68. Secure cipher code
    ~~~~~~~~~~~~~~~~~~
One of the most fascinating stories of World War II concerns how British
Intelligence were able to crack the codes generated by the German "Enigma"
cipher machines. Thanks to a simple but elegant idea from James Slater, you
too can produce very secure code using your Beeb. The method relies on the
fact that you can "seed" the random number generator to start at different
points in the pseudo-random sequence, and the message can't be decoded unless
you know the "key" integer number used to seed the generator in the first
place. The ASCII code is shifted by a random amount each time, and if you
type a continuous string of A's, you will see the result!

Note that the random number generator on the Master/Compact is coded
differently from that on a BBC B/B+, so there is no compatibility between
them when using this technique.

The more digits you use in your key number, the harder it will be to crack
the code by chance. The message may contain any keyboard character, including
<Space>, but not <Return>. I am sure this cipher would not be considered
secure in the context of giant business computers, which can try umpteen
millions of combinations in a short time, but I reckon it could be a useful
way of protecting private data on micros.

If you make the alterations shown underneath, only capitals will be allowed,
and the test is forced into 5-letter groups for extra security.

Try it, it's great fun, but NOT over the air - it's strictly against the
rules!

100 MODE6:INPUT''"Enter CODE KEY... "n%
110 randomize=RND(-(ABS(n%)))
120 PRINT'"Decipher or Encipher (D/E) ? ";
130 REPEATg$=CHR$(GETAND&DF):c%=INSTR("DE",g$)*2-3:UNTILc%<>-3
140 PRINTg$''"Please type your MESSAGE"'
150 REPEAT:REPEATg%=GET:UNTILg%>31ANDg%<127
160 IFPOS>38THENPRINT''
170 k%=g%-32+c%*(RND(95)-1):VDUg%,10,8(k%+95)MOD95+32,11
180 UNTILFALSE

Alternative lines for 5-letter groups, capitals only:

150 REPEAT:REPEATg%=GET:UNTILg%>64ANDg%<91 
160 IFPOS>35THENPRINT'''" ";ELSEIFPOS MOD6=0 THEN PRINT" ";
170 k%=g%-65+c%*(RND(25)-1):VDUg%,10,8(k%+25)MOD25+65,11