It doesn't have be using a truly random key, (it could be the Enigma) but the attack must be based on XORing two parts of ciphertext.

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    $\begingroup$ en.wikipedia.org/wiki/Venona_project $\endgroup$ – Biv Feb 25 '16 at 9:37
  • $\begingroup$ The Wiki page gives a highly significant real example. But how "simple" was that attack? It would be fine if there exists a paper detailing how one should advantageously proceed in practice to deal with a two-time pad IMHO. $\endgroup$ – Mok-Kong Shen Feb 27 '16 at 9:22
  • $\begingroup$ @Mok-KongShen Solving it using a modern computer is easy. Doing it manually is pretty impressive. $\endgroup$ – CodesInChaos Oct 13 '16 at 19:49
  • $\begingroup$ @CodesInChaos: I am an ignorant. Could you please give a link to a good computer code that decrypts two messages that were encypted with the same one-time pad? $\endgroup$ – Mok-Kong Shen Oct 14 '16 at 9:57
  • $\begingroup$ @Mok-KongShen Such a program can not exist for general messages (on the scale of the halting problem). You simply XOR two ciphertexts and then have the XOR of the plaintexts. Getting the original messages from that depends entirely on the kind of message, the language, etc. $\endgroup$ – tylo Oct 14 '16 at 10:28

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