I was unable to get cowpatty working with a packet capture that actually
contains a four-way handshake of a WPA session.
I got it working like this:
First, download cowpatty 4.6 right here, within the source directory of cowpatty.
Then build couwpatty just with 'make' and 'make install'.
I created a test setup with a known password. However, this patched
version did not find the passphrase using a dictionary file.
I then used genpmk to create a precomputed hash database like this:
genpmk -f ./aircrack-ng-1.0/test/password.lst -d hashes -s default
Please note that I added the correct passphrase to this password list to
make sure that cowpatty works.
Finally, using the -d option on the hash file, cowpatty
managed to crack the PSK.