The Cambridge Multiple-Access System was first introduced into service experimentally in March 1967, and has been fully operational since 1968.
It runs on the Titan computer which was the prototype Atlas 2.
A conventional password system for logging-in is used, but is has associated with it a feature, due to R. M. Needham, that greatly improves its effectiveness.
This feature is easily implemented and deserves to be better known.
It consists in storing in the computer, not the original password, but a scrambled form of it.
The scrambling is done by an algorithm resembling one for generating pseudo-random numbers.
When first set, the password is scrambled and the result stored; when the user attempts to log-in and types his password, what he types is again scrambled and compared with the original version.
Only if they agree, is logging-in permitted.
Since the scrambling algorithm is virtually irreversible, at any rate without a great deal of computer time being used, no harm is done if the list of scrambled passwords gets printed out by inadvertence.
It is not even necessary to keep the scrambling algorithm secret.
The security of the system is further improved by the fact that users can set and change their own passwords from their consoles.
Thus, it is not necessary for a password to be communicated to a second party, or even written down.
