Project 25 (P25) is the standard used by police and emergency first-responders across the US, Australia and New Zealand. OP25 is a free software project we've initiated that allows us to receive, analyse and transmit P25 traffic using the USRP/GNUradio software-defined radio suite. This presentation will give an introduction to the basics of software-defined radio using the GNURadio framework, provide a detailed analysis of some of the security flaws present in P25 and show how these can be exploited to conduct targeted denial-of-service attacks and key-recovery attacks.