When all security mechanisms inluding the user supplied password are used a stolen key is virtually of no use to anybody. Thus the extended key file format should be used on most systems. Key Length vpnd uses by default a 576 bit key. If by legal requirements the key length has to be reduced vpnd will still generate keys of 576 bit. Security vpnd uses the shared master key only for setup of a whitening buffer and the exchange of the first session key when the TCP or serial line connection is established.
Session keys consist of random data.
As you can see the master key is only used to transfer random data. Thus it is impossible to retrieve the master key by snooping network traffic. The shared master key between two vpnd peers is one major difference to many other software packages like ssh . The default and maximum key length of 576 bits is more than enough. It has to be remembered that this is the key length of a symmetric cipher.