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

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+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