RosettaCodeData/Task/Caesar-cipher/Yabasic/caesar-cipher.basic

REM *** By changing the key and pattern, an encryption system that is difficult to break can be achieved. ***

text$ = "You are encouraged to solve this task according to the task description, using any language you may know."
key$ = "desoxirribonucleic acid" // With a single character you get a Caesar encryption. With more characters the key is much more difficult to discover.
CIPHER = 1 : DECIPHER = -1

print text$ : print
encrypted$ = criptex$(text$, key$, CIPHER)
a = open("cryptedText.txt", "w") : if a then print #a encrypted$ : close #a end if
print encrypted$ : print
print criptex$(encrypted$, key$, DECIPHER)

sub criptex$(text$, key$, mode)
   local i, k, delta, longtext, longPattern, longkey, pattern$, res$

   pattern$ = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890 .,;:()-"

   longPattern = len(pattern$)
   longtext = len(text$)
   longkey = len(key$)

   for i = 1 to longtext
      k = k + 1 : if k > longkey k = 1
      delta = instr(pattern$, mid$(text$, i, 1))
      delta = delta + (mode * instr(pattern$, mid$(key$, k, 1)))
      if delta > longPattern then
         delta = delta - longPattern
      elseif delta < 1 then
         delta = longPattern + delta
      end if
      res$ = res$ + mid$(pattern$, delta, 1)
   next i

   return res$
end sub