use one cypher pads there hard to break (so the story goes)
what do i mean?
work out what letters your allowed to use
A-Z,0-9, make space maybe _
now make a line with each letter say 6 times
AAAAAABBBBBBCCCCCC etc etc
now jumble them up
DJSUIEWK3S5D2_ODK8A6S etc etc
save that to a file and repeat the process 1000 times for 1000 lines in the file.
you now have a encryption and decryption key file, give it ONLY to those who are ment to have it
to encode a message like "HI DAVE!"
get the 1st letter H and the first line of the file and a random number from 1 to 6
%letter = H
%line = $read(filename,1)
%num = $rand(1,6)
now get a value from them
get the location of the %num occurance of %letter in the %line
$pos(%line,%letter,%num), that value represnts the H, repeat the process using each letter of the message and the next line of the file.
anyone decoding just looks up the corresponding letter in in each line of the identical file.
you can repeat the mesaage any number of times and it wont look the same, due to there being 6 numbers for each letter, and those numbers change for each position in the message., you end up sending thinsg like
23 567 34 125 345 7 63 43
and next time
45 45 23 32 87 432 2 45
no one can even tell you said the same thing twice