# A Simple Introduction to One-Way Functions

## A Simple Introduction to One-Way Functions

This is the first article of the series that I have planned to write explaining the Cryptography behindBlockchain.

To understand the need for one-way functions, let me first help you understand Two-Way functions.

### Two-Way Functions

Let us assume that we have a function f(x)=y. Whenever we give input as “x”, we get an output “y”.

Now let us look at the following table:

If we observe the above Input and Output values, by close observation we find that output is always the square of the input, i.e y=x*x. Or f(x)=x*x.

Now, let us say that the function generates an output as “121”. Since I know the function, I can easily tell that the input would have been “11”. Functions wherein we can determine the input with the help of its output are known as Two-way functions .

As much as we have enjoyed cracking this function we should realize that if we incorporate them in the Systems or Application Design, any hacker with can crack open the application with a simple brute force attack .

This particular vulnerability of Two-way functions led to the adoption of One-way functions.

### One-Way Functions

Let us consider the SHA256 of the following numbers.