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2 Internet Engineering Task Force A. Popov
3 Internet-Draft M. Nystroem
4 Intended status: Standards Track Microsoft Corp.
5 Expires: December 28, 2018 D. Balfanz, Ed.
6 A. Langley
7 N. Harper
8 Google Inc.
9 J. Hodges
10 PayPal
11 June 26, 2018
13 Token Binding over HTTP
14 draft-ietf-tokbind-https-18
16 Abstract
18 This document describes a collection of mechanisms that allow HTTP
19 servers to cryptographically bind security tokens (such as cookies
20 and OAuth tokens) to TLS connections.
22 We describe both first-party and federated scenarios. In a first-
23 party scenario, an HTTP server is able to cryptographically bind the
24 security tokens it issues to a client, and which the client
25 subsequently returns to the server, to the TLS connection between the
26 client and server. Such bound security tokens are protected from
27 misuse since the server can generally detect if they are replayed
28 inappropriately, e.g., over other TLS connections.
30 Federated token bindings, on the other hand, allow servers to
31 cryptographically bind security tokens to a TLS connection that the
32 client has with a different server than the one issuing the token.
34 This document is a companion document to The Token Binding Protocol.
36 Status of This Memo
38 This Internet-Draft is submitted in full conformance with the
39 provisions of BCP 78 and BCP 79.
41 Internet-Drafts are working documents of the Internet Engineering
42 Task Force (IETF). Note that other groups may also distribute
43 working documents as Internet-Drafts. The list of current Internet-
44 Drafts is at https://datatracker.ietf.org/drafts/current/.
46 Internet-Drafts are draft documents valid for a maximum of six months
47 and may be updated, replaced, or obsoleted by other documents at any
48 time. It is inappropriate to use Internet-Drafts as reference
49 material or to cite them other than as "work in progress."
51 This Internet-Draft will expire on December 28, 2018.
53 Copyright Notice
55 Copyright (c) 2018 IETF Trust and the persons identified as the
56 document authors. All rights reserved.
58 This document is subject to BCP 78 and the IETF Trust's Legal
59 Provisions Relating to IETF Documents
60 (https://trustee.ietf.org/license-info) in effect on the date of
61 publication of this document. Please review these documents
62 carefully, as they describe your rights and restrictions with respect
63 to this document. Code Components extracted from this document must
64 include Simplified BSD License text as described in Section 4.e of
65 the Trust Legal Provisions and are provided without warranty as
66 described in the Simplified BSD License.
68 Table of Contents
70 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
71 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
72 2. The Sec-Token-Binding HTTP Request Header Field . . . . . . . 4
73 2.1. HTTPS Token Binding Key Pair Scoping . . . . . . . . . . 5
74 3. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . . . 6
75 4. First-Party Use Cases . . . . . . . . . . . . . . . . . . . . 6
76 5. Federation Use Cases . . . . . . . . . . . . . . . . . . . . 7
77 5.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 7
78 5.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 8
79 5.3. HTTP Redirects . . . . . . . . . . . . . . . . . . . . . 10
80 5.4. Negotiated Key Parameters . . . . . . . . . . . . . . . . 12
81 5.5. Federation Example . . . . . . . . . . . . . . . . . . . 12
82 6. Implementation Considerations . . . . . . . . . . . . . . . . 15
83 7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
84 7.1. Security Token Replay . . . . . . . . . . . . . . . . . . 15
85 7.2. Sensitivity of the Sec-Token-Binding Header . . . . . . . 15
86 7.3. Securing Federated Sign-On Protocols . . . . . . . . . . 17
87 8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 19
88 8.1. Scoping of Token Binding Key Pairs . . . . . . . . . . . 19
89 8.2. Lifetime of Token Binding Key Pairs . . . . . . . . . . . 20
90 8.3. Correlation . . . . . . . . . . . . . . . . . . . . . . . 20
91 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
92 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
93 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
94 11.1. Normative References . . . . . . . . . . . . . . . . . . 21
95 11.2. Informative References . . . . . . . . . . . . . . . . . 23
97 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
99 1. Introduction
101 The Token Binding Protocol [I-D.ietf-tokbind-protocol] defines a
102 Token Binding ID for a TLS connection between a client and a server.
103 The Token Binding ID of a TLS connection is constructed using the
104 public key of a private-public key pair. The client proves
105 possession of the corresponding private key. This Token Binding key
106 pair is long-lived. I.e., subsequent TLS connections between the
107 same client and server have the same Token Binding ID, unless
108 specifically reset, e.g., by the user. When issuing a security token
109 (e.g., an HTTP cookie or an OAuth token [RFC6749]) to a client, the
110 server can include the Token Binding ID in the token, thus
111 cryptographically binding the token to TLS connections between that
112 particular client and server, and inoculating the token against abuse
113 (re-use, attempted impersonation, etc.) by attackers.
115 While the Token Binding Protocol [I-D.ietf-tokbind-protocol] defines
116 a message format for establishing a Token Binding ID, it does not
117 specify how this message is embedded in higher-level protocols. The
118 purpose of this specification is to define how TokenBindingMessages
119 are embedded in HTTP (both versions 1.1 [RFC7230] and 2 [RFC7540]).
120 Note that TokenBindingMessages are only defined if the underlying
121 transport uses TLS. This means that Token Binding over HTTP is only
122 defined when the HTTP protocol is layered on top of TLS (commonly
123 referred to as HTTPS [RFC2818]).
125 HTTP clients establish a Token Binding ID with a server by including
126 a special HTTP header field in HTTP requests. The HTTP header field
127 value is a base64url-encoded TokenBindingMessage.
129 TokenBindingMessages allow clients to establish multiple Token
130 Binding IDs with the server, by including multiple TokenBinding
131 structures in the TokenBindingMessage. By default, a client will
132 establish a provided Token Binding ID with the server, indicating a
133 Token Binding ID that the client will persistently use with the
134 server. Under certain conditions, the client can also include a
135 referred Token Binding ID in the TokenBindingMessage, indicating a
136 Token Binding ID that the client is using with a different server
137 than the one that the TokenBindingMessage is sent to. This is useful
138 in federation scenarios.
