Complexity: NP-intermediate as of 2020:
The basis of RSA: RSA. But not proved NP-complete, which leads to:
This is natural question because both integer factorization and discrete logarithm are the basis for the most popular public-key cryptography systems as of 2020 (RSA and Diffie-Hellman key exchange respectively), and both are NP-intermediate. Why not use something more provenly hard?