Computer Science (0478) | Encryption

Welcome to the Encryption Chapter!

Hello future Computer Scientists! In the previous sections, we learned how data is broken down into packets and transmitted across networks. But when your sensitive data (like passwords or bank details) travels the internet, it’s vulnerable to hackers.

This chapter is all about how we put a digital lock on those packets. You will learn the purpose of encryption and the two main methods used to keep your online life secret and safe. Don’t worry if this sounds complicated—we'll break it down into simple, manageable steps!

2.3 Understanding the Need and Purpose of Encryption

What is Encryption?

Imagine you write a secret message on a postcard. If a postman (or a hacker!) sees it, they can read it instantly. Encryption is the process of scrambling that message so that only the intended recipient can unscramble and read it.

• Plaintext: This is the original, readable data or message. (Example: "My password is P@ss123")
• Encryption: This is the mathematical process (using an algorithm and a key) that turns plaintext into a scrambled form.
• Ciphertext: This is the scrambled, unreadable data that is transmitted across the network. (Example: "h8sT0dY&k$z")
• Decryption: This is the process of reversing the encryption, turning the ciphertext back into readable plaintext, usually requiring the correct key.

Why Do We Need Encryption? (The Purpose)

The primary purpose of encryption is to ensure confidentiality and security, especially during data transmission.

When data travels over a network (like the internet), it might pass through many routers and cables. Without encryption, if a third party intercepts the data packet (a cyber security threat known as Data Interception), they can read the contents immediately.

Encryption ensures that even if a hacker intercepts the data, all they get is meaningless ciphertext. They cannot understand the message without the correct decryption key.

Symmetric Encryption (Single Key)

Symmetric Encryption is the simplest form of encryption, characterized by the use of only one key for both encrypting and decrypting data.

How Symmetric Encryption Works:

Think of symmetric encryption like a diary locked with a single physical key:

1. Preparation: Both the sender (Alice) and the receiver (Bob) must agree on the same secret key beforehand. This key is often called the Secret Key or Session Key.
2. Encryption: Alice uses the Secret Key to encrypt the plaintext message, turning it into ciphertext.
3. Transmission: Alice sends the ciphertext to Bob.
4. Decryption: Bob uses the exact same Secret Key to decrypt the ciphertext back into readable plaintext.

The Main Challenge of Symmetric Encryption

The biggest issue is securely sharing the Secret Key. If Alice and Bob are communicating over a secure channel, it's fine. But if they have to send the key itself over the insecure internet, a hacker could intercept the key and then decrypt all future messages!

Did you know? Despite this challenge, symmetric encryption is very fast and efficient. It is often used to encrypt large amounts of data quickly after a secure connection has been established using other methods.

Asymmetric Encryption (Two Keys)

Asymmetric Encryption (also known as Public Key Encryption) solves the key sharing problem by using two mathematically related keys: a Public Key and a Private Key.

This method is crucial for secure initial communication over the internet, such as when you connect to a banking website (HTTPS).

Key Roles in Asymmetric Encryption:

1. The Public Key

• The Public Key is shared freely and openly. It is like an open email address or a postal mailbox slot.
• Anyone who wants to send a secret message to the key owner uses the Public Key to encrypt the data.
• The Public Key cannot be used to decrypt the data it encrypted.

2. The Private Key

• The Private Key is kept absolutely secret by the owner. It is like the physical key to open your locked postal mailbox.
• Only the owner can use the Private Key to decrypt the data that was encrypted using their corresponding Public Key.

How Asymmetric Encryption Works (Step-by-Step):

Let's say Alice wants to send a secret message to Bob.

1. Sharing: Bob sends his Public Key to Alice (and anyone else).
2. Encryption: Alice takes her plaintext message and uses Bob's Public Key to encrypt it into ciphertext.
3. Transmission: Alice sends the ciphertext. Even if a hacker intercepts Bob’s Public Key and the ciphertext, they cannot decrypt it.
4. Decryption: When Bob receives the ciphertext, he uses his Private Key (which only he has) to decrypt the message back into plaintext.

Encouragement: Don't worry if this seems tricky at first. The key idea is that the Public Key is used to lock the box, and the Private Key is the only thing that can unlock it.