to extend this key escrow requirement to public use of cryptography was highly controversial. TEMPEST – protecting plaintext from compromise by electronic, acoustic or other emanations. Tamper resistance, tamper-evident, self-destruct – ensuring security even if encryption systems are physically accessed without authorization or are captured. Meeting military specifications for size, weight, power consumption, MTBF and ruggedness to fit in mobile platforms. Electromagnetic pulse hardening – protecting against nuclear explosion effects, particularly electromagnetic pulse. Ensuring compatibility with military and commercial communication standards. Controlling cost – making sure encryption is affordable so units that need it have it. There are many costs beyond the initial purchase price, including the manpower to operate and maintain the systems and to ensure their security and the cost of key distribution. Enabling secure communication with NATO, allied and coalition forces without compromising secret methods. Five generations of NSA encryption The large number of encryption systems that NSA has developed in its half century of operation can be grouped into five generations (decades given are very approximate): First generation: electromechanical First generation NSA systems were introduced in the 1950s and were built on the legacy of NSA's World War II predecessors and used rotor machines derived from the SIGABA design for most high level encryption; for example, the KL-7. Key distribution involved distribution of paper key lists that described the rotor arrangements, to be changed each day (the cryptoperiod) at midnight, GMT. The highest level traffic was sent using one-time tape systems, including the British 5-UCO, that required vast amounts of paper tape keying material. Second generation: vacuum tubes Second generation systems (1970s) were all electronic designs based on vacuum tubes and transformer logic. Algorithms appear to be based on linear-feedback shift registers, perhaps with some non-linear elements thrown in to make them more difficult to cryptanalyze. Keys were loaded by placing a punched card in a locked reader on the front panel. The cryptoperiod was still usually one day. These systems were introduced in the late 1960s and stayed in use until the mid-1980s. They required a great deal of care and maintenance, but were not vulnerable to EMP. The discovery of the Walker spy ring provided an impetus for their retirement, along with remaining first generation systems. Third generation: integrated circuits Third generation systems (1980s) were transistorized and based on integrated circuits and likely used stronger algorithms. They were smaller and more reliable. Field maintenance