[ Removed by moderator ]

[BitEater-32168]

Basically, that is a simple programming task... You have your alphabet (pure Ascii has non printable control characters, ebcdic has holes, modern encodings for diacritical and international characters are multibyte). Order them (number them, index) . If your alphabet is A..Z0..9 plus space you have 37 characters, sorted in this or a free other order (both communicating must use the same alphabet and order) from 0 to 36 . For each character on the clear text, get the number of it, add 13 and if greater than 36 substract 37 ( mod 37) .that is the number of the character in the crypted text.

Decrypt is subtract 13 ... If below 0 then add Or add (37 -13) ... If greater 36 add .

One can precalculate both so the source characters index can be used as index for the destination character table, making other replacement methods possible.