OAuth                                                         W. Denniss
Internet-Draft                                                    Google
Intended status: Standards Track                              J. Bradley
Expires: January 18, February 2, 2019                                  Ping Identity
                                                                M. Jones
                                                           H. Tschofenig
                                                             ARM Limited
                                                           July 17,
                                                          August 1, 2018

  OAuth 2.0 Device Flow for Browserless and Input Constrained Devices


   This OAuth 2.0 authorization flow for browserless and input input-
   constrained devices, often referred to as the device flow, enables
   OAuth clients to request user authorization from devices that have an
   Internet connection, but don't have an easy input method (such as a
   smart TV, media console, picture frame, or printer), or lack a
   suitable browser for a more traditional OAuth flow.  This
   authorization flow instructs the user to perform the authorization
   request on a secondary device, such as a smartphone.  There is no
   requirement for communication between the constrained device and the
   user's secondary device.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 18, February 2, 2019.

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   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Protocol  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Device Authorization Request  . . . . . . . . . . . . . .   5
     3.2.  Device Authorization Response . . . . . . . . . . . . . .   6
     3.3.  User Interaction  . . . . . . . . . . . . . . . . . . . .   7
       3.3.1.  Non-textual Verification URI Optimization . . . . . .   8
     3.4.  Device Access Token Request . . . . . . . . . . . . . . .   9
     3.5.  Device Access Token Response  . . . . . . . . . . . . . .  10
   4.  Discovery Metadata  . . . . . . . . . . . . . . . . . . . . .  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
     5.1.  User Code Brute Forcing . . . . . . . . . . . . . . . . .  11
     5.2.  Device Trustworthiness  . . . . . . . . . . . . . . . . .  12
     5.3.  Remote Phishing . . . . . . . . . . . . . . . . . . . . .  12
     5.4.  Session Spying  . . . . . . . . . . . . . . . . . . . . .  13
     5.5.  Non-confidential Clients  . . . . . . . . . . . . . . . .  13
     5.6.  Non-Visual Code Transmission  . . . . . . . . . . . . . .  13
   6.  Usability Considerations  . . . . . . . . . . . . . . . . . .  13
     6.1.  User Code Recommendations . . . . . . . . . . . . . . . .  13  14
     6.2.  Non-Browser User Interaction  . . . . . . . . . . . . . .  14
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14  15
     7.1.  OAuth URI Registration  . . . . . . . . . . . . . . . . .  14  15
       7.1.1.  Registry Contents . . . . . . . . . . . . . . . . . .  14  15
     7.2.  OAuth Extensions Error Registration . . . . . . . . . . .  15
       7.2.1.  Registry Contents . . . . . . . . . . . . . . . . . .  15
     7.3.  OAuth 2.0 Authorization Server Metadata . . . . . . . . .  15  16
       7.3.1.  Registry Contents . . . . . . . . . . . . . . . . . .  15  16
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  16
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  16  17
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18  19

1.  Introduction

   This OAuth 2.0 [RFC6749] protocol flow for browserless and input input-
   constrained devices, often referred to as the device flow, enables
   OAuth clients to request user authorization from devices that have an
   internet connection, but don't have an easy input method (such as a
   smart TV, media console, picture frame, or printer), or lack a
   suitable browser for a more traditional OAuth flow.  This
   authorization flow instructs the user to perform the authorization
   request on a secondary device, such as a smartphone.

   The device flow is not intended to replace browser-based OAuth in
   native apps on capable devices (like smartphones).  Those apps should
   follow the practices specified in OAuth 2.0 for Native Apps OAuth 2.0
   for Native Apps

   The only requirements to use this flow are that the device is
   connected to the Internet, and able to make outbound HTTPS requests,
   be able to display or otherwise communicate a URI and code sequence
   to the user, and that the user has a secondary device (e.g., personal
   computer or smartphone) from which to process the request.  There is
   no requirement for two-way communication between the OAuth client and
   the user-agent, enabling a broad range of use-cases.

   Instead of interacting with the end-user's user-agent, end user's user agent, the client
   instructs the end-user end user to use another computer or device and connect
   to the authorization server to approve the access request.  Since the
   client cannot receive incoming requests, it polls the authorization
   server repeatedly until the end-user end user completes the approval process.

