Manually Request access to history when joining a restricted room that allows it.

[BillCarsonFr]

Related to this epic

Inspired from the knocking join rule, when joining a room a user should be able to request access to history

Existing members of the room with enough power level should be able to manually accept and forward history keys

By requiring an existing member to manually accept we limit the process (extracting historical keys) and potential surface of attack.

Might require Spec work, to send keys in batch instead of one by one:

• Restricted join rules are incompatible with E2EE history matrix-org/matrix-spec#1
• Implement in iOS
• Implement in android
• Implement on web
• Implement on Rust

[karlabbott]

This would be an excellent feature to have in my opinion!

[dkasak]

This only makes sense iff:

• Device signing is mandated, as in MSC4153, and
• Either:
  • The user was invited into the room via a MSC3917-style mechanism, or
  • The user that's being invited was previously verified by the inviter, or
  • The user that's being invited had their cryptographic identity previously pinned via a TOFU mechanism (but in this case, we offer a much reduced level of protection in a malicious homeserver environment)