2
$\begingroup$

So for a Diffie-Hellman problem, I am given the prime $p$ for the Diffie-Hellman exchange. I am also given $g$, secret number for machine as $A$, secret number for station as $B$, a Diffie-Hellman shielded Login Name $V$, and Diffie-Hellman shielded password $W$.

For this problem, I am given three users and see which one accessed files to which they had no clearance to. So I computed $x=g^A\pmod p$ and $y=g^B\pmod p$, then $x^A\pmod p$ and $y^B\pmod p$ which gave me my secret common key. Where I am confused is as to how to now unshield the DHS key and how I can use $V$ and $W$ to do so.

What do I do next?

According to a textbook I am reading, it says that the equation $DHS*u = 1\pmod p$ has a solution in $\mathbb N_p$ and this is the solution for $UDHS$, or the unshielding of DHS. But I am confused by what this means. Help is appreciated.

$\endgroup$
  • 2
    $\begingroup$ Actually, you should be computing $x^B \bmod p$ and $y^A \bmod p$ to derive the shared secret. $\endgroup$ – poncho Oct 22 '13 at 19:26
  • $\begingroup$ Within the DH protocol, there's no standard way to do "Diffie-Hellman shielded Logic name" and "password". It is certainly possible to design a protocol that uses DH which does it, however the details of that protocol are outside the Diffie-Hellman protocol. $\endgroup$ – poncho Nov 4 '13 at 17:14