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2 OAuth W. Denniss
3 Internet-Draft Google
4 Intended status: Standards Track J. Bradley
5 Expires: April 22, 2019 Ping Identity
6 M. Jones
7 Microsoft
8 H. Tschofenig
9 ARM Limited
10 October 19, 2018
12 OAuth 2.0 Device Flow for Browserless and Input Constrained Devices
13 draft-ietf-oauth-device-flow-13
15 Abstract
17 This OAuth 2.0 authorization flow is designed for devices that either
18 lack a browser to perform a user-agent based OAuth flow, or are
19 input-constrained to the extent that requiring the user to input a
20 lot of text (like their credentials to authenticate with the
21 authorization server) is impractical. It enables OAuth clients on
22 such devices (like smart TVs, media consoles, digital picture frames,
23 and printers) to obtain user authorization to access protected
24 resources without using an on-device user-agent, provided that they
25 have an Internet connection.
27 Status of This Memo
29 This Internet-Draft is submitted in full conformance with the
30 provisions of BCP 78 and BCP 79.
32 Internet-Drafts are working documents of the Internet Engineering
33 Task Force (IETF). Note that other groups may also distribute
34 working documents as Internet-Drafts. The list of current Internet-
35 Drafts is at https://datatracker.ietf.org/drafts/current/.
37 Internet-Drafts are draft documents valid for a maximum of six months
38 and may be updated, replaced, or obsoleted by other documents at any
39 time. It is inappropriate to use Internet-Drafts as reference
40 material or to cite them other than as "work in progress."
42 This Internet-Draft will expire on April 22, 2019.
44 Copyright Notice
46 Copyright (c) 2018 IETF Trust and the persons identified as the
47 document authors. All rights reserved.
49 This document is subject to BCP 78 and the IETF Trust's Legal
50 Provisions Relating to IETF Documents
51 (https://trustee.ietf.org/license-info) in effect on the date of
52 publication of this document. Please review these documents
53 carefully, as they describe your rights and restrictions with respect
54 to this document. Code Components extracted from this document must
55 include Simplified BSD License text as described in Section 4.e of
56 the Trust Legal Provisions and are provided without warranty as
57 described in the Simplified BSD License.
59 Table of Contents
61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
62 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
63 3. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 5
64 3.1. Device Authorization Request . . . . . . . . . . . . . . 5
65 3.2. Device Authorization Response . . . . . . . . . . . . . . 6
66 3.3. User Interaction . . . . . . . . . . . . . . . . . . . . 7
67 3.3.1. Non-textual Verification URI Optimization . . . . . . 9
68 3.4. Device Access Token Request . . . . . . . . . . . . . . . 9
69 3.5. Device Access Token Response . . . . . . . . . . . . . . 10
70 4. Discovery Metadata . . . . . . . . . . . . . . . . . . . . . 12
71 5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
72 5.1. User Code Brute Forcing . . . . . . . . . . . . . . . . . 12
73 5.2. Device Code Brute Forcing . . . . . . . . . . . . . . . . 13
74 5.3. Device Trustworthiness . . . . . . . . . . . . . . . . . 13
75 5.4. Remote Phishing . . . . . . . . . . . . . . . . . . . . . 13
76 5.5. Session Spying . . . . . . . . . . . . . . . . . . . . . 14
77 5.6. Non-confidential Clients . . . . . . . . . . . . . . . . 14
78 5.7. Non-Visual Code Transmission . . . . . . . . . . . . . . 14
79 6. Usability Considerations . . . . . . . . . . . . . . . . . . 14
80 6.1. User Code Recommendations . . . . . . . . . . . . . . . . 15
81 6.2. Non-Browser User Interaction . . . . . . . . . . . . . . 16
82 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
83 7.1. OAuth Parameters Registration . . . . . . . . . . . . . . 16
84 7.1.1. Registry Contents . . . . . . . . . . . . . . . . . . 16
85 7.2. OAuth URI Registration . . . . . . . . . . . . . . . . . 16
86 7.2.1. Registry Contents . . . . . . . . . . . . . . . . . . 16
87 7.3. OAuth Extensions Error Registration . . . . . . . . . . . 16
88 7.3.1. Registry Contents . . . . . . . . . . . . . . . . . . 17
89 7.4. OAuth 2.0 Authorization Server Metadata . . . . . . . . . 17
90 7.4.1. Registry Contents . . . . . . . . . . . . . . . . . . 17
91 8. Normative References . . . . . . . . . . . . . . . . . . . . 17
92 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 18
93 Appendix B. Document History . . . . . . . . . . . . . . . . . . 19
94 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
96 1. Introduction
98 This OAuth 2.0 [RFC6749] protocol flow for browserless and input-
99 constrained devices, often referred to as the device flow, enables
100 OAuth clients to request user authorization from applications on
101 devices that have an Internet connection, but don't have an easy
102 input method (such as a smart TV, media console, picture frame, or
103 printer), or lack a suitable browser for a more traditional OAuth
104 flow. This authorization flow instructs the user to perform the
105 authorization request on a secondary device, such as a smartphone.
