Encryption is a very important technology for securing networks. In this module, we'll cover an overview of encryption technologies and describe some common uses of encryption on networks. At the end of this module, you should be able to understand how encryption algorithms work, identify the characteristics of symmetrical key encryption, understand the characteristics of how asymmetrical encryption works, understand how SSL or TLS protects data transmissions, and understand how IPSec protects data transmissions.

The idea of encryption is to scramble information in a way that makes it unreadable to unauthorised people. When we encrypt information, we're using an encryption algorithm and an encryption key, and encryption algorithms are also known as ciphers. The encryption algorithm is a mathematical formula which takes the original information, known as the plain text, and uses the encryption key to create a scrambled representation of that information called a ciphertext. The ciphertext can be unscrambled using a decryption algorithm and a decryption key. There are many different algorithms used for this. One of the most popular ones is the Advanced Encryption Standard or AES, and the algorithms can make use of symmetrical or asymmetrical keys.

With symmetrical key encryption, the same key is used to both encrypt and decrypt the data. Symmetrical key encryption is fast, but it has challenges with key management. Imagine if I encrypted some data using symmetrical key encryption, only someone with the key would be able to decrypt the ciphertext. But, if I want one of my friends to be able to decrypt, how do I get a copy of that key to them safely? If I send the key to them via the internet, for example, what happens if someone intercepts the key? Also, because there's only one key to encrypt and decrypt, I'd need to make sure I use a different key every time I wanted to keep encrypted data secure, when sharing it with different people.

Most symmetrical key encryption is based on block ciphers. With block ciphers, data is encrypted in chunks. A block cipher breaks down plain text messages into fixed-sized blocks before converting them into ciphertext using the key. So, AES is the most common used block cipher. The applications of block ciphers include things like Wi-Fi, file and disk encryption, Virtual Private Networks, Transport Layer Security, and digital signatures.

Symmetrical key encryption can also be used with stream ciphers. A stream cipher breaks a plain text message down into single bits, which are then converted individually into ciphertext using key bits. Some virtual private networks and implementations of TLS use stream ciphers.

With asymmetrical encryption, there are two different keys. The keys are mathematically related, but in a way where one key can't be used to work out the other. With asymmetrical encryption, when information is encrypted with one key, it can only be decrypted with the other corresponding key. Asymmetrical keys are usually known as public-private keys. The private key is kept secret to an individual or a system, and it's never exposed to anyone else. The public key can be made freely accessible to anyone that needs it. Now, if someone encrypts some information with my public key, the only thing that can be used to decrypt that information is my private key. If only I have my private key, then only I can decrypt the ciphertext. Asymmetrical key encryption is slow compared to symmetrical, so it's not usually used for protecting large amounts of data. However, where it is widely used is as a way to securely send a symmetrical key. So, if I encrypt some data with a symmetrical key, I can now send that symmetrical key to my friend securely by encrypting it as plain text with their public key. If I then send that encrypted ciphertext to my friend, nobody can access it because it can only be decrypted with their private key. To decrypt the message, my friend uses the private key and that then gives them the symmetrical key I used to encrypt the data, so now my friend can decrypt the data with that symmetrical key, and this is how most encryption systems work.

We can also use public and private keys the other way around. If I encrypt something with my private key, only my public key can decrypt it. Now, remember that my public key is available to everyone, which means that anyone could decrypt the message. So, you might think, 'Well, what's the point?'. Well, only I have my private key, so if something can be decrypted with my public key, it must have been encrypted with my private key, and therefore, it must have been encrypted by me. And we call this a digital signature, and it's used to prove where something came from, and we'll talk more about this later.

This slide lists some examples of popular asymmetric key techniques. One of the oldest and widely used for securing data transmission is RSA, which gets its name from its inventors, Rivest, Shamir and Adleman, who published their method in 1977. An equivalent system was developed secretly in 1973 at government communication headquarters, GCHQ, by the English mathematician Clifford Cocks, but that was kept secret until 1997.

The popularity of cryptocurrencies like Bitcoin has raised the profile of distributed ledger technology. A distributed ledger is a storage system that can't have existing information changed, but it can have new information added. The ledger is stored at multiple occasions on a shared network. Some ledgers are public, which means they can be accessed by anyone, while others are private and can be used by companies for their own individual use. A distributed ledger will often store data in the form of a blockchain, a type of data structure consisting of blocks of data with a strict sequential ordering. The blockchain records transactions and the order in which they took place. Distributed ledger technologies use encryption and hashing technologies. Hashing is used to confirm the integrity of the information and prove it hasn't been tampered with. Digital signatures are used to prove the authenticity of the information, and encryption is used to protect it.

Let's take a look at the example on the slide, which shows how blockchain is used when people transfer cryptocurrency like Bitcoin. So, Alice wants to send some of her cryptocurrency to Bob. Now, cryptocurrency isn't real, so she can't just give him the money, but we need some way to be able to record that she's done it, and we call that a transaction. The transaction information is stored in a block, that block is then sent to the entire network of computers. To confirm the transaction's genuine, we need a consensus from the network, so the network agrees that the transaction's okay and approves it. Then the transaction, or block, is added to the existing chain. This records all transactions in the order that they occurred. Once the block's been added to the chain, the transaction's complete.

Have you ever heard of the Enigma machine? The Enigma machine is an electro-mechanical device which was designed to encrypt messages and was used by the German military. Enigma is an encryption cipher. There were different models of the machine, and they looked like typewriters. The machines had a lamp board above the keys with a lamp for each letter. The sending operator pressed the key for the plain text letter of the message and the encrypted version of the letter lit up on the lamp board. The receiving operator pressed the key for the encrypted letter on their keyboard and the plain text letter lit up. The machine contained a series of interchangeable rotors which rotated every time a key was pressed, to keep the cipher changing continuously, and this was combined with a plugboard on the front of the machine where pairs of letters were transposed. These two systems combined offered 103 sextillion possible settings to choose from, which the Germans believed made Enigma unbreakable. During the Second World War, a team of researchers, including Alan Turing, secretly broke the Enigma code. They were able to decipher the German messages, and this had a significant impact on the outcome of the war.

In this lab, your challenge is to use a simulator of the Enigma machine to see if it can encrypt and decrypt messages. The machine's quite difficult to set up and you'll need to follow the instructions carefully. The instructions will guide you through setting up the Enigma machine with some initial settings, and these settings will then be saved. You'll then open another copy of the machine and load up the same settings. During the war, the machines were set up with a daily setting and every machine needed to use common settings for that day. Now, once you have two machines set up, you'll then follow some instructions as a sender to create an encrypted message, and then enter that message on the other machine to see if you can decrypt it. Now, don't worry if you can't get it to work first time, you can start again from the beginning and give it another try. The images on the slide are also shown in the lab instructions. You'll need to know how to open the lid of the machine, insert the rotors and access the plugboard. The emulator also has a help section. Now, you'll need to have your lab computer connected to the internet network to access this. And finally, before you begin, you'll need to enter the lab access code. The code is 96272. Good luck!

Module 3: Encryption

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