add NTT implementation, and use it for the negacyclic poly ring multiplication, more details on the NTT can be found at https://github.com/arnaucube/math/blob/master/notes_ntt.pdf .

README.md

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+# fhe-study
+Code done while studying some FHE papers.
+
+- arithmetic: contains $\mathbb{Z}_q$ and $\mathbb{Z}_q[X]/(X^N+1)$ arithmetic implementations, together with the NTT implementation.