If you want to study elementary number theory systematically, you can read vinogradov's Basic Number Theory (this exercise is difficult) or Min Sihe's Elementary Number Theory or Pan Cheng's Elementary Number Theory (this exercise is relatively large, nearly 1000) (just choose one of these three books); If you still want to know the application and cryptography of elementary number theory, read Rosen's Elementary Number Theory and Its Application. Of course, it is also good to read Hua's Introduction to Number Theory after laying a solid foundation (Hua Lao's book has basically no exercises and is more difficult. It is written in semi-classical Chinese, so don't read it without some suggestions on number theory and analysis). It's not bad to read Dudley's Basic Number Theory and then Chen Jingrun's Elementary Number Theory.