[[seoane]]

Y para que no digais que falta el codigo, y aunque ando un poco apurado ...

Código Delphi [-]
uses AES, Base64;

function Cifrar(Str,Clave: String): String;
var
  Src: TStringStream;
  Dst: TMemoryStream;
  Size: Integer;
  Key: TAESKey;
  ExpandedKey: TAESExpandedKey;
begin
  Result:= EmptyStr;
  Src:= TStringStream.Create(Str);
  try
    Dst:= TMemoryStream.Create;
    try
      FillChar(Key,Sizeof(Key),#0);
      if Length(Clave) > Sizeof(Key) then
        move(PChar(Clave)^,Key,Sizeof(key))
      else
        move(PChar(Clave)^,Key,Length(Clave));
      AEsExpandKey(ExpandedKey,Key);
      // Guardamos el tamaño del archivo
      Size:= Src.Size;
      Dst.WriteBuffer(Size,Sizeof(Size));
      AESEncryptStreamECB(Src,Dst,ExpandedKey);
      Result:= BinToStr(Dst.Memory,Dst.Size);
    finally
      Dst.Free;
    end;
  finally
    Src.Free;
  end;
end;

function Descifrar(Str,Clave: String): String;
var
  Src: TMemoryStream;
  Dst: TStringStream;
  Size: Integer;
  Key: TAESKey;
  ExpandedKey: TAESExpandedKey;
begin
  Result:= EmptyStr;
  Src:= TMemoryStream.Create;
  try
    Dst:= TStringStream.Create(Str);
    try
      StrToStream(Str,Src);
      Src.Position:= 0;
      FillChar(Key,Sizeof(Key),#0);
      if Length(Clave) > Sizeof(Key) then
        move(PChar(Clave)^,Key,Sizeof(key))
      else
        move(PChar(Clave)^,Key,Length(Clave));
      AESExpandKey(ExpandedKey,Key);
      // Guardamos el tamaño del archivo
      Src.ReadBuffer(Size,Sizeof(Size));
      AESDecryptStreamECB(Src,Dst,ExpandedKey);
      Dst.Size:= Size;
      Result:= Dst.DataString;
    finally
      Dst.Free;
    end;
  finally
    Src.Free;
  end;
end;

// Ejemplo
ShowMessage(Descifrar(Cifrar('Hola mundo','1234'),'1234'));