It is convenient to consider the ways in which IT may be used to threaten or to protect the safety and security of our lives and property under four heads: safety of life, invasion of privacy, crime and war. Malign intent is not the principal reason for concern in the Western democracies, but risks can arise when IT systems themselves are faulty. In the early days, failures were mainly caused by hardware breakdowns; today, silicon chips are highly reliable, and the emphasis has moved to data errors and software ‘bugs’.

Input data may be incorrectly scanned by automatic readers, and human operators make mistakes when entering data through a keyboard; checks are therefore made to reveal the presence of errors. The simplest, but not the cheapest, check is for two independent operators to enter the data and then to compare their versions; if they agree then it is highly likely that no mistake was made. Otherwise, the entry is repeated until they do agree. Or, a batch of items of numerical data may be added together on a pocket calculator and the total entered as a ‘sum check’ to be compared with the result of the same addition by the computer once input is complete. It is not necessary for the total to have a meaning. Thus, the birth dates of a group of staff can be totalled for comparison; such a total is known as a ‘hash total’.