TIGRESS Threat Model
draft-lassey-tigress-threat-model-01

Transfer dIGital cREdentialS Securely                          B. Lassey
Internet-Draft                                                    Google
Intended status: Informational                                  C. Astiz
Expires: 4 December 2023                                    D. Vinokurov
                                                                   Apple
                                                             2 June 2023

                          TIGRESS Threat Model
                  draft-lassey-tigress-threat-model-01

Abstract

   This document describes a threat model by which the working group can
   evaluate potential solutions to the problems laid out in the TIGRESS
   charter (https://datatracker.ietf.org/doc/charter-ietf-tigress/).

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://bslassey.github.io/tigress-threat-model/draft-lassey-tigress-
   threat-model.html.  Status information for this document may be found
   at https://datatracker.ietf.org/doc/draft-lassey-tigress-threat-
   model/.

   Discussion of this document takes place on the Transfer dIGital
   cREdentialS Securely Working Group mailing list
   (mailto:tigress@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/tigress/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/tigress/.

   Source for this draft and an issue tracker can be found at
   https://github.com/bslassey/tigress-threat-model.

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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   Internet-Drafts are draft documents valid for a maximum of six months
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Privacy goals:  . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Security goals: . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  Functional goals: . . . . . . . . . . . . . . . . . . . .   3
   2.  Threat Model  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Assets and Data . . . . . . . . . . . . . . . . . . . . .   4
       2.1.1.  Credential  . . . . . . . . . . . . . . . . . . . . .   4
       2.1.2.  Intermediary data . . . . . . . . . . . . . . . . . .   4
       2.1.3.  Credential transfer invitation  . . . . . . . . . . .   4
   3.  Users . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Sender  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Receiver  . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Credential Authority  . . . . . . . . . . . . . . . . . .   4
   4.  Attackers and Motivations . . . . . . . . . . . . . . . . . .   5
   5.  Threats and mitigations . . . . . . . . . . . . . . . . . . .   5
     5.1.  If an intermediary server is used . . . . . . . . . . . .   6
     5.2.  Mitigations.  . . . . . . . . . . . . . . . . . . . . . .   6
       5.2.1.  User authentication at the time of transfer
               initiation  . . . . . . . . . . . . . . . . . . . . .   6
       5.2.2.  Secret to be sent securily  . . . . . . . . . . . . .   6
       5.2.3.  Transfer control  . . . . . . . . . . . . . . . . . .   6
       5.2.4.  Limited time-to-live for mailbox storage  . . . . . .   7
       5.2.5.  Separation of shareURL and secret . . . . . . . . . .   7
       5.2.6.  Group transfer warning  . . . . . . . . . . . . . . .   7
       5.2.7.  Encrypted mailbox content . . . . . . . . . . . . . .   7

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       5.2.8.  Mailbox size limit and TTL  . . . . . . . . . . . . .   7
   6.  Conventions and Definitions . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   The TIGRESS Working Group is chartered
   (https://datatracker.ietf.org/doc/charter-ietf-tigress/) to deliver a
   protocol for transferring copies of digital credentials.  The charter
   specifies certain goals:

1.1.  Privacy goals:

   *  The intermediate server should not see sensitive details of the
      Provisioning Information [Tigress-req-03]

   *  The intermediate server should not be able to provision the
      credential itself, acting as an intermediary for the recipient
      (person-in-the-middle, impersonation attack)

   *  Aside from network-level metadata, the intermediate server should
      not learn information about the sender or receiver

1.2.  Security goals:

   *  Allow for ensuring that only the intended recipient is able to
      provision the credential

   *  Allow for ensuring that the credential can only be provisioned
      once (anti-replay)

   *  Allow for ensuring that the sender has the intent to transfer
      (proof of the fact that the initiation of the credential transfer
      is attributed to a valid device and a user)

1.3.  Functional goals:

   *  Allow a sender to initiate a credential transfer and select an
      intermediary server

   *  Allow a recipient to view the transfer request with Provisioning
      Information [Tigress-req-03], and provision the credential
      information associated with it upon receipt

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   *  Allow a sender and a recipient to perform multiple round trip
      communications within a limited time frame

   *  Not require that both the sender and recipient have connectivity
      to the intermediary server at the same time

   *  Support opaque message content based on the credential type

   *  Support a variety of types of credentials, to include those
      adhering to public standards (e.g., Car Connectivity Consortium)
      and proprietary (i.e., non-public or closed community) formats

   From these goals we can derive a threat model for the general problem
   space.

2.  Threat Model

2.1.  Assets and Data

2.1.1.  Credential

   The credential or key that is being transferred via this protocol.

2.1.2.  Intermediary data

   Data that is transferred over the course of the transaction.

2.1.3.  Credential transfer invitation

   The initial data containing Provisioning Information [Tigress-req-03]
   transmetted to the receiver which represents an invitation to accept
   the transferred credential.

3.  Users

3.1.  Sender

   The user who initiates the credential transfer.

3.2.  Receiver

   The user who is the intended recipient and accepts the invitation
   with the transferred credential.

3.3.  Credential Authority

   The Provisioning Entity [Tigress-req-03] that manages the lifecycle
   of a credential on a device.