      +----------+                                +----------------+
      |          |>---(A)-- Client Identifier --->|                |
      |          |                                |                |
      |          |<---(B)-- Verification Code, --<|                |
      |          |              User Code,        |                |
      |          |         & Verification URI     |                |
      |  Device  |                                |                |
      |  Client  |         Client Identifier &    |                |
      |          |>---(E)-- Verification Code --->|                |
      |          |    polling...                  |                |
      |          |>---(E)-- Verification Code --->|                |
      |          |                                |  Authorization |
      |          |<---(F)-- Access Token --------<|     Server     |
      +----------+  (w/ Optional Refresh Token)   |                |
            v                                     |                |
            :                                     |                |
           (C) User Code & Verification URI       |                |
            :                                     |                |
            v                                     |                |
      +----------+                                |                |
      | End-user End user |                                |                |
      |    at    |<---(D)-- User authenticates -->|                |
      |  Browser |                                |                |
      +----------+                                +----------------+

                          Figure 1: Device Flow.

   The device flow illustrated in Figure 1 includes the following steps:

      (A) The client requests access from the authorization server and
      includes its client identifier in the request.

      (B) The authorization server issues a verification code, an end-
      user code, and provides the end-user verification URI.

      (C) The client instructs the end-user end user to use its user-agent user agent
      (elsewhere) and visit the provided end-user verification URI.  The
      client provides the end-user user with the end-user code to enter in order
      to grant access.

      (D) The authorization server authenticates the end-user end user (via the
      user agent) and prompts the end-user user to grant the client's access
      request.  If the end-user user agrees to the client's access request, the end-user
      user enters the end-user user code provided by the client.  The
      authorization server validates the end-user user code provided by the
      end-user. user.

      (E) While the end-user end user authorizes (or denies) the client's request
      (step D), the client repeatedly polls the authorization server to
      find out if the end-user user completed the end-user user authorization step.  The
      client includes the verification code and its client identifier.

      (F) Assuming the end-user end user granted access, the authorization server
      validates the verification code provided by the client and
      responds back with the access token.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119]. BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Device Authorization Endpoint:
      The authorization server's endpoint capable of issuing device
      verification codes, user codes, and verification URLs.

   Device Verification Code:
      A short-lived token representing an authorization session.

   End-User Verification Code:
      A short-lived token which the device displays to the end user, is
      entered by the end-user user on the authorization server, and is thus used
      to bind the device to the end-user. user.

3.  Protocol

3.1.  Device Authorization Request

   This specification defines a new OAuth endpoint, the device
   authorization endpoint.  This is separate from the OAuth
   authorization endpoint defined in [RFC6749] with which the user
   interacts with via a user-agent (i.e., a browser).  By comparison,
   when using the device authorization endpoint, the OAuth client on the
   device interacts with the authorization server directly without
   presenting the request in a user-agent, and the end user authorizes
   the request on a separate device.  This interaction is defined as

   The client initiates the flow by requesting a set of verification
   codes from the authorization server by making an HTTP "POST" request
   to the device authorization endpoint.

   All requests from the device MUST use the Transport Layer Security
   (TLS) [RFC5246] protocol and implement the best practices of

   The client constructs the request with the following parameters,
   encoded with the "application/
   x-www-form-urlencoded" "application/x-www-form-urlencoded" content type:

      REQUIRED.  The client identifier as described in Section 2.2 of

      OPTIONAL.  The scope of the access request as described by
      Section 3.3 of [RFC6749].

   For example, the client makes the following HTTPS request (line
   breaks are for display purposes only):

      POST /device_authorization HTTP/1.1
      Host: server.example.com
      Content-Type: application/x-www-form-urlencoded


   Parameters sent without a value MUST be treated as if they were
   omitted from the request.  The authorization server MUST ignore
   unrecognized request parameters.  Request and response parameters
   MUST NOT be included more than once.

   Due to the polling nature of this protocol, to avoid unneeded
   requests on the token endpoint, the client SHOULD only commence a
   device authorization request when prompted by the user, and not
   automatically such as when the app starts.

3.2.  Device Authorization Response

   In response, the authorization server generates a device verification
   code and an end-user code that are valid for a limited time and
   includes them in the HTTP response body using the "application/json"
   format [RFC8259] with a 200 (OK) status code.  The response contains
   the following parameters:

      REQUIRED.  The device verification code.

