In this module you will learn about Atomic swap and why it is used. First, you will get acquainted with crosschain, chainID, explore the basics of cryptography and look at several hashing algorithms. This knowledge will allow you to write a Solidity smart contract for implementing atomic swap. Next, we will discuss an audit of a smart contract and instruments to perform it. Ensuring the security of a contract is essential, and we will look at mechanisms for attacking contracts as well as tools for protecting them. At the end of the module you will be able to conduct various testing of smart contracts, find vulnerabilities and prevent them.