140 1.1. Requirements Language
142 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
143 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
144 "OPTIONAL" in this document are to be interpreted as described in BCP
145 14 [RFC2119] [RFC8174] when, and only when, they appear in all
146 capitals, as shown here.
148 2. The Sec-Token-Binding HTTP Request Header Field
150 Once a client and server have negotiated the Token Binding Protocol
151 with HTTP/1.1 or HTTP/2 (see [I-D.ietf-tokbind-protocol] and
152 [I-D.ietf-tokbind-negotiation]), clients MUST include a Sec-Token-
153 Binding header field in their HTTP requests, and MUST include only
154 one such header field per HTTP request. Also, The Sec-Token-Binding
155 header field MUST NOT be included in HTTP responses. The ABNF of the
156 Sec-Token-Binding header field is (in [RFC7230] style, see also
157 Section 8.3 of [RFC7231]):
159 Sec-Token-Binding = EncodedTokenBindingMessage
161 The header field name is Sec-Token-Binding and its single value,
162 EncodedTokenBindingMessage, is a base64url encoding of a single
163 TokenBindingMessage, as defined in [I-D.ietf-tokbind-protocol]. The
164 base64url encoding uses the URL- and filename-safe character set
165 described in Section 5 of [RFC4648], with all trailing padding
166 characters '=' omitted and without the inclusion of any line breaks,
167 whitespace, or other additional characters.
169 For example:
171 Sec-Token-Binding: AIkAAgBBQFzK4_bhAqLDwRQxqJWte33d7hZ0hZWHwk-miKPg4E\
172 9fcgs7gBPoz-9RfuDfN9WCw6keHEw1ZPQMGs9CxpuHm-YAQM_j\
173 aOwwej6a-cQBGU7CJpUHOvXG4VvjNq8jDsvta9Y8_bPEPj25Gg\
174 mKiPjhJEtZA6mJ_9SNifLvVBTi7fR9wSAAAA
176 (Note that the backslashes and line breaks are provided to ease
177 readability, they are not part of the actual encoded message.)
179 If the server receives more than one Sec-Token-Binding header field
180 in an HTTP request, then the server MUST reject the message with a
181 400 (Bad Request) HTTP status code. Additionally, the Sec-Token-
182 Binding header field:
184 SHOULD NOT be stored by origin servers on PUT requests,
186 MAY be listed by a server in a Vary response header field, and,
188 MUST NOT be used in HTTP trailers.
190 The TokenBindingMessage MUST contain exactly one TokenBinding
191 structure with TokenBindingType of provided_token_binding, which MUST
192 be signed with the Token Binding private key used by the client for
193 connections between itself and the server that the HTTP request is
194 sent to (clients use different Token Binding key pairs for different
195 servers, see Section 2.1 below). The Token Binding ID established by
196 this TokenBinding is called a Provided Token Binding ID.
198 The TokenBindingMessage MAY also contain exactly one TokenBinding
199 structure with TokenBindingType of referred_token_binding, as
200 specified in Section 5.3. In addition to the latter, or rather than
201 the latter, the TokenBindingMessage MAY contain other TokenBinding
202 structures. This is use case-specific, and such use cases are
203 outside the scope of this specification.
205 A TokenBindingMessage is validated by the server as described in
206 Section 4.2 ("Server Processing Rules") of
207 [I-D.ietf-tokbind-protocol]. If validation fails and a Token Binding
208 is rejected, any associated bound tokens MUST also be rejected by the
209 server. HTTP requests containing invalid tokens MUST be rejected.
210 In this case, the server application MAY return HTTP status code 400
211 (Bad Request) or proceed with an application-specific invalid token
212 response (e.g., directing the client to re-authenticate and present a
213 different token), or terminate the connection.
215 In HTTP/2, the client SHOULD use Header Compression [RFC7541] to
216 avoid the overhead of repeating the same header field in subsequent
217 HTTP requests.
219 2.1. HTTPS Token Binding Key Pair Scoping
221 HTTPS is used in conjunction with various application protocols and
222 application contexts, in various ways. For example, general-purpose
223 Web browsing is one such HTTP-based application context. Within that
224 context, HTTP cookies [RFC6265] are typically utilized for state
225 management, including client authentication. A related, though
226 distinct, example of other HTTP-based application contexts is where
227 OAuth tokens [RFC6749] are utilized to manage authorization for
228 third-party application access to resources. The token scoping rules
229 of these two examples can differ: the scoping rules for cookies are
230 concisely specified in [RFC6265], whereas OAuth is a framework and
231 defines various token types with various scopings, some of which are
232 determined by the encompassing application.
234 The scoping of Token Binding key pairs generated by Web browsers for
235 the purpose of binding HTTP cookies MUST be no wider than the
236 granularity of a "registered domain" (also known as "effective top-
237 level domain + 1", or "eTLD+1"). An origin's "registered domain" is
238 the origin's host's public suffix plus the label to its left, with
239 the term "public suffix" being defined in a note in Section 5.3 of
240 [RFC6265] as "a domain that is controlled by a public registry". For
241 example, for "https://www.example.com", the public suffix (eTLD) is
242 "com", and the registered domain (eTLD+1) is "example.com". User
243 agents SHOULD use an up-to-date public suffix list, such as the one
244 maintained by Mozilla [PSL].
246 This means that in practice the scope of a Token Binding key pair is
247 no larger than the scope of a cookie allowed by a Web browser. If a
248 Web browser restricts cookies to a narrower scope than registered
249 domains, the scope of Token Binding key pairs MAY also be more
250 narrow. This applies to the use of Token Binding key pairs in first-
251 party use cases, as well as in federation use cases defined in this
252 specification (Section 5).
254 Key pairs used to bind other application tokens, such as OAuth tokens
255 or OpenID Connect ID Tokens, SHOULD adhere to the above eTLD+1
256 scoping requirement for those tokens being employed in first-party or
257 federation scenarios. Applications other than Web browsers MAY use
258 different key pair scoping rules. See also Section 8.1, below.
