Internet-Draft | Secure Credential Transfer | December 2021 |
Vinokurov, et al. | Expires 10 June 2022 | [Page] |
This document describes a mechanism to transfer digital credentials securely between two devices. Secure credentials may represent a digital key to a hotel room, a digital key to a door lock in a house or a digital key to a car. Devices that share credentials may belong to the same or two different platforms (e.g. iOS and Android). Secure transfer may include one or more write and read operations. Credential transfer needs to be performed securely due to the sensitive nature of the information.¶
This note is to be removed before publishing as an RFC.¶
Source for this draft and an issue tracker can be found at https://github.com/dimmyvi/secure-credential-transfer.¶
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This Internet-Draft will expire on 10 June 2022.¶
Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
Today, there is no standard way of transferring digital credentials securely between two devices belonging to the same platform or two different platforms. This document proposes a solution to this problem by introducing a Relay server which allows two devices to exchange encrypted Provisioning Information securely. The Relay server solves this problem by creating and managing temporary mailbox storage.¶
Each mailbox can be referenced by devices using a unique mailbox identifier in a URL. The URL pointing to encrypted Provisioning Information is to be passed between devices directly over various channels (e.g. SMS, email, messaging applications). The Security Considerations section provides recommendations on passing the URL and the Secret securely.¶
This document describes a Hypertext (HTTP) Application Programming Interface (API) that allows Sender and Receiver devices to interact with a Relay server in order to perform secure credential transfer.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
General terms:¶
API parameters:¶
We define two flows for credential transfer: 1. Stateless (Relay server facilitates a single credential data transfer: Sender -> Relay -> Receiver) and 2. Stateful (Relay server facilitates additional data transfers - there are multiple data transfers in this flow to prepare credential data for registering or provisioning by Receiver). Relay server does not limit the number of such data tranfsfers between Sender and Receiver devices. The details are provided below.¶
Both stateless and stateful share the following common steps. The processes start with a Sender device composing a set of Provisioning Information, encrypting it with a Secret and storing encrypted Provisioning Information on a Relay server in a mailbox. A unique Mailbox Identifier is generated by the Relay server as a part of CreateMailbox call, created using a good source of entropy (preferably hardware-based entropy). Sender device generates a unique token - a Sender Device Claim - and stores it to the mailbox. Device Claim allows the Sender device presenting it to read and write data to / from the mailbox, thus binding it to the mailbox.¶
Sender device calls CreateMailbox API endpoint on a Relay server in order to create a mailbox. Once a mailbox is created, it has limited time to live. When expired, the mailbox SHALL be deleted - refer to DeleteMailbox endpoint. TimeToLive mailbox configuration in the request is required to use with the CreateMailbox call (refer to mailboxConfiguration request parameter). Relay server is responsible to periodically check for mailboxes with expired TimeToLive and delete them.¶
Relay server builds a unique URL link to a mailbox (for example, "http://relayserver.com/m/1234567890") and returns it to the Sender device, which sends the link directly to the Receiver device over communication channel (e.g. SMS, email, iMessage). Please refer to section "Security Considerations" for more details.¶
Receiver device, having obtained both the URL link and the Secret, generates a unique token - a Receiver Device Claim - and passes it to the Relay server in order to read the encrypted Provisioning Information from the mailbox.¶
Relay server now binds a given pair of Sender and Receiver devices to the mailbox by provided Sender and Receiver Device Claims. Only bound devices are allowed to read or write data to the mailbox or to delete the mailbox.¶
The stateless workflow completes the common steps described in "Credential transfer workflows" section, then finishes the transfer completing the following steps. Receiver device, having read the encrypted Provisioning Information from the Relay mailbox, decrypts it with the Secret received from the Sender and starts credential registering or provisioning process on the device. Once the Receiver device has successfully provisioned credentials, it deletes the mailbox by sending a DeleteMailbox call to the Relay server.¶
The stateful workflow completes the common steps described in "Credential transfer workflows" section, then finishes the transfer completing the following steps.¶
Then the Receiver device, having downloaded the encrypted Provisioning Information from the mailbox by URL and decrypted it with the Secret, generates a new structure of Provisioning Information, e.g. a digital key, and encrypts it with the same Secret, received from the Sender device. It then stores the payload in the same mailbox on the Relay server. In addition to the encrypted payload, Receiver stores a Receiver Notification Token in the given mailbox.¶
Having received the encrypted Provisioning Information, the Relay server sends a Notification to the Sender device using the Sender Notification Token.¶
Sender device, having received the notification from the Relay server, reads secure content from the mailbox and decrypts all using the same Secret. Sender device generates new Provisioning Information, encrypts all fields using the Secret and stores all data in the same mailbox on the Relay server.¶
Relay server, having stored the data above, sends a notification to the Receiver device using Receiver Notification Token. Receiver device, having received the notification, reads the encrypted Provisioning Information, decrypts the data using the same Secret and uses this data to finalize credential registration or provisioning on device.¶
