Internet-Draft tigress-requirements November 2022
Vinokurov, et al. Expires 13 May 2023 [Page]
Intended Status:
D. Vinokurov
Apple Inc
C. Astiz
Apple Inc
A. Pelletier
Apple Inc
J. L. Giraud
Apple Inc
A. Bulgakov
Apple Inc
M. Byington
Apple Inc
N. Sha
Alphabet Inc
M. Gerster
Mercedes-Benz AG

Transfer Digital Credentials Securely - Requirements


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 of the corresponding Tigress Internet-draft [Tigress-00].

About This Document

This note is to be removed before publishing as an RFC.

The latest revision of this draft can be found at Status information for this document may be found at

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This Internet-Draft will expire on 13 May 2023.

1. Introduction

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.

Tigress allows for a sender and receiver to communicate in order to facilitate a secure credential transfer between two digital wallets. Tigress also specifies certain privacy requirements in order to maintain a high level of user privacy.

2. General Setting

When sharing digital secure credentials, there are several actors involved. While the Tigress working group's solution will focus on sharing information between two digital wallets, potentially through an intermediary server, there are a couple more actors involved.

The companies that are providing the digital credential for consumption by a digital wallet are the provisioning partners. They are in control of the provisioning information and the lifecycle of the credentials. Each digital wallet has a preexisting trust relationship between itself and the Provisioning Partner.

The interface between the devices and the Provisioning Partner can be proprietary or a part of published specifications such as the [CCC-Digital-Key-30]. The sender obtains provisioning information from the provisioning partner, then shares it to the recipient via Tigress. The recipient then takes that data and sends it to the Provisioning Partner to redeem a credential for consumption in a digital wallet.

For some credential types the Provisioning Partner who mints new credentials is actually the sender. 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.

3. 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 [RFC2119] [RFC8174] 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 Partner to provision it to the Receiver device.
  • Provisioning - A process of adding a new credential to the device.
  • Provisioning Partner - an entity which facilitates Credential Information lifecycle on a device. 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 intermediary server that provides a standardized and platform-independent way of transferring provisioning information between Sender and Receiver devices.
  • 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.

4. Use Cases

  • Let's say Ben owns a vehicle that supports digital keys which comply with the CCC specification [CCC-Digital-Key-30]. Ben would like to let Ryan borrow the car for the weekend. Ryan and Ben are using two different mobile phones 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. The data structure shared between the two participants is defined in the [CCC-Digital-Key-30]. In addition, the [CCC-Digital-Key-30] requires the receiver to generate required 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 Partner). 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 Partner to receive new credential information.

5. Relationships

mermaid sequenceDiagram actor S as Sender participant I as Intermediary actor R as Receiver S ->> I : upload credential data break Generic messaging channel S ->> R : send invite end Loop Provision credential R ->> I : request credential data I ->> R : deliver credential data end

6. Assumptions

  • Original credential information (with cryptographic key material) MUST NOT be sent or shared. Instead, sender SHALL be transferring its approval token for Receiver to acquire new credential information.
  • Provisioning Partner SHALL NOT allow for two users to use the same credential / cryptographic keys.
  • Security: Communication between Sender / Receiver and Provisioning Partner SHOULD be trusted.
  • The choice of intermediary SHALL be defined by the application initiating the credential transfer.
  • Sender and Receiver SHALL both be able to manage the shared credential at any point by communicating with the Provisioning Partner. Credential lifecycle management is out of scope for this proposal.
  • Any device OEM with a digital credential implementation adherent to Tigress [Tigress-00] SHALL be able to receive shared provisioning information, whether or not they can originate provisioning information themselves or host their own intermediary.
  • Provisioning a credential on the Receiver MAY require multiple round trips.

