There are several points where the security of the authentication protocol can be attacked. We address each below:
as it is sent over the network from the server to
U. In this scenario, E learns two pieces of
information: the name of one of the file system users ( 
) and the random number R. This information is
only useful to E if it can generate a signed version of the
challenge.
To do so requires that either E have a copy of 
or that the Digital Signature Algorithm is
insecure. Since the Digital Signature Algorithm uses the
computationally secure [13] cryptographic hash function
SHA-1 for message digests and the RawDSA algorithm for
signing data, the protocol is as secure as these algorithms.
and sends it to the server, masquerading as U. A
protection against this ``man-in-the-middle'' attack is to encrypt
all messages in the authentication protocol so E cannot
interpret any of the communication.
as it is
sent from the server to U. Again, applying encryption
to the authentication protocol avoids this man-in-the-middle
attack. . This attack is
extremely unlikely. Since is 64 bits
long, the chances that E correctly guesses are 1 in 
. The randomness of
is dependent on the
quality of the random numbers that SecureRandom
provides.