Re: cbc mode - why IV and not random first block

This a joke, right? Your "completely random first block" is the
definition of what an iv is (in cbc mode).

No joke.
The IV is usually not added to the encrypted text as first block, but
only used for the encryption.
To decrypt, the first block of the encrypted plaintext is decrypted and
only then XORed with the IV. The IV is not content of the encrypted
text.

My suggestion was to add 256 bits of random data in front of the
plaintext, encrypt all, and after cbc-decryption, to remove the first
256 bits from the front of the decrypted text.

This is a small difference, and I wonder if there are any flaws.

Thanks,
Jens

.

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