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It seems that CTB-Locker make a lot of victims nowadays, and yet, the full encryption scheme of it is now publicly known [1,2].

Would any of you could find a weakness to exploit in this encryption scheme that would be able to take it down once for all ? The point would be to recover the Master key or a way to circumvent the encryption.

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No. One of the most important principles of cryptography is that knowing the encryption scheme cannot help someone attempting to decrypt the material without the key. The encryption used seems to be a reasonably well-written implementation of several standard algorithms, for which no practical attacks are known. Finding a way to crack these would be a major undertaking, one which many skilled cryptographers have already failed at.

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  • $\begingroup$ I didn't mean "breaking ECC" or "breaking AES", I was more thinking about an error in the way the things have been arranged to build the whole encryption scheme or specificities that are linked to the very peculiar way the encryption is used here. For example, if you have a partial back-up of your file, you will have access to a reasonable quantity of cleartext/ciphertext couple that you may use. Or, is there a flaw in the entropy of the chosen keys... But, of course, I believe in the security of ECC and AES... I am not that naive ! $\endgroup$
    – perror
    Commented Feb 18, 2015 at 21:56
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    $\begingroup$ Then it goes to a probably not. The encryption scheme is pretty simple ECDH to compute a shared secret and then using a hash of that as an AES key. The implementation seems sound using the MS Crypto API where possible (AES, SHA-256, and the CSPRNG), and there are no known-plaintext or chosen-plaintext attacks on the ciphers used. The keys are generated with the MS Crypto API's CryptGenRandom, which is a good CSPRNG. I don't know about their implementation of curve25519, but that was designed to be easy to implement securely. $\endgroup$
    – SAI Peregrinus
    Commented Feb 19, 2015 at 1:14

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