Improvement to AES-GCM about js-encryptedfs

[CMCDragonkai]

Hey @robert-cronin I think you mentioned this is not necessary in our previous review. Can you justify it here, and close this issue?

[robert-cronin]

Yeah so AES-GCM-SIV is primarily used to prevent the same init vector from encrypting the same data more than once, i.e. a nonce being reused. This is a concern in encryption as an attacker can observe repeat encryptions with the same init vector and message.
I believe this to be redundant in EFS because the init vector we use for chunk level encryption is randomised every time a chunk is encrypted so reusing of a nonce doesn't occur.