June 2018 Update

Task/Vigen-re-cipher/Applesoft-BASIC/vigen-re-cipher.applesoft

@@ -0,0 +1,28 @@
+  100 :
+  110  REM  VIGENERE CIPHER
+  120 :
+  200  REM  SET-UP
+  210 K$ = "LEMON": PRINT "KEY: "; K$
+  220 PT$ = "ATTACK AT DAWN": PRINT "PLAIN TEXT: ";PT$
+  230  DEF  FN MOD(A) = A -  INT (A / 26) * 26
+  300  REM  ENCODING
+  310 K = 1
+  320  FOR I = 1 TO  LEN (PT$)
+  330  IF  ASC ( MID$ (PT$,I,1)) < 65
+       OR  ASC ( MID$ (PT$,I,1)) > 90 THEN  NEXT I
+  340 TV =  ASC ( MID$ (PT$,I,1)) - 65
+  350 KV =  ASC ( MID$ (K$,K,1)) - 65
+  360 CT$ = CT$ +  CHR$ ( FN MOD(TV + KV) + 65)
+  370 K = K + 1: IF K >  LEN (K$) THEN K = 1
+  380  NEXT I
+  390  PRINT "CIPHER TEXT: ";CT$
+  400  REM  DECODING
+  410 K = 1
+  420  FOR I = 1 TO  LEN (CT$)
+  430 TV =  ASC ( MID$ (CT$,I,1)) - 65
+  440 KV =  ASC ( MID$ (K$,K,1)) -  65
+  450 T = TV - KV: IF T < 0 THEN T = T + 26
+  460 DT$ = DT$ +  CHR$ (T + 65)
+  470 K = K + 1: IF K >  LEN (K$) THEN K = 1
+  480  NEXT I
+  490  PRINT "DECRYPTED TEXT: ";DT$