Internet Engineering Task Force A. Popov
Internet-Draft M. Nystroem
Intended status: Standards Track Microsoft Corp.
Expires: February 27, 2017 D. Balfanz, Ed.
A. Langley
Google Inc.
J. Hodges
Paypal
August 26, 2016
Token Binding over HTTP
draft-ietf-tokbind-https-06
Abstract
This document describes a collection of mechanisms that allow HTTP
servers to cryptographically bind authentication tokens (such as
cookies and OAuth tokens) to TLS [RFC5246] connections.
We describe both _first-party_ and _federated_ scenarios. In a
first-party scenario, an HTTP server is able to cryptographically
bind the security tokens it issues to a client, and which the client
subsequently returns to the server, to the TLS connection between the
client and server. Such bound security tokens are protected from
misuse since the server can generally detect if they are replayed
inappropriately, e.g., over other TLS connections.
Federated token bindings, on the other hand, allow servers to
cryptographically bind security tokens to a TLS 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 [I-D.ietf-tokbind-protocol]
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
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 27, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. The Sec-Token-Binding Header Field . . . . . . . . . . . . . 4
2.1. HTTPS Token Binding Key Pair Scoping . . . . . . . . . . 4
3. First-party Use Cases . . . . . . . . . . . . . . . . . . . . 5
4. Federation Use Cases . . . . . . . . . . . . . . . . . . . . 5
4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 6
4.3. HTTP Redirects . . . . . . . . . . . . . . . . . . . . . 7
4.4. Negotiated Key Parameters . . . . . . . . . . . . . . . . 9
4.5. Federation Example . . . . . . . . . . . . . . . . . . . 10
5. Implementation Considerations . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6.1. Security Token Replay . . . . . . . . . . . . . . . . . . 12
6.2. Triple Handshake Vulnerability in TLS 1.2 and Older TLS
Versions . . . . . . . . . . . . . . . . . . . . . . . . 13
6.3. Sensitivity of the Sec-Token-Binding Header . . . . . . . 13
6.4. Securing Federated Sign-On Protocols . . . . . . . . . . 14
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 16
7.1. Scoping of Token Binding Keys . . . . . . . . . . . . . . 16
7.2. Life Time of Token Binding Keys . . . . . . . . . . . . . 16
7.3. Correlation . . . . . . . . . . . . . . . . . . . . . . . 17
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. Normative References . . . . . . . . . . . . . . . . . . 18
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10.2. Informative References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
The Token Binding Protocol [I-D.ietf-tokbind-protocol] 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
token to TLS connections between that particular client and server,
and inoculating the token against abuse (re-use, attempted
impersonation, etc.) by attackers.
While the Token Binding Protocol [I-D.ietf-tokbind-protocol] defines
a message format for establishing a Token Binding ID, it does not
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 [RFC7230] and 2 [RFC7540]).
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 field in HTTP requests. The HTTP header field
value is a base64url-encoded 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].
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2. The Sec-Token-Binding Header Field
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
[I-D.ietf-tokbind-negotiation]), clients MUST include the Sec-Token-
Binding header field in their HTTP requests. The ABNF of the Sec-
Token-Binding header field is (in [RFC7230] style, see also [RFC7231]
Section 8.3):
Sec-Token-Binding = EncodedTokenBindingMessage
The header field name is "Sec-Token-Binding" and its value is a
base64url encoding of the TokenBindingMessage defined in
[I-D.ietf-tokbind-protocol] using the URL- and filename-safe
character set described in Section 5 of [RFC4648], with all trailing
pad characters '=' omitted and without the inclusion of any line
breaks, whitespace, or other additional characters.
For example:
Sec-Token-Binding: <base64url-encoded TokenBindingMessage>
The TokenBindingMessage MUST contain one TokenBinding structure with
TokenBindingType of provided_token_binding, which MUST be signed with
the Token Binding private 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, see
Section 2.1 below). The Token Binding ID established by this
TokenBinding is called a _Provided Token Binding ID_.
The TokenBindingMessage MAY also contain one TokenBinding structure
with TokenBindingType of referred_token_binding, as specified in
Section 4.3. In addition to the latter, or rather than the latter,
the TokenBindingMessage MAY contain other TokenBinding structures.
This is use case-specific, and such use cases are outside the scope
of this specification.
In HTTP/2, the client SHOULD use Header Compression [RFC7541] to
avoid the overhead of repeating the same header field in subsequent
HTTP requests.
