B

Task/Caesar-cipher/BBC-BASIC/caesar-cipher.bbc

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+      plaintext$ = "Pack my box with five dozen liquor jugs"
+      PRINT plaintext$
+
+      key% = RND(25)
+      cyphertext$ = FNcaesar(plaintext$, key%)
+      PRINT cyphertext$
+
+      decyphered$ = FNcaesar(cyphertext$, 26-key%)
+      PRINT decyphered$
+
+      END
+
+      DEF FNcaesar(text$, key%)
+      LOCAL I%, C%
+      FOR I% = 1 TO LEN(text$)
+        C% = ASC(MID$(text$,I%))
+        IF (C% AND &1F) >= 1 AND (C% AND &1F) <= 26 THEN
+          C% = (C% AND &E0) OR (((C% AND &1F) + key% - 1) MOD 26 + 1)
+          MID$(text$, I%, 1) = CHR$(C%)
+        ENDIF
+      NEXT
+      = text$