PROGRAM "caesarcipher"
VERSION "0.0001"

DECLARE FUNCTION Entry()
INTERNAL FUNCTION Encrypt$(p$, key%%)
INTERNAL FUNCTION Decrypt$(p$, key%%)

FUNCTION Entry()
  txt$ = "The five boxing wizards jump quickly"
  key%% = 3
  PRINT "Original:  "; txt$
  enc$ = Encrypt$(txt$, key%%)
  PRINT "Encrypted: "; enc$
  PRINT "Decrypted: "; Decrypt$(enc$, key%%)
END FUNCTION

FUNCTION Encrypt$(p$, key%%)
  e$ = p$ ' In order to avoid iterative concatenations
  FOR i = 0 TO LEN(p$) - 1
    t@@ = p${i} ' or (slower) t@@ = ASC(p$, i + 1)
    SELECT CASE TRUE
      CASE (t@@ >= 'A') AND (t@@ <= 'Z'):
        t@@ = t@@ + key@@
        IF t&& > 'Z' THEN
          t@@ = t@@ - 26
        END IF
      CASE (t@@ >= 'a') AND (t@@ <= 'z'):
        t@@ = t@@ + key%%
        IF t@@ > 'z' THEN
          t@@ = t@@ - 26
        END IF
    END SELECT
    e${i} = t@@
  NEXT i
END FUNCTION e$

FUNCTION Decrypt$(p$, key%%)
  e$ = p$ ' In order to avoid iterative concatenations
  FOR i = 0 TO LEN(p$) - 1
    t@@ = p${i} ' or (slower) t@@ = ASC(p$, i + 1)
    SELECT CASE TRUE
      CASE (t@@ >= 'A') AND (t@@ <= 'Z'):
        t@@ = t@@ - key%%
        IF t@@ < 'A' THEN
          t@@ = t@@ + 26
        END IF
      CASE (t@@ >= 'a') AND (t@@ <= 'z'):
        t@@ = t@@ - key%%
        IF t@@ < 'a' THEN
          t@@ = t@@ + 26
        END IF
    END SELECT
    e${i} = t@@
  NEXT i
END FUNCTION e$
END PROGRAM