MIMI Transport

Introduction This document describes an HTTP-based transport layer protocol for use with the Delivery Service protocol specified in draft-robert-mimi-delivery-service, as well as other MIMI-relevant components such as contact discovery.

Transport Security and Authentication All HTTP queries described in this document MUST use TLS with version 1.3 or higher to protect confidentiality and authenticity of the payloads. Delivery Service payloads are typically authenticated by the sender through the use of signatures and rely on HTTPS to authenticate the recipient. To prevent forwarding attacks, the payloads of the Delivery Service include both sender and recipient. The provider should thus always verify that the recipient corresponds to its own provider name.

Endpoint Discovery A messaging provider that wants to query the endpoint of another messaging provider first has to discover the fully qualified domain name under which Delivery Service of that provider can be reached. It does so by performing a GET request to [provider.com](http://provider.com)/.well-known/mimi/ds-domain. provider.com could for example answer by providing the domain [ds.provider.com](http://ds.provider.com) (assuming that this is where it responds to the REST endpoints defined below).

REST Endpoints The following REST endpoints can then be used to access the different functionalities of the Delivery Service. As the Delivery Service relies on TLS encoded structs, all requests to endpoints described below should be marked as Content-type: application/octet-stream. All structs and concepts referred to below are defined in draft-robert-mimi-delivery-service, where their underlying functionality is defined in more detail.

Process Group Message This REST endpoint provides access to all operations associated with an existing MLS group on the Delivery Service such as delivering application messages, adding group members, removing group members, updating key material, etc. The payloads for this endpoint are generally provided (and signed) by a member of the corresponding group rather than the service provider of that member. The exact operation, as well as the target group ID is determined by the payload itself rather than an HTTP header, the path or any other query parameter.

Welcome Information Through this endpoint, a provider can obtain information required to join the group for clients that have already received a Welcome message. The DS responds with the group’s RatchetTree, as well as authentication information of existing group members.

External Commit Information Guest providers can use this endpoint to obtain information that allows a client to join a group without a Welcome message from an existing group member.

Verification Key This allows guest providers to obtain the verification key of this provider. This allows other providers to authenticate queries originating from this provider.

Deliver Connection Request This endpoint lets other providers deliver connection establishment request to clients of this provider.

Deliver Message An owning provider can deliver messages from one of its owned groups to this endpoint, if one of the group’s clients is associated with this provider.

Connection KeyPackage Retrieval Allows another provider to retrieve KeyPackages for use during the connection establishment process between two users.

Group KeyPackage Retrieval Allows another provider to retrieve KeyPackages that can be used to add another user or one of its clients to an existing group.

Rate-limiting The MIMI transport protocol itself doesn’t include any rate-limiting measures. However, traditional rate-limiting (e.g. based on sender IP) can be applied, as well as rate-limiting based on information in the message body such as Group ID (e.g. in the case of the /welcome_information endpoint) or group member (in the case of the /group_operation endpoint). More fine-grained rate-limiting can be applied through the use of the emerging Privacy Pass protocol (draft-ietf-privacypass-auth-scheme).