E2E encrypted attachments hashes should cover IV & key data too

We include a H(ciphertext) in the metadata of our encrypted attachments to ensure that the media repo doesn't swap out the ciphertext on us. However, we don't protect the IV & key data, which is a problem for antivirus scanners which need to be sure they have the right IV & key data to decrypt a file in order to scan it. Instead we should include H(ciphertext, IV, key) - or something, but it needs to be properly thought through

Start by locating the Matrix specification's encrypted-attachment metadata definition and reviewing how ciphertext, IV, and key data are represented. Determine the integrity design needed by antivirus scanners, then document the agreed protocol behavior; the issue notes that this requires careful thought and has no linked implementation or test.

Written by the indexing model from the issue text.