2.1. HTTPS Token Binding Key Pair Scoping
HTTPS is used in conjunction with various application protocols, and
application contexts, in various ways. For example, general purpose
Web browsing is one such HTTP-based application context. Within the
latter context, HTTP cookies [RFC6265] are typically utilized for
state management, including client authentication. A related, though
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distinct, example of other HTTP-based application contexts is where
OAuth tokens [RFC6749] are utilized to manage authorization for
third-party application access to resources. The token scoping rules
of these two examples can differ: the scoping rules for cookies are
concisely specified in [RFC6265], whereas OAuth is a framework and
defines various token types with various scopings, some of which are
determined by the encompassing application.
The Token Binding key pair scoping for those key pairs generated in
the context of the first-party and federation use cases defined in
this specification (below), and to be used for binding HTTP cookies
MUST be at the granularity of "effective top-level domain (public
suffix) + 1" (eTLD+1), i.e., at the same granularity at which cookies
can be set (see [RFC6265]). Key pairs used to bind other application
tokens, such as OAuth tokens, SHOULD adhere to the above eTLD+1
scoping requirement for those tokens being employed in first-party or
federation scenarios as described below, e.g., OAuth refresh tokens
or Open ID Connect "ID Tokens". See also Section 7.1, below.
Scoping rules for other HTTP-based application contexts are outside
the scope of this specification.
3. First-party Use Cases
In a first-party use case, an 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 later time, e.g., in order to
authenticate. Binding the security token to the TLS connection
between client and server protects the security token from misuse
since the server can detect if the security token is replayed
inappropriately, e.g., over other TLS connections.
See [I-D.ietf-tokbind-protocol] Section 6 for general guidance
regarding binding of security tokens and their subsequent validation.
4. Federation Use Cases
4.1. 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.
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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) signals to the client that it should
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 [I-D.ietf-tokbind-protocol], and includes a
TokenBinding structure in the Sec-Token-Binding HTTP header field
defined above. What differs between the various mechanisms is _how_
the Token Consumer signals to the client that it should 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.
4.2. 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).
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 and 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
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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
[I-D.ietf-tokbind-protocol], 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 [I-D.ietf-tokbind-protocol].
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 requests the client to do
so.
Below, we specify how Token Consumers can signal this request in
redirect-based federation protocols. Note that this assumes that the
federated sign-on flow starts at the Token Consumer, or at the very
least include a redirect from Token Consumer to Token Provider. It
is outside the scope of this document to specify similar mechanisms
for flows that do not include such redirects.
4.3. 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-
Referred-Token-Binding-ID HTTP response header field in its HTTP
response. The ABNF of the Include-Referred-Token-Binding-ID header
is (in [RFC7230] style, see also [RFC7231] Section 8.3):
Include-Referred-Token-Binding-ID = "true"
Where the header field name is "Include-Referred-Token-Binding-ID",
and the field-value of "true" is case-insensitive. For example:
Include-Referred-Token-Binding-ID: true
Including this response header field signals to the client that it
should reveal, to the Token Provider, the Token Binding ID used
between itself and the Token Consumer. In the absence of this
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response header field, the client will not disclose any information
about the Token Binding used between the client and the Token
Consumer to the Token Provider.
As illustrated in Section 4.5, when a client receives this header
field, it should take the TokenBindingID of the provided TokenBinding
from the referrer and create a referred TokenBinding with it to
include in the TokenBindingMessage on the redirect request. In other
words, the Token Binding message in the redirect request to the Token
Provider now includes one provided binding and one referred binding,
the latter constructed from the binding between the client and the
Token Consumer.
When a client receives the Include-Referred-Token-Binding-ID header,
it includes the referred token binding even if both the Token
Provider and the Token Consumer fall under the same eTLD+1 and the
provided and referred token binding IDs are the same. Note that the
referred token binding is sent only on the request resulting from the
redirect and not on any subsequent requests to the Token Provider.
If the Include-Referred-Token-Binding-ID header field is received in
response to a request that did not include the Token-Binding header
field, the client MUST ignore the Include-Referred-Token-Binding-ID
header field.
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
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 Sec-Token-
Binding header field to the Token Provider with each HTTP request
(see above).
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-Referred-Token-
Binding-ID response header field). 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 Provider (more specifically, the web origin that
the token request is being sent to).
