]> Apple Inc

dvinokurov@apple.com

Apple Inc

castiz@apple.com

Apple Inc

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Alphabet Inc

lassey@google.com

ART TIGRESS Internet-Draft This document describes the use cases necessitating the secure transfer of digital credentials, residing in a digital wallet, between two devices and defines general assumptions, requirements and the scope for possible solutions to this problem. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Source for this draft and an issue tracker can be found at .

Introduction In this document we are identifying a problem of transferring digital credentials (e.g. a digital car key, a digital key a a hotel room or a digital key to a private house) from a wallet on one device (smartphone) to another, particularly, if these devices belong to two different platforms (e.g. one is iOS, another - Android). Today, there is no widely accepted way of transferring digital credentials securely between two digital wallets independent of hardware and software manufacturer. This document describes the problem space and the requirements for the solution the working group creates. A Working Group, called Tigress has been established to find a solution to the problem described above. Within the WG an initial solution was presented (https://datatracker.ietf.org/doc/draft-art-tigress). The community decided to generalize the requirements to the solution and consider alternative solutions within the WG. This document presents the general requirements to possible solutions and specifies certain privacy requirements in order to maintain a high level of user privacy.

General Setting When sharing digital secure credentials, there are several actors involved. This document will focus on sharing information between two digital wallets, directly or through an intermediary server. Digital credentials provide access to property owned and / or operated by 3-rd party entities, such as hotel or residential building owners. The entity that is providing the digital credential for consumption by a digital wallet are referred to as Provisioning Entities. For some kind of credentials Provisioning Entity require to have control over digital credential issuance and life time management - for example, hotel is the owner of the rooms and allow guests to access them for the time of thier stay only. Digital wallet is a combination of software and hardware in a smartphone device, there are two devices involved in credentail transfer process - Sender and Receiver. They are defined in terms of which one is a transfer initiator (Sender) and which device is eventually consuming transferred credentials (Receiver). Device roles can change based on the transfer direction - in some transfers a device can act as a Sender, in other - as a Receiver. The interface between the device and the Provisioning Entity can be proprietary or a part of published specifications. The sender wallet obtains provisioning information from the Provisioning Entity, then shares it to the recipient using a solution defined in Tigress WG. The recipient then takes that provisioning information and sends it to the Provisioning Entity to redeem for credential for consumption in a digital wallet. For some credential types the Provisioning Entity who issues new credentials is actually the sender wallet. In that scenario the receiver will generate a new key material at the request of the sender, and then communicate with the sender over Tigress to have its key material signed by the sender. New credential, with the key material generated by receiver device and signed by sender device, will finally be added (provisioned) into a digital wallet on sender device.

Credential transfer without Provisioning Entity mermaid sequenceDiagram actor S as Sender participant I as Intermediary actor R as Receiver S ->> I : upload Sharing Invititation ({{CCC-Digital-Key-30}}) break Generic messaging channel S ->> R : send invite end break Credential Provisioning flow {{CCC-Digital-Key-30}} R ->> I : upload Key Signing Request ({{CCC-Digital-Key-30}}) S ->> I : read Key Signing Request, generate and upload Key Import Request ({{CCC-Digital-Key-30}}) R ->> I : read Key Import Request and Import Key Data ({{CCC-Digital-Key-30}}) end

Conventions and Definitions 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 when, and only when, they appear in all capitals, as shown here. General terms:

• Credential information - data used to authenticate the user with an access point.
• Provisioning information - data transferred from Sender to Receiver device that is both necessary and sufficient for the Receiver to request a new credential from Provisioning Entity to provision it to the Receiver device.
• Provisioning - A process of adding a new credential to the device.
• Provisioning Entity - an entity which facilitates Credential Information lifecycle on a device - for some types of access credentials (e.g. hotels, corporate access). Lifecycle may include provisioning of credential, credential termination, credential update.
• Sender (device) - a device initiating a transfer of Provisioning Information to a Receiver that can provision this credential.
• Receiver (device) - a device that receives Provisioning Information and uses it to provision a new credential.
• Intermediary (server) - an optional intermediary server that provides a standardized and platform-independent way of transferring provisioning information between Sender and Receiver devices, acting as a temporary store and forward service between Sender and Receiver.
• Digital Wallet - A device, service, and/or software that faciliates transactions either online or in-person via a technology like NFC. Digital Wallet's typically support payments, drivers licenses, loyalty cards, access credentials and more.

Use Cases

• Let's say Ben owns a vehicle that supports digital car keys. Ben would like to let Ryan borrow the car for the weekend. Ryan and Ben are using two different devices (smartphones) with different operating systems. In order for Ben to share his digital car key to Ryan for a weekend, he must transfer some data to the receiver device. Receiver device generates new key material and return it to the sender to sign and return back to the receiver. At this point, the receiver now has a token that will allow them to provision their new key with the car.
• Bob booked a room at a hotel for the weekend, but will be arriving late at night. Alice, his partner, comes to the hotel first, so Bob wants to share his digital room key with Alice. Bob and Alice are using two different mobile phones with different operating systems. In order for Bob to share his digital room key to Alice for a weekend, he must transfer some data to her device. The data structure shared between the two participants is proprietary to the given hotel chain (or Provisioning Entity). This data transfer is a one-time, unidirectional transfer from Bob's device to Alice's. Once Alice receives this data, she can provision a new key to her digital wallet, making a call to Provisioning Entity to receive new credential information.

