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A2 · Practice questions · Digital signatures and digital certificates

Signed, sealed, verified.

Six original Cambridge-style questions on digital certificates and digital signatures, with mark-scheme solutions.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Hash, sign, compare.

01
Explain
[4 marks]

Describe how an organisation acquires a digital certificate.

  • The organisation generates a public / private key pair. ✓
  • It sends its public key and identifying details (name, domain, address) to a certificate authority. ✓
  • The CA verifies the organisation’s identity. ✓
  • The CA creates the certificate, signs it with the CA’s private key and issues it. ✓
02
Recall
[3 marks]

State three items of data contained in a digital certificate.

  • The owner’s public key. ✓
  • The owner’s name / organisation / domain name; the issuing CA’s name. ✓
  • Serial number, validity dates or the CA’s digital signature (any one per mark, max 3). ✓
03
Explain
[4 marks]

Explain how a digital signature is produced for an email.

  • A hashing algorithm is applied to the email. ✓
  • This produces a fixed-length message digest. ✓
  • The digest is encrypted using the sender’s private key. ✓
  • The encrypted digest is the digital signature, sent with the email (and the sender’s certificate). ✓
04
Explain
[4 marks]

Explain how the receiver of the email uses the digital signature and the sender’s digital certificate to check the email.

  • The receiver checks the certificate is valid using the CA’s public key and takes the sender’s public key from it. ✓
  • The signature is decrypted with the sender’s public key to give the original digest. ✓
  • The received email is hashed with the same algorithm to give a new digest. ✓
  • If the two digests match, the email is from the sender and has not been altered. ✓
05
Apply
[2 marks]

A key pair uses n = 55, public key e = 3 and private key d = 27. A received signature is 2 and the receiver hashes the received message to get 9. Determine whether the message is verified.

  • Decrypt the signature with the public key: 23 MOD 55 = 8. ✓
  • 8 is not equal to 9, so the message is not verified: it has been altered or was not signed with the matching private key. ✓
06
Analysis
[3 marks]

A software company signs its updates digitally. Explain why a user can trust a signed update, and why the signature alone does not stop a competitor reading the update