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If for some deployment-specific reason the initial Token Provider
("TP1") needs to redirect the client to another Token Provider
("TP2"), rather than directly back to the Token Consumer, it can be
accomodated using the header fields defined in this specification in
the following fashion ("the redirect-chain approach"):
Initially, the client is redirected to TP1 by the Token Consumer
("TC"), as described above. Upon receiving the client's request,
containing a TokenBindingMessage which contains both provided and
referred TokenBindings (for TP1 and TC, respectively), TP1
responds to the client with a redirect response containing the
Include-Referred-Token-Binding-ID header field and directing the
client to send a request to TP2. This causes the client to follow
the same pattern and send a request containing a
TokenBindingMessage which contains both provided and referred
TokenBindings (for TP2 and TP1, respectively) to TP2. Note that
this pattern can continue to further Token Providers. In this
case, TP2 issues a security token, bound to the client's
TokenBinding with TP1, and sends a redirect response to the client
pointing to TP1. TP1 in turn constructs a security token for the
Token Consumer, bound to the TC's referred TokenBinding which had
been conveyed earlier, and sends a redirect response pointing to
the TC, containing the bound security token, to the client.
The above is intended as only a non-normative example. Details are
specific to deployment contexts. Other approaches are possible, but
are outside the scope of this specification.
4.4. Negotiated Key Parameters
The TLS Extension for Token Binding Protocol Negotiation
[I-D.ietf-tokbind-negotiation] allows the server and client to
negotiate the parameters (signature algorithm, length) of the Token
Binding key. 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 key parameters.
The client MUST use the key parameters negotiated with the Token
Consumer in the referred_token_binding TokenBinding of the
TokenBindingMessage, even if those key parameters are different from
the ones negotiated with the origin that the header field is sent to.
Token Providers SHOULD support all the Token Binding key parameters
specified in the [I-D.ietf-tokbind-protocol]. If a token provider
does not support the key parameters specified in the
referred_token_binding TokenBinding in the TokenBindingMessage, it
MUST NOT issue a bound token.
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4.5. Federation Example
The diagram below shows a typical HTTP Redirect-based Web Browser SSO
Profile (no artifact, no callbacks), featuring binding of, e.g., a
TLS Token Binding ID into an OpenID Connect "ID Token".
Legend:
+------------+------------------------------------------------------+
| EKM: | TLS Exported Keying Material [RFC5705] |
| {EKMn}Ksm: | EKM for server "n", signed by private key of TBID |
| | "m", where "n" must represent server receiving the |
| | ETBMSG, if a conveyed TB's type is |
| | provided_token_binding, then m = n, else if TB's |
| | type is referred_token_binding, then m != n. E.g., |
| | see step 1b in diagram below. |
| ETBMSG: | "Sec-Token-Binding" HTTP header field conveying an |
| | EncodedTokenBindingMessage, in turn conveying |
| | TokenBinding (TB)struct(s), e.g.: ETBMSG[[TB]] or |
| | ETBMSG[[TB1],[TB2]] |
| ID Token: | the "ID Token" in OIDC, it is the semantic |
| | equivalent of a SAML "authentication assertion". "ID |
| | Token w/TBIDn" denotes a "token bound" ID Token |
| | containing TBIDn. |
| Ks & Kp: | private (aka secret) key, and public key, |
| | respectively, of client-side Token Binding key pair |
| OIDC: | Open ID Connect |
| TB: | TokenBinding struct containing signed EKM, TBID, and |
| | TB type, e.g.: |
| | [{EKM1}Ks1,TBID1,provided_token_binding] |
| TBIDn: | Token Binding ID for client and server n's token- |
| | bound TLS association. TBIDn contains Kpn. |
+------------+------------------------------------------------------+
Client, Token Consumer, Token Provider,
aka: aka: aka:
User Agent OpenID Client, OpenID Provider,
OIDC Relying Party, OIDC Provider,
SAML Relying Party SAML Identity Provider
[ server "1" ] [ server "2" ]
+--------+ +----+ +-----+
| Client | | TC | | TP |
+--------+ +----+ +-----+
| | |
| | |
| | |
| 0. Client interacts w/TC | |
| over HTTPS, establishes Ks1 & Kp1, TBID1 |
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| ETBMSG[[{EKM1}Ks1,TBID1,provided_token_binding]] |
|------------------------------>| |
| | |
| | |
| | |
| 1a. OIDC ID Token request, aka| |
| "Authentication Request", conveyed with |
| HTTP response header field of: |
| Include-Referred-Token-Binding-ID:true |
| any security-relevant cookies | |
| should contain TBID1 | |
+<- - - - - - - - - - - - - - - - | |
. | (redirect to TP via 301, 302, | |
. | 303, 307, or 308) | |
. | | |
+------------------------------------------------------->|
| 1b. opens HTTPS w/TP, |
| establishes Ks2, Kp2, TBID2; |
| sends GET or POST with |
| ETBMSG[[{EKM2}Ks2,TBID2,provided_token_binding], |
| [{EKM2}Ks1,TBID1,referred_token_binding]] |
| as well as the ID Token request |
| | |
| | |
| | |
| 2. user authentication (if applicable, |
| methods vary, particulars are out of scope) |
|<====================================================>|
| (TP generates ID Token for TC containing TBID1, may |
| also set cookie(s) containing TBID2 and/or TBID1, |
| details vary, particulars are out of scope) |
| | |
| | |
| | |
| 3a. ID Token containing Kp1, issued for TC, |
| conveyed via OIDC "Authentication Response" |
+<- - - - - - - - - - - - - - - - - - - - - - - - - - - -|
. | (redirect to TC) | |
. | | |
. | | |
+-------------------------------->| |
| 3b. HTTPS GET or POST with |
| ETBMSG[[{EKM1}Ks1,TBID1,provided_token_binding]] |
| conveying Authn Reponse containing |
| ID Token w/TBID1, issued for TC |
| | |
| | |
| | |
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| 4. user is signed-on, any security-relevant cookie(s)|
| that are set SHOULD contain TBID1 |
|<------------------------------| |
| | |
| | |
5. Implementation Considerations
HTTPS-based applications may have multi-party use cases other than,
or in addition to, the HTTP redirect-based signaling-and-conveyance
of referred token bindings, as presented above in Section 4.3.