107 The device flow is not intended to replace browser-based OAuth in
108 native apps on capable devices (like smartphones). Those apps should
109 follow the practices specified in OAuth 2.0 for Native Apps
110 [RFC8252].
112 The operating requirements to be able to use this authorization flow
113 are:
115 (1) The device is already connected to the Internet.
117 (2) The device is able to make outbound HTTPS requests.
119 (3) The device is able to display or otherwise communicate a URI and
120 code sequence to the user.
122 (4) The user has a secondary device (e.g., personal computer or
123 smartphone) from which they can process the request.
125 As the device flow does not require two-way communication between the
126 OAuth client and the user-agent (unlike other OAuth 2 flows), it
127 supports several use cases that cannot be served by those other
128 approaches.
130 Instead of interacting with the end user's user agent, the client
131 instructs the end user to use another computer or device and connect
132 to the authorization server to approve the access request. Since the
133 client cannot receive incoming requests, it polls the authorization
134 server repeatedly until the end user completes the approval process.
136 The device typically chooses the set of authorization servers to
137 support (i.e., its own authorization server, or those by providers it
138 has relationships with). It is not uncommon for the device
139 application to support only a single authorization server, such as
140 with a TV application for a specific media provider that supports
141 only that media provider's authorization server. The user may not
142 have an established relationship yet with that authorization
143 provider, though one can potentially be set up during the
144 authorization flow.
146 +----------+ +----------------+
147 | |>---(A)-- Client Identifier --->| |
148 | | | |
149 | |<---(B)-- Verification Code, --<| |
150 | | User Code, | |
151 | | & Verification URI | |
152 | Device | | |
153 | Client | Client Identifier & | |
154 | |>---(E)-- Verification Code --->| |
155 | | polling... | |
156 | |>---(E)-- Verification Code --->| |
157 | | | Authorization |
158 | |<---(F)-- Access Token --------<| Server |
159 +----------+ (w/ Optional Refresh Token) | |
160 v | |
161 : | |
162 (C) User Code & Verification URI | |
163 : | |
164 v | |
165 +----------+ | |
166 | End user | | |
167 | at |<---(D)-- User authenticates -->| |
168 | Browser | | |
169 +----------+ +----------------+
171 Figure 1: Device Flow.
173 The device flow illustrated in Figure 1 includes the following steps:
175 (A) The client requests access from the authorization server and
176 includes its client identifier in the request.
178 (B) The authorization server issues a verification code, an end-
179 user code, and provides the end-user verification URI.
181 (C) The client instructs the end user to use its user agent
182 (elsewhere) and visit the provided end-user verification URI. The
183 client provides the user with the end-user code to enter in order
184 to grant access.
186 (D) The authorization server authenticates the end user (via the
187 user agent) and prompts the user to grant the client's access
188 request. If the user agrees to the client's access request, the
189 user enters the user code provided by the client. The
190 authorization server validates the user code provided by the user.
192 (E) While the end user authorizes (or denies) the client's request
193 (step D), the client repeatedly polls the authorization server to
194 find out if the user completed the user authorization step. The
195 client includes the verification code and its client identifier.
197 (F) Assuming the end user granted access, the authorization server
198 validates the verification code provided by the client and
199 responds back with the access token.
201 2. Terminology
203 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
204 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
205 "OPTIONAL" in this document are to be interpreted as described in BCP
206 14 [RFC2119] [RFC8174] when, and only when, they appear in all
207 capitals, as shown here.
209 Device Authorization Endpoint:
210 The authorization server's endpoint capable of issuing device
211 verification codes, user codes, and verification URLs.
213 Device Verification Code:
214 A short-lived token representing an authorization session.
216 End-User Verification Code:
217 A short-lived token which the device displays to the end user, is
218 entered by the user on the authorization server, and is thus used
219 to bind the device to the user.