260 Scoping rules for other HTTP-based application contexts are outside
261 the scope of this specification.
263 3. TLS Renegotiation
265 Token Binding over HTTP/1.1 [RFC7230] can be performed in combination
266 with TLS renegotiation. In this case, renegotiation MUST only occur
267 between a client's HTTP request and the server's response, the client
268 MUST NOT send any pipelined requests, and the client MUST NOT
269 initiate renegotiation. (I.e., the client may only send a
270 renegotiation ClientHello in response to the server's HelloRequest.)
271 These conditions ensure that both the client and the server can
272 clearly identify which TLS Exported Keying Material value [RFC5705]
273 to use when generating or verifying the TokenBindingMessage. This
274 also prevents a TokenBindingMessage from being split across TLS
275 renegotiation boundaries. (I.e., due to TLS message fragmentation -
276 see Section 6.2.1 of [RFC5246].)
278 4. First-Party Use Cases
280 In a first-party use case (also known as a "same-site" use case), an
281 HTTP server issues a security token such as a cookie (or similar) to
282 a client, and expects the client to return the security token at a
283 later time, e.g., in order to authenticate. Binding the security
284 token to the TLS connection between client and server protects the
285 security token from misuse, since the server can detect if the
286 security token is replayed inappropriately, e.g., over other TLS
287 connections.
289 See Section 5 of [I-D.ietf-tokbind-protocol] for general guidance
290 regarding binding of security tokens and their subsequent validation.
292 5. Federation Use Cases
294 5.1. Introduction
296 For privacy reasons, clients use different Token Binding key pairs to
297 establish Provided Token Binding IDs with different servers. As a
298 result, a server cannot bind a security token (such as an OAuth token
299 or an OpenID Connect ID Token [OpenID.Core]) to a TLS connection that
300 the client has with a different server. This is, however, a common
301 requirement in federation scenarios: For example, an Identity
302 Provider may wish to issue an identity token to a client and
303 cryptographically bind that token to the TLS connection between the
304 client and a Relying Party.
306 In this section, we describe mechanisms to achieve this. The common
307 idea among these mechanisms is that a server (called the Token
308 Consumer in this document) signals to the client that it should
309 reveal the Provided Token Binding ID that is used between the client
310 and itself to another server (called the Token Provider in this
311 document). Also common across the mechanisms is how the Token
312 Binding ID is revealed to the Token Provider: The client uses the
313 Token Binding Protocol [I-D.ietf-tokbind-protocol], and includes a
314 TokenBinding structure in the Sec-Token-Binding HTTP header field
315 defined above. What differs between the various mechanisms is how
316 the Token Consumer signals to the client that it should reveal the
317 Token Binding ID to the Token Provider. Below, we specify one such
318 mechanism, which is suitable for redirect-based interactions between
319 Token Consumers and Token Providers.
321 Client Token Consumer Token Provider
322 +--------+ +----+ +-----+
323 | Client | | TC | | TP |
324 +--------+ +----+ +-----+
325 | | |
326 | | |
327 | | |
328 | Client interacts w/TC | |
329 | using TokenBindingID TBID1: | |
330 | TBMSG[[provided_token_binding,| |
331 | TBID1, signature]] | |
332 |------------------------------>| |
333 | | |
334 | Client interacts w/TP |
335 | using TokenBindingID TBID2: |
336 | TBMSG[[provided_token_binding, |
337 | TBID2, signature]] |
338 |----------------------------------------------------->|
339 | |
340 | | |
341 | TC signals permission to | |
342 | reveal TBID1 to TP | |
343 |<------------------------------| |
344 | | |
345 | |
346 | Client interacts w/TP |
347 | using TokenBindingID TBID1 and TBID2: |
348 | TBMSG[[provided_token_binding, |
349 | TBID2, signature], |
350 | [referred_token_binding, |
351 | TBID1, signature]] |
352 |----------------------------------------------------->|
353 | |
354 | | |
355 | | |
357 5.2. Overview
359 In a Federated Sign-On protocol, an Identity Provider issues an
360 identity token to a client, which sends the identity token to a
361 Relying Party to authenticate itself. Examples of this include
362 OpenID Connect (in which the identity token is called an "ID Token")
363 and SAML [OASIS.saml-core-2.0-os] (in which the identity token is a
364 SAML assertion).
366 To better protect the security of the identity token, the Identity
367 Provider may wish to bind the identity token to the TLS connection
368 between the client and the Relying Party, thus ensuring that only
369 said client can use the identity token. The Relying Party will
370 compare the Token Binding ID (or a cryptographic hash of it) in the
371 identity token with the Token Binding ID (or a hash thereof) of the
372 TLS connection between this Relying Party and the client.
374 This is an example of a federation scenario, which more generally can
375 be described as follows:
377 o A Token Consumer causes the client to issue a token request to the
378 Token Provider. The goal is for the client to obtain a token and
379 then use it with the Token Consumer.
381 o The client delivers the token request to the Token Provider.
383 o The Token Provider issues the token. The token is issued for the
384 specific Token Consumer who requested it (thus preventing
385 malicious Token Consumers from using tokens with other Token
386 Consumers). The token is, however, typically a bearer token,
387 meaning that any client can use it with the Token Consumer, not
388 just the client to which it was issued.
390 o Therefore, in the previous step, the Token Provider may want to
391 include in the token the Token Binding ID (or a cryptographic hash
392 of it) that the client uses when communicating with the Token
393 Consumer, thus binding the token to the client's Token Binding key
394 pair. The client proves possession of the private key when
395 communicating with the Token Consumer through the Token Binding
396 Protocol [I-D.ietf-tokbind-protocol], and uses the corresponding
397 public key of this key pair as a component of the Token Binding
398 ID. Comparing the Token Binding ID from the token to the Token
399 Binding ID established with the client allows the Token Consumer
400 to verify that the token was sent to it by the legitimate client.