Thus, generic Token Binding implementations intended to support any
HTTPS-based client-side application (e.g., so-called "native
applications"), should provide means for applications to have Token
Binding messages, containing Token Binding IDs of various
application-specified Token Binding types and for application-
specified TLS connections, conveyed over an application-specified
HTTPS connection, i.e., within the TokenBindingMessage conveyed by
the Sec-Token-Binding header field.
However, such applications MUST only convey Token Binding IDs to
other servers if the server associated with a Token Binding ID
explicitly signals to do so, e.g., by returning an Include-Referred-
Token-Binding-ID HTTP response header field.
NOTE: See Section 7 "Privacy Considerations", for privacy guidance
regarding the use of this functionality.
6. Security Considerations
6.1. 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.
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6.2. Triple Handshake Vulnerability in TLS 1.2 and Older TLS Versions
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
vulnerability in TLS 1.2 and older TLS versions, allowing the
attacker to synchronize keying material between TLS connections. The
attacker can then successfully replay bound tokens. For this reason,
the Token Binding protocol MUST NOT be negotiated with these TLS
versions, unless the Extended Master Secret [RFC7627] TLS extension
has also been negotiated. In addition, TLS renegotiation MUST NOT be
initiated or allowed, unless the Renegotiation Indication [RFC5746]
TLS extension has been negotiated.
6.3. Sensitivity of the Sec-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 Sec-Token-Binding header field, 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 field,
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 misuse of the cookie.
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 Sec-Token-Binding header field 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, did not successfully bind the
cookie to the client.
Likewise, we need integrity protection of the Sec-Token-Binding
header field: A client should not be tricked into sending a Sec-
Token-Binding header field to a server that contains Token Binding
messages about key pairs that the client does not 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 is 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
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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 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
field to S, it would appear to S as if C controls a certain 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
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
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 Sec-Token-Binding
header field. One origin should not be able to set the Sec-Token-
Binding header field (through a DOM API or otherwise) that the User
Agent uses with another origin. Employing the "Sec-" header field
prefix helps to meet this requirement by denoting the header field
name to be a "forbidden header name", see [fetch-spec].
6.4. 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 has not
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done so yet (in other words, we are 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
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 Sec-Token-Binding header field that
matches the TLS connection between the man-in-the-middle and the
Token Provider; or it can choose to leave the Sec-Token-Binding
header field 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
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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
Sec-Token-Binding header field (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 Sec-Token-Binding header field in the federated sign-
on use case contains both, a proof of possession of the provided
Token Binding key, as well as a proof of possession of the referred
Token Binding key.
7. Privacy Considerations
7.1. Scoping of Token Binding Keys
Clients use different Token Binding key pairs for different servers,
so as to not allow Token Binding to become a tracking tool across
different servers. However, the scoping of the Token Binding key
pairs to servers varies according to the scoping rules of the
application protocol ([I-D.ietf-tokbind-protocol] section 4.1).
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
effective top-level domains, and clients therefore should use the
same Token Binding key across such subdomains. This will 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 token
binding of the cookie. See Section 2.1, above.
7.2. Life Time of Token Binding Keys
Token Binding keys do not have an expiration time. This means that
they can potentially be used by a server to track a user across an
extended period of time (similar to a long-lived cookie). HTTPS
clients such as web user agents should therefore provide a user
interface for discarding Token Binding keys (similar to the
affordances provided to delete cookies).
