Circa 1975, IBM proposed the cipher now called DES, the Data Encryption Standard. It became a worldwide standard for secret key encryption. As IBM originally designed it, DES had a 64-bit key. The National Security Agency (NSA) required that the key be reduced from 64 bits to 56 bits, with the other 8 bits used as a checksum. This made no sense. If a checksum were really needed, then the key could be increased from 64 to 72 bits. It was widely believed that the real reason the NSA made this demand was that it knew how to crack messages using a 56-bit key, but not messages using a 64-bit key. This proved to be true.

Secret Key Cryptography by Frank Rubin