all hash functions have the following basic characteristics: if two hash values are different (according to the same function), the original inputs of the two hash values are also different. This characteristic is the result of the certainty of the hash function. On the other hand, the input and output of the hash function are not one-to-one. If the two hash values are the same, the two input values are likely to be the same. However, it is not absolutely certain that the two are equal. If you input some data to calculate the hash value, and then change the input value partially, a hash function with strong confusion will produce a completely different hash value.

Typical hash functions have infinite definition fields, such as byte strings of arbitrary length, and finite value fields, such as bit strings of fixed length. In some cases, hash functions can be designed to have one-to-one correspondence between the definition fields and the value fields with the same size. The goal of constructing hash function is to make the hash addresses evenly distributed in the hash space as much as possible, and to make the calculation as simple as possible to save calculation time. The method of taking the keyword K itself or the keyword plus a certain numerical constant C as the hash address, and the corresponding hash function: h(K)=K+C One-way hash function is an algorithm for data encryption/encoding

One-way hash function is generally used to generate message digest, key encryption, etc., and the common ones are:

MD5(Message Digest Algorithm 5): it is a one-way hash algorithm developed by RSA Data Security Company, MDD.