221 3. Protocol
223 3.1. Device Authorization Request
225 This specification defines a new OAuth endpoint, the device
226 authorization endpoint. This is separate from the OAuth
227 authorization endpoint defined in [RFC6749] with which the user
228 interacts with via a user-agent (i.e., a browser). By comparison,
229 when using the device authorization endpoint, the OAuth client on the
230 device interacts with the authorization server directly without
231 presenting the request in a user-agent, and the end user authorizes
232 the request on a separate device. This interaction is defined as
233 follows.
235 The client initiates the authorization flow by requesting a set of
236 verification codes from the authorization server by making an HTTP
237 "POST" request to the device authorization endpoint.
239 The client constructs the request with the following parameters, sent
240 as the body of the request, encoded with the "application/x-www-form-
241 urlencoded" encoding algorithm defined by Section 4.10.22.6 of
242 [HTML5]:
244 client_id
245 REQUIRED. The client identifier as described in Section 2.2 of
246 [RFC6749].
248 scope
249 OPTIONAL. The scope of the access request as described by
250 Section 3.3 of [RFC6749].
252 For example, the client makes the following HTTPS request:
254 POST /device_authorization HTTP/1.1
255 Host: server.example.com
256 Content-Type: application/x-www-form-urlencoded
258 client_id=459691054427
260 All requests from the device MUST use the Transport Layer Security
261 (TLS) [RFC8446] protocol and implement the best practices of BCP 195
262 [RFC7525].
264 Parameters sent without a value MUST be treated as if they were
265 omitted from the request. The authorization server MUST ignore
266 unrecognized request parameters. Request and response parameters
267 MUST NOT be included more than once.
269 Due to the polling nature of this protocol, to avoid unneeded
270 requests on the token endpoint, the client SHOULD only commence a
271 device authorization request when prompted by the user, and not
272 automatically such as when the app starts.
274 3.2. Device Authorization Response
276 In response, the authorization server generates a unique device
277 verification code and an end-user code that are valid for a limited
278 time and includes them in the HTTP response body using the
279 "application/json" format [RFC8259] with a 200 (OK) status code. The
280 response contains the following parameters:
282 device_code
283 REQUIRED. The device verification code.
285 user_code
286 REQUIRED. The end-user verification code.
288 verification_uri
289 REQUIRED. The end-user verification URI on the authorization
290 server. The URI should be short and easy to remember as end users
291 will be asked to manually type it into their user-agent.
293 verification_uri_complete
294 OPTIONAL. A verification URI that includes the "user_code" (or
295 other information with the same function as the "user_code"),
296 designed for non-textual transmission.
298 expires_in
299 REQUIRED. The lifetime in seconds of the "device_code" and
300 "user_code".
302 interval
303 OPTIONAL. The minimum amount of time in seconds that the client
304 SHOULD wait between polling requests to the token endpoint. If no
305 value is provided, clients MUST use 5 as the default.
307 For example:
309 HTTP/1.1 200 OK
310 Content-Type: application/json
311 Cache-Control: no-store
313 {
314 "device_code": "GmRhmhcxhwAzkoEqiMEg_DnyEysNkuNhszIySk9eS",
315 "user_code": "WDJB-MJHT",
316 "verification_uri": "https://example.com/device",
317 "verification_uri_complete":
318 "https://example.com/device?user_code=WDJB-MJHT",
319 "expires_in": 1800,
320 "interval": 5
321 }
323 3.3. User Interaction
325 After receiving a successful Authorization Response, the client
326 displays or otherwise communicates the "user_code" and the
327 "verification_uri" to the end user and instructs them to visit the
328 URI in a user agent on a secondary device (for example, in a browser
329 on their mobile phone), and enter the user code.
331 +-----------------------------------------------+
332 | |
333 | Using a browser on another device, visit: |
334 | https://example.com/device |
335 | |
336 | And enter the code: |
337 | WDJB-MJHT |
338 | |
339 +-----------------------------------------------+
341 Figure 2: Example User Instruction
343 The authorizing user navigates to the "verification_uri" and
344 authenticates with the authorization server in a secure TLS-protected
345 ([RFC8446]) session. The authorization server prompts the end user
346 to identify the device authorization session by entering the
347 "user_code" provided by the client. The authorization server should
348 then inform the user about the action they are undertaking and ask
349 them to approve or deny the request. Once the user interaction is
350 complete, the server MAY inform the user to return to their device.
352 During the user interaction, the device continuously polls the token
353 endpoint with the "device_code", as detailed in Section 3.4, until
354 the user completes the interaction, the code expires, or another
355 error occurs. The "device_code" is not intended for the end user
356 directly, and thus should not be displayed during the interaction to
357 avoid confusing the end user.