      REQUIRED.  The end-user verification code.

      REQUIRED.  The end-user verification URI on the authorization
      server.  The URI should be short and easy to remember as end-users end users
      will be asked to manually type it into their user-agent.

      OPTIONAL.  A verification URI that includes the "user_code" (or
      other information with the same function as the "user_code"),
      designed for non-textual transmission.

      REQUIRED.  The lifetime in seconds of the "device_code" and

      OPTIONAL.  The minimum amount of time in seconds that the client
      SHOULD wait between polling requests to the token endpoint.  If no
      value is provided, clients MUST use 5 as the default.

   For example:

      HTTP/1.1 200 OK
      Content-Type: application/json
      Cache-Control: no-store

        "expires_in" : 1800,
        "interval": 5

3.3.  User Interaction

   After receiving a successful Authorization Response, the client
   displays or otherwise communicates the "user_code" and the
   "verification_uri" to the end-user end user and instructs them to visit the
   URI in a user agent on a secondary device (for example, in a browser
   on their mobile phone), and enter the user code.

                  |                                               |
                  |  Using a browser on another device, visit:    |
                  |  https://example.com/device                   |
                  |                                               |
                  |  And enter the code:                          |
                  |  WDJB-MJHT                                    |
                  |                                               |

                    Figure 2: Example User Instruction

   The authorizing user navigates to the "verification_uri" and
   authenticates with the authorization server in a secure TLS-protected
   ([RFC5246]) session.  The authorization server prompts the end-user end user
   to identify the device authorization session by entering the
   "user_code" provided by the client.  The authorization server should
   then inform the user about the action they are undertaking and ask
   them to approve or deny the request.  Once the user interaction is
   complete, the server
   informs MAY inform the user to return to their device.

   During the user interaction, the device continuously polls the token
   endpoint with the "device_code", as detailed in Section 3.4, until
   the user completes the interaction, the code expires, or another
   error occurs.  The "device_code" is not intended for the end-user end user
   directly, and
   MUST NOT thus should not be displayed or communicated. during the interaction to
   avoid confusing the end user.

   Authorization servers supporting this specification MUST implement a
   user interaction sequence that starts with the user navigating to
   "verification_uri" and continues with them supplying the "user_code"
   at some stage during the interaction.  Other than that, the exact
   sequence and implementation of the user interaction is up to the
   authorization server and is out of scope of this specification.

   It is NOT RECOMMENDED for authorization servers to include the user
   code in the verification URI ("verification_uri"), as this increases
   the length and complexity of the URI that the user must type.  The
   next section documents user interaction with
   "verification_uri_complete", which is designed to carry this

3.3.1.  Non-textual Verification URI Optimization

   When "verification_uri_complete" is included in the Authorization
   Response (Section 3.2), clients MAY present this URI in a non-textual
   manner using any method that results in the browser being opened with
   the URI, such as with QR (Quick Response) codes or NFC, NFC (Near Field
   Communication), to save the user typing the URI.

   For usability reasons, it is RECOMMENDED for clients to still display
   the textual verification URI ("verification_uri") for users not able
   to use such a shortcut.  Clients MUST still display the "user_code",
   as the authorization server may still require the user to confirm it
   to disambiguate devices, or as a remote phishing mitigation (See
   Section 5.3).

                  |                                                 |
                  |  Using  Scan the QR code, or using     +------------+  |
                  |  a browser on another     +------------+  |
                  | device, visit:   |[_]..  . [_]|  |
                  |  https://example.com/device  visit:                         | .  ..   . .|  |
                  |  https://example.com/device     | . .  . ....|  |
                  |                                 |.   . . .   |  |
                  |  And enter the code:            |[_]. ... .  |  |
                  |  WDJB-MJHT                      +------------+  |
                  |                                                 |

   Figure 3: Example User Instruction with QR Code Representation of the
                         Complete Verification URI

3.4.  Device Access Token Request

   After displaying instructions to the user, the client makes an Access
   Token Request to the token endpoint with a "grant_type" of
   "urn:ietf:params:oauth:grant-type:device_code".  This is an extension
   grant type (as defined by Section 4.5 of [RFC6749]) with the
   following parameters:

      REQUIRED.  Value MUST be set to

      REQUIRED.  The device verification code, "device_code" from the
      Device Authorization Response, defined in Section 3.2.

