# How does one find out ciphers/passwords without knowing what cryptographic algorithms are used?

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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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