**What does the word hash mean ?**

Hash function is a hash function that, with data of arbitrary size, associates an image of fixed size with data of arbitrary size, and whose essential property is that it is practically impossible to go back, if the function is calculated very efficiently, the reverse calculation of an input data with a certain value in image form is practically impossible. For this reason, such a function is called a one-way function.

Because of their omnipresence, these one-way functions have been called the working tools of modern cryptography. The input data of these functions is often called a message. The output value is often called hash value, digital fingerprint, fingerprint or hash (in English, a condensed message).

An ideal cryptographic hash function has the following four properties:

- The hash value of a message is calculated « very quickly »;
- By definition, it is impossible, for a given hash value, to build a message with this hash value;
- By definition, it is impossible to modify a message without changing its hash value;
- By definition, it is impossible to find two different messages with the same hash value.

Cryptographic hash functions are a symmetric cryptographic primitive, but constitute a basic brick widely used in the design of cryptographic schemes (symmetric and asymmetric), including digital signatures, message authentication codes and other forms of authentication.