“Correspondences Regarding Cryptography between John Nash and the NSA”, 1955 (; backlinks; similar):
[In 1955, well-known mathematician John Nash was in correspondence with the United States National Security Agency. In these letters, Nash proposes a novel enciphering scheme. He also sets forth an important cryptographic principle that now underpin modern computational complexity theory and cryptography. In particular, he proposes a natural definition for “[security] in a practical sense”—that exponential computational effort is required for an enemy to recovery a secret key. Nash further conjectures that this property holds for any suitable enciphering mechanism.
These correspondences, recently declassified by the NSA1; this document is the NSA scan with original images and drawings of Nash’s handwritten letters.]