Instead of passing safe prime bit length which is a kind of an
implementation detail, we should pass a public key bit length. Public
key is a Paillier modulus defining plaintext space. This information is
more important than the safe prime value/bit length used under the hood.
`4` and `120*time.Seconds` were just some magic numbers in the code.
We decided to move them to separate value declarations to make the
code more self-explanatory.
Piotr Dyraga