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4.  Attackers and Motivations

5.  Threats and mitigations

    +=============================+============+========+=============+
    | Threat Description          | Likelihood | Impact | Mitigations |
    +=============================+============+========+=============+
    | An Attacker with physical   | MED        | HIGH   | Section     |
    | access to the victim's      |            |        | 5.2.1       |
    | phone initiates the         |            |        |             |
    | transfer of a Credential to |            |        |             |
    | the the Attacker's device   |            |        |             |
    +-----------------------------+------------+--------+-------------+
    | Attacker intercepts or      | HIGH       | HIGH   | Section     |
    | eavesdrops on sharing       |            |        | 5.2.2       |
    | message                     |            |        |             |
    +-----------------------------+------------+--------+-------------+
    | Sender mistakenly sends to  | HIGH       | HIGH   | Section     |
    | the wrong Receiver          |            |        | 5.2.3       |
    +-----------------------------+------------+--------+-------------+
    | Sender device compromised   | MED        | HIGH   | Section     |
    |                             |            |        | 5.2.3       |
    +-----------------------------+------------+--------+-------------+
    | Attacker compromises        | LOW        | HIGH   | None        |
    | Credential Authority        |            |        |             |
    +-----------------------------+------------+--------+-------------+
    | Credential Authority can    | HIGH       | LOW    | None        |
    | recognize and track Sender  |            |        |             |
    | across shares               |            |        |             |
    +-----------------------------+------------+--------+-------------+
    | Credential Authority can    | HIGH       | LOW    | None        |
    | recognize and track         |            |        |             |
    | Receiver across shares      |            |        |             |
    +-----------------------------+------------+--------+-------------+
    | Sender can recognize and    | HIGH       | LOW    | None        |
    | track Receiver across       |            |        |             |
    | shares                      |            |        |             |
    +-----------------------------+------------+--------+-------------+
    | Receiver can recognize and  | HIGH       | LOW    | None        |
    | track Sender across shares  |            |        |             |
    +-----------------------------+------------+--------+-------------+

                                  Table 1

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5.1.  If an intermediary server is used

   Some designs may rely on an intermediary server to facilitate the
   transfer of material.  Below are threats and mitigations assuming
   that there is an intermediary server hosting encrypted content at an
   "unguessable" location.

    +============================+============+========+=============+
    | Threat Description         | Likelihood | Impact | Mitigations |
    +============================+============+========+=============+
    | Attacker brute forces      | LOW        | LOW    | Section     |
    | "unguessable" location     |            |        | 5.2.4       |
    +----------------------------+------------+--------+-------------+
    | Attacker intercepts        | MED        | MED    | Section     |
    | encryption key             |            |        | 5.2.5       |
    +----------------------------+------------+--------+-------------+
    | Attacker intercepts        | MED        | HIGH   | Section     |
    | encryption key and         |            |        | 5.2.6       |
    | unguessable location       |            |        |             |
    +----------------------------+------------+--------+-------------+
    | Attacker compromises       | LOW        | LOW    | Section     |
    | intermediary server        |            |        | 5.2.7       |
    +----------------------------+------------+--------+-------------+
    | Attacker uses intermediary | HIGH       | LOW    | Section     |
    | server to store unrelated  |            |        | 5.2.8       |
    | items (i.e. cat pictures)  |            |        |             |
    +----------------------------+------------+--------+-------------+

                                 Table 2

5.2.  Mitigations.

5.2.1.  User authentication at the time of transfer initiation

   Implementers SHOULD take sufficient precautions to ensure that the
   device owner is in possession of the device when initiating a
   transfer such as requiring authentication at the time of initition.

5.2.2.  Secret to be sent securily

   Solution should require an end-to-end encrypted messaging channel or
   otherwise specify a way to send a secret out of band.

5.2.3.  Transfer control

   Implementers should ensure any initiated attempts of credential
   transfer can be withdrawn or revoked at any time.

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5.2.4.  Limited time-to-live for mailbox storage

   Limited TTL of storage, rate limiting of requests.

5.2.5.  Separation of shareURL and secret

   Separate transmission of encryption key and unguessable location.

5.2.6.  Group transfer warning

   Implementor should warn users about transferring credentials to
   groups.

5.2.7.  Encrypted mailbox content

   Content on the server is encrypted.

5.2.8.  Mailbox size limit and TTL

   Intermediary server should have tight size limits and TTLS to
   discourage misuse

6.  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.

7.  IANA Considerations

   This document has no IANA actions.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

8.2.  Informative References

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   [Tigress-req-03]
              Vinokurov, D., Pelletier, A., Astiz, C., Lassey, B.,
              Karandikar, Y., and B. Lassey, "Tigress requirements",
              April 2023,
              <https://github.com/dimmyvi/tigress-requirements/>.

Acknowledgments

   This document took as inspiration the threat model
   (https://github.com/dimmyvi/tigress-sample-implementation/blob/main/
   draft-tigress-sample-implementation.md#threat-model) that was part of
   Dmitry Vinokurov's sample implementation document.

Authors' Addresses

   Brad Lassey
   Google
   Email: lassey@google.com

   Casey Astiz
   Apple
   Email: castiz@apple.com

   Dmitry Vinokurov
   Apple
   Email: dvinokurov@apple.com

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