Once the Receiver device has successfully registered or provisioned credentials, it deletes the mailbox by sending a DeleteMailbox call to the Relay server. Sender device may terminate the secure credential transfer by deleting the mailbox it created at any time. Deletion of the mailbox on the Relay server stops any on-going credential transfer process.¶
The Relay server API endpoint MUST be accessed over HTTP using an https URI [RFC2818] and SHOULD use the default https port. Request and response bodies SHALL be formatted as either JSON or HTML (based on the API endpoint). The communication protocol used for all interfaces SHALL be HTTPs. All Strings SHOULD be UTF-8 encoded (Unicode Normalization Form C (NFC)). An API version SHOULD be included in the URI for all interfaces. The version at the time of this document's latest update is v1. The version SHALL be incremented by 1 for major API changes or backward incompatible iterations on existing APIs.¶
All requests to and from Relay server will have an HTTP header "Mailbox-Request-ID". The corresponding response to the API will have the same HTTP header, which SHALL echo the value in the request header. This is used to identify the request associated to the response for a particular API request and response pair. The value SHOULD be a UUID [RFC4122]. The request originator SHALL match the value of this header in the response with the one sent in the request. If response is not received, caller may retry sending the request with the same value of "Mailbox-Request-ID". Relay server SHOULD store the value of the last successfully processed "Mailbox-Request-ID" for each device based on the caller's Device Claim. A key-value pair of "Device Claim" to "Mailbox-Request-ID" is suggested to store the last successfully processed request for each device. In case of receiving a request with duplicated "Mailbox-Request-ID", Relay SHOULD respond to the caller with status code 201, ignoring the duplicate request body content.¶
An application running on a remote device can invoke this API on Relay Server to create a mailbox and store secure data content to it (encrypted data specific to a provisioning partner). MailboxIdentifier is created by the Relay server as an UUID [RFC4122], using cryptographic entropy. A URL to the created mailbox to be returned to the caller in the response.¶
Path parameters¶
Header parameters¶
This API call consumes the following media types via the Content-Type request header: application/json
¶
This API call produces the following media types via the Content-Type response header: application/json
¶
Request body is a complex structure, including the following fields:¶
payload (Object, Required) - for the purposes of Secure Credential Transfer API, this is a data structure, describing Provisioning Information specific to Credential Provider. It consists of the following 2 key-value pairs:¶
displayInformation (String, Required) - for the purposes of the Secure Credential Transfer API, this is a JSON data blob. It allows an application running on a receiving device to build a visual representation of the credential to show to user. The data structure contains the following fields:¶
notificationToken (Object, Optional) - optional notification token used to notify an appropriate remote device that the mailbox data has been updated. Data structure includes the following (if notificationToken is provided it should include both fields):¶
mailboxConfiguration (Object, Optional) - optional mailbox configuration, defines access rights to the mailbox, mailbox expirationTime. Required at the time of the mailbox creation. OEM device may provide this data in the request, Relay server shall define a default configuration, if it is not provided in the incoming request. Data structure includes the following:¶
200
Status: "200" (OK)¶
ResponseBody:¶
201
Status: "201" (Created) - response to a duplicated request (duplicated "Mailbox-Request-Id"). Relay server SHALL respond to duplicated requests with 201 without creating a new mailbox. "Mailbox-Request-Id" passed in the first CreateMailbox request's header SHOULD be stored by the Relay server and compared to the same value in the subsequent requests to identify duplicated requests. If duplicate is found, Relay SHALL not create a new mailbox, but respond with 201 instead. The value of "Mailbox-Request-Id" of the last successfully completed request SHOULD be stored based on the Device Claim passed by the caller.¶
400
Bad Request - invalid request has been passed (can not parse or required fields missing).¶
401
Unauthorized - calling device is not authorized to create a mailbox. E.g. a device presented the incorrect deviceClaim or mailbox with the provided mailboxIdentifier already exists.¶
An application running on a remote device can invoke this API on Relay Server to update secure data content in an existing mailbox (encrypted data specific to a Provisioning Partner). The update effectively overwrites the secure payload previously stored in the mailbox.¶
Path parameters:¶
Header parameters:¶
This API call consumes the following media types via the Content-Type request header: application/json
¶
Request body is a complex structure, including the following fields:¶
payload (Object, Required) - for the purposes of Secure Credential Transfer API, this is a data structure, describing Provisioning Information specific to Credential Provider. It consists of the following 2 key-value pairs:¶
notificationToken (Object, Optional) - optional notification token used to notify an appropriate remote device that the mailbox data has been updated. Data structure includes the following (if notificationToken is provided it should include both fields):¶
ResponseBody:¶
200
Status: "200" (OK)¶
201
Status: "201" (Created) - response to a duplicate request (duplicate "Mailbox-Request-Id"). Relay server SHALL respond to duplicate requests with 201 without performing mailbox update. "Mailbox-Request-Id" passed in the first UpdateMailbox request's header SHALL be stored by the Relay server and compared to the same value in the subsequent requests to identify duplicate requests. If duplicate is found, Relay SHALL not perform mailbox update, but respond with 201 instead.