359 Authorization servers supporting this specification MUST implement a
360 user interaction sequence that starts with the user navigating to
361 "verification_uri" and continues with them supplying the "user_code"
362 at some stage during the interaction. Other than that, the exact
363 sequence and implementation of the user interaction is up to the
364 authorization server, for example, the authorization server may
365 enable new users to sign up for an account during the authorization
366 flow, or add additional security verification steps.
368 It is NOT RECOMMENDED for authorization servers to include the user
369 code in the verification URI ("verification_uri"), as this increases
370 the length and complexity of the URI that the user must type. While
371 the user must still type the same number of characters with the
372 user_code separated, once they successfully navigate to the
373 verification_uri, any errors in entering the code can be highlighted
374 by the authorization server to improve the user experience. The next
375 section documents user interaction with "verification_uri_complete",
376 which is designed to carry both pieces of information.
378 3.3.1. Non-textual Verification URI Optimization
380 When "verification_uri_complete" is included in the Authorization
381 Response (Section 3.2), clients MAY present this URI in a non-textual
382 manner using any method that results in the browser being opened with
383 the URI, such as with QR (Quick Response) codes or NFC (Near Field
384 Communication), to save the user typing the URI.
386 For usability reasons, it is RECOMMENDED for clients to still display
387 the textual verification URI ("verification_uri") for users not able
388 to use such a shortcut. Clients MUST still display the "user_code",
389 as the authorization server will require the user to confirm it to
390 disambiguate devices, or as a remote phishing mitigation (See
391 Section 5.4).
393 If the user starts the user interaction by browsing to
394 "verification_uri_complete", then the user interaction described in
395 Section 3.3 is still followed, but with the optimization that the
396 user does not need to type the "user_code". The server SHOULD
397 display the "user_code" to the user and ask them to verify that it
398 matches the "user_code" being displayed on the device, to confirm
399 they are authorizing the correct device. As before, in addition to
400 taking steps to confirm the identity of the device, the user should
401 also be afforded the choice to approve or deny the authorization
402 request.
404 +-------------------------------------------------+
405 | |
406 | Scan the QR code, or using +------------+ |
407 | a browser on another device, |[_].. . [_]| |
408 | visit: | . .. . .| |
409 | https://example.com/device | . . . ....| |
410 | |. . . . | |
411 | And enter the code: |[_]. ... . | |
412 | WDJB-MJHT +------------+ |
413 | |
414 +-------------------------------------------------+
416 Figure 3: Example User Instruction with QR Code Representation of the
417 Complete Verification URI
419 3.4. Device Access Token Request
421 After displaying instructions to the user, the client makes an Access
422 Token Request to the token endpoint (as defined by Section 3.2 of
423 [RFC6749]) with a "grant_type" of
424 "urn:ietf:params:oauth:grant-type:device_code". This is an extension
425 grant type (as defined by Section 4.5 of [RFC6749]) created by this
426 specification, with the following parameters:
428 grant_type
429 REQUIRED. Value MUST be set to
430 "urn:ietf:params:oauth:grant-type:device_code".
432 device_code
433 REQUIRED. The device verification code, "device_code" from the
434 Device Authorization Response, defined in Section 3.2.
436 client_id
437 REQUIRED, if the client is not authenticating with the
438 authorization server as described in Section 3.2.1. of [RFC6749].
440 For example, the client makes the following HTTPS request (line
441 breaks are for display purposes only):
443 POST /token HTTP/1.1
444 Host: server.example.com
445 Content-Type: application/x-www-form-urlencoded
447 grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Adevice_code
448 &device_code=GmRhmhcxhwAzkoEqiMEg_DnyEysNkuNhszIySk9eS
449 &client_id=459691054427
451 If the client was issued client credentials (or assigned other
452 authentication requirements), the client MUST authenticate with the
453 authorization server as described in Section 3.2.1 of [RFC6749].
454 Note that there are security implications of statically distributed
455 client credentials, see Section 5.6.
457 The response to this request is defined in Section 3.5. Unlike other
458 OAuth grant types, it is expected for the client to try the Access
459 Token Request repeatedly in a polling fashion, based on the error
460 code in the response.
462 3.5. Device Access Token Response
464 If the user has approved the grant, the token endpoint responds with
465 a success response defined in Section 5.1 of [RFC6749]; otherwise it
466 responds with an error, as defined in Section 5.2 of [RFC6749].
