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Practise 25 exam-style questions on Encryption for Cambridge IGCSE Computer Science (Paper 1). Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.
Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.
An encryption system uses keys that are 12 bits long.
State how many different keys are possible.
2¹² = 4096 different keys
Each bit can be 0 or 1, so every extra bit doubles the number of possible keys: n bits give 2ⁿ keys. 2¹⁰ = 1024, so 2¹² = 1024 × 4 = 4096.
4096
accept 2¹² if evaluated as 4096; do not accept 2¹² unevaluated, 12 or 24
A Caesar cipher replaces each letter with the letter a fixed number of places later in the alphabet. The number of places is the key, and the alphabet wraps round from Z back to A.
The ciphertext CWFD was produced using a Caesar cipher with a key of 5.
Decrypt the ciphertext to find the original word.
Move each letter 5 places back:
C → X (wrapping back past A to Z)
W → R
F → A
D → Y (wrapping back past A to Z)
Plaintext: XRAY
To decrypt, the same key is applied in reverse: move each letter 5 places back. C and D are near the start of the alphabet, so counting back wraps round past A to the end of the alphabet: C, B, A, Z, Y, X. Forgetting to wrap round is the usual mistake.
Any two or three letters correct and in the correct positions (X R A Y)
e.g. XRAI or HRAY (a wrap-around error) scores 1 mark
XRAY (all four letters correct)
XRAY scores both marks; accept X-RAY, lower case or spaces between the letters
The diagram shows data being sent from one computer to another using symmetric encryption.
Identify A, B and C.
A is the plaintext.
B is the ciphertext.
C is the (secret) key, the same key being used by both the encryption and the decryption algorithm.
Plaintext goes into the encryption algorithm, which uses a key to produce ciphertext; the ciphertext is transmitted and the decryption algorithm turns it back into plaintext. In symmetric encryption one key, C, is used for both steps, so the receiver must also have a copy of it.
A: plaintext
accept 'plain text'; accept 'original/unencrypted data'; do not accept 'data' or 'message' on its own
B: ciphertext
accept 'cipher text'; accept 'encrypted data'
C: (secret / symmetric) key
accept 'encryption key' or 'the shared key'; do not accept 'public key' or 'private key'
Compare symmetric encryption and asymmetric encryption.
Both methods use an encryption algorithm and a key to turn plaintext into ciphertext, so intercepted data is meaningless. Symmetric encryption uses one key for both encryption and decryption, whereas asymmetric encryption uses a matching pair of keys: a public key to encrypt and a private key to decrypt. With symmetric encryption the key has to be sent to the recipient, so it could be intercepted and used to decrypt the data, whereas with asymmetric encryption the private key is never sent, which makes it more secure.
A comparison needs both methods in each point, such as 'symmetric uses one key whereas asymmetric uses two'. The biggest difference is how the key reaches the recipient: the symmetric key must be shared, but the asymmetric private key never leaves its owner.
Similarity: both use an encryption algorithm and key(s) to turn plaintext into ciphertext / both make intercepted data meaningless
max 4; max 1 mark for similarities
Symmetric uses one key / the same key to encrypt and decrypt, whereas asymmetric uses two different keys (public and private)
a difference must state both sides, or a clear comparative, to earn the mark
In symmetric encryption the key must be sent to the recipient, whereas in asymmetric encryption the private key is never sent / shared
If the symmetric key is intercepted the data can be decrypted, whereas intercepting a public key does not allow the data to be decrypted
Asymmetric encryption is more secure (as the decryption key is never transmitted)
Symmetric encryption is quicker / needs less processing than asymmetric encryption
Ready to try the rest? Every answer you write is marked against the mark scheme, so you can see exactly where the marks are.
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