In Multi-party communication, how can I find that one encrypted value is negative or not?
Assume that there is a such function $f$, that can determine the sign of the number within the ciphertext .

Let the message space $P$
Using the binary technique, then an adversary can actually find the message as; where $c$ is the ciphertext

if  $f( c - E_p( |P|/2 ))$ &gt; 0 
  if $f( c - E_p( |P| ))$ &gt; 0 
else if $f( c - E_p( 0 ))$ &gt; 0...


As usual binary search.