I am an engineer on the infosec team. If you are interested in any aspect of computer security (from hardware/low level stuff to backend infrastructure, writing secure web application or working on Android/iOS) you should really consider talking to our team!
I love working at Square. We are helping people grow their businesses. More important for me, I find my peers to be very altruistic: you can walk up to any engineer and ask them questions or schedule a pair-programming session with them, bounce ideas off each other, etc. I have never seen this kind of healthy engineering culture at any other tech company, we have something special going on.
Square has a strong open source culture and the infosec team has contributed several projects over the years. For example:
Drop me an email if you are interested!