13 - Computationally Secure Single-Key Cryptosystems

Summary

Secure block cyphers in the real world

We have seen that the addition cypher is not secure (unless used as a onetime pad). If you stick to one key and use the addition cypher as a block cypher, this cryptosystem is subject to a plaintext–cyphertext attack. Other attacks are effective as well.

There are lots of relatively secure cryptosystems, however. The most famous is DES (Data Encryption Standard). DES came out of an effort in 1970 by the National Bureau of Standards (NBS) to select a standard cryptosystem for use with non-classified data. In 1974, in response to the NBS's public appeal for a cryptosystem, IBM submitted a cryptosystem called Lucifer that they had developed earlier that decade. NBS then went to the National Security Agency (NSA) for help in evaluating the cryptosystem. NSA modified the system somewhat, including a reduction of the key size from the 112 bits used by Lucifer to only 56 bits (a compromise, as NSA had tried to reduce it to 48 bits). The resulting system was then certified by NBS for use. Institutions that needed to communicate privately with the government were expected to use DES (unless the material was classified). In 1979 the American Bankers Association recommended use of DES for encryption. Thus DES achieved very broad use.