      REQUIRED, if the client is not authenticating with the
      authorization server as described in Section 3.2.1. of [RFC6749].

   For example, the client makes the following HTTPS request (line
   breaks are for display purposes only):

      POST /token HTTP/1.1
      Host: server.example.com
      Content-Type: application/x-www-form-urlencoded


   If the client was issued client credentials (or assigned other
   authentication requirements), the client MUST authenticate with the
   authorization server as described in Section 3.2.1 of [RFC6749].
   Note that there are security implications of statically distributed
   client credentials, see Section 5.5.

   The response to this request is defined in Section 3.5.  Unlike other
   OAuth grant types, it is expected for the client to try the Access
   Token Request repeatedly in a polling fashion, based on the error
   code in the response.

3.5.  Device Access Token Response

   If the user has approved the grant, the token endpoint responds with
   a success response defined in Section 5.1 of [RFC6749]; otherwise it
   responds with an error, as defined in Section 5.2 of [RFC6749].

   In addition to the error codes defined in Section 5.2 of [RFC6749],
   the following error codes are specified by the device flow for use in
   token endpoint responses:

      The authorization request is still pending as the end-user end user hasn't
      yet completed the user interaction steps (Section 3.3).  The
      client should SHOULD repeat the Access Token Request to the token

      The end-user denied
      endpoint (a process known as polling).  Before each new request
      the authorization request.

      The client is MUST wait at least the number of seconds specified by
      the "interval" parameter of the Device Authorization Response (see
      Section 3.2), or 5 seconds if none was provided, and respect any
      increase in the polling too quickly interval required by the "slow_down"

      A variant of "authorization_pending", the authorization request is
      still pending and polling should back off at a
      reasonable rate. continue, but the interval MUST
      be increased by 5 seconds for this and all subsequent requests.

      The end user denied the authorization request.

      The "device_code" has expired. expired and the device flow authorization
      session has concluded.  The client will need to make MAY commence a new Device
      Authorization Request.

   The error codes "authorization_pending" and "slow_down" are
   considered soft errors.  The client should continue to poll the token
   endpoint by repeating the Device Token Request (Section 3.4) when but SHOULD wait for user interaction before
      restarting to avoid unnecessary polling.

   A client receiving soft errors, increasing the time between polls if a
   "slow_down" error is received.  Other an error codes are considered hard
   errors; the client should response as defined in Section 5.2 of
   [RFC6749] MUST stop polling and SHOULD react accordingly, for
   example, by displaying an error to the user.

   If the verification codes have expired, the server SHOULD respond
   with user, except for the error code "expired_token".  Clients MAY then choose to
   start a new device authorization session.

   The interval at
   codes "authorization_pending" and "slow_down" which the client polls MUST NOT are be more frequent than
   the "interval" parameter returned in the Device Authorization
   Response (see Section 3.2).  If no interval was provided, the client
   MUST use a reasonable default polling interval. processed
   as described above.

   The assumption of this specification is that the secondary device the
   user is authorizing the request on does not have a way to communicate
   back to the OAuth client.  Only a one-way channel is required to make
   this flow useful in many scenarios.  For example, an HTML application
   on a TV that can only make outbound requests.  If a return channel
   were to exist for the chosen user interaction interface, then the
   device MAY wait until notified on that channel that the user has
   completed the action before initiating the token request. request (as an
   alternative to polling).  Such behavior is, however, outside the
   scope of this specification.

4.  Discovery Metadata

   Support for the device flow MAY be declared in the OAuth 2.0
   Authorization Server Metadata [RFC8414] with the following metadata:

      OPTIONAL.  URL of the authorization server's device authorization
      endpoint defined in Section 3.1.

5.  Security Considerations

5.1.  User Code Brute Forcing

   Since the user code is typed by the user, shorter codes are more
   desirable for usability reasons.  This means the entropy is typically
   less than would be used for the device code or other OAuth bearer
   token types where the code length does not impact usability.  It is
   therefore recommended that the server rate-limit user code attempts.
   The user code SHOULD have enough entropy that when combined with rate
   limiting and other mitigations makes a brute-force attack infeasible.

