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National Cryptologic Museum Opens New Exhibit on Dr. John Nash, NSA 2012. Die Stelle auf S. 4 des Briefs von 1955 lautet: Now my general conjecture is as follows: for almost all sufficiently complex types of enciphering, especially where the instructions given by different portions of the key interact complexly with each other in the determination of their ultimate effects on the enciphering, the mean key computation length increases exponentially with the length of the key, or in other words, the information content of the key. The significance of this general conjecture, assuming its truth, is easy to see. It means that it is quite feasible to design ciphers that are effectively unbreakable. […] The nature of this conjecture is such that I cannot prove it, even for a special type of ciphers. Nor do I expect it to be proven.
