Let me answer your second question. No matter at home or abroad, the basic course of cryptography will not change. More importantly, there are not many master's courses in the United States. Only by being prepared can we avoid trouble. Computational number theory, algebra and cryptography are all basic courses. For Chinese reference books, please refer to the following websites. You are only a sophomore now. You can read Computational Number Theory in your spare time, both in Chinese and English. After studying modern algebra, you can teach yourself Jing Tian (by Rudolf liddell and harald Niederrett). Then enjoy the classic cryptology textbook Cryptography: Theory and Practice (by Douglas R. Stinson). In addition, you can usually pay attention to programming and algorithms.

I hope these will help you in your future development. Good luck!