I have came up with another simple formula, that allows 65535 possible combinations to decrypt it, while not bloating the string 1 bit.
ASSERT(isnum(key) && isnum(inc))
for(var/i = 1 to length(string))
var/char = text2ascii(string,i))
key += inc
while(key > 255)
key -= 255
char ^= key
. += char
So it allows 255 possible keys that can be incremented bh 255 different values allowing 65535 combinations. I think this is pretty good since it doesent increase the length of the string and is pretty difficult to crack.