7. Requirements

  • (Req-Connectivity) Sender and Receiver SHALL be allowed to be online at different times. Sender and Receiver SHALL never need to be online at the same time.
  • (Req-init) Solution SHOULD allow Sender to send the share invitation to Receiver over any messaging channel, with various degrees of security.
  • (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-Security) Solution SHOULD provide security of the provisioning data transferred (confidentiality, integrity and availability).
  • (Req-Revoke) Solution SHALL maintain access control, allowing Sender to revoke before the share has been accepted, and for Receiver to end transfer at any time.
  • (Req-ArbitraryFormat) The solution SHALL support arbitrary message formats to support both digital keys that implement public standards like [CCC-Digital-Key-30] as well as proprietary implementations of digital keys.
  • (Req-RoundTrips) Solution SHALL allow for stateful requests between Sender and Receiver to support stateful actions like key signing requests.
  • (Req-Preview) Solution SHOULD allow for receiver to know what is being added to their digital wallet.

7.1. Intermediary server requirments

If the solution requires an intermediary server, it should have the following requirements.

  • (Req-Privacy) An Intermediary server SHALL not be able to correlate users between exchanges, or create a social graph. Intermediary server shall not be an arbiter of Identity.
  • (Req-Notify) Solution SHOULD support a notification mechanism to inform devices on the content update on Intermediary server.
  • (Req-Opaque) Message content between Sender and Receiver MUST be opaque to an Intermediary.
  • (Req-IntermediaryAttestation) An Intermediary SHALL implement mechanisms to prevent abuse by share initiating device, verifying that the 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).
  • (Req-ReceiverTrust) The Receiver device SHOULD be able to evaluate the trustworthiness of the Intermediary using a list of trusted/approved intermediaries.

8. Review of existing solutions

A number of existing solutions / protocols have been reviewed in order to be used for secure credential transfer based on the requirements: GSS-API, Kerberos, AWS S3, email, Signal. None of the existing protocols comply with the requirements; the effort of modifying the existing protocols has been accessed to be significantly higher than introducing a new solution to solve this problem. The goal of the Tigress draft [Tigress-00] is not to define a new encryption or secure message exchange protocol, but rather a standardized mechanism of exchanging access-specific encrypted credential information.

8.1. Arbitrary Messaging Channel (Email / WhatsApp / SMS / Signal / etc.)

The Provisioning Information MAY be sent from Sender to Receiver over an arbitrary messaging channel that supports binary file transfer, but this would not support provisioning flows which require multiple round trips as requied by (Req-RoundTrips). The same requirement applies to Signal protocol outside of the Signal app, as the Req-RoundTrips would likely be difficult and add a lot of friction for the user.

8.2. GSS-API, Kerberos

GSS-API [RFC2078] and Kerberos [RFC4120] are authentication technologies which could be used to authenticate Sender, Receiver and intermediary. However, as they provide strong authentication, they would allow the Intermediary server to build a social graph in violation of (Req-Privacy). Their setup also require strong coordination between the actors of the system which seems overly costly for the intended system. AWS S3 could be used as an Intermediary server but it would force all participants to use a specific cloud service which is in violation of (Req-AnyPlatorm).

9. Out of Scope

  • Identification and Authorization - solution shall not require strong identification and authentication from user (e.g. using PKI certificates).
  • Fully stopping people from sharing malicious content ("cat pictures").
  • Solving problem of sharing to groups.
  • Detailing how credentials are provisioned either on a device or with a provisioning partner.

11. IANA Considerations

This document has no IANA actions.

12. Normative References

Car Connectivity Consortium, "Digital Key Release 3", , <>.
Linn, J., "Generic Security Service Application Program Interface, Version 2", RFC 2078, DOI 10.17487/RFC2078, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos Network Authentication Service (V5)", RFC 4120, DOI 10.17487/RFC4120, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Vinokurov, D., Byington, M., Lerch, M., Pelletier, A., and N. Sha, "Transfer Digital Credentials Securely", , <>.


TODO acknowledge.

Authors' Addresses

Dmitry Vinokurov
Apple Inc
Casey Astiz
Apple Inc
Alex Pelletier
Apple Inc
Jean-Luc Giraud
Apple Inc
Alexey Bulgakov
Apple Inc
Matt Byington
Apple Inc
Nick Sha
Alphabet Inc
Manuel Gerster
Mercedes-Benz AG