That is brutal mate. I've had that mission a handful of times, and the target has always given it up on the first go. As far as I know, the code is a random set of numbers so I don't think anyone will be able to help you in that respect.
I have seen posts about this mission before in the bug forum.
A developer/admin posted the % rate of getting the code. I think he said it was 40%. But it was a while ago ie years. I remember it being under 50% for sure.
The code changes each time. I know because I've done it twice. The first time, the guy had a 1 digit, 1 digit, 2 digit, and the second time was all 2 digits.
Dang same here. Every time you get a mission that is super vague and does not have anyone to specifically attack, ASK SOMEONE. Also, the help wiki should list missions, unless Ched doesn't want us to figure it out.