How did Kang get the last Cerberus code?

It was mentioned in the movie that a brute-force attack to crack all 3 Cerberus codes would take an awful long time (not sure about the exact time-period). So Kang could not employ a brute-force attack to get the codes. However, once he had 2 codes, it was just a matter of time (forgive the pun) before he was able to break through, without essentially having the 3rd access code as the system's security was compromised.

It is safe to assume that Kang knew from the beginning that he would be able to access Cerberus once he had at least 2 access codes. His entire play was to make President Asher believe that he needed all 3 codes and it would all come down to the President at the end. This made the President give away the first 2 codes rather easily since he was confident that there was nothing Kang could do that would make him give up the 3rd code.

TL;DR Kang's Plan A was to get all 3 access codes, using Connor (the President's son) as the leverage for the President's code. His contingency plan was to acquire the other 2 codes and then attack the already compromised Cerberus.

Ok I looked in to this by watching the movie a few times. What I think is that they were in the room for pretty much the entire movie, so once they took control of the room and the computer, they started working on the codes. The code breaker had been split up into three different sets, splitting up the power. The President pretty much gave them what they needed. With 2 of the codes unlocked, the software was able to focus all its power to break the last code. Which in fact it had already been working on for pretty much the entire time.

If that guy is smart enough to infiltrate the South Korean security team and be the biggest terrorist on the planet, then he is smart enough to have backup plans. Like having the next-gen weaponry in place and covering the White House in C4. And also knowing about the interiors of the White House. Breaking one code, though it was random, should be pretty easy.