Cryptography is a big topic. We are going to cover the basics, to give you a basic understanding.

It is used by a wide range of applications, almost everywhere in the internet.

Why we need cryptography?

Without encryption communications over the internet will be very insecure, and it would be very easy for someone to see your data. Fortunately this isn't true, and almost all of the data you get/send over the internet is encrypted and cannot be seen in plain text by someone who got access to it. Now let's look into cryptography in more detail.

There are two types of cryptography :

• Symmetric
• Asymmetric

What is symmetric cryptography?

Let's say Alice wants to send message to Bob, but she doesn't want to send the message in plain text because anyone can read it.

She  has a recipe how to mess up the letters in a way that no one can understand what it means(that is called encryption). Bob has the same recipe but he uses it to order them so it will become readable again(that is called decryption), and he will understand what Alice wrote. The recipe in cryptography is called a key.  In symmetric cryptography both users have the same key to encrypt and decrypt the messages.

What is asymmetric cryptography?

In asymmetric cryptography the encryption and decryption keys are different. The encryption key can be used only for encryption, not for decryption and vice versa. The encryption key is called Public key and the decryption key is called Private key.

Why is asymmetric cryptography used?

Well..because it's more secure! If you are using symmetric cryptography and someone takes your key he can encrypt and decrypt messages that you send. While in asymmetric cryptography he can't decrypt the messages you encrypted with the public key. Only the holder of the Private key can decrypt them. Let's give an example :

Alice wants to talk to Google

Google gives Alice the Public key.

Alice encrypts her message with the public key and sends it to Google.

Google decrypts the message with his private key and sends back what Alice wanted.

And that's how the conversation goes...

Note : Symmetric cryptography is faster than asymmetric, but asymmetric is more secure.

Now let's see what you've learned!

Hashes are a long strings of letters and numbers generated by hashing algorithms. They take plain text and make it a hash.

The important thing about hashes is that they are not reversible. There's no way to decrypt/decode a hash.

Popular hashing algorithms are : MD5 (Message Digest 5) and SHA (Secure Hash Algorithm).

That's how the word "hello" looks as MD5 hash : 5d41402abc4b2a76b9719d911017c592

What are hashes used for?

The most popular use of hashes is for file identification and storing sensitive data, like passwords. When you create an account on a website your password is converted to hash and this hash is stored in the server's database. So when you login the password that you type in will be converted to a hash, the server will take it and compare it with the hash in it's database, if it's the same that means your password is correct and the server will let you in.

MD5 hashes are also used to ensure the data integrity of files. Because the MD5 hashing algorithm always produces the same output for the same given input, it can be used to compare a hash of the source file with a newly created hash of the destination file to check that it is intact and unmodified.

There are plenty of other hashing algorithms out there, but this room is intended for beginners so we're not going to cover them here.

There's difference between encoding and encrypting. Let's say you have an encrypted file, the only way to decrypt it is using key. While encoded data can be decoded immediately, without keys. It's NOT a form of encryption, it just a way of representing data.

A very popular encoding is Base64. Here's how "hi there" looks with Base64 encoding : aGkgdGhlcmU=