I applied online. The process took 1 week. I interviewed at Red Balloon Security
Interview
They inserted some code into your resume and expected you to solve it, two stages, which consists of 70% of the technical interview. You need to send your write-up and code back to them and wait for their evaluation. I completed both stages, but got an email saying impressed but not match this time, "they want very specific people".
Interview questions [1]
Question 1
Some Python code related to the Russian doll, and one programming assignment, cryptography and multi-thread programming get involved.