Anything sent across a network can be intercepted. Encryption cannot stop that, but it can make sure that whoever grabs the data cannot understand it. Learn the two ways of doing it, and the key problem each one solves.
Encryption scrambles plaintext into ciphertext using an algorithm and a key, so that if data is intercepted it is meaningless to anyone without the key. Encryption does not stop interception. In symmetric encryption the same key encrypts and decrypts, so the key has to be shared, and it could be intercepted too. In asymmetric encryption there is a matching pair of keys: data encrypted with the receiver's public key can only be decrypted with the receiver's private key, which is never sent.
Fig. 1: anyone can encrypt with Bob's public key, but only Bob's private key decrypts
Section 01
Why encrypt?
Data travelling across networks passes through many devices and can be intercepted by a hacker. Encryption protects what the data means.
Plaintext: the original, readable data.
Ciphertext: the scrambled data produced by encryption.
Key: the value used by the encryption algorithm to encrypt or decrypt the data.
Purpose: if ciphertext is intercepted, it cannot be understood without the key, so sensitive data such as passwords and bank details stays confidential.
Examiner trap
Encryption does not stop data being intercepted or stolen. It makes intercepted data meaningless (unreadable) without the key. Write exactly that.
Section 02
Symmetric: one key.
The lab starts with a Caesar cipher, a very simple symmetric method: shift every letter along the alphabet by the key. Try encrypting a word, then decrypting it.
One key: the same key is used to encrypt and to decrypt.
Sharing: the sender must send the key to the receiver before they can decrypt.
Weakness: if the key is intercepted while it is being shared, the hacker can decrypt every message.
Worked example Caesar shift
Encrypt HELLO using a Caesar cipher with a key of 3.
Shift each letter 3 places along the alphabet.
H → K, E → H, L → O, L → O, O → R.
Answer: KHOOR
Your turn
The ciphertext MJQQT was made with a Caesar shift of 5. Decrypt it.
Shift each letter 5 places back: HELLO.
Examiner trap
Real symmetric encryption uses very long keys, not a shift of 3. The Caesar cipher only shows the idea: the same key locks and unlocks.
Stage 1 · Learn
Check what the sim just showed you
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What is ciphertext?
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Using a Caesar shift of 2, what is DATA encrypted to?
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In the lab's asymmetric mode, which key decrypts the message?
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What is the main weakness of symmetric encryption?
Section 03
Asymmetric: two keys.
Switch the lab to asymmetric mode. The receiver publishes a public key and keeps a private key secret.
1. The receiver has a matching pair of keys: a public key and a private key.
2. The receiver sends its public key to the sender. Anyone can have it.
3. The sender encrypts the plaintext using the receiver's public key and sends the ciphertext.
4. The receiver decrypts the ciphertext using its own private key, which is never shared.
5. A hacker who intercepts the public key and the ciphertext still cannot decrypt it, because the public key cannot decrypt.
Worked example Compare
Explain why asymmetric encryption is more secure than symmetric encryption when sending data to a new contact.
In symmetric encryption the one key must be sent to the contact, and it could be intercepted.
In asymmetric encryption only the public key is sent; it can encrypt but not decrypt.
The private key, the only key that can decrypt, never leaves the receiver.
Answer: The decrypting key never has to be transmitted
Examiner trap
The sender uses the receiver's public key, not its own. The answer "the message is encrypted with a private key and decrypted with a public key" is the wrong way round for sending secret data.
Stage 2 · Exam
Exam-style questions
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What is the purpose of encrypting data before it is transmitted?
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Kai wants to send Mia a secret file using asymmetric encryption. Which key does Kai use to encrypt it?
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Which statement about symmetric encryption is correct?
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A hacker intercepts a message and the public key used to encrypt it. Why can they not read it?
Skill unlocked
Encryption, mastered.
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