   A successful brute forcing of the user code would enable the attacker
   to authenticate with their own credentials and make an authorization
   grant to the device.  This is the opposite scenario to an OAuth
   bearer token being brute forced, whereby the attacker gains control
   of the victim's authorization grant.  In some applications this
   attack  Such attacks may not always
   make much economic sense, for example for a video app,
   the owner of app the device owner may
   then be able to purchase movies with using the attacker's account, however there are still though
   a privacy considerations in
   that case as well as other risk would still remain and thus is important to protect
   against.  Furthermore, some uses of the device flow whereby give the granting
   account may be able the ability to perform sensitive actions such as controlling the victim's device.
   device, which needs to be protected.

   The precise length of the user code and the entropy contained within
   is at the discretion of the authorization server, which needs to
   consider the sensitivity of their specific protected resources, the
   practicality of the code length from a usability standpoint, and any
   mitigations that are in place such as rate-limiting, when determining
   the user code format.

5.2.  Device Trustworthiness

   Unlike other native application OAuth 2.0 flows, the device
   requesting the authorization is not the same as the device that the
   user grants access from.  Thus, signals from the approving user's
   session and device are not relevant to the trustworthiness of the
   client device.

   Note that if an authorization server used with this flow is
   malicious, then it could man-in-the middle man-in-the-middle the backchannel flow to
   another authorization server.  In this scenario, the man-in-the-
   middle is not completely hidden from sight, as the end-user end user would end
   up on the authorization page of the wrong service, giving them an
   opportunity to notice that the authorization being requested is
   wrong.  For this to be possible, the device manufacturer must either
   directly be the attacker, shipping a device intended to perform the
   man-in-the-middle attack, or be using an authorization server that is
   controlled by an attacker, possibly because the attacker compromised
   the authorization server used by the device.  In part, the person
   purchasing the device is counting on it and its business partners to
   be trustworthy.

5.3.  Remote Phishing

   It is possible for the device flow to be initiated on a device in an
   attacker's possession.  For example, the an attacker might send an email
   instructing the target user to visit the verification URL and enter
   the user code.  To mitigate such an attack, it is RECOMMENDED to
   inform the user that they are authorizing a device during the user
   interaction step (see Section 3.3), and to confirm that the device is
   in their possession.  The authorization server SHOULD display
   information about the device so that the person can notice if a
   software client was attempting to impersonating a hardware device.

   For authorization servers that support the option specified in
   Section 3.3.1 for the client to append the user code to the
   authorization URI, it is particularly important to confirm that the
   device is in the user's possession, as the user no longer has to type
   the code manually.  One possibility is to display the code during the
   authorization flow and asking the user to verify that the same code
   is being displayed on the device they are setting up.

   The user code needs to have a long enough lifetime to be useable
   (allowing the user to retrieve their secondary device, navigate to
   the verification URI, login, etc.), but should be sufficiently short
   to limit the usability of a code obtained for phishing.  This doesn't
   prevent a phisher presenting a fresh token, particularly in the case
   they are interacting with the user in real time, but it does limit
   the viability of codes sent over email or SMS.

5.4.  Session Spying

   While the device is pending authorization, it may be possible for a
   malicious user to spy on the device user interface and hijack the
   session by completing the authorization faster than the user that
   initiated it.  Devices SHOULD take into account the operating
   environment when considering how to communicate the code to the user
   to reduce the chances it will be observed by a malicious user.

5.5.  Non-confidential Clients

   Most device clients are incapable of being confidential clients, as
   secrets that are statically included as part of an app distributed to
   multiple users cannot be considered confidential.  For such clients,
   the recommendations of Section 5.3.1 of [RFC6819] and Section 8.5 of
   [RFC8252] apply.

5.6.  Non-Visual Code Transmission

   There is no requirement that the user code be displayed by the device
   visually.  Other methods of one-way communication can potentially be
   used, such as text-to-speech audio, or Bluetooth Low Energy.  To
   mitigate an attack in which a malicious user can bootstrap their
   credentials on a device not in their control, it is RECOMMENDED that
   any chosen communication channel only be accessible by people in
   close proximity.  E.g., users who can see, or hear the device, or
   within range of a short-range wireless signal. device.

