A Level Computer Science (A2) chapter 5: Security MCQs with answers
By Team Megalecture · Updated 4 Nov 2026
29 exam-style questions on Security, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.
Encryption, Encryption Protocols and Digital Certificates29 questions
Ten Security questions with answers
Try each one, then open the answer. These ten stay the same; the test draws new ones every time.
1. The cipher text 10011100 was produced by XOR with the key 00110101. What is the plain text?
A
10101001
B
00010100
C
10111101
D
01010110
Show answer
Answer: A. XOR with the same key reverses the encryption, because (P XOR K) XOR K = P. 10011100 XOR 00110101 = 10101001.
2. An online shop publishes its public key on its website and customers encrypt their card details with it. Why can an eavesdropper who has copied the public key still not read the data?
A
The public key can only be used to decrypt data, never to encrypt it.
B
The data can only be decrypted with the shop's matching private key, which is never published.
C
Each customer encrypts with their own private key, so the eavesdropper needs the customer.
D
The public key changes after every purchase, so the copy is useless.
Show answer
Answer: B. Data encrypted with a public key can only be decrypted with the matching private key, which the shop keeps secret. Knowing the public key does not reveal the private key.
3. Which statement about symmetric key cryptography is correct?
A
A public key encrypts the data and a different private key decrypts it.
B
The same secret key is used to encrypt the data and to decrypt it.
C
Two linked keys are used and both of them may safely be published.
D
No key is needed because the algorithm itself is kept secret.
Show answer
Answer: B. In symmetric cryptography one shared secret key does both jobs. Using a public key to encrypt and a private key to decrypt describes asymmetric cryptography.
4. Aisha wants to send Bilal a message that only Bilal can read, using asymmetric encryption. Which key should Aisha use to encrypt it?
A
Bilal's private key
B
Aisha's private key
C
Aisha's public key
D
Bilal's public key
Show answer
Answer: D. To keep the message private it is encrypted with the recipient's public key, so that only the recipient's private key, which nobody else holds, can decrypt it.
5. A bank wants customers to be sure that a notice really came from the bank. Using asymmetric cryptography, which key does the bank use to produce the signature on the notice?
A
the bank's private key
B
the bank's public key
C
the customer's public key
D
the customer's private key
Show answer
Answer: A. A signature must be something only the sender could produce, so it is created with the sender's private key. Anyone can then check it with the bank's published public key.
6. Why is asymmetric encryption often used only to exchange a symmetric key rather than to encrypt all of the data sent in a session?
A
It cannot be used to encrypt plain text, only to create signatures.
B
Its cipher text can be decrypted by anyone who holds the public key.
C
It is much slower than symmetric encryption for large amounts of data.
D
It needs both parties to keep the same private key hidden.
Show answer
Answer: C. Asymmetric algorithms need far more processing than symmetric ones, so they protect the small session key, and the fast symmetric algorithm then encrypts the bulk data. Asymmetric encryption can encrypt plain text, and each party keeps its own private key.
7. In encryption, what is meant by cipher text?
A
the scrambled data produced when an algorithm is applied to plain text
B
the original readable message before any encryption takes place
C
the secret value that the algorithm uses to scramble the data
D
a fixed-length value used to check that a message is unaltered
Show answer
Answer: A. Cipher text is the output of encryption. The readable input is plain text, the secret value is the key, and a fixed-length check value is a hash (digest).
8. Which is a drawback of quantum cryptography?
A
It needs specialised, expensive equipment and works over only limited distances.
B
An eavesdropper can copy the photons without the two ends noticing.
C
The keys it produces are always shorter than those of public key methods.
D
It can only be used to encrypt text, not images or video.
Show answer
Answer: A. Quantum key distribution needs dedicated hardware such as photon sources and detectors, and signals degrade over distance in fibre. Eavesdropping is detectable, and the resulting key can be used with a normal cipher for any data.
9. Aisha wants to send Bilal a message that only Bilal can read and that Bilal can be sure came from her, using asymmetric cryptography. Which two keys does Aisha need to apply to the message?
A
Aisha's public key and Bilal's private key
B
Aisha's private key and Bilal's private key
C
Aisha's public key and Bilal's public key
D
Aisha's private key and Bilal's public key
Show answer
Answer: D. Confidentiality needs the recipient's public key, because only Bilal's private key can then decrypt the message. Proof of origin needs the sender's private key, because only Aisha holds it and anyone can check the result with her public key. Neither key of the pair can do both jobs.
10. What is the main drawback of symmetric key cryptography when two people who have never met wish to communicate?
A
Encryption and decryption are far slower than with asymmetric methods.
B
Anyone who intercepts the cipher text can immediately work out the key.
C
The secret key has to be shared with the other person securely before use.
D
It cannot be used to encrypt any file larger than a few kilobytes.
Show answer
Answer: C. Symmetric methods are fast but both parties need the same secret key, so the key must first travel to the other person by a safe route. That key-distribution problem is what asymmetric methods help to solve.
29, each with the answer and a short explanation, across 1 topics: Encryption, Encryption Protocols and Digital Certificates. Every chapter test draws ten of them, so you can take it many times.
Is the A Level Computer Science (A2) chapter 5 test free?
Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.
How is the chapter test marked?
The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.
