Code and ciphers: Julius Caesar, the Enigma and the internet

12
Public key cryptography
Historical background
The first general purpose computers were built in the 1940s. They were
large, filling big rooms. They used hundreds of valves and consumed
many kilowatts of electricity. They performed about a thousand instructions
a second, which was considered amazing at that time, and they were
popularly referred to as ‘giant brains’. A few people, including Alan
Turing, discussed ‘whether machines could think’ and laid bets as to
whether a machine would defeat the World Chess Champion in the next
25 years. The former question remains a matter for debate; the latter was
settled about 45 years later when a World Chess Champion did lose a
match to a computer.
These early machines had very small direct access memories, only a
thousand or so ‘words’, based upon cathode ray tubes or mercury delay
lines. They rarely functioned for more than a few minutes before breaking
down. Their input and output were primitive: paper tape or punched
cards and a typewriter. They also cost a great deal of money; £100 000 in
1948 which was equivalent to several millions 30 years later. Very few
people knew how to write programs for them. There was virtually no software
(as it later became known) and all programs had to be written in
‘absolute machine code’.
Even the instruction codes of these machines were very limited. The
first machine at Manchester University in 1948, for example, had no division
instruction [12.2], so division had to be programmed by repeated
subtraction. Programmers, who were usually mathematics, science or
engineering graduates, were very skilful and competed with each other at
finding elegant and efficient ways of carrying out various processes, such
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