IPSEC/VPN - PSK Cracking using IKE Aggressive Mode

In a presentation about VPN hacking Michael Thunmann and Enno Rey talked about the process of cracking pre-shared keys in certain IPSEC/VPN environments. They were able to capture and crack successfully PSKs of a cisco router due to the issue that the cisco router switches automatically to aggressive mode if the initiating clients demands it. Key-Recovery was done with the help of ikecrack and good old tcpdump.