Internet DRAFT - draft-lassey-tigress-threat-model
draft-lassey-tigress-threat-model
Transfer dIGital cREdentialS Securely B. Lassey
Internet-Draft Google
Intended status: Informational C. Astiz
Expires: 7 December 2023 D. Vinokurov
Y. Karandikar
Apple
5 June 2023
TIGRESS Threat Model
draft-lassey-tigress-threat-model-02
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
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Source for this draft and an issue tracker can be found at
https://github.com/bslassey/tigress-threat-model.
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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 . . . . . . . . . . . . . . . . . . 5
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 securely . . . . . . . . . . . . . 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
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.
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.
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3.3. Credential Authority
The Provisioning Entity [Tigress-req-03] that manages the lifecycle
of a credential on a device.
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 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.
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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., and
Y. Karandikar, "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
Yogesh Karandikar
Apple
Email: ykarandikar@apple.com
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