402 o To allow the Token Provider to include the Token Binding ID in the
403 token, the Token Binding ID between client and Token Consumer must
404 therefore be communicated to the Token Provider along with the
405 token request. Communicating a Token Binding ID involves proving
406 possession of a private key and is described in the Token Binding
407 Protocol [I-D.ietf-tokbind-protocol].
409 The client will perform this last operation only if the Token
410 Consumer requests the client to do so.
412 Below, we specify how Token Consumers can signal this request in
413 redirect-based federation protocols. Note that this assumes that the
414 federated sign-on flow starts at the Token Consumer, or at the very
415 least, includes a redirect from the Token Consumer to the Token
416 Provider. It is outside the scope of this document to specify
417 similar mechanisms for flows that do not include such redirects.
419 5.3. HTTP Redirects
421 When a Token Consumer redirects the client to a Token Provider as a
422 means to deliver the token request, it SHOULD include an Include-
423 Referred-Token-Binding-ID HTTP response header field in its HTTP
424 response. The ABNF of the Include-Referred-Token-Binding-ID header
425 is (in [RFC7230] style, see also Section 8.3 of [RFC7231]):
427 Include-Referred-Token-Binding-ID = "true"
429 Where the header field name is "Include-Referred-Token-Binding-ID",
430 and the field-value of "true" is case-insensitive. For example:
432 Include-Referred-Token-Binding-ID: true
434 Including this response header field signals to the client that it
435 should reveal, to the Token Provider, the Token Binding ID used
436 between itself and the Token Consumer. In the absence of this
437 response header field, the client will not disclose any information
438 about the Token Binding used between the client and the Token
439 Consumer to the Token Provider.
441 As illustrated in Section 5.5, when a client receives this header
442 field, it should take the TokenBindingID of the provided TokenBinding
443 from the referrer and create a referred TokenBinding with it to
444 include in the TokenBindingMessage on the redirect request. In other
445 words, the Token Binding message in the redirect request to the Token
446 Provider now includes one provided binding and one referred binding,
447 the latter constructed from the binding between the client and the
448 Token Consumer.
450 When a client receives the Include-Referred-Token-Binding-ID header,
451 it includes the referred token binding even if both the Token
452 Provider and the Token Consumer fall under the same eTLD+1 and the
453 provided and referred token binding IDs are the same.
455 The referred token binding is sent only on the initial request
456 resulting from the HTTP response that included the Include-Referred-
457 Token-Binding-ID header. Should the response to that initial request
458 be a further redirect, the original referred token binding is no
459 longer included in subsequent requests. (A new referred token
460 binding may be included if the redirecting endpoint itself responded
461 with a Include-Referred-Token-Binding-ID response header.)
462 If the Include-Referred-Token-Binding-ID header field is received in
463 response to a request that did not include the Token-Binding header
464 field, the client MUST ignore the Include-Referred-Token-Binding-ID
465 header field.
467 This header field has only meaning if the HTTP status code is a
468 redirection code (300-399), and MUST be ignored by the client for any
469 other status codes. If the client supports the Token Binding
470 Protocol, and has negotiated the Token Binding Protocol with both the
471 Token Consumer and the Token Provider, it already sends the Sec-
472 Token-Binding header field to the Token Provider with each HTTP
473 request (as described in Section 2 above).
475 The TokenBindingMessage included in the redirect request to the Token
476 Provider SHOULD contain a TokenBinding with TokenBindingType
477 referred_token_binding. If included, this TokenBinding MUST be
478 signed with the Token Binding private key used by the client for
479 connections between itself and the Token Consumer (more specifically,
480 the server that issued the Include-Referred-Token-Binding-ID response
481 header field). The Token Binding ID established by this TokenBinding
482 is called a Referred Token Binding ID.
484 As described above, the TokenBindingMessage MUST additionally contain
485 a Provided Token Binding ID, i.e., a TokenBinding structure with
486 TokenBindingType of provided_token_binding, which MUST be signed with
487 the Token Binding private key used by the client for connections
488 between itself and the Token Provider (more specifically, the server
489 that the token request is being sent to).
491 If, for some deployment-specific reason, the initial Token Provider
492 ("TP1") needs to redirect the client to another Token Provider
493 ("TP2"), rather than directly back to the Token Consumer, it can be
494 accommodated using the header fields defined in this specification in
495 the following fashion ("the redirect-chain approach"):
497 Initially, the client is redirected to TP1 by the Token Consumer
498 ("TC"), as described above. Upon receiving the client's request,
499 containing a TokenBindingMessage which contains both provided and
500 referred TokenBindings (for TP1 and TC, respectively), TP1
501 responds to the client with a redirect response containing the
502 Include-Referred-Token-Binding-ID header field and directing the
503 client to send a request to TP2. This causes the client to follow
504 the same pattern and send a request containing a
505 TokenBindingMessage which contains both provided and referred
506 TokenBindings (for TP2 and TP1, respectively) to TP2. Note that
507 this pattern can continue to further Token Providers. In this
508 case, TP2 issues a security token, bound to the client's
509 TokenBinding with TP1, and sends a redirect response to the client
510 pointing to TP1. TP1 in turn constructs a security token for the
511 Token Consumer, bound to the TC's referred TokenBinding which had
512 been conveyed earlier, and sends a redirect response pointing to
513 the TC, containing the bound security token, to the client.
515 The above is intended as only a non-normative example. Details are
516 specific to deployment contexts. Other approaches are possible, but
517 are outside the scope of this specification.
519 5.4. Negotiated Key Parameters
521 The TLS Extension for Token Binding Protocol Negotiation
522 [I-D.ietf-tokbind-negotiation] allows the server and client to
523 negotiate the parameters (signature algorithm, length) of the Token
524 Binding key pair. It is possible that the Token Binding ID used
525 between the client and the Token Consumer, and the Token Binding ID
526 used between the client and Token Provider, use different key
527 parameters. The client MUST use the key parameters negotiated with
528 the Token Consumer in the referred_token_binding TokenBinding of the
529 TokenBindingMessage, even if those key parameters are different from
530 the ones negotiated with the server that the header field is sent to.
