Unlocks in Mission 10: Imperial City
This achievement is related to the story and will unlock when completeing mission objective 2/3. You have to get into the vault and get a decoder.
How to reach vault:
After you get to the black imperial I.S.O building you walk into the building and take the elevator to the upper floor.
To reach the top floor you need a key card. The card is hidden in the upper floor, located in 2 small rooms with officers. Once you use the buttons in those rooms you will open a vent and through that vent you get to the key card room.