#### This is the first article of the series that I have planned to write explaining the Cryptography behind **Blockchain**.

In this article, I will be covering **One-Way Functions.**

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 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.

SHA256(1) = 6B86B273FF34FCE19D6B804EFF5A3F5747ADA4EAA22F1D49C01E52DDB7875B4B

SHA256(2) = D4735E3A265E16EEE03F59718B9B5D03019C07D8B6C51F90DA3A666EEC13AB35

SHA256(3) = 4E07408562BEDB8B60CE05C1DECFE3AD16B72230967DE01F640B7E4729B49FCE

If you observe the above three outputs, they do not follow a pattern. Thus, there is no way we can identify the input of the function. When these functions are used in applications, it is **not an easy task** for a hacker to penetrate into the application. Such functions are known as **One-Way Functions.**

In the upcoming articles of this series, I will be explaining **Hashing Algorithms **and how they are incorporated in **Blockchain Design**. Stay tuned for the updates! Thank you for reading my article.

*If you have any questions, please feel free to send me an **email**. You can also contact me via **Linkedin**.*

A Simple Introduction to One-Way Functions was originally published in Hacker Noon on Medium, where people are continuing the conversation by highlighting and responding to this story.