532 Token Providers SHOULD support all the Token Binding key parameters
533 specified in [I-D.ietf-tokbind-protocol]. If a token provider does
534 not support the key parameters specified in the
535 referred_token_binding TokenBinding in the TokenBindingMessage, it
536 MUST NOT issue a bound token.
538 5.5. Federation Example
540 The diagram below shows a typical HTTP Redirect-based Web Browser SSO
541 Profile (no artifact, no callbacks), featuring binding of, e.g., a
542 TLS Token Binding ID into an OpenID Connect ID Token.
544 Legend:
546 +------------+------------------------------------------------------+
547 | EKM: | TLS Exported Keying Material [RFC5705] |
548 | {EKMn}Ksm: | EKM for server "n", signed by private key of TBID |
549 | | "m", where "n" must represent server receiving the |
550 | | ETBMSG. If a conveyed TB's type is |
551 | | provided_token_binding, then m = n, else if TB's |
552 | | type is referred_token_binding, then m != n. E.g., |
553 | | see step 1b in diagram below. |
554 | ETBMSG: | "Sec-Token-Binding" HTTP header field conveying an |
555 | | EncodedTokenBindingMessage, in turn conveying |
556 | | TokenBinding (TB)struct(s), e.g.: ETBMSG[[TB]] or |
557 | | ETBMSG[[TB1],[TB2]] |
558 | ID Token: | the ID Token in OpenID Connect, it is the semantic |
559 | | equivalent of a SAML "authentication assertion". "ID |
560 | | Token w/TBIDn" denotes a "token bound" ID Token |
561 | | containing TBIDn. |
562 | Ks & Kp: | private (aka secret) key, and public key, |
563 | | respectively, of client-side Token Binding key pair |
564 | OIDC: | OpenID Connect |
565 | TB: | TokenBinding struct containing signed EKM, TBID, and |
566 | | TB type, e.g.: |
567 | | [{EKM1}Ks1,TBID1,provided_token_binding] |
568 | TBIDn: | Token Binding ID for client and server n's token- |
569 | | bound TLS association. TBIDn contains Kpn. |
570 +------------+------------------------------------------------------+
572 Client, Token Consumer, Token Provider,
573 aka: aka: aka:
574 User Agent OpenID Client, OpenID Provider,
575 OIDC Relying Party, OIDC Provider,
576 SAML Relying Party SAML Identity Provider
577 [ server "1" ] [ server "2" ]
578 +--------+ +----+ +-----+
579 | Client | | TC | | TP |
580 +--------+ +----+ +-----+
581 | | |
582 | | |
583 | | |
584 | 0. Client interacts w/TC | |
585 | over HTTPS, establishes Ks1 & Kp1, TBID1 |
586 | ETBMSG[[{EKM1}Ks1,TBID1,provided_token_binding]] |
587 |------------------------------>| |
588 | | |
589 | | |
590 | | |
591 | 1a. OIDC ID Token request, aka| |
592 | "Authentication Request", conveyed with |
593 | HTTP response header field of: |
594 | Include-Referred-Token-Binding-ID:true |
595 | any security-relevant cookies | |
596 | should contain TBID1 | |
597 +<- - - - - - - - - - - - - - - - | |
598 . | (redirect to TP via 301, 302, | |
599 . | 303, 307, or 308) | |
600 . | | |
601 +------------------------------------------------------->|
602 | 1b. opens HTTPS w/TP, |
603 | establishes Ks2, Kp2, TBID2; |
604 | sends GET or POST with |
605 | ETBMSG[[{EKM2}Ks2,TBID2,provided_token_binding], |
606 | [{EKM2}Ks1,TBID1,referred_token_binding]] |
607 | as well as the ID Token request |
608 | | |
609 | | |
610 | | |
611 | 2. user authentication (if applicable, |
612 | methods vary, particulars are out of scope) |
613 |<====================================================>|
614 | (TP generates ID Token for TC containing TBID1, may |
615 | also set cookie(s) containing TBID2 and/or TBID1, |
616 | details vary, particulars are out of scope) |
617 | | |
618 | | |
619 | | |
620 | 3a. ID Token containing Kp1, issued for TC, |
621 | conveyed via OIDC "Authentication Response" |
622 +<- - - - - - - - - - - - - - - - - - - - - - - - - - - -|
623 . | (redirect to TC) | |
624 . | | |
625 . | | |
626 +-------------------------------->| |
627 | 3b. HTTPS GET or POST with |
628 | ETBMSG[[{EKM1}Ks1,TBID1,provided_token_binding]] |
629 | conveying Authn Response containing |
630 | ID Token w/TBID1, issued for TC |
631 | | |
632 | | |
633 | | |
634 | 4. user is signed-on, any security-relevant cookie(s)|
635 | that are set SHOULD contain TBID1 |
636 |<------------------------------| |
637 | | |
638 | | |
640 6. Implementation Considerations
642 HTTPS-based applications may have multi-party use cases other than,
643 or in addition to, the HTTP redirect-based signaling-and-conveyance
644 of referred token bindings, as presented above in Section 5.3.
646 Thus, Token Binding implementations should provide APIs for such
647 applications to generate Token Binding messages containing Token
648 Binding IDs of various application-specified Token Binding types, to
649 be conveyed by the Sec-Token-Binding header field.
651 However, Token Binding implementations MUST only convey Token Binding
652 IDs to servers if signaled to do so by an application. For example,
653 a server can return an Include-Referred-Token-Binding-ID HTTP
654 response header field to an application, which then signals to the
655 Token Binding implementation that it intends to convey the Token
656 Binding ID used with this server to another server. Other signaling
657 mechanisms are possible, and are specific to the application layer
658 protocol, but are outside the scope of this specification.
660 NOTE: See Section 8 ("Privacy Considerations"), for privacy guidance
661 regarding the use of this functionality.
