SDR Capture The Flag Competition Tutorial

During several hacker and security themed conferences (Shmoocon, Bsides, Derbycon, Defcon, etc) organizers from Wireless Village have been setting up competitive WiFi and SDR themed capture the flag (CTF) games. In the competition the organizers broadcast a signal and the competitors are required to complete various tasks (capturing flags) such as determining the centre frequency of the transmission, demodulating the signal and finding any meta data contained within the signal such as codec flags in DVB-T signals and RDS data in FM signals. The team which captures the most flags wins a prize. The process of capturing flags often requires the use of some sort of software defined radio like the RTL-SDR, HackRF or BladeRF.

Recently, Russell one of the CTF organizers wrote in to let us know about a SDR CTF training resource that he has put together. The site contains various exercises/tutorials that allow participants to practice the skills needed to compete in the competition. Most exercises involve using a Raspberry Pi together with PiFM for transmitting a simulated competition signal, then receiving and demodulating the signal with a SDR. The exercises include running rtl_power, setting the PPM offset, decoding morse code, AFSK, RDS, ASK/OOK, DVB-T, POCSAG, MotoTRBO, SSTV and decoding numbers stations.