468 In addition to the error codes defined in Section 5.2 of [RFC6749],
469 the following error codes are specified by the device flow for use in
470 token endpoint responses:
472 authorization_pending
473 The authorization request is still pending as the end user hasn't
474 yet completed the user interaction steps (Section 3.3). The
475 client SHOULD repeat the Access Token Request to the token
476 endpoint (a process known as polling). Before each new request
477 the client MUST wait at least the number of seconds specified by
478 the "interval" parameter of the Device Authorization Response (see
479 Section 3.2), or 5 seconds if none was provided, and respect any
480 increase in the polling interval required by the "slow_down"
481 error.
483 slow_down
484 A variant of "authorization_pending", the authorization request is
485 still pending and polling should continue, but the interval MUST
486 be increased by 5 seconds for this and all subsequent requests.
488 access_denied
489 The end user denied the authorization request.
491 expired_token
492 The "device_code" has expired and the device flow authorization
493 session has concluded. The client MAY commence a new Device
494 Authorization Request but SHOULD wait for user interaction before
495 restarting to avoid unnecessary polling.
497 A client receiving an error response as defined in Section 5.2 of
498 [RFC6749] MUST stop polling and SHOULD react accordingly, for
499 example, by displaying an error to the user, except for the error
500 codes "authorization_pending" and "slow_down" which are processed as
501 described above.
503 The assumption of this specification is that the secondary device the
504 user is authorizing the request on does not have a way to communicate
505 back to the OAuth client. Only a one-way channel is required to make
506 this flow useful in many scenarios. For example, an HTML application
507 on a TV that can only make outbound requests. If a return channel
508 were to exist for the chosen user interaction interface, then the
509 device MAY wait until notified on that channel that the user has
510 completed the action before initiating the token request (as an
511 alternative to polling). Such behavior is, however, outside the
512 scope of this specification.
514 4. Discovery Metadata
516 Support for the device flow MAY be declared in the OAuth 2.0
517 Authorization Server Metadata [RFC8414] with the following metadata:
519 device_authorization_endpoint
520 OPTIONAL. URL of the authorization server's device authorization
521 endpoint defined in Section 3.1.
523 5. Security Considerations
525 5.1. User Code Brute Forcing
527 Since the user code is typed by the user, shorter codes are more
528 desirable for usability reasons. This means the entropy is typically
529 less than would be used for the device code or other OAuth bearer
530 token types where the code length does not impact usability. It is
531 therefore recommended that the server rate-limit user code attempts.
533 The user code SHOULD have enough entropy that when combined with rate
534 limiting and other mitigations makes a brute-force attack infeasible.
535 For example, it's generally held that 128-bit symmetric keys for
536 encryption are seen as good enough today because an attacker has to
537 put in 2^96 work to have a 2^-32 chance of guessing correctly via
538 brute force. The rate limiting and finite lifetime on the user code
539 places an artificial limit on the amount of work an attacker can
540 "do", so if, for instance, one uses a 8-character base-20 user code
541 (with roughly 34.5 bits of entropy), the rate-limiting interval and
542 validity period would need to only allow 5 attempts in order to get
543 the same 2^-32 probability of success by random guessing.
545 A successful brute forcing of the user code would enable the attacker
546 to authenticate with their own credentials and make an authorization
547 grant to the device. This is the opposite scenario to an OAuth
548 bearer token being brute forced, whereby the attacker gains control
549 of the victim's authorization grant. Such attacks may not always
550 make economic sense, for example for a video app the device owner may
551 then be able to purchase movies using the attacker's account, though
552 a privacy risk would still remain and thus is important to protect
553 against. Furthermore, some uses of the device flow give the granting
554 account the ability to perform actions such as controlling the
555 device, which needs to be protected.
557 The precise length of the user code and the entropy contained within
558 is at the discretion of the authorization server, which needs to
559 consider the sensitivity of their specific protected resources, the
560 practicality of the code length from a usability standpoint, and any
561 mitigations that are in place such as rate-limiting, when determining
562 the user code format.
564 5.2. Device Code Brute Forcing
566 An attacker who guesses the device code would be able to potentially
567 obtain the authorization code once the user completes the flow. As
568 the device code is not displayed to the user and thus there are
569 usability considerations on the length, a very high entropy code
570 SHOULD be used.
572 5.3. Device Trustworthiness
574 Unlike other native application OAuth 2.0 flows, the device
575 requesting the authorization is not the same as the device that the
576 user grants access from. Thus, signals from the approving user's
577 session and device are not relevant to the trustworthiness of the
578 client device.
