How to detect block cipher mode

aes ecb block-cipher cbc-mode
Xantra picture Xantra · Jun 29, 2013 · Viewed 10.9k times · Source

How to detect if a message was crypt by CBC or ECB mode?

I have made a function who encrypt in AES 128 CBC or ECB randomly, and I do hamming between clear text and cipher text, but seams not correlated to cipher mode.

How can I detect the block cipher mode?

Thank you in advance

Answer

crishoj picture crishoj · Dec 21, 2013

The answer is pretty much given in the problem statement:

Remember that the problem with ECB is that it is stateless and deterministic; the same 16 byte plaintext block will always produce the same 16 byte cipher text.

Thus, with the assumption that some repeated plaintext blocks occur at the same ciphertext block offsets, we can simply go ahead and look for repeated ciphertext blocks of various lengths.

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