663 7. Security Considerations
665 7.1. Security Token Replay
667 The goal of the Federated Token Binding mechanisms is to prevent
668 attackers from exporting and replaying tokens used in protocols
669 between the client and Token Consumer, thereby impersonating
670 legitimate users and gaining access to protected resources. Although
671 bound tokens can still be replayed by any malware present in clients
672 (which may be undetectable by a server), in order to export bound
673 tokens to other machines and successfully replay them, attackers also
674 need to export the corresponding Token Binding private keys. Token
675 Binding private keys are therefore high-value assets and SHOULD be
676 strongly protected, ideally by generating them in a hardware security
677 module that prevents key export.
679 This consideration is a special case of the Security Token Replay
680 security consideration laid out in the The Token Binding Protocol
681 [I-D.ietf-tokbind-protocol] specification.
683 7.2. Sensitivity of the Sec-Token-Binding Header
685 The purpose of the Token Binding protocol is to convince the server
686 that the client that initiated the TLS connection controls a certain
687 key pair. For the server to correctly draw this conclusion after
688 processing the Sec-Token-Binding header field, certain secrecy and
689 integrity requirements must be met.
691 For example, the client's Token Binding private key must be kept
692 secret by the client. If the private key is not secret, then another
693 actor in the system could create a valid Token Binding header field,
694 impersonating the client. This can render the main purpose of the
695 protocol - to bind bearer tokens to certain clients - moot.
696 Consider, for example, an attacker who obtained (perhaps through a
697 network intrusion) an authentication cookie that a client uses with a
698 certain server. Consider further that the server bound that cookie
699 to the client's Token Binding ID precisely to thwart misuse of the
700 cookie. If the attacker were to come into possession of the client's
701 private key, he could then establish a TLS connection with the server
702 and craft a Sec-Token-Binding header field that matches the binding
703 present in the cookie, thus successfully authenticating as the
704 client, and gaining access to the client's data at the server. The
705 Token Binding protocol, in this case, did not successfully bind the
706 cookie to the client.
708 Likewise, we need integrity protection of the Sec-Token-Binding
709 header field. A client should not be tricked into sending a Sec-
710 Token-Binding header field to a server that contains Token Binding
711 messages about key pairs that the client does not control. Consider
712 an attacker A that somehow has knowledge of the exported keying
713 material (EKM) for a TLS connection between a client C and a server
714 S. (While that is somewhat unlikely, it is also not entirely out of
715 the question, since the client might not treat the EKM as a secret -
716 after all, a pre-image-resistant hash function has been applied to
717 the TLS master secret, making it impossible for someone knowing the
718 EKM to recover the TLS master secret. Such considerations might lead
719 some clients to not treat the EKM as a secret.) Such an attacker A
720 could craft a Sec-Token-Binding header field with A's key pair over
721 C's EKM. If the attacker could now trick C into sending such a
722 header field to S, it would appear to S as if C controls a certain
723 key pair, when in fact it does not (the attacker A controls the key
724 pair).
726 If A has a pre-existing relationship with S (perhaps has an account
727 on S), it now appears to the server S as if A is connecting to it,
728 even though it is really C. (If the server S does not simply use
729 Token Binding IDs to identify clients, but also uses bound
730 authentication cookies, then A would also have to trick C into
731 sending one of A's cookies to S, which it can do through a variety of
732 means - inserting cookies through Javascript APIs, setting cookies
733 through related-domain attacks, etc.) In other words, A tricked C
734 into logging into A's account on S. This could lead to a loss of
735 privacy for C, since A presumably has some other way to also access
736 the account, and can thus indirectly observe C's behavior (for
737 example, if S has a feature that lets account holders see their
738 activity history on S).
740 Therefore, we need to protect the integrity of the Sec-Token-Binding
741 header field. One eTLD+1 should not be able to set the Sec-Token-
742 Binding header field (through a DOM API or otherwise) that the User
743 Agent uses with another eTLD+1. Employing the "Sec-" header field
744 prefix helps to meet this requirement by denoting the header field
745 name to be a "forbidden header name", see [fetch-spec].
747 7.3. Securing Federated Sign-On Protocols
749 As explained above, in a federated sign-in scenario, a client will
750 prove possession of two different Token Binding private keys to a
751 Token Provider: One private key corresponds to the "provided" Token
752 Binding ID (which the client normally uses with the Token Provider),
753 and the other is the Token Binding private key corresponding to the
754 "referred" Token Binding ID (which the client normally uses with the
755 Token Consumer). The Token Provider is expected to issue a token
756 that is bound to the referred Token Binding ID.
758 Both proofs (that of the provided Token Binding private key and that
759 of the referred Token Binding private key) are necessary. To show
760 this, consider the following scenario:
762 o The client has an authentication token with the Token Provider
763 that is bound to the client's Token Binding ID used with that
764 Token Provider.
766 o The client wants to establish a secure (i.e., free of men-in-the-
767 middle) authenticated session with the Token Consumer, but has not
768 done so yet (in other words, we are about to run the federated
769 sign-on protocol).
771 o A man-in-the-middle is allowed to intercept the connection between
772 client and Token Consumer or between Client and Token Provider (or
773 both).
775 The goal is to detect the presence of the man-in-the-middle in these
776 scenarios.
778 First, consider a man-in-the-middle between the client and the Token
779 Provider. Recall that we assume that the client possesses a bound
780 authentication token (e.g., cookie) for the Token Provider. The man-
781 in-the-middle can intercept and modify any message sent by the client
782 to the Token Provider, and any message sent by the Token Provider to
783 the client. (This means, among other things, that the man-in-the-
784 middle controls the Javascript running at the client in the origin of
785 the Token Provider.) It is not, however, in possession of the
786 client's Token Binding private key. Therefore, it can either choose
787 to replace the Token Binding ID in requests from the client to the
788 Token Provider, and create a Sec-Token-Binding header field that
789 matches the TLS connection between the man-in-the-middle and the
790 Token Provider, or it can choose to leave the Sec-Token-Binding
791 header field unchanged. If it chooses the latter, the signature in
792 the Token Binding message (created by the original client on the
793 exported keying material (EKM) for the connection between client and
794 man-in-the-middle) will not match a signature on the EKM between man-
795 in-the-middle and the Token Provider. If it chooses the former (and
796 creates its own signature, using its own Token Binding private key,
797 over the EKM for the connection between itself, the man-in-the-
798 middle, and Token Provider), then the Token Binding message will
799 match the connection between man-in-the-middle and Token Provider,
800 but the Token Binding ID in the message will not match the Token
801 Binding ID that the client's authentication token is bound to.
802 Either way, the man-in-the-middle is detected by the Token Provider,
803 but only if the proof of possession of the provided Token Binding
804 private key is required in the protocol (as is done above).
