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If one does not reveal what cryptographic algorithms are used, how does a cracker find out a password/cipher that is used to communicate data? I heard that because there are methods to try cracking without revealed algorithms, one should never create his own cryptographic algorithms, as they might be vulnerable.

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The solution for problems like this is called differential cryptoanalysis. With differential cryptoanalysis you try to find patterns between a known plaintext and the resulting ciphertext without or with little knowledge of the algorithm used.

Also any code can be read if the executable is available... So hiding the algorithm itself is nearly impossible (see also security by obscurity).

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This isn't right. You certainly require knowledge of the algorithm to perform differential cryptanalysis. – Thomas Nov 26 '12 at 8:13
Not completely. You can do differential cryptoanalysis without knowing the algorithm. Or what do you mean what knowledge is needed to do cryptoanalysis? – Uwe Plonus Nov 26 '12 at 8:19
With differential cryptanalysis you need to have not only the plaintexts but also the corresponding ciphertexts. If you are just provided a ciphertext encrypted with an unknown algorithm then how are you going to obtain the needed ciphertexts for your chosen plaintexts when you don't know the algorithm? From the wiki you linked to: Differential cryptanalysis is usually a chosen plaintext attack, meaning that the attacker must be able to obtain encrypted ciphertexts for some set of plaintexts of his choosing. – Sani Huttunen Nov 26 '12 at 11:05
That's the reason why I stated that you can find patterns between plaintext and ciphertext. But Thomas mentioned that you have to know the algorithm which is not true for differential cryptoanalysis. – Uwe Plonus Nov 26 '12 at 12:34
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@UwePlonus You must be mistaken - differential cryptanalysis tracks a differential characteristic throughout the algorithm (or part of it). You must know the algorithm to perform differential cryptanalysis. – Thomas Nov 27 '12 at 3:19
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