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-
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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
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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.
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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.
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* 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.
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* 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.
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* (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
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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, .
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[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
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Manuel Gerster
Mercedes-Benz AG
Email: manuel.gerster@mercedes-benz.com
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