FORMAT

File Format of Encrypted Journals in Mémoires 4
===============================================

Position  Length  Description
    0      8      Identification string ("MEM_encr")
    8      1      Format version (currently 1)
    9      1      scrypt log(N)
   10      1      scrypt log(r)
   11      1      scrypt log(p)
   12     32      scrypt salt
   44     16      Encryption IV
   60     32      Header hash         BLAKE2b(0..60)
   92     32      Header MAC          BLAKE2b(MAC_key, 0..92)
  124      *      Encrypted content   AES-256-CBC(Encryption_key, iv, 124..*)
 *-32     32      MAC                 BLAKE2b(MAC_Key, 0..*-32)


Key derivation:

   MAC_key, Encryption_key = scrypt(salt, password, N, r, p, 64)

   Length of MAC_key: 32 bytes
   Length of Encryption_key: 32 bytes

Hashing and MAC:

    Header hash: BLAKE2b with 32-byte output.
    Header MAC and content MAC: keyed BLAKE2b with 32-byte output.

Encryption:

    AES-256 in CBC mode with randomly generated IV.
    PKCS#7 padding.