Internet Engineering Task Force A. Popov
Internet-Draft M. Nystroem
Intended status: Standards Track Microsoft Corp.
Expires: December 28, 2018 D. Balfanz, Ed.
A. Langley
N. Harper
Google Inc.
J. Hodges
PayPal
June 26, 2018
Token Binding over HTTP
draft-ietf-tokbind-https-18
Abstract
This document describes a collection of mechanisms that allow HTTP
servers to cryptographically bind security tokens (such as cookies
and OAuth tokens) to TLS 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 document is a companion document to The Token Binding 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 https://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 December 28, 2018.
Copyright Notice
Copyright (c) 2018 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
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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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 HTTP Request Header Field . . . . . . . 4
2.1. HTTPS Token Binding Key Pair Scoping . . . . . . . . . . 5
3. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . . . 6
4. First-Party Use Cases . . . . . . . . . . . . . . . . . . . . 6
5. Federation Use Cases . . . . . . . . . . . . . . . . . . . . 7
5.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 7
5.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 8
5.3. HTTP Redirects . . . . . . . . . . . . . . . . . . . . . 10
5.4. Negotiated Key Parameters . . . . . . . . . . . . . . . . 12
5.5. Federation Example . . . . . . . . . . . . . . . . . . . 12
6. Implementation Considerations . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
7.1. Security Token Replay . . . . . . . . . . . . . . . . . . 15
7.2. Sensitivity of the Sec-Token-Binding Header . . . . . . . 15
7.3. Securing Federated Sign-On Protocols . . . . . . . . . . 17
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 19
8.1. Scoping of Token Binding Key Pairs . . . . . . . . . . . 19
8.2. Lifetime of Token Binding Key Pairs . . . . . . . . . . . 20
8.3. Correlation . . . . . . . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
11.1. Normative References . . . . . . . . . . . . . . . . . . 21
11.2. Informative References . . . . . . . . . . . . . . . . . 23
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
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 constructed using the
public key of a private-public key pair. The client proves
possession of the corresponding private key. This Token Binding key
pair is long-lived. I.e., subsequent TLS connections between the
same client and server have the same Token Binding ID, unless
specifically reset, e.g., by the user. When issuing a security token
(e.g., an HTTP cookie or an OAuth token [RFC6749]) 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 [RFC2818]).
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", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
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14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. The Sec-Token-Binding HTTP Request 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 a Sec-Token-
Binding header field in their HTTP requests, and MUST include only
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
Sec-Token-Binding header field is (in [RFC7230] style, see also
Section 8.3 of [RFC7231]):
Sec-Token-Binding = EncodedTokenBindingMessage
The header field name is Sec-Token-Binding and its single value,
EncodedTokenBindingMessage, is a base64url encoding of a single
TokenBindingMessage, as defined in [I-D.ietf-tokbind-protocol]. The
base64url encoding uses the URL- and filename-safe character set
described in Section 5 of [RFC4648], with all trailing padding
characters '=' omitted and without the inclusion of any line breaks,
whitespace, or other additional characters.
For example:
Sec-Token-Binding: AIkAAgBBQFzK4_bhAqLDwRQxqJWte33d7hZ0hZWHwk-miKPg4E\
9fcgs7gBPoz-9RfuDfN9WCw6keHEw1ZPQMGs9CxpuHm-YAQM_j\
aOwwej6a-cQBGU7CJpUHOvXG4VvjNq8jDsvta9Y8_bPEPj25Gg\
mKiPjhJEtZA6mJ_9SNifLvVBTi7fR9wSAAAA
(Note that the backslashes and line breaks are provided to ease
readability, they are not part of the actual encoded message.)
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
400 (Bad Request) HTTP status code. Additionally, the Sec-Token-
Binding header field:
SHOULD NOT be stored by origin servers on PUT requests,
MAY be listed by a server in a Vary response header field, and,
MUST NOT be used in HTTP trailers.
The TokenBindingMessage MUST contain exactly one TokenBinding
structure with TokenBindingType of provided_token_binding, which MUST
be signed with the Token Binding private key used by the client for
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connections between itself and the server that the HTTP request is
sent to (clients use different Token Binding key pairs 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 exactly one TokenBinding
structure with TokenBindingType of referred_token_binding, as
specified in Section 5.3. 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.
A TokenBindingMessage is validated by the server as described in
Section 4.2 ("Server Processing Rules") of
[I-D.ietf-tokbind-protocol]. If validation fails and a Token Binding
is rejected, any associated bound tokens MUST also be rejected by the
server. HTTP requests containing invalid tokens MUST be rejected.
