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Data Transmission, Error Checking and Encryption: Question 1

Syllabus 2.3

Multiple choice 1 mark

An online tax-filing service lets any user download a public encryption key from its website and use it to scramble their tax return before sending it to the server. Only the tax service holds the second, secret key needed to unscramble the file, and this secret key is never published or transmitted anywhere.

Which type of encryption is being used to protect the tax return?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall the difference between symmetric and asymmetric encryption

  • Symmetric encryption uses one shared secret key for both encrypting and decrypting data, and this same key must somehow be exchanged safely with the receiver in advance.
  • Asymmetric encryption uses two mathematically linked keys: a public key, which can be published and given to anyone, and a private key, which is kept secret by the receiver.

Step 2: Match the scenario to the correct type

  • The tax service’s public key is downloaded openly, so anyone can use it to scramble (“encrypt”) a tax return.
  • Only the tax service’s second, secret key can reverse this process, and that key is never shared with anyone.
  • This “one key to lock, a different key to unlock” pattern is the defining feature of asymmetric encryption. It is not symmetric encryption, which would need the very same single key shared between both parties beforehand, and it is not an error-detection method like parity checking or a check digit at all.

Step 3: Confirm the match

Only asymmetric encryption fits a scheme built around a freely published key and a separate, secret key held by just one party.

Final answer

The scenario describes asymmetric encryption, option B.