls the redirect URL and can tamper with any
any redirect URL issued by the Token Consumer (as well as 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 Javascript running in the origin of the Token Consumer). The goal of
the man-in-the-middle is to trick the Token Provider to issue a token the man-in-the-middle is to trick the Token Provider into issuing a
bound to _its_ Token Binding ID, not to the Token Binding ID of the token bound to its Token Binding ID, not to the Token Binding ID of
legitimate client. To thwart this goal of the man-in-the-middle, the the legitimate client. To thwart this goal of the man-in-the-middle,
client's referred Token Binding ID must be communicated to the Token the client's referred Token Binding ID must be communicated to the
Producer in a manner that can not be affected by the man-in-the- Token Producer in a manner that cannot be affected by the man-in-the-
middle (who, as we recall, can modify redirect URLs and Javascript at middle (who, as we recall, can modify redirect URLs and Javascript at
the client). Including the referred Token Binding structure in the the client). Including the referred Token Binding structure in the
Sec-Token-Binding header field (as opposed to, say, including the Sec-Token-Binding header field (as opposed to, say, including the
referred Token Binding ID in an application-level message as part of referred Token Binding ID in an application-level message as part of
the redirect URL) is one way to assure that the man-in-the-middle the redirect URL) is one way to assure that the man-in-the-middle
between client and Token Consumer cannot affect the communication of between client and Token Consumer cannot affect the communication of
the referred Token Binding ID to the Token Provider. the referred Token Binding ID to the Token Provider.
Therefore, the Sec-Token-Binding header field in the federated sign- Therefore, the Sec-Token-Binding header field in the federated sign-
on use case contains both: a proof of possession of the provided on use case contains both: a proof of possession of the provided
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Token Binding key. Token Binding key.
8. Privacy Considerations 8. Privacy Considerations
8.1. Scoping of Token Binding Key Pairs 8.1. Scoping of Token Binding Key Pairs
Clients use different Token Binding key pairs for different servers, Clients use different Token Binding key pairs for different servers,
so as to not allow Token Binding to become a tracking tool across so as to not allow Token Binding to become a tracking tool across
different servers. However, the scoping of the Token Binding key different servers. However, the scoping of the Token Binding key
pairs to servers varies according to the scoping rules of the pairs to servers varies according to the scoping rules of the
application protocol ([I-D.ietf-tokbind-protocol] section 4.1). application protocol (Section 4.1 of [I-D.ietf-tokbind-protocol]).
In the case of HTTP cookies, servers may use Token Binding to secure In the case of HTTP cookies, servers may use Token Binding to secure
their cookies. These cookies can be attached to any sub-domain of their cookies. These cookies can be attached to any sub-domain of
effective top-level domains (eTLDs), and clients therefore should use effective top-level domains (eTLDs), and clients therefore should use
the same Token Binding key pair across such subdomains. This will the same Token Binding key pair across such subdomains. This will
ensure that any server capable of receiving the cookie will see the ensure that any server capable of receiving the cookie will see the
same Token Binding ID from the client, and thus be able to verify the same Token Binding ID from the client, and thus be able to verify the
token binding of the cookie. See Section 2.1, above. token binding of the cookie. See Section 2.1, above.
If the client application is not a Web browser, it may have If the client application is not a Web browser, it may have
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servers. For example, the client application might be aware of the servers. For example, the client application might be aware of the
fact that two servers play the role of Relying Party and Identity fact that two servers play the role of Relying Party and Identity
Provider in a federated sign-on protocol, and that they therefore Provider in a federated sign-on protocol, and that they therefore
share the identity of the user. In such cases, it is permissible to share the identity of the user. In such cases, it is permissible to
use different Token Binding key pair scoping rules, such as using the use different Token Binding key pair scoping rules, such as using the
same Token Binding key pair for both the Relying Party and the same Token Binding key pair for both the Relying Party and the
Identity Provider. Absent such special knowledge, conservative key- Identity Provider. Absent such special knowledge, conservative key-
scoping rules should be used, assuring that clients use different scoping rules should be used, assuring that clients use different
Token Binding key pairs with different servers. Token Binding key pairs with different servers.
8.2. Life Time of Token Binding Key Pairs 8.2. Lifetime of Token Binding Key Pairs
Token Binding key pairs do not have an expiration time. This means Token Binding key pairs do not have an expiration time. This means
that they can potentially be used by a server to track a user across that they can potentially be used by a server to track a user for an
an extended period of time (similar to a long-lived cookie). HTTPS extended period of time (similar to a long-lived cookie). HTTPS
clients such as web user agents should therefore provide a user clients such as Web user agents should therefore provide a user
interface for discarding Token Binding key pairs (similar to the interface for discarding Token Binding key pairs (similar to the
affordances provided to delete cookies). affordances provided to delete cookies).
If a user agent provides modes such as private browsing mode in which If a user agent provides modes such as private browsing mode in which
the user is promised that browsing state such as cookies are the user is promised that browsing state such as cookies are
discarded after the session is over, the user agent should also discarded after the session is over, the user agent should also
discard Token Binding key pairs from such modes after the session is discard Token Binding key pairs from such modes after the session is
over. Generally speaking, users should be given the same level of over. Generally speaking, users should be given the same level of
control over life time of Token Binding key pairs as they have over control over lifetime of Token Binding key pairs as they have over
cookies or other potential tracking mechanisms. cookies or other potential tracking mechanisms.
8.3. Correlation 8.3. Correlation
An application's various communicating endpoints, that receive Token An application's various communicating endpoints that receive Token
Binding IDs for TLS connections other than their own, obtain Binding IDs for TLS connections other than their own, obtain
information about the application's other TLS connections (in this information about the application's other TLS connections. (In this
context, "an application" is a combination of client-side and server- context, "an application" is a combination of client-side and server-
side components, communicating over HTTPS, where the client side may side components, communicating over HTTPS, where the client side may
be either or both web browser-based or native application-based). be either or both Web browser-based or native application-based.)
