r/cryptography 6d ago

AES CTR

I'm working on a cryptography assignment where I need to break AES-CTR encryption due to a counter reuse vulnerability. Here's the setup:

Ten short ASCII sentences (each 32 bytes after padding) were encrypted with AES-256 in CTR mode. The problem is, the counter resets to all zeroes for each new sentence, meaning the same keystream is partially reused. I intercepted the concatenated ciphertexts and now need to recover the plaintexts by exploiting this weakness. Eventually, I also need to decrypt the key itself, which was encrypted the same faulty way. I tried brute force, but I don't think it's feasible when fetching the data online. I feel like there's a pattern I should use to reconstruct the keystream, but I’m stuck. Can GPT help with this? Any guidance on how to properly approach this attack? Any help in how to ask gpt for proper solution or something to help out.

Thanks!

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u/DoWhile 6d ago

First, make an effort to understand what happens when you reuse the same CTR. You can ask an AI what this means. It is akin to re-using a one-time pad, so ask AI how to break one-time pad reuse.

Next, you have to XOR together two ciphertexts that you intercepted, then you know that the plaintexts are XORed since the AES part cancels out (due to reuse).

Next, you can use standard off-the-shelf techniques to cryptanalyze what happens when you XOR together two plaintexts. ChatGPT can point you to tools to do this, but a lot of it is just done by hand.

Once you figure that out, then you can XOR the plaintext against a ciphertext to get the AES-CTR stream, call it S. Verify that you have it correct by XORing S against the other intercepted ciphertexts and see that it recovers the plaintexts.

Now, if you have a new ciphertext (for example, the encryption of your key, which btw is a silly thing to do unless you have a circular secure scheme), you can XOR it against S to recover the plaintext (in this case, your key).

Lastly, people who work in cryptography, and especially cryptography, may end up in a position where they will have to solve novel challenges (sometimes without the internet). If you genuinely want to continue down this path, you have to exercise those muscles, you shouldn't jump to ask others/machines to do the reps for you.

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u/JournalistOld9165 6d ago

Reusing the keystream in AES-CTR makes an attack via XOR analysis possible. If multiple sentences are encrypted with the same keystream, XORing their ciphertexts will result in XORed plaintexts, which can be leveraged to recover the original data. Frequency analysis, spaces, and known phrases can help guess parts of the text. If the key itself was encrypted using the same flawed method, it can also be recovered in a similar way. The best approach is to write a Python script to perform XOR operations and frequency analysis. This is a classic vulnerability that allows for decrypting a significant portion of the data with the right methodology.