Re: Newbie question about AES encryption

Say you have a 128 bit block encrypted with AES. If you change as
little as a single bit of that block, and then decrypt it, the result
will be totally random. It will not look anything like the original
plaintext block.

So, as I understand it, the full answer is as follows.

(1) Block ciphers (like AES) have "modes".

(2) If you encrypt in ECB mode, you can decrypt each block seperately
from any other block.

(3) Point (2) is generally considered to be a /dis/advantage - not an
advantage.

(4) If you encrypt in other modes, you can /not/ decrypt each block
seperately.

(5) In any case, regardless of mode, if you make /any change/ to the
bits within a single block, the decryption of that block will look like
random data. It will not look anything like the original plaintext
block.

I'm sure that someone else will jump in here, if any of that is wrong.

This about sums it up. One additional note though: if you encrypt in
other modes, you cannot decrypt each block separately. But you _can_
successfully decrypt the first blocks of cipher text without knowing
the last few. So the TS's example can still occur.

As already mentioned, the correct way to find out about it is to
include a MAC. Either send the MAC of the original message as a
separate block with the cipher text, or concatenate it to the plain
text before encryption. Then don't forget to check whether the MAC is
still valid after decryption.

Good luck!
Peter

.

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