6.  Usability Considerations

   This section is a non-normative discussion of usability

6.1.  User Code Recommendations

   For many users, their nearest Internet-connected device will be their
   mobile phone, and typically these devices offer input methods that
   are more time consuming than a computer keyboard to change the case
   or input numbers.  To improve usability (improving entry speed, and
   reducing retries), these limitations should be taken into account
   when selecting the user-code character set.

   One way to improve input speed is to restrict the character set to
   case-insensitive A-Z characters, with no digits.  These characters
   can typically be entered on a mobile keyboard without using modifier
   keys.  Further removing vowels to avoid randomly creating words
   results in the base-20 character set: "BCDFGHJKLMNPQRSTVWXZ".  Dashes
   or other punctuation may be included for readability.

   An example user code following this guideline, guideline containing 8
   significant characters and dashes added for end-user readability,
   with an a resulting entropy of 20^8: "WDJB-MJHT".

   Pure numeric codes are also a good choice for usability, especially
   for clients targeting locales where A-Z character keyboards are not
   used, though their length needs to be longer to maintain a high

   An example numeric user code, code containing 9 significant digits and
   dashes added for end-user readability, with an entropy of 10^9:


   When processing the inputted user code, the server should ignore any characters like strip
   dashes and other punctuation that are not
   in it added for readability (making the user-code character set.  Provided
   inclusion of that punctuation by the character set
   doesn't include user optional).  For codes using
   only characters of different case, in the comparison A-Z range as with the base-20 charset defined
   above, the user's input should be upper-cased before comparison to
   account for the fact that the user may input the equivalent lower-
   case insensitive. characters.  Further stripping of all characters outside the
   user_code charset is recommended to reduce instances where an
   errantly typed character (like a space character) invalidates
   otherwise valid input.

6.2.  Non-Browser User Interaction

   Devices and authorization servers MAY negotiate an alternative code
   transmission and user interaction method in addition to the one
   described in Section 3.3.  Such an alternative user interaction flow
   could obviate the need for a browser and manual input of the code,
   for example, by using Bluetooth to transmit the code to the
   authorization server's companion app.  Such interaction methods can
   utilize this protocol, as ultimately, the user just needs to identify
   the authorization session to the authorization server; however, user
   interaction other than via the verification URI is outside the scope
   of this specification.

7.  IANA Considerations

7.1.  OAuth URI Registration

   This specification registers the following values in the IANA "OAuth
   URI" registry [IANA.OAuth.Parameters] established by [RFC6755].

7.1.1.  Registry Contents

   o  URN: urn:ietf:params:oauth:grant-type:device_code
   o  Common Name: Device flow grant type for OAuth 2.0
   o  Change controller: IESG
   o  Specification Document: Section 3.1 of [[ this specification ]]

7.2.  OAuth Extensions Error Registration

   This specification registers the following values in the IANA "OAuth
   Extensions Error Registry" registry [IANA.OAuth.Parameters]
   established by [RFC6749].

7.2.1.  Registry Contents

   o  Error name: authorization_pending
   o  Error usage location: Token endpoint response
   o  Related protocol extension: [[ this specification ]]
   o  Change controller: IETF
   o  Specification Document: Section 3.5 of [[ this specification ]]

   o  Error name: access_denied
   o  Error usage location: Token endpoint response
   o  Related protocol extension: [[ this specification ]]
   o  Change controller: IETF
   o  Specification Document: Section 3.5 of [[ this specification ]]

   o  Error name: slow_down
   o  Error usage location: Token endpoint response
   o  Related protocol extension: [[ this specification ]]
   o  Change controller: IETF
   o  Specification Document: Section 3.5 of [[ this specification ]]

   o  Error name: expired_token
   o  Error usage location: Token endpoint response
   o  Related protocol extension: [[ this specification ]]
   o  Change controller: IETF
   o  Specification Document: Section 3.5 of [[ this specification ]]

7.3.  OAuth 2.0 Authorization Server Metadata

   This specification registers the following values in the IANA "OAuth
   2.0 Authorization Server Metadata" registry [IANA.OAuth.Parameters]
   established by [RFC8414].

