yesterday, i read chapter 1 of An Introduction to Cryptography by Jeffrey Hoffstein, Jill Pipher, J.H. Silverman done with these sections: 
>Simple substitution ciphers 
>Divisibility and greatest common divisors 
>Modular arithmetic 

read a bit on finite fields and i also watched some videos

> read prime numbers, unique factorization, and finite fields
> made a representation of the extended Euclidean algorithm in rust
https://github.com/g4titanx/gcd_euclidean_algorithm.git
>solved some exercises