How to detect block cipher mode

Question

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 1

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.

Answer 2

I am doing the same problem set and just finished this problem (using clojure).

My first hint is, it will be more clear what you need to do if you are using a language which supports first class functions/lambdas.

Anyways, let's break down the problem a bit:

First, just write a function which validates that a blackbox is encrypting data with ecb. How would you do this?

It might look something like (pseudocode below)

function boolean isEcbBlackbox(func f) 
{   //what input can I use to determine this?
    result = f("chosen input")
    if(result ...) {//what property of result should I look for?
        true
    } else {
        false
    }
}

Remember, the key weakness of ECB is identical blocks of plaintext will be encrypted to identical blocks of ciphertext.

EDIT: The challenges are now public, so I will link to my solution(s):

https://github.com/dustinconrad/crypto-tutorial/blob/master/src/crypto_tutorial/lib/block.clj#L118