7.3.1.  Registry Contents

   o  Metadata name: device_authorization_endpoint
   o  Metadata Description: The Device Authorization Endpoint.
   o  Change controller: IESG
   o  Specification Document: Section 4 of [[ this specification ]]

8.  Normative References

              IANA, "OAuth Parameters",

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 2119, 5246,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>. 10.17487/RFC5246, August 2008,

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,

   [RFC6755]  Campbell, B. and H. Tschofenig, "An IETF URN Sub-Namespace
              for OAuth", RFC 6755, DOI 10.17487/RFC6755, October 2012,

   [RFC6819]  Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
              Threat Model and Security Considerations", RFC 6819,
              DOI 10.17487/RFC6819, January 2013,

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8252]  Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps",
              BCP 212, RFC 8252, DOI 10.17487/RFC8252, October 2017,

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,

   [RFC8414]  Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", RFC 8414,
              DOI 10.17487/RFC8414, June 2018,

Appendix A.  Acknowledgements

   The starting point for this document was the Internet-Draft draft-
   recordon-oauth-v2-device, authored by David Recordon and Brent
   Goldman, which itself was based on content in draft versions of the
   OAuth 2.0 protocol specification removed prior to publication due to
   a then lack of sufficient deployment expertise.  Thank you to the
   OAuth working group members who contributed to those earlier drafts.

   This document was produced in the OAuth working group under the
   chairpersonship of Rifaat Shekh-Yusef and Hannes Tschofenig with
   Benjamin Kaduk, Kathleen Moriarty, and Eric Rescorla serving as
   Security Area Directors.

   The following individuals contributed ideas, feedback, and wording
   that shaped and formed the final specification:

   Brian Campbell, Roshni Chandrashekhar, Eric Fazendin, Torsten
   Lodderstedt, James Manger, Breno de Medeiros, Simon Moffatt, Stein
   Myrseth, Justin Richer, Nat Sakimura, Andrew Sciberras, Marius
   Scurtescu, Ken Wang, and Steven E. Wright.

Appendix B.  Document History

   [[ to be removed by the RFC Editor before publication as an RFC ]]


   o  Set a default polling interval to 5s explicitly.

   o  Defined the slow_down behavior that it should increase the current
      interval by 5s.
   o  expires_in now REQUIRED
   o  Other changes in response to review feedback.


   o  Updated reference to OAuth 2.0 Authorization Server Metadata.


   o  Added a missing definition of access_denied for use on the token
   o  Corrected text documenting which error code should be returned for
      expired tokens (it's "expired_token", not "invalid_grant").
   o  Corrected section reference to RFC 8252 (the section numbers had
      changed after the initial reference was made).
   o  Fixed line length of one diagram (was causing xml2rfc warnings).
   o  Added line breaks so the URN grant_type is presented on an
      unbroken line.
   o  Typos fixed and other stylistic improvements.


   o  Addressed review comments by Security Area Director Eric Rescorla
      about the potential of a confused deputy attack.


   o  Expanded the User Code Brute Forcing section to include more
      detail on this attack.


   o  Replaced the "user_code" URI parameter optimization with
      verification_uri_complete following the IETF99 working group
   o  Added security consideration about spying.
   o  Required that device_code not be shown.
   o  Added text regarding a minimum polling interval.


   o  Clarified usage of the "user_code" URI parameter optimization
      following the IETF98 working group discussion.

   o  response_type parameter removed from authorization request.
   o  Added option for clients to include the user_code on the
      verification URI.
   o  Clarified token expiry, and other nits.


   o  Security & Usability sections.  OAuth Discovery Metadata.


   o  device_code is now a URN.  Added IANA Considerations


   o  Added token request & response specification.


   o  Applied spelling and grammar corrections and added the Document
      History appendix.


   o  Initial working group draft based on draft-recordon-oauth-

Authors' Addresses

   William Denniss
   1600 Amphitheatre Pkwy
   Mountain View, CA  94043

   Email: wdenniss@google.com
   URI:   http://wdenniss.com/device-flow

   John Bradley
   Ping Identity

   Email: ve7jtb@ve7jtb.com
   URI:   http://www.thread-safe.com/
   Michael B. Jones

   Email: mbj@microsoft.com
   URI:   http://self-issued.info/

   Hannes Tschofenig
   ARM Limited

   Email: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at