Relationships

Credential transfer with intermediary server mermaid sequenceDiagram actor S as Sender participant I as Intermediary actor R as Receiver S ->> I : upload provisioning information break Generic messaging channel S ->> R : send invite end loop Provision credential R ->> I : request additional provisioning information I ->> R : deliver additional provisioning information end

Credential transfer without intermediary mermaid sequenceDiagram actor S as Sender actor R as Receiver break secure messaging channel S ->> R : transfer provisioning information E2E end loop Provision credential R ->> S : request additional provisioning information S ->> R : deliver additional provisioning information end

Assumptions

• Depending on credential type, there are 3 possible scenarios for transferred credential cryptographic key material: 1) An existing cryptographic key is being copied and sent from Sender to receiver and provisioned to digital wallet as it is. In this case two credentails on both devices are indistinguashable. This scenario is currently used by a very limited number of entities. 2) Instead of the original cryptographic key, Sender device fetches a provisiong token from Provisiong entity approval and sends it to Receiver so that Receiver can acquire new credential information from Provisiong Entity presenting given provisiong token to it and receiving new cryptographic key material. In this case receiver device may have just a subset of the sender's device access rights or receiver's device credential may be revoked independently of the sender's device credential. As a result, Sender and Receiver devices have 2 different keys, potentially linked to the same logical "user account". Also, the Provisiong Entity may not allow the same cryptographic key to be added to different physical devices. 3) When Provisioning Entity does not exist in the transfer flow. For example, for a transfer of a digital car key both sender and receiver devices are used to generate new credential's cryptographic key matherial and sign over the new cryptographic key for it to be trusted by the access point.
• Security: Communication between sender or receiver devices and Provisioning Entity should be trusted. Since new credential's key material is generated by Provisioning Entity, the channel between the device and Provisioning Entity shall be secure and trusted by both parties.
• In case of an intermediary server, used during the credential transfer from sender device to receiver device, the choice of intermediary shall be defined by the application initiating the credential transfer. Digital wallet or another application that manages credentials on sender device shall make the decision regarding the channel to be used to sent the Provisioning Information.
• Sender and Receiver shall both be able to manage the shared credential at any point in transfer or lifecycle: a) the process of credential transfer can be stopped at any time before the credential is provisioned to the receiver device by either sender or receiver device (e.g. making a call to intermediate server to delete a temporary mailbox); b) or after credential has been provisioned - by ether "manage credential" call issued from sender device to Provisiong Entity (or from Provisiong Entity initiating "manage credential" API).
• Any device OEM with a digital credential implementation adherent to Tigress solution shall be able to receive shared provisioning information, whether or not they can originate provisioning information themselves. We define the digital credential transfer as platform-independant; therefore, if the receiver device can recognize the data format of the received Provisioning Information, it should be able to provision the new credential to the Digital Wallet.
• Provisioning new credential on the receiver may require multiple round trips. In case the Provisioning Entity is not used for a certain type of credentials (e.g. for a transfer of a digital car key), both sender and receiver devices are used to generate new credential's cryptographic key matherial and sign on the new cryptographic key for it to be trusted by the access point.

Requirements

• (Req-XPlatform) Solution shall support transfer of digital credential across different platforms (e.g. from Android to iOS).
• (Req-P2P) A goal of credential transfer covered in this document shall include transfer from one device to another (group sharing shall not be a goal).
• (Req-CredentialType) The solution shall support transfer of various digital credential types, based on symmetric and asymmetric cryptography, public and proprietary standards.
• (Req-Security) Solution should provide security of the provisioning data transferred (confidentiality, integrity and availability of provisioning information in transit).
• (Req-Privacy) Transport protocol used to transfer provisioning information ( e.g. secure E2E transfer protocol or intermediary server) shall prevent from correlating users between exchanges or create a social graph of users involved into transfer. Intermediary server shall not be an arbiter of identity. User identities shall not be collected, stored and used for purpose other then the credential transfer itself.
• (Req-Connectivity) Sender and Receiver shall be allowed to be online at different times. Sender and Receiver shall not need to be online at the same time. This requirement allows devices to connect to network to only exchange the portion of information required during the transfer, allowing them upload or download data in turns to network servers.
• (Req-RoundTrips) Solution shall allow for multiple data exchanges between sender and receiver devices in the process of credential transfer. This requirement shall alighn with (Req-Connectivity) above.
• (Req-Opaque) In the case when an intermediary server is used to facilitate the credential transfer, message content between sender and receiver must be opaque to an intermediary, intermediary server shall not be able to recognize the content of provisioning information or use it to provision digital credential on its own.
• (Req-SenderTrust) In the case when an intermediary server is used to facilitate the credential transfer, sender device should establish trusted relationship with the intermediary server. Intermediary server shall be able to verify that the sender device is in good standing and content generated by the sender device can be trusted by the intermediary. The trust mechanism could be proprietary or publicly verifiable ( e.g. WebAuthN). This is important because intermediary server shall have no visibility to the content of the provisiong information sent through it (Req-Opaque).
• (Req-ReceiverTrust) In the case when an intermediary server is used to facilitate the credential transfer, receiver device should be able to evaluate the trustworthiness of the intermediary based on agreed criteria.

Security Considerations TODO Security

IANA Considerations This document has no IANA actions.

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References Car Connectivity Consortium

Acknowledgments TODO acknowledge.