9618 · 17.1

Encryption, Encryption Protocols and Digital Certificates — practice questions

Practice and worked examples for 9618 Encryption, Encryption Protocols and Digital Certificates. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

A message SECURE is to be encrypted using a symmetric Caesar cipher with a key of +3. Show the encryption process and then decrypt the resulting ciphertext to retrieve the original message.

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Encryption Process: Each letter in the plaintext is shifted 3 places forward in the alphabet.

S + 3 -> V
E + 3 -> H
C + 3 -> F
U + 3 -> X
R + 3 -> U
E + 3 -> H

Plaintext: SECURE Ciphertext: VHFXUH

Decryption Process: To decrypt, we apply the reverse operation, shifting each letter 3 places backward.

V - 3 -> S
H - 3 -> E
F - 3 -> C
X - 3 -> U
U - 3 -> R
H - 3 -> E

Original Plaintext Recovered: SECURE

Worked example 2

Alice wants to send a confidential message to Bob's Bank using asymmetric encryption. The bank's website has a public key and a corresponding private key. Outline the steps required for Alice to send the message securely.

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Key Acquisition: Alice accesses Bob's Bank's website. Her browser automatically obtains the bank's public key, likely from its digital certificate. [1 mark]
Encryption: Alice writes her confidential message (plaintext). She then uses the bank's public key to encrypt the message. The result is ciphertext. [1 mark]
Transmission: Alice sends the ciphertext to Bob's Bank over the internet. Even if intercepted, the message is unreadable without the corresponding private key. [1 mark]
Decryption: Bob's Bank receives the ciphertext. It uses its own private key, which has been kept secret and secure, to decrypt the message back into the original plaintext. [1 mark]

Note: Only the bank's private key can decrypt a message encrypted with its public key, ensuring confidentiality.

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