my_des [OPTION] [msg] key [encrypted msg]

Note: Key argument: must be a hexadecimal string and size must be equal to 16 characters
--decrypt : add for decrypt the message
-h, --help : print a help message

for encrypt a message:
  "./my_des message key"
for decrypt a message: 
  "./my_des --decrypt key encrypted_message"

  ./my_des fooBar 0123456789ABCDEF
  equal: A591268739E2E7B3

  ./my_des --decrypt 0123456789ABCDEF A591268739E2E7B3
  equal: fooBar