TIGRESS D. Vinokurov Internet-Draft C. Astiz Intended status: Informational A. Pelletier Expires: 13 May 2023 J. L. Giraud A. Bulgakov M. Byington Apple Inc N. Sha Alphabet Inc M. Gerster Mercedes-Benz AG 9 November 2022 Transfer Digital Credentials Securely - Requirements draft-tigress-requirements-04 Abstract 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 https://datatracker.ietf.org/doc/draft-tigress-requirements/. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-tigress-requirements/. Source for this draft and an issue tracker can be found at https://github.com/dimmyvi/tigress-requirements. 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Vinokurov, et al. Expires 13 May 2023 [Page 1] Internet-Draft tigress-requirements November 2022 Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 13 May 2023. Copyright Notice Copyright (c) 2022 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. General Setting . . . . . . . . . . . . . . . . . . . . . . . 3 3. Conventions and Definitions . . . . . . . . . . . . . . . . . 3 4. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5. Relationships . . . . . . . . . . . . . . . . . . . . . . . . 5 6. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 5 7. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 6 7.1. Intermediary server requirments . . . . . . . . . . . . . 6 8. Review of existing solutions . . . . . . . . . . . . . . . . 7 8.1. Arbitrary Messaging Channel (Email / WhatsApp / SMS / Signal / etc.) . . . . . . . . . . . . . . . . . . . . . 7 8.2. GSS-API, Kerberos . . . . . . . . . . . . . . . . . . . . 7 9. Out of Scope . . . . . . . . . . . . . . . . . . . . . . . . 8 10. Security Considerations . . . . . . . . . . . . . . . . . . . 8 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 12. Normative References . . . . . . . . . . . . . . . . . . . . 8 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 Vinokurov, et al. Expires 13 May 2023 [Page 2] Internet-Draft tigress-requirements November 2022 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. Vinokurov, et al. Expires 13 May 2023 [Page 3] Internet-Draft tigress-requirements November 2022 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. Vinokurov, et al. Expires 13 May 2023 [Page 4] Internet-Draft tigress-requirements November 2022 * 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. Vinokurov, et al. Expires 13 May 2023 [Page 5] Internet-Draft tigress-requirements November 2022 * 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. Vinokurov, et al. Expires 13 May 2023 [Page 6] Internet-Draft tigress-requirements November 2022 * (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 Vinokurov, et al. Expires 13 May 2023 [Page 7] Internet-Draft tigress-requirements November 2022 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. 10. Security Considerations TODO Security 11. IANA Considerations This document has no IANA actions. 12. Normative References [CCC-Digital-Key-30] Car Connectivity Consortium, "Digital Key Release 3", July 2022, . [RFC2078] Linn, J., "Generic Security Service Application Program Interface, Version 2", RFC 2078, DOI 10.17487/RFC2078, January 1997, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos Network Authentication Service (V5)", RFC 4120, DOI 10.17487/RFC4120, July 2005, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . Vinokurov, et al. Expires 13 May 2023 [Page 8] Internet-Draft tigress-requirements November 2022 [Tigress-00] Vinokurov, D., Byington, M., Lerch, M., Pelletier, A., and N. Sha, "Transfer Digital Credentials Securely", November 2022, . Acknowledgments TODO acknowledge. Authors' Addresses Dmitry Vinokurov Apple Inc Email: dvinokurov@apple.com Casey Astiz Apple Inc Email: castiz@apple.com Alex Pelletier Apple Inc Email: a_pelletier@apple.com Jean-Luc Giraud Apple Inc Email: jgiraud@apple.com Alexey Bulgakov Apple Inc Email: abulgakov@apple.com Matt Byington Apple Inc Email: mbyington@apple.com Nick Sha Alphabet Inc Email: nicksha@google.com Vinokurov, et al. Expires 13 May 2023 [Page 9] Internet-Draft tigress-requirements November 2022 Manuel Gerster Mercedes-Benz AG Email: manuel.gerster@mercedes-benz.com Vinokurov, et al. Expires 13 May 2023 [Page 10]