Format: [ "Version" uint16 big endian ] [ "Id" 16 random bytes ]
Quoting SECURITY.md:
* Every file has a header that contains a 16-byte random *file id*
* Each block uses the file id and its block number as GCM *authentication data*
* This means the position of the blocks is protected as well. The blocks
can not be reordered or copied between different files without
causing an decryption error.
