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I have two files, one in plain-text and one encrypted. I don't know what kind of encryption is used. But based on my limited knowledge, I guess it is possible to compare them.

Now I have a third encrypted file; however, this one does not have a corresponding plain-text file. So I need to figure the encryption out between the first two files to unlock the third. I know the encryption used is the same as with the other one.

I have heard about a known plain-text attack, but how do you do it?

How can I do this? Are there any software for it?

Thank you. :)

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  • $\begingroup$ There are very few ways to distinguish between functions as one popular security property to target is that the function is indistinguishable from a random function. If you also know the key, then you may trial encrypt/decrypt the plain/cipher texts to compare equality. Otherwise, no, you'll also need to brute force the keys... $\endgroup$ – cypherfox Mar 23 '18 at 12:17
  • $\begingroup$ Chosen Plaintext Attacks (CPA) also require you know the function. $\endgroup$ – cypherfox Mar 23 '18 at 12:20
  • $\begingroup$ My two files are .cipher files and the plain message file is a .plain file, Am I even on the right track, if you know from this limited information? It is also said to be using a very old and insecure form of encryption.. :) $\endgroup$ – mjoerck Mar 23 '18 at 12:24
  • $\begingroup$ Here is a snippet of the encrypted version of the plain text: Z fZS ff. ffS ffO .c fff fZd .c n fZm $\endgroup$ – mjoerck Mar 23 '18 at 12:25
  • $\begingroup$ Known to be an old and insecure function? Okay, maybe this is enough to break it. But as a CTF or homework task, you'll need to look into many old functions and learn about their weeknesses. I certainly should, but I tend to focus on modern cryptography. Certainly plenty to learn from the mistakes immortalized as history. $\endgroup$ – cypherfox Mar 23 '18 at 12:30

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