The value of "Mailbox-Request-Id" of the last successfully completed request SHALL be stored based on the Device Claim passed by the caller.¶
400
Bad Request - invalid request has been passed (can not parse or required fields missing).¶
401
Unauthorized - calling device is not authorized to update the mailbox. E.g. a device presented the incorrect deviceClaim.¶
404
Not Found - mailbox with provided mailboxIdentifier not found.¶
An application running on a remote device can invoke this API on Relay Server to close the existing mailbox after it served its purpose. Receiver or Sender device needs to present a deviceClaim in order to close the mailbox.¶
Path parameters:¶
Header parameters:¶
200
Status: "200" (OK)¶
401
Unauthorized - calling device is not authorized to delete a mailbox. E.g. a device presented the incorrect deviceClaim.¶
404
Not Found - mailbox with provided mailboxIdentifier not found. Relay server may respond with 404 if the Mailbox Identifier passed by the caller is invalid or mailbox has already been deleted (as a result of duplicate DeleteMailbox request).¶
An application running on a remote device can invoke this API on Relay Server to retrieve public display information content from a mailbox. Display Information shall be returned in OpenGraph format (please refer to https://ogp.me for details). OpenGraph-formatted display information is required to display a preview of credential in a messaging application, e.g. iMessage or WhatsApp.¶
Path parameters:¶
An application running on a remote device can invoke this API on Relay Server to retrieve secure payload content from a mailbox (encrypted data specific to a Provisioning Information Provider).¶
Path parameters:¶
Header parameters:¶
This API call produces the following media types via the Content-Type response header: application/json
¶
200
Status: "200" (OK)¶
ResponseBody :¶
401
Unauthorized - calling device is not authorized to read the secure content of the mailbox. E.g. a device presented the incorrect deviceClaim.¶
404
Not Found - mailbox with provided mailboxIdentifier not found.¶
An application running on a remote device can invoke this API on Relay Server to relinquish their ownership of the mailbox. Receiver device needs to present the currently established Receiver deviceClaim in order to relinquish their ownership of the mailbox. Once relinquished, the mailbox can be bound to a different Receiver device that presents its deviceClaim in a ReadSecureContentFromMailbox call.¶
Path parameters:¶
Header parameters:¶
200
Status: "200" (OK)¶
201
Status: "201" (Created) - response to a duplicate request (duplicate "Mailbox-Request-Id"). Relay server SHALL respond to duplicate requests with 201 without performing mailbox relinquish. "Mailbox-Request-Id" passed in the first RelinquishMailbox request's header SHALL be stored by the Relay server and compared to the same value in the subsequent requests to identify duplicate requests. If duplicate is found, Relay SHALL not perform mailbox relinquish, but respond with 201 instead.
The value of "Mailbox-Request-Id" of the last successfully completed request SHALL be stored based on the Device Claim passed by the caller.¶
401
Unauthorized - calling device is not authorized to relinquish a mailbox. E.g. a device presented the incorrect deviceClaim, or the device is not bound to the mailbox.¶
404
Not Found - mailbox with provided mailboxIdentifier not found. Relay server may respond with 404 if the Mailbox Identifier passed by the caller is invalid.¶
The encrypted payload (Provisioning Information) should be a data structure having the following key-value pairs: "type", which defines the encryption algorithm and mode used and "data", which contains BASE-64 encoded binary value of ciphertext.¶
Please refer to [RFC5116] for the details of the encryption algorithm.¶
The following algorithms and modes are mandatory to implement:¶
The following threats and mitigations have been considered:¶
Sender shares with the wrong receiver¶
Malicious receiver forwards the share to 3rd party without redeeming it or the Receiver's device is compromised.¶
Malicious receiver attempts re-use share¶
Share URL accidental disclosure. (e.g. share URL sent as a message which gets displayed on a locked screen)¶
Network attacks¶
This document registers a new header, "Mailbox-Request-ID", in the "Permanent Message Header Field Names" <https://www.iana.org/assignments/message-headers>.¶
The following people provided substantive contributions to this document:¶
TODO acknowledge.¶