The door decoder is a gadget obtained from Dr. Doak in the Arkangelsk chemical warfare facility. It is needed on the secret agent and 00 agent difficulties to open the security door leading to the bottling room. Once you approach the door, you need to use your firing key on it, for the gadget to decode the security door.
In the XBLA remastered version, however, this item is obtained by Dr Rakhmanov, instead.