Fix Paillier encryption function to sample r from n

[nmajid2]

Goal

we only need to select r < n for encryption. In the existing code, r is sampled from n^2 and then raised to n mod n^2. Thus, the goal is to pick r from n.

##Resources

• Public-Key Cryptosystems Based on Composite Degree Residuosity Classes

Acceptance criteria

1. Well documented code
2. Tests pass with more than 80% test coverage
3. Static analysis should pass
4. Code should be reviewed by two engineers