In this article, we are going to explain addresses and hash functions. Unlike the examples of commonly used cryptosystems such as signing contracts or encrypting files, hash functions can be a little difficult to understand because they are not presented as a direct example to the user. However, since it plays a very important role in the construction of the blockchain, it is a concept that must be understood for the safe management of assets. Since the content can be long, it is composed of two parts. In this first part, we will look at the necessity and characteristics of a hash function.