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

part of the story of what has been one of the most remarkable technological
advances of all time; see, for example, [12.2], [12.3], [12.4] and [12.5].
Security issues
Security became an issue from the start. Initially it was mainly a question
of physical security: protecting the equipment from physical damage such
as fire or flooding. Within a few years however there were reports of disgruntled
employees causing havoc by modifying or destroying key programs.
One often cited case, possibly apocryphal but certainly credible,
was of a programmer who had written a payroll program and who noticed
that his employers had a habit of sacking their employees at very short
notice. He therefore inserted a section of code into the program to check
that his name was still on the payroll and, if it wasn’t, to delete the entire
program. In due course he was sacked and the payroll program deleted
itself. The story went on to say that the programmer had to be re-hired at
an enhanced salary to put things right. This may or may not be true but it
draws attention to a serious possibility, known later as a ‘Trojan horse’
attack, and the need for a system of checking that programs had not been
‘modified’ which, in turn, led to the development of ‘anti-virus software’.
These problems still remain and computer viruses are regularly reported;
many of these are just a nuisance but some are potentially disastrous.
Protection of programs and data
Public key cryptography 163
The integrity of programs and data can be protected either with or
without encipherment. Programs consist of blocks of code. Data are
usually stored, or transmitted, in blocks or packets. If every block is converted
into a set of numbers which are then subjected to some mathematical
function the final result can be either left as it is or enciphered. In
either case the final value of the function can be stored, as a check sum, at
the end of the block. If someone wishes to change anything in the block he
must be able to compute both the mathematical function and, where relevant,
its encipherment for the new block. If he can’t do this the check sum
will fail and it will be obvious that the block has been modified. An early,
and very simple, version of this approach was the use of parity checks on
programs and data stored on magnetic tape on computers in the 1950s.In
this case there was no encipherment involved. A check sum, known as the
lateral parity check, was automatically generated for every six-bit (later,