In this case, the server application MAY return HTTP status code 400
(Bad Request) or proceed with an application-specific invalid token
response (e.g., directing the client to re-authenticate and present a
different token), or terminate the connection.
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 that
context, HTTP cookies [RFC6265] are typically utilized for state
management, including client authentication. A related, though
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 scoping of Token Binding key pairs generated by Web browsers for
the purpose of binding HTTP cookies MUST be no wider than the
granularity of a "registered domain" (also known as "effective top-
level domain + 1", or "eTLD+1"). An origin's "registered domain" is
the origin's host's public suffix plus the label to its left, with
the term "public suffix" being defined in a note in Section 5.3 of
[RFC6265] as "a domain that is controlled by a public registry". For
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example, for "https://www.example.com", the public suffix (eTLD) is
"com", and the registered domain (eTLD+1) is "example.com". User
agents SHOULD use an up-to-date public suffix list, such as the one
maintained by Mozilla [PSL].
This means that in practice the scope of a Token Binding key pair is
no larger than the scope of a cookie allowed by a Web browser. If a
Web browser restricts cookies to a narrower scope than registered
domains, the scope of Token Binding key pairs MAY also be more
narrow. This applies to the use of Token Binding key pairs in first-
party use cases, as well as in federation use cases defined in this
specification (Section 5).
Key pairs used to bind other application tokens, such as OAuth tokens
or OpenID Connect ID Tokens, SHOULD adhere to the above 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
the scope of this specification.
3. TLS Renegotiation
Token Binding over HTTP/1.1 [RFC7230] can be performed in combination
with TLS renegotiation. In this case, renegotiation MUST only occur
between a client's HTTP request and the server's response, the client
MUST NOT send any pipelined requests, and the client MUST NOT
initiate renegotiation. (I.e., the client may only send a
renegotiation ClientHello in response to the server's HelloRequest.)
These conditions ensure that both the client and the server can
clearly identify which TLS Exported Keying Material value [RFC5705]
to use when generating or verifying the TokenBindingMessage. This
also prevents a TokenBindingMessage from being split across TLS
renegotiation boundaries. (I.e., due to TLS message fragmentation -
see Section 6.2.1 of [RFC5246].)
4. First-Party Use Cases
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
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.
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See Section 5 of [I-D.ietf-tokbind-protocol] for general guidance
regarding binding of security tokens and their subsequent validation.
5. Federation Use Cases
5.1. Introduction
For privacy reasons, clients use different Token Binding key pairs 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 ID Token [OpenID.Core]) to a TLS connection that
the client has with a different server. This is, however, a common
requirement in federation scenarios: For example, an Identity
Provider may wish to issue an identity token to a client and
cryptographically bind that token to the TLS connection between the
client and a Relying Party.
In this section, we describe mechanisms to achieve this. The common
idea among these mechanisms is that a server (called the Token
Consumer in this document) 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.
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Client Token Consumer Token Provider
+--------+ +----+ +-----+
| Client | | TC | | TP |
+--------+ +----+ +-----+
| | |
| | |
| | |
| Client interacts w/TC | |
| using TokenBindingID TBID1: | |
| TBMSG[[provided_token_binding,| |
| TBID1, signature]] | |
|------------------------------>| |
| | |
| Client interacts w/TP |
| using TokenBindingID TBID2: |
| TBMSG[[provided_token_binding, |
| TBID2, signature]] |
|----------------------------------------------------->|
| |
| | |
| TC signals permission to | |
| reveal TBID1 to TP | |
|<------------------------------| |
| | |
| |
| Client interacts w/TP |
| using TokenBindingID TBID1 and TBID2: |
| TBMSG[[provided_token_binding, |
| TBID2, signature], |
| [referred_token_binding, |
| TBID1, signature]] |
|----------------------------------------------------->|
| |
| | |
| | |
5.2. 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 (in which the identity token is called an "ID Token")
and SAML [OASIS.saml-core-2.0-os] (in which 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
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said client can use the identity token. The Relying Party will
compare the Token Binding ID (or a cryptographic hash of it) in the
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
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 ID (or a cryptographic hash
of it) that the client uses when communicating with the Token
Consumer, thus binding the token to the client's Token Binding key
pair. 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 uses the corresponding
public key of this key pair as a component of the Token Binding
ID. Comparing the Token Binding ID from the token to the Token
Binding ID established with the client 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 ID 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 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, includes a redirect from the Token Consumer to the Token
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Provider. It is outside the scope of this document to specify
similar mechanisms for flows that do not include such redirects.
