IGCSE · Practice questions · Symmetric and asymmetric encryption

Lock, key, pair.

Six original Cambridge-style questions on encryption, with mark-point solutions.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Who holds which key?

Purpose: intercepted data is meaningless without the key; it does not stop interception.
Plaintext to ciphertext: using an algorithm and a key.
Symmetric: one key encrypts and decrypts; the key must be shared and could be intercepted.
Asymmetric: encrypt with the receiver's public key; decrypt with the receiver's private key, which is never sent.

Explain

[2 marks]

Explain the purpose of encrypting data before it is transmitted.

If the data is intercepted it cannot be understood / is meaningless. ✓
Because it cannot be decrypted without the key, so the data stays confidential (encryption does not stop interception). ✓

Recall

[2 marks]

State the meaning of the terms plaintext and ciphertext.

Plaintext: the original data before encryption. ✓
Ciphertext: the data after it has been encrypted. ✓

Apply

[2 marks]

A Caesar cipher with a key of 4 is used. (a) Encrypt the word SECRET. (b) State which type of encryption a Caesar cipher is.

(a) WIGVIX. ✓
(b) Symmetric: the same key (shift of 4) is used to encrypt and decrypt. ✓

Explain

[3 marks]

Describe how data is encrypted and decrypted using symmetric encryption, and give one drawback.

The plaintext is encrypted using an algorithm and a key to produce ciphertext. ✓
The receiver decrypts the ciphertext using the same key. ✓
Drawback: the key has to be sent to the receiver, and if it is intercepted the data can be decrypted. ✓

Explain

[4 marks]

Priya wants to send her bank a confidential message using asymmetric encryption. Describe how this is done.

The bank has a public key and a matching private key. ✓
The bank's public key is sent to / made available to Priya. ✓
Priya encrypts the message using the bank's public key. ✓
The bank decrypts the ciphertext using its private key, which is never shared. ✓

Analysis

[2 marks]

A hacker intercepts both the public key and the encrypted message in question 5. Explain why the message is still secure.

The public key can only be used to encrypt the message, not to decrypt it. ✓
Only the matching private key can decrypt it, and that key is kept by the bank and never transmitted. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.