806 Next, consider the presence of a man-in-the-middle between client and
807 Token Consumer. That man-in-the-middle can intercept and modify any
808 message sent by the client to the Token Consumer and any message sent
809 by the Token Consumer to the client. The Token Consumer is the party
810 that redirects the client to the Token Provider. In this case, the
811 man-in-the-middle controls the redirect URL and can tamper with any
812 redirect URL issued by the Token Consumer (as well as with any
813 Javascript running in the origin of the Token Consumer). The goal of
814 the man-in-the-middle is to trick the Token Provider into issuing a
815 token bound to its Token Binding ID, not to the Token Binding ID of
816 the legitimate client. To thwart this goal of the man-in-the-middle,
817 the client's referred Token Binding ID must be communicated to the
818 Token Producer in a manner that cannot be affected by the man-in-the-
819 middle (who, as we recall, can modify redirect URLs and Javascript at
820 the client). Including the referred Token Binding structure in the
821 Sec-Token-Binding header field (as opposed to, say, including the
822 referred Token Binding ID in an application-level message as part of
823 the redirect URL) is one way to assure that the man-in-the-middle
824 between client and Token Consumer cannot affect the communication of
825 the referred Token Binding ID to the Token Provider.
827 Therefore, the Sec-Token-Binding header field in the federated sign-
828 on use case contains both: a proof of possession of the provided
829 Token Binding key, as well as a proof of possession of the referred
830 Token Binding key.
832 Note that the presence of Token Binding does not relieve the Token
833 Provider and Token Consumer from performing various checks to ensure
834 the security of clients during federated sign-on protocols. These
835 include the following:
837 o The Token Provider should not issue tokens to Token Consumers that
838 have been shown to act maliciously. To aid in this, the
839 federation protocol should identify the Token Consumer to the
840 Token Provider (e.g., through OAuth client IDs or similar
841 mechanisms), and the Token Provider should ensure that tokens are
842 indeed issued to the Token Consumer identified in the token
843 request (e.g., by verifying that the redirect URI is associated
844 with the OAuth client ID.)
846 o The Token Consumer should verify that the tokens were issued for
847 it, and not some other token consumer. To aid in this, the
848 federation protocol should include an audience parameter in the
849 token response, or apply equivalent mechanisms (the implicit OAuth
850 flow requires Token Consumers to identify themselves when they
851 exchange OAuth authorization codes for OAuth refresh tokens,
852 leaving it up to the Token Provider to verify that the OAuth
853 authorization was delivered to the correct Token Consumer).
855 8. Privacy Considerations
857 8.1. Scoping of Token Binding Key Pairs
859 Clients use different Token Binding key pairs for different servers,
860 so as to not allow Token Binding to become a tracking tool across
861 different servers. However, the scoping of the Token Binding key
862 pairs to servers varies according to the scoping rules of the
863 application protocol (Section 4.1 of [I-D.ietf-tokbind-protocol]).
865 In the case of HTTP cookies, servers may use Token Binding to secure
866 their cookies. These cookies can be attached to any sub-domain of
867 effective top-level domains (eTLDs), and clients therefore should use
868 the same Token Binding key pair across such subdomains. This will
869 ensure that any server capable of receiving the cookie will see the
870 same Token Binding ID from the client, and thus be able to verify the
871 token binding of the cookie. See Section 2.1, above.
873 If the client application is not a Web browser, it may have
874 additional knowledge about the relationship between different
875 servers. For example, the client application might be aware of the
876 fact that two servers play the role of Relying Party and Identity
877 Provider in a federated sign-on protocol, and that they therefore
878 share the identity of the user. In such cases, it is permissible to
879 use different Token Binding key pair scoping rules, such as using the
880 same Token Binding key pair for both the Relying Party and the
881 Identity Provider. Absent such special knowledge, conservative key-
882 scoping rules should be used, assuring that clients use different
883 Token Binding key pairs with different servers.
885 8.2. Lifetime of Token Binding Key Pairs
887 Token Binding key pairs do not have an expiration time. This means
888 that they can potentially be used by a server to track a user for an
889 extended period of time (similar to a long-lived cookie). HTTPS
890 clients such as Web user agents SHOULD therefore provide a user
891 interface for discarding Token Binding key pairs (similar to the
892 affordances provided to delete cookies).
894 If a user agent provides modes such as private browsing mode in which
895 the user is promised that browsing state such as cookies are
896 discarded after the session is over, the user agent MUST also discard
897 Token Binding key pairs from such modes after the session is over.
898 Generally speaking, users should be given the same level of control
899 over lifetime of Token Binding key pairs as they have over cookies or
900 other potential tracking mechanisms.
902 8.3. Correlation
904 An application's various communicating endpoints that receive Token
905 Binding IDs for TLS connections other than their own obtain
906 information about the application's other TLS connections. (In this
907 context, "an application" is a combination of client-side and server-
908 side components, communicating over HTTPS, where the client side may
909 be either or both Web browser-based or native application-based.)
910 These other Token Binding IDs can serve as correlation handles for
911 the endpoints of the other connections. If the receiving endpoints
912 are otherwise aware of these other connections, then no additional
913 information is being exposed. For instance, if in a redirect-based
914 federation protocol, the Identity Provider and Relying Party already
915 possess URLs for one another, also having Token Binding IDs for these
916 connections does not provide additional correlation information. If
917 not, then, by providing the other Token Binding IDs, additional
918 information is exposed that can be used to correlate the other
919 endpoints. In such cases, a privacy analysis of enabled correlations
920 and their potential privacy impacts should be performed as part of
921 the application design decisions of how, and whether, to utilize
922 Token Binding.
924 Also, Token Binding implementations must take care to only reveal
925 Token Binding IDs to other endpoints if the application associated
926 with a Token Binding ID signals to do so, see Section 6
927 ("Implementation Considerations").