If a user agent provides modes such as private browsing mode in which
the user is promised that browsing state such as cookies are
discarded after the session is over, the user agent should also
discard Token Binding keys from such modes after the session is over.
Generally speaking, users should be given the same level of control
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over life time of Token Binding keys as they have over cookies or
other potential tracking mechanisms.
7.3. Correlation
An application's various communicating endpoints, that receive Token
Binding IDs for TLS connections other than their own, obtain
information about the application's other TLS connections (in this
context, "an application" is a combination of client-side and server-
side components, communicating over HTTPS, where the client side may
be either or both web browser-based or native application-based).
These other Token Binding IDs can serve as correlation handles for
the endpoints of the other connections. If the receiving endpoints
are otherwise aware of these other connections, then no additional
information is being exposed. For instance, if in a redirect-based
federation protocol, the Identity Provider and Relying Party already
possess URLs for one another, also having Token Binding IDs for these
connections does not provide additional correlation information. If
not, then, by providing the other Token Binding IDs, additional
information is exposed that can be used to correlate the other
endpoints. In such cases, a privacy analysis of enabled correlations
and their potential privacy impacts should be performed as part of
the application design decisions of how, and whether, to utilize
Token Binding.
Also, applications must take care to only reveal Token Binding IDs to
other endpoints if the server associated with a Token Binding ID
explicitly signals to do so, see Section 5
"Implementation Considerations".
Finally, care should be taken to ensure that unrelated applications
do not obtain information about each other's Token Bindings. For
instance, a Token Binding implementation shared between multiple
applications on a given system should prevent unrelated applications
from obtaining each other's Token Binding information. This may be
accomplished by using techniques such as application isolation and
key segregation, depending upon system capabilities.
8. IANA Considerations
Below are the Internet Assigned Numbers Authority (IANA) Permanent
Message Header Field registration information per [RFC3864].
Header field name: Sec-Token-Binding
Applicable protocol: HTTP
Status: standard
Author/Change controller: IETF
Specification document(s): this one
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Header field name: Include-Referred-Token-Binding-ID
Applicable protocol: HTTP
Status: standard
Author/Change controller: IETF
Specification document(s): this one
[[TODO: possibly add further considerations wrt the behavior of the
above header fields, per <https://tools.ietf.org/html/
rfc7231#section-8.3>]]
9. Acknowledgements
This document incorporates comments and suggestions offered by Eric
Rescorla, Gabriel Montenegro, Martin Thomson, Vinod Anupam, Anthony
Nadalin, Michael B. Jones, Bill Cox, Nick Harper, Brian Campbell,
and others.
10. References
10.1. Normative References
[fetch-spec]
WhatWG, "Fetch", Living Standard ,
<https://fetch.spec.whatwg.org/>.
[I-D.ietf-tokbind-negotiation]
Popov, A., Nystrom, M., Balfanz, D., and A. Langley,
"Transport Layer Security (TLS) Extension for Token
Binding Protocol Negotiation", draft-ietf-tokbind-
negotiation-03 (work in progress), July 2016.
[I-D.ietf-tokbind-protocol]
Popov, A., Nystrom, M., Balfanz, D., Langley, A., and J.
Hodges, "The Token Binding Protocol Version 1.0", draft-
ietf-tokbind-protocol-08 (work in progress), July 2016.
[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>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<http://www.rfc-editor.org/info/rfc3864>.
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[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<http://www.rfc-editor.org/info/rfc4648>.
[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>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<http://www.rfc-editor.org/info/rfc6265>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<http://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<http://www.rfc-editor.org/info/rfc7231>.
[RFC7541] Peon, R. and H. Ruellan, "HPACK: Header Compression for
HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
<http://www.rfc-editor.org/info/rfc7541>.
10.2. Informative References
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
<http://www.rfc-editor.org/info/rfc5746>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<http://www.rfc-editor.org/info/rfc6749>.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750,
DOI 10.17487/RFC6750, October 2012,
<http://www.rfc-editor.org/info/rfc6750>.
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[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<http://www.rfc-editor.org/info/rfc7540>.
[RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
Langley, A., and M. Ray, "Transport Layer Security (TLS)
Session Hash and Extended Master Secret Extension",
RFC 7627, DOI 10.17487/RFC7627, September 2015,
<http://www.rfc-editor.org/info/rfc7627>.
[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
Adam Langley
Google Inc.
USA
Email: agl@google.com
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Jeff Hodges
Paypal
USA
Email: Jeff.Hodges@paypal.com
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