{

   Inspired from http://delphi.about.com/b/2010/08/30/vigenere-cipher-algorithm-delphi-implementation-comments-by-alan-lloyd.htm

   which looked arcane enough to be worthy of a unit test



   This implementation is rather fragile, don't reuse ;)



}

function Vigenere(src, key : String; encrypt : boolean) : String;

begin

   const cOrdMinChar : integer = Ord('A');

   const cOrdMaxChar : integer = Ord('Z');

   const cCharRangeCount = cOrdMaxChar - cOrdMinChar + 1;



   var i, j, keyLen, keyInc, srcOrd, cryptOrd : integer;

   var srcA : String;





   keyLen := Length(key);

   SetLength(srcA, Length(src));

   if encrypt then begin

      // transfer only included characters to srcA for encryption

      j := 1;

      for i := 1 to Length(src) do begin

         case src[i] of

            'A'..'Z': begin

               srcA[j] := src[i];

               j:=j+1;

            end;

         end;

      end;

      SetLength(srcA, j - 1);

   end;



   if encrypt then begin

      // encrypt to Result

      SetLength(Result, Length(srcA));

      for i := 1 to Length(srcA) do begin

         srcOrd := Ord(srcA[i]) - cOrdMinChar;

         keyInc := Ord(key[(i-1) mod keyLen + 1]) - cOrdMinChar;

         cryptOrd := ((srcOrd + keyInc) mod cCharRangeCount) + cOrdMinChar;

         Result[i] := Chr(cryptOrd);

      end;

   end else begin

      // Decrypt to Result

      SetLength(Result, Length(src));

      for i := 1 to Length(src) do begin

         srcOrd := Ord(src[i]) - cOrdMinChar;

         keyInc := Ord(key[(i-1) mod keyLen + 1]) - cOrdMinChar;

         cryptOrd := ((srcOrd - keyInc + cCharRangeCount) mod cCharRangeCount) + cOrdMinChar;

         // keyInc may be larger than srcOrd

         Result[i] := Chr(cryptOrd);

      end;

   end;

end;



const key : String = 'MYKEY';

var buf : String;



buf:=Vigenere('HELLO WORLD', key, True);



PrintLn(buf);



buf:=Vigenere(buf, key, False);



PrintLn(buf);