580 Note that if an authorization server used with this flow is
581 malicious, then it could man-in-the-middle the backchannel flow to
582 another authorization server. In this scenario, the man-in-the-
583 middle is not completely hidden from sight, as the end user would end
584 up on the authorization page of the wrong service, giving them an
585 opportunity to notice that the URL in the browser's address bar is
586 wrong. For this to be possible, the device manufacturer must either
587 directly be the attacker, shipping a device intended to perform the
588 man-in-the-middle attack, or be using an authorization server that is
589 controlled by an attacker, possibly because the attacker compromised
590 the authorization server used by the device. In part, the person
591 purchasing the device is counting on it and its business partners to
592 be trustworthy.
594 5.4. Remote Phishing
596 It is possible for the device flow to be initiated on a device in an
597 attacker's possession. For example, an attacker might send an email
598 instructing the target user to visit the verification URL and enter
599 the user code. To mitigate such an attack, it is RECOMMENDED to
600 inform the user that they are authorizing a device during the user
601 interaction step (see Section 3.3), and to confirm that the device is
602 in their possession. The authorization server SHOULD display
603 information about the device so that the person can notice if a
604 software client was attempting to impersonating a hardware device.
606 For authorization servers that support the option specified in
607 Section 3.3.1 for the client to append the user code to the
608 authorization URI, it is particularly important to confirm that the
609 device is in the user's possession, as the user no longer has to type
610 the code manually. One possibility is to display the code during the
611 authorization flow and asking the user to verify that the same code
612 is being displayed on the device they are setting up.
614 The user code needs to have a long enough lifetime to be useable
615 (allowing the user to retrieve their secondary device, navigate to
616 the verification URI, login, etc.), but should be sufficiently short
617 to limit the usability of a code obtained for phishing. This doesn't
618 prevent a phisher presenting a fresh token, particularly in the case
619 they are interacting with the user in real time, but it does limit
620 the viability of codes sent over email or SMS.
622 5.5. Session Spying
624 While the device is pending authorization, it may be possible for a
625 malicious user to spy on the device user interface and hijack the
626 session by completing the authorization faster than the user that
627 initiated it. Devices SHOULD take into account the operating
628 environment when considering how to communicate the code to the user
629 to reduce the chances it will be observed by a malicious user.
631 5.6. Non-confidential Clients
633 Most device clients are incapable of being confidential clients, as
634 secrets that are statically included as part of an app distributed to
635 multiple users cannot be considered confidential. For such clients,
636 the recommendations of Section 5.3.1 of [RFC6819] and Section 8.5 of
637 [RFC8252] apply.
639 5.7. Non-Visual Code Transmission
641 There is no requirement that the user code be displayed by the device
642 visually. Other methods of one-way communication can potentially be
643 used, such as text-to-speech audio, or Bluetooth Low Energy. To
644 mitigate an attack in which a malicious user can bootstrap their
645 credentials on a device not in their control, it is RECOMMENDED that
646 any chosen communication channel only be accessible by people in
647 close proximity. E.g., users who can see, or hear the device.
649 6. Usability Considerations
651 This section is a non-normative discussion of usability
652 considerations.
654 6.1. User Code Recommendations
656 For many users, their nearest Internet-connected device will be their
657 mobile phone, and typically these devices offer input methods that
658 are more time consuming than a computer keyboard to change the case
659 or input numbers. To improve usability (improving entry speed, and
660 reducing retries), these limitations should be taken into account
661 when selecting the user-code character set.
663 One way to improve input speed is to restrict the character set to
664 case-insensitive A-Z characters, with no digits. These characters
665 can typically be entered on a mobile keyboard without using modifier
666 keys. Further removing vowels to avoid randomly creating words
667 results in the base-20 character set: "BCDFGHJKLMNPQRSTVWXZ". Dashes
668 or other punctuation may be included for readability.
670 An example user code following this guideline containing 8
671 significant characters and dashes added for end-user readability,
672 with a resulting entropy of 20^8: "WDJB-MJHT".
674 Pure numeric codes are also a good choice for usability, especially
675 for clients targeting locales where A-Z character keyboards are not
676 used, though their length needs to be longer to maintain a high
677 entropy.
679 An example numeric user code containing 9 significant digits and
680 dashes added for end-user readability, with an entropy of 10^9:
681 "019-450-730".
683 When processing the inputted user code, the server should strip
684 dashes and other punctuation it added for readability (making the
685 inclusion of that punctuation by the user optional). For codes using
686 only characters in the A-Z range as with the base-20 charset defined
687 above, the user's input should be upper-cased before comparison to
688 account for the fact that the user may input the equivalent lower-
689 case characters. Further stripping of all characters outside the
690 user_code charset is recommended to reduce instances where an
691 errantly typed character (like a space character) invalidates
692 otherwise valid input.
