Corrected entry: Why does Henry Van Statten need to "bypass" the security codes to his own base?
Corrected entry: Any security system worth its salt would stop accepting codes after too many incorrect ones have been entered. This is pretty much standard for security software, even today. No security system of the future would be written so badly as to just allow millions of combinations to be sent one after the other so quickly. The Dalek should really never have been able to get out as the door panel should have blocked after three or four invalid codes were entered.
Correction: Technically true, but Van Statten was so arrogant that he never thought that anyone could possibly hack the code anyway. Character mistake.
Correction: The Dalek changed the codes when it smashed the computer.