929 Finally, care should be taken to ensure that unrelated applications
930 do not obtain information about each other's Token Bindings. For
931 instance, a Token Binding implementation shared between multiple
932 applications on a given system should prevent unrelated applications
933 from obtaining each other's Token Binding information. This may be
934 accomplished by using techniques such as application isolation and
935 key segregation, depending upon system capabilities.
937 9. IANA Considerations
939 Below are the Internet Assigned Numbers Authority (IANA) Permanent
940 Message Header Field registration information per [RFC3864].
942 Header field name: Sec-Token-Binding
943 Applicable protocol: HTTP
944 Status: standard
945 Author/Change controller: IETF
946 Specification document(s): this one
948 Header field name: Include-Referred-Token-Binding-ID
949 Applicable protocol: HTTP
950 Status: standard
951 Author/Change controller: IETF
952 Specification document(s): this one
954 10. Acknowledgements
956 This document incorporates comments and suggestions offered by Eric
957 Rescorla, Gabriel Montenegro, Martin Thomson, Vinod Anupam, Anthony
958 Nadalin, Michael B. Jones, Bill Cox, Brian Campbell, and others.
960 This document was produced under the chairmanship of John Bradley and
961 Leif Johansson. The area directors included Eric Rescorla, Kathleen
962 Moriarty and Stephen Farrell.
964 11. References
966 11.1. Normative References
968 [I-D.ietf-tokbind-negotiation]
969 Popov, A., Nystrom, M., Balfanz, D., and A. Langley,
970 "Transport Layer Security (TLS) Extension for Token
971 Binding Protocol Negotiation", draft-ietf-tokbind-
972 negotiation-14 (work in progress), May 2018.
974 [I-D.ietf-tokbind-protocol]
975 Popov, A., Nystrom, M., Balfanz, D., Langley, A., and J.
976 Hodges, "The Token Binding Protocol Version 1.0", draft-
977 ietf-tokbind-protocol-19 (work in progress), May 2018.
979 [PSL] Mozilla, "Public Suffix List, https://publicsuffix.org/",
980 .
982 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
983 Requirement Levels", BCP 14, RFC 2119,
984 DOI 10.17487/RFC2119, March 1997,
985 .
987 [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
988 DOI 10.17487/RFC2818, May 2000,
989 .
991 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
992 Procedures for Message Header Fields", BCP 90, RFC 3864,
993 DOI 10.17487/RFC3864, September 2004,
994 .
996 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
997 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
998 .
1000 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
1001 (TLS) Protocol Version 1.2", RFC 5246,
1002 DOI 10.17487/RFC5246, August 2008,
1003 .
1005 [RFC5705] Rescorla, E., "Keying Material Exporters for Transport
1006 Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
1007 March 2010, .
1009 [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
1010 DOI 10.17487/RFC6265, April 2011,
1011 .
1013 [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
1014 Protocol (HTTP/1.1): Message Syntax and Routing",
1015 RFC 7230, DOI 10.17487/RFC7230, June 2014,
1016 .
1018 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
1019 Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
1020 DOI 10.17487/RFC7231, June 2014,
1021 .
1023 [RFC7541] Peon, R. and H. Ruellan, "HPACK: Header Compression for
1024 HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
1025 .
1027 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
1028 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
1029 May 2017, .
1031 11.2. Informative References
1033 [fetch-spec]
1034 WhatWG, "Fetch", Living Standard ,
1035 .
1037 [I-D.ietf-tokbind-tls13]
1038 Harper, N., "Token Binding for Transport Layer Security
1039 (TLS) Version 1.3 Connections", draft-ietf-tokbind-
1040 tls13-01 (work in progress), May 2018.
1042 [OASIS.saml-core-2.0-os]
1043 Cantor, S., Kemp, J., Philpott, R., and E. Maler,
1044 "Assertions and Protocol for the OASIS Security Assertion
1045 Markup Language (SAML) V2.0", OASIS Standard saml-core-
1046 2.0-os, March 2005, .
1049 [OpenID.Core]
1050 Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
1051 C. Mortimore, "OpenID Connect Core 1.0", August 2015,
1052 .
1054 [RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
1055 "Transport Layer Security (TLS) Renegotiation Indication
1056 Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
1057 .
1059 [RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
1060 RFC 6749, DOI 10.17487/RFC6749, October 2012,
1061 .
1063 [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
1064 Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
1065 DOI 10.17487/RFC7540, May 2015,
1066 .
1068 [RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
1069 Langley, A., and M. Ray, "Transport Layer Security (TLS)
1070 Session Hash and Extended Master Secret Extension",
1071 RFC 7627, DOI 10.17487/RFC7627, September 2015,
1072 .
1074 [TRIPLE-HS]
1075 Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti,
1076 A., and P. Strub, "Triple Handshakes and Cookie Cutters:
1077 Breaking and Fixing Authentication over TLS. IEEE
1078 Symposium on Security and Privacy", 2014.
1080 Authors' Addresses
1082 Andrei Popov
1083 Microsoft Corp.
1084 USA
1086 Email: andreipo@microsoft.com
1088 Magnus Nystroem
1089 Microsoft Corp.
1090 USA
1092 Email: mnystrom@microsoft.com
1094 Dirk Balfanz (editor)
1095 Google Inc.
1096 USA
1098 Email: balfanz@google.com
1100 Adam Langley
1101 Google Inc.
1102 USA
1104 Email: agl@google.com
1106 Nick Harper
1107 Google Inc.
1108 USA
1110 Email: nharper@google.com
1111 Jeff Hodges
1112 PayPal
1113 USA
1115 Email: Jeff.Hodges@paypal.com