5.3. HTTP Redirects
When a Token Consumer redirects the client to a Token Provider as a
means to deliver the token request, it SHOULD include an 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 Section 8.3 of [RFC7231]):
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
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 5.5, 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.
The referred token binding is sent only on the initial request
resulting from the HTTP response that included the Include-Referred-
Token-Binding-ID header. Should the response to that initial request
be a further redirect, the original referred token binding is no
longer included in subsequent requests. (A new referred token
binding may be included if the redirecting endpoint itself responded
with a Include-Referred-Token-Binding-ID response header.)
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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 a
redirection code (300-399), 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 (as described in Section 2 above).
The TokenBindingMessage included in the redirect request to the Token
Provider SHOULD contain a TokenBinding with TokenBindingType
referred_token_binding. If included, this TokenBinding MUST be
signed with the Token Binding private key used by the client for
connections between itself and the Token Consumer (more specifically,
the server 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 of provided_token_binding, which MUST be signed with
the Token Binding private key used by the client for connections
between itself and the Token Provider (more specifically, the server
that the token request is being sent to).
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
accommodated 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
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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.
5.4. 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 pair. 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 server that the header field is sent to.
Token Providers SHOULD support all the Token Binding key parameters
specified in [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.
5.5. 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.
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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 OpenID Connect, 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: | OpenID 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 |
| ETBMSG[[{EKM1}Ks1,TBID1,provided_token_binding]] |
|------------------------------>| |
| | |
| | |
| | |
| 1a. OIDC ID Token request, aka| |
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| "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 Response containing |
| ID Token w/TBID1, issued for TC |
| | |
| | |
| | |
| 4. user is signed-on, any security-relevant cookie(s)|
| that are set SHOULD contain TBID1 |
|<------------------------------| |
| | |
| | |
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6. 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 5.3.
Thus, Token Binding implementations should provide APIs for such
applications to generate Token Binding messages containing Token
Binding IDs of various application-specified Token Binding types, to
be conveyed by the Sec-Token-Binding header field.
However, Token Binding implementations MUST only convey Token Binding
IDs to servers if signaled to do so by an application. For example,
a server can return an Include-Referred-Token-Binding-ID HTTP
response header field to an application, which then signals to the
Token Binding implementation that it intends to convey the Token
Binding ID used with this server to another server. Other signaling
mechanisms are possible, and are specific to the application layer
protocol, but are outside the scope of this specification.
NOTE: See Section 8 ("Privacy Considerations"), for privacy guidance
regarding the use of this functionality.
7. Security Considerations
7.1. 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. Although
bound tokens can still be replayed by any malware present in clients
(which may be undetectable by a server), in order to export bound
tokens to other machines and successfully replay them, attackers also
need to export the corresponding Token Binding private keys. Token
Binding private keys are therefore high-value assets and SHOULD be
strongly protected, ideally by generating them in a hardware security
module that prevents key export.
This consideration is a special case of the Security Token Replay
security consideration laid out in the The Token Binding Protocol
[I-D.ietf-tokbind-protocol] specification.
7.2. 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
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processing the Sec-Token-Binding header field, certain secrecy and
integrity requirements must be met.
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
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
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 into sending 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 IDs 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
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the account, and can thus indirectly observe C'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 eTLD+1 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 eTLD+1. 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].
7.3. Securing Federated Sign-On Protocols
As explained above, in a federated sign-in scenario, a client will
prove possession of two different Token Binding private keys to a
Token Provider: One private key corresponds to the "provided" Token
Binding ID (which the client normally uses with the Token Provider),
and the other is the Token Binding private key corresponding to the
"referred" Token Binding ID (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 ID.
Both proofs (that of the provided Token Binding private key and that
of the referred Token Binding private 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 ID used with that
Token Provider.
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
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-
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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 private key. Therefore, it can either choose
to replace the Token Binding ID 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 a signature on the EKM between man-
in-the-middle and the Token Provider. If it chooses the former (and
creates its own signature, using its own Token Binding private key,
over the EKM for the connection between itself, the 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 ID in the message will not match the Token
Binding ID 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 possession of the provided Token Binding
private key is required in the protocol (as is done 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 Provider into issuing a
token bound to its Token Binding ID, not to the Token Binding ID of
the legitimate client. To thwart this goal of the man-in-the-middle,
the client's referred Token Binding ID must be communicated to 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
the client). Including the referred Token Binding structure in the
Sec-Token-Binding header field (as opposed to, say, including the
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
between client and Token Consumer cannot affect the communication of
the referred Token Binding ID 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.
