Bugs in NTTFriendlyPrimesGenerator

[JoowonLee1209]

Hello,

I found that ring.NewNTTFriendlyPrimesGenerator does not check if NextPrime and PrevPrime are prime numbers in v6.

[lehugueni]

Hi!

I think the names are slightly misleading, NextPrime and PrevPrime are the next values of the form 2^BitSize + k*NThRoot + 1 to be checked for primality. When calling the methods to generate primes (e.g. NextUpstreamPrime), we check whether NextPrime is actually prime and we increment NextPrime by NThRoot.