In Fallout 3, security terminals are located outside of the entrance to buildings, vaults and guarded rooms. Terminals are used to disable security bots, gain information or unlock safes. To hack the security terminals in the game, you need to have a specific password or a high "Science" skill. As passwords are rarely used, you manually hack into the security terminals most of the time. You play a password mini-game to gain access to the security terminal.
Walk up to a security terminal and press the "Activate" button. If your "Science" level is not high enough, you will not be able to access the terminal. The hardest security terminals require a "Science" skill of 100.
Select a word from the list of passwords. All of the potential words are the same length. The higher the difficulty, the longer the words. Once you select a word, you receive a message that says, "X out of Y correct." X is the number of correct letters in the word you selected, and Y is the total letters in the word.
Select another word with the same number of correct letters. If it is correct, you gain access to the terminal. If it is not correct, you receive the "X out of Y correct" message again. You have four total attempts before you are locked out of the terminal.
Select clumps of nonsensical characters within parenthesis or brackets to remove duds or to reset your attempts.
Exit out of the terminal after your third attempt to prevent being locked out. You can immediately retry to hack the terminal with a different set of words.
If you are locked out of a terminal, you will be unable to access it for the remainder of the game.