TheCodeBreakers
sum tacked onto the end of the keynumber, forming part of it, and the
process repeated with these digits. For example, with the priming key
396 4, 3 and 9 are 12, which is listed as 2, since all addition is
noncarrying and tens digits are dropped; 9 and 6 are 5, and
6 and 4 are 0. These three figures join the key at its tail: 3964250.
The process is then continued with 4 and 2, making 6, which is put on
after the 0, with 2 and 5, making a 7 which is put on after the 6, and so
on: 39642506
7 5 6 3 2 1 .... More complex methods are possible. The computer
might multiply a base keynumber for the day by a message keynumber
to ten places, then multiply the product by the basic key to ten places,
that product again by the basic key to ten places, and so on, each time
extracting the last four digits of each product as the final key.
Modern algebraic techniques now enable the key generation system to
be made so complex that, given a portion of the key, the rest of it cannot
annaabi.ee 
