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Applications and Real-Time Transfer dIGital cREdentialS Securely tigress threat model TODO Abstract About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Transfer dIGital cREdentialS Securely Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction The TIGRESS Working Group is chartered to deliver a protocol for transferring copies of digital credentials. The charter specifies certain goals:

Privacy goals:

• The relay server should not see sensitive details of the share
• The relay 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 relay server should not learn information about the sender or receiver

Security goals:

• Ensure only the intended recipient is able to provision the credential
• Ensure the credential can only be provisioned once (anti-replay)
• Ensure the sender has the intent to transfer (proof of the fact that the share initiation is attributed to a valid device and a user)

Functional goals:

• Allow a sender to initiate a share and select a relay server
• Allow a recipient to view the share request, and provision the credential associated with the share 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 relay 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.

Threat Model ## Assets and Data ### Credential The credential or key that is being shared via this protocol. ### Intermediary data Data that is shared over the course of the transaction. ### Share invitation The initial data shared with the reciever which represents an invitation to share a credential. # Users ## Sender The user who initiates the share. ## Receiver The user who is the intended recipient and accepts the invitation to share a credential. # Attackers and Motivations # Threats and mitigations

Threat Description	Likelihood	Impact	Mitigations
An Attacker with physical access to the victim's phone initiates a share of a Credential to the the Attacker's device	MED	HIGH	Implementors SHOULD take sufficient precautions to ensure that the device owner is in possession of the device when initiating a share such as requiring authentication at share time
Attacker intercepts or eavesdrops on sharing message	HIGH	HIGH
Sender mistakenly sends to the wrong Receiver	HIGH	HIGH	Implementors should ensure any initiated shares can be withdrawn or revoked at any time.
Sender device compromised	MED	HIGH

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 "unguessible" location.

Threat Description	Likelihood	Impact	Mitigations
Attacker brute forces "unguessible" location	LOW	LOW	Limited TTL of storage, rate limiting of requests
Attacker intercepts encryption key	MED	MED	Seperate transimission of encryption key and unguessible location
Attacker intercepts encryption key and unguessible location	MED	HIGH	Implementor should warn users about sharing credentials to groups
Attacker compromises intermediary server	LOW	LOW	Content on the server is encrypted
Attacker uses intermediary server to store unrelated items (i.e. cat pictures)	HIGH	LOW	intermediary server should have tight size limits and TTLS to discourage misuse

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 when, and only when, they appear in all capitals, as shown here.

IANA Considerations This document has no IANA actions.

Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.

Acknowledgments This document took as inspiration the threat model that was part of Dmitry Vinokurov's sample implementation document.