Sistema criptográfico propuesto por Nash ¿El nacimiento de P vs NP?
What this is, is a recently declassified correspondence between John Nash and the NSA from January 1955. In it, John Nash makes the distinction between polynomial time and exponential time, conjectures that there are problems that -cannot- be solved faster than in exponential time, and uses this conjecture as the basis on which the security of a cryptosystem (of his own design) relies. He also anticipates that proving complexity lower bounds is a difficult mathematical problem:
"The nature of this conjecture is such that I cannot prove it, even for a special type of cipher. Nor do I expect it to be proven. But this does not destroy its significance. The probability of the truth of the conjecture can be guessed at on the basis of experiencing with enciphering and deciphering"
