| Internet-Draft | TIGRESS Threat Model | June 2023 |
| Lassey, et al. | Expires 7 December 2023 | [Page] |
- Workgroup:
- Transfer dIGital cREdentialS Securely
- Internet-Draft:
- draft-lassey-tigress-threat-model-02
- Published:
- Intended Status:
- Informational
- Expires:
TIGRESS Threat Model
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.¶
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-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
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 7 December 2023.¶
Copyright Notice
Copyright (c) 2023 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.¶
1. Introduction
The TIGRESS Working Group is chartered 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¶
- 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
A digital credential [Tigress-req-03] is composed of Cryptographic material and other data that enables an user to access a property.¶
2.1.2. Intermediary data
Data that is exchanged over the course of credential transfer.¶
2.1.3. Credential transfer invitation
The initial data containing Provisioning Information [Tigress-req-03] sent to the receiver. It represents an invitation to accept the transfer of the credential.¶
5. Threats and mitigations
| Threat Description | Likelihood | Impact | Mitigations |
|---|---|---|---|
| An Attacker with physical access to the victim's phone initiates the transfer of a Credential to the the Attacker's device | MED | HIGH | Section 5.2.1 |
| Attacker intercepts or eavesdrops on sharing message | HIGH | HIGH | Section 5.2.2 |
| Sender mistakenly sends to the wrong Receiver | HIGH | HIGH | Section 5.2.3 |
| Sender device compromised | MED | HIGH | Section 5.2.3 |
| Attacker compromises Credential Authority | LOW | HIGH | None |
| Credential Authority can recognize and track Sender across shares | HIGH | LOW | None |
| Credential Authority can recognize and track Receiver across shares | HIGH | LOW | None |
| Sender can recognize and track Receiver across shares | HIGH | LOW | None |
| Receiver can recognize and track Sender across shares | HIGH | LOW | None |
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 "unguessable" location | LOW | LOW | Section 5.2.4 |
| Attacker intercepts encryption key | MED | MED | Section 5.2.5 |
| Attacker intercepts encryption key and unguessable location | MED | HIGH | Section 5.2.6 |
| Attacker compromises intermediary server | LOW | LOW | Section 5.2.7 |
| Attacker uses intermediary server to store unrelated items (i.e. cat pictures) | HIGH | LOW | Section 5.2.8 |
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 initiation.¶
5.2.2. Secret to be sent securely
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.¶
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, , <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, , <https://www.rfc-editor.org/rfc/rfc8174>.
8.2. Informative References
- [Tigress-req-03]
- Vinokurov, D., Pelletier, A., Astiz, C., Lassey, B., and Y. Karandikar, "Tigress requirements", , <https://github.com/dimmyvi/tigress-requirements/>.
Acknowledgments
This document took as inspiration the threat model that was part of Dmitry Vinokurov's sample implementation document.¶
