Configurable Key Forwarding strategy in SDK

[BillCarsonFr]

Objective

Clarify cross platform message key forward strategy, allowing to share more or less depending on needs and taking advantage of cross-signing.

Prosposal

Define a KeyForwardingStrategy configuration in Matrix SDK:

Level 0 -> No Forwarding Level 1 -> Forward Re-share if it was previously shared Level 2 -> Level 1 + Forward to current user verified sessions Level 3 -> Level 2 + Forward if the key was initially shared with another device belonging to the same verified user identity

[BillCarsonFr]

Useful doc https://docs.google.com/drawings/d/18v2_PhNM87BXNclju_iSj6oJc1WR79EjF1oGHBF5k8k/edit

https://raw.githubusercontent.com/matrix-org/matrix-rust-sdk/main/contrib/key-sharing-algorithm/model.png