These other Token Binding IDs can serve as correlation handles for These other Token Binding IDs can serve as correlation handles for
the endpoints of the other connections. If the receiving endpoints the endpoints of the other connections. If the receiving endpoints
are otherwise aware of these other connections, then no additional are otherwise aware of these other connections, then no additional
information is being exposed. For instance, if in a redirect-based information is being exposed. For instance, if in a redirect-based
federation protocol, the Identity Provider and Relying Party already federation protocol, the Identity Provider and Relying Party already
possess URLs for one another, also having Token Binding IDs for these possess URLs for one another, also having Token Binding IDs for these
connections does not provide additional correlation information. If connections does not provide additional correlation information. If
not, then, by providing the other Token Binding IDs, additional not, then, by providing the other Token Binding IDs, additional
information is exposed that can be used to correlate the other information is exposed that can be used to correlate the other
endpoints. In such cases, a privacy analysis of enabled correlations endpoints. In such cases, a privacy analysis of enabled correlations
and their potential privacy impacts should be performed as part of and their potential privacy impacts should be performed as part of
the application design decisions of how, and whether, to utilize the application design decisions of how, and whether, to utilize
Token Binding. Token Binding.
Also, Token Binding implementations must take care to only reveal Also, Token Binding implementations must take care to only reveal
Token Binding IDs to other endpoints if the application associated Token Binding IDs to other endpoints if the application associated
with a Token Binding ID signals to do so, see Section 6 with a Token Binding ID signals to do so, see Section 6
"Implementation Considerations". ("Implementation Considerations").
Finally, care should be taken to ensure that unrelated applications Finally, care should be taken to ensure that unrelated applications
do not obtain information about each other's Token Bindings. For do not obtain information about each other's Token Bindings. For
instance, a Token Binding implementation shared between multiple instance, a Token Binding implementation shared between multiple
applications on a given system should prevent unrelated applications applications on a given system should prevent unrelated applications
from obtaining each other's Token Binding information. This may be from obtaining each other's Token Binding information. This may be
accomplished by using techniques such as application isolation and accomplished by using techniques such as application isolation and
key segregation, depending upon system capabilities. key segregation, depending upon system capabilities.
9. IANA Considerations 9. IANA Considerations
skipping to change at page 19, line 49 skipping to change at page 20, line 9
11.1. Normative References 11.1. Normative References
[fetch-spec] [fetch-spec]
WhatWG, "Fetch", Living Standard , WhatWG, "Fetch", Living Standard ,
<https://fetch.spec.whatwg.org/>. <https://fetch.spec.whatwg.org/>.
[I-D.ietf-tokbind-negotiation] [I-D.ietf-tokbind-negotiation]
Popov, A., Nystrom, M., Balfanz, D., and A. Langley, Popov, A., Nystrom, M., Balfanz, D., and A. Langley,
"Transport Layer Security (TLS) Extension for Token "Transport Layer Security (TLS) Extension for Token
Binding Protocol Negotiation", draft-ietf-tokbind- Binding Protocol Negotiation", draft-ietf-tokbind-
negotiation-06 (work in progress), November 2016. negotiation-07 (work in progress), February 2017.
[I-D.ietf-tokbind-protocol] [I-D.ietf-tokbind-protocol]
Popov, A., Nystrom, M., Balfanz, D., Langley, A., and J. Popov, A., Nystrom, M., Balfanz, D., Langley, A., and J.
Hodges, "The Token Binding Protocol Version 1.0", draft- Hodges, "The Token Binding Protocol Version 1.0", draft-
ietf-tokbind-protocol-11 (work in progress), November ietf-tokbind-protocol-13 (work in progress), February
2016. 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864, Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004, DOI 10.17487/RFC3864, September 2004,
<http://www.rfc-editor.org/info/rfc3864>. <http://www.rfc-editor.org/info/rfc3864>.
skipping to change at page 21, line 7 skipping to change at page 21, line 11
Protocol (HTTP/1.1): Semantics and Content", RFC 7231, Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014, DOI 10.17487/RFC7231, June 2014,
<http://www.rfc-editor.org/info/rfc7231>. <http://www.rfc-editor.org/info/rfc7231>.
[RFC7541] Peon, R. and H. Ruellan, "HPACK: Header Compression for [RFC7541] Peon, R. and H. Ruellan, "HPACK: Header Compression for
HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015, HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
<http://www.rfc-editor.org/info/rfc7541>. <http://www.rfc-editor.org/info/rfc7541>.
11.2. Informative References 11.2. Informative References
[OpenID.Core]
Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
C. Mortimore, "OpenID Connect Core 1.0", August 2015,
<http://openid.net/specs/openid-connect-core-1_0.html>.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov, [RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication "Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010, Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
<http://www.rfc-editor.org/info/rfc5746>. <http://www.rfc-editor.org/info/rfc5746>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", [RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012, RFC 6749, DOI 10.17487/RFC6749, October 2012,
<http://www.rfc-editor.org/info/rfc6749>. <http://www.rfc-editor.org/info/rfc6749>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
skipping to change at page 22, line 17 skipping to change at page 22, line 23
Email: balfanz@google.com Email: balfanz@google.com
Adam Langley Adam Langley
Google Inc. Google Inc.
USA USA
Email: agl@google.com Email: agl@google.com
Jeff Hodges Jeff Hodges
Paypal PayPal
USA USA
Email: Jeff.Hodges@paypal.com Email: Jeff.Hodges@paypal.com
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