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When writing to an encrypted file, a cryptographic hash (secure one-way function) of the key and the initialization vector are pre-pended to the output file. This allows mi2b2 to verify if the encrypted key is the same as the key users have chosen when opening an encrypted image without divulging the real key.

mi2b2 uses SHA-256 to hash the key, and SHA-256 produces a hash that is 256 bits (32 bytes) long. The initialization vector is the length of key (128 bits, or 16 bytes) plus 2 more bytes. This means every encrypted image file is pre-pended with 50 bytes of information. Otherwise the encrypted image files are exactly the same size as the unencrypted ones.

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