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Note that the presence of Token Binding does not relieve the Token
Provider and Token Consumer from performing various checks to ensure
the security of clients during federated sign-on protocols. These
include the following:
o The Token Provider should not issue tokens to Token Consumers that
have been shown to act maliciously. To aid in this, the
federation protocol should identify the Token Consumer to the
Token Provider (e.g., through OAuth client IDs or similar
mechanisms), and the Token Provider should ensure that tokens are
indeed issued to the Token Consumer identified in the token
request (e.g., by verifying that the redirect URI is associated
with the OAuth client ID.)
o The Token Consumer should verify that the tokens were issued for
it, and not some other token consumer. To aid in this, the
federation protocol should include an audience parameter in the
token response, or apply equivalent mechanisms (the implicit OAuth
flow requires Token Consumers to identify themselves when they
exchange OAuth authorization codes for OAuth refresh tokens,
leaving it up to the Token Provider to verify that the OAuth
authorization was delivered to the correct Token Consumer).
8. Privacy Considerations
8.1. Scoping of Token Binding Key Pairs
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 (Section 4.1 of [I-D.ietf-tokbind-protocol]).
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 (eTLDs), and clients therefore should use
the same Token Binding key pair 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.
If the client application is not a Web browser, it may have
additional knowledge about the relationship between different
servers. For example, the client application might be aware of the
fact that two servers play the role of Relying Party and Identity
Provider in a federated sign-on protocol, and that they therefore
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
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same Token Binding key pair for both the Relying Party and the
Identity Provider. Absent such special knowledge, conservative key-
scoping rules should be used, assuring that clients use different
Token Binding key pairs with different servers.
8.2. Lifetime of Token Binding Key Pairs
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 for 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 key pairs (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 MUST also discard
Token Binding key pairs from such modes after the session is over.
Generally speaking, users should be given the same level of control
over lifetime of Token Binding key pairs as they have over cookies or
other potential tracking mechanisms.
8.3. 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, Token Binding implementations must take care to only reveal
Token Binding IDs to other endpoints if the application associated
with a Token Binding ID signals to do so, see Section 6
("Implementation Considerations").
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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.
9. 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
Header field name: Include-Referred-Token-Binding-ID
Applicable protocol: HTTP
Status: standard
Author/Change controller: IETF
Specification document(s): this one
10. Acknowledgements
This document incorporates comments and suggestions offered by Eric
Rescorla, Gabriel Montenegro, Martin Thomson, Vinod Anupam, Anthony
Nadalin, Michael B. Jones, Bill Cox, Brian Campbell, and others.
This document was produced under the chairmanship of John Bradley and
Leif Johansson. The area directors included Eric Rescorla, Kathleen
Moriarty and Stephen Farrell.
11. References
11.1. Normative References
[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-14 (work in progress), May 2018.
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[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-19 (work in progress), May 2018.
[PSL] Mozilla, "Public Suffix List, https://publicsuffix.org/",
<https://publicsuffix.org/>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<https://www.rfc-editor.org/info/rfc2818>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://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,
<https://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, <https://www.rfc-editor.org/info/rfc5705>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://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,
<https://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,
<https://www.rfc-editor.org/info/rfc7231>.
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[RFC7541] Peon, R. and H. Ruellan, "HPACK: Header Compression for
HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
<https://www.rfc-editor.org/info/rfc7541>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
11.2. Informative References
[fetch-spec]
WhatWG, "Fetch", Living Standard ,
<https://fetch.spec.whatwg.org/>.
[I-D.ietf-tokbind-tls13]
Harper, N., "Token Binding for Transport Layer Security
(TLS) Version 1.3 Connections", draft-ietf-tokbind-
tls13-01 (work in progress), May 2018.
[OASIS.saml-core-2.0-os]
Cantor, S., Kemp, J., Philpott, R., and E. Maler,
"Assertions and Protocol for the OASIS Security Assertion
Markup Language (SAML) V2.0", OASIS Standard saml-core-
2.0-os, March 2005, <http://docs.oasis-
open.org/security/saml/v2.0/saml-core-2.0-os.pdf>.
[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,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
<https://www.rfc-editor.org/info/rfc5746>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
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[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,
<https://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
Nick Harper
Google Inc.
USA
Email: nharper@google.com
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Jeff Hodges
PayPal
USA
Email: Jeff.Hodges@paypal.com
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