key backup: Trust on Decrypt

[manuroe]

Trust a backup if the user is able to recover/decrypt keys from it.

With cross-signing, Trust on Decrypt will be not required as key backups will be created with 2 signatures:

• the device signature
• the cross-signing Self-Signed Key

As this SSK will be shared across all cross-signed devices, all these device will trust the key backup.

Without cross-signing, as discussed here, one solution is to add a signature to the key backup from the device on which the recovery/decryption succeeds.
But this new behaviour requires:

• Spec update to add a new endpoint (PUT /room_keys/version/{version})
• Server implementation
• Client implementation

[manuroe]

Spec proposal: uhoreg/matrix-doc#1

[manuroe]

Server part done in matrix-org/synapse#4580