694 It is RECOMMENDED to avoid character sets that contain two or more
695 characters that can easily be confused with each other like "0" and
696 "O", or "1", "l" and "I". Furthermore, the extent practical, where a
697 character set contains one character that may be confused with
698 characters outside the character set the character outside the set
699 MAY be substituted with the one in the character set that it is
700 commonly confused with (for example, "O" for "0" when using a
701 numerical 0-9 character set).
703 6.2. Non-Browser User Interaction
705 Devices and authorization servers MAY negotiate an alternative code
706 transmission and user interaction method in addition to the one
707 described in Section 3.3. Such an alternative user interaction flow
708 could obviate the need for a browser and manual input of the code,
709 for example, by using Bluetooth to transmit the code to the
710 authorization server's companion app. Such interaction methods can
711 utilize this protocol, as ultimately, the user just needs to identify
712 the authorization session to the authorization server; however, user
713 interaction other than via the verification URI is outside the scope
714 of this specification.
716 7. IANA Considerations
718 7.1. OAuth Parameters Registration
720 This specification registers the following values in the IANA "OAuth
721 Parameters" registry [IANA.OAuth.Parameters] established by
722 [RFC6749].
724 7.1.1. Registry Contents
726 o Parameter name: device_code
727 o Parameter usage location: token request
728 o Change controller: IESG
729 o Specification Document: Section 3.1 of [[ this specification ]]
731 7.2. OAuth URI Registration
733 This specification registers the following values in the IANA "OAuth
734 URI" registry [IANA.OAuth.Parameters] established by [RFC6755].
736 7.2.1. Registry Contents
738 o URN: urn:ietf:params:oauth:grant-type:device_code
739 o Common Name: Device flow grant type for OAuth 2.0
740 o Change controller: IESG
741 o Specification Document: Section 3.1 of [[ this specification ]]
743 7.3. OAuth Extensions Error Registration
745 This specification registers the following values in the IANA "OAuth
746 Extensions Error Registry" registry [IANA.OAuth.Parameters]
747 established by [RFC6749].
749 7.3.1. Registry Contents
751 o Error name: authorization_pending
752 o Error usage location: Token endpoint response
753 o Related protocol extension: [[ this specification ]]
754 o Change controller: IETF
755 o Specification Document: Section 3.5 of [[ this specification ]]
757 o Error name: access_denied
758 o Error usage location: Token endpoint response
759 o Related protocol extension: [[ this specification ]]
760 o Change controller: IETF
761 o Specification Document: Section 3.5 of [[ this specification ]]
763 o Error name: slow_down
764 o Error usage location: Token endpoint response
765 o Related protocol extension: [[ this specification ]]
766 o Change controller: IETF
767 o Specification Document: Section 3.5 of [[ this specification ]]
769 o Error name: expired_token
770 o Error usage location: Token endpoint response
771 o Related protocol extension: [[ this specification ]]
772 o Change controller: IETF
773 o Specification Document: Section 3.5 of [[ this specification ]]
775 7.4. OAuth 2.0 Authorization Server Metadata
777 This specification registers the following values in the IANA "OAuth
778 2.0 Authorization Server Metadata" registry [IANA.OAuth.Parameters]
779 established by [RFC8414].
781 7.4.1. Registry Contents
783 o Metadata name: device_authorization_endpoint
784 o Metadata Description: The Device Authorization Endpoint.
785 o Change controller: IESG
786 o Specification Document: Section 4 of [[ this specification ]]
788 8. Normative References
790 [HTML5] IANA, "HTML5",
791 .
793 [IANA.OAuth.Parameters]
794 IANA, "OAuth Parameters",
795 .
797 [RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
798 RFC 6749, DOI 10.17487/RFC6749, October 2012,
799 .
801 [RFC6755] Campbell, B. and H. Tschofenig, "An IETF URN Sub-Namespace
802 for OAuth", RFC 6755, DOI 10.17487/RFC6755, October 2012,
803 .
805 [RFC6819] Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
806 Threat Model and Security Considerations", RFC 6819,
807 DOI 10.17487/RFC6819, January 2013,
808 .
810 [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
811 "Recommendations for Secure Use of Transport Layer
812 Security (TLS) and Datagram Transport Layer Security
813 (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
814 2015, .
816 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
817 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
818 May 2017, .
820 [RFC8252] Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps",
821 BCP 212, RFC 8252, DOI 10.17487/RFC8252, October 2017,
822 .
824 [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
825 Interchange Format", STD 90, RFC 8259,
826 DOI 10.17487/RFC8259, December 2017,
827 .
829 [RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
830 Authorization Server Metadata", RFC 8414,
831 DOI 10.17487/RFC8414, June 2018,
832 .
834 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
835 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
836 .
838 Appendix A. Acknowledgements
840 The starting point for this document was the Internet-Draft draft-
841 recordon-oauth-v2-device, authored by David Recordon and Brent
842 Goldman, which itself was based on content in draft versions of the
843 OAuth 2.0 protocol specification removed prior to publication due to
844 a then lack of sufficient deployment expertise. Thank you to the
845 OAuth working group members who contributed to those earlier drafts.
847 This document was produced in the OAuth working group under the
848 chairpersonship of Rifaat Shekh-Yusef and Hannes Tschofenig with
849 Benjamin Kaduk, Kathleen Moriarty, and Eric Rescorla serving as
850 Security Area Directors.
852 The following individuals contributed ideas, feedback, and wording
853 that shaped and formed the final specification:
855 Adam Roach, Alissa Cooper, Ben Campbell, Brian Campbell, Benjamin
856 Kaduk, Roshni Chandrashekhar, Eric Fazendin, Torsten Lodderstedt,
857 James Manger, Breno de Medeiros, Simon Moffatt, Stein Myrseth, Justin
858 Richer, Nat Sakimura, Andrew Sciberras, Marius Scurtescu, Ken Wang,
859 and Steven E. Wright.
861 Appendix B. Document History
863 [[ to be removed by the RFC Editor before publication as an RFC ]]
865 -13
867 o Added a longer discussion about entropy, proposed by Benjamin
868 Kaduk.
869 o Added device_code to OAuth IANA registry.
870 o Expanded explanation of "case insensitive".
871 o Added security section on Device Code Brute Forcing.
872 o application/x-www-form-urlencoded normativly referenced.
873 o Editatorial improvements.
875 -12
877 o Set a default polling interval to 5s explicitly.
878 o Defined the slow_down behavior that it should increase the current
879 interval by 5s.
880 o expires_in now REQUIRED
881 o Other changes in response to review feedback.
883 -11
885 o Updated reference to OAuth 2.0 Authorization Server Metadata.
887 -10
889 o Added a missing definition of access_denied for use on the token
890 endpoint.
892 o Corrected text documenting which error code should be returned for
893 expired tokens (it's "expired_token", not "invalid_grant").
894 o Corrected section reference to RFC 8252 (the section numbers had
895 changed after the initial reference was made).
896 o Fixed line length of one diagram (was causing xml2rfc warnings).
897 o Added line breaks so the URN grant_type is presented on an
898 unbroken line.
899 o Typos fixed and other stylistic improvements.
901 -09
903 o Addressed review comments by Security Area Director Eric Rescorla
904 about the potential of a confused deputy attack.
906 -08
908 o Expanded the User Code Brute Forcing section to include more
909 detail on this attack.
911 -07
913 o Replaced the "user_code" URI parameter optimization with
914 verification_uri_complete following the IETF99 working group
915 discussion.
916 o Added security consideration about spying.
917 o Required that device_code not be shown.
918 o Added text regarding a minimum polling interval.
920 -06
922 o Clarified usage of the "user_code" URI parameter optimization
923 following the IETF98 working group discussion.
925 -05
927 o response_type parameter removed from authorization request.
928 o Added option for clients to include the user_code on the
929 verification URI.
930 o Clarified token expiry, and other nits.
932 -04
934 o Security & Usability sections. OAuth Discovery Metadata.
936 -03
938 o device_code is now a URN. Added IANA Considerations
939 -02
941 o Added token request & response specification.
943 -01
945 o Applied spelling and grammar corrections and added the Document
946 History appendix.
948 -00
950 o Initial working group draft based on draft-recordon-oauth-
951 v2-device.
953 Authors' Addresses
955 William Denniss
956 Google
957 1600 Amphitheatre Pkwy
958 Mountain View, CA 94043
959 USA
961 Email: wdenniss@google.com
962 URI: http://wdenniss.com/device-flow
964 John Bradley
965 Ping Identity
967 Email: ve7jtb@ve7jtb.com
968 URI: http://www.thread-safe.com/
970 Michael B. Jones
971 Microsoft
973 Email: mbj@microsoft.com
974 URI: http://self-issued.info/
976 Hannes Tschofenig
977 ARM Limited
978 Austria
980 Email: Hannes.Tschofenig@gmx.net
981 URI: http://www.tschofenig.priv.at