Which part are you wanting to understand?
Are you familiar with the concept of a digital signature?
Are you familiar with the concept of a hash?
Are you familiar with the concept of proof-of-work?
That's all the cryptography that bitcoin uses. Once you understand those three concepts, you understand bitcoin's cryptography.
If you want an introduction to how these concepts are used by bitcoin, I suggest reading the bitcoin whitepaper originally posted by Satoshi Nakamoto:
https://bitcoin.org/bitcoin.pdf