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

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chapter 12
eight-bit) character in the block and another check sum, the longitudinal
parity check, based upon the number of 1s in each of the six (or eight) separate
streams, was generated at the end of the block. These check sums were
typically constructed so that each stream of 0s and 1s had an odd number
of 1s in it. This was sufficient to detect and correct a single error in the
block. If there were two or more errors they could not be corrected and,
possibly, not even detected. The tape drives would carry out the checks
automatically and re-read any block that failed the parity test. Dust on the
tape could cause a misread and the tapes would frequently be seen to
oscillate backwards and forwards several times before moving on. Since
the computer itself generated the parity check bits there was no difficulty
in changing the data if a genuine single error was detected. The parity
checks were not designed to beat crooks, who might have managed to
change both the data and the check sums; they were there only to protect
the data from dust, including cigarette ash, and machine malfunctions.
Smoking in the machine room, prevalent in the early days, was often
found to be the cause of tape errors and was subsequently banned.
Occasionally there was nothing wrong with the data on the magnetic tape
and it was the checking circuitry in the tape drives themselves that was
faulty. The computer operators soon discovered this and also found that,
in the USA, a quarter-dollar inserted into the appropriate slot on the tape
drive caused the checking circuits to be overridden.
When it became possible to connect computers together so that ‘messages’,
which might be programs or data, could be passed between them
the question of how to ensure that the messages did not get ‘garbled’,
either accidentally or deliberately, en route had to be considered. Whilst
parity checking, particularly if more sophisticated error correcting codes
such as those described in [1.1], [1.2] and [1.3] were used, could provide
protection against accidental corruption, a crook would be able to generate
the appropriate check sums for any block that he had changed and so
escape detection. It thus came to be realised that encipherment, using a
system which a crook couldn’t replicate, was necessary.
Encipherment of programs, data and messages
By the beginning of the 1970s most major businesses, government offices
and academic institutions were using computers, and computer networks
were spreading. A typical installation would have one or more large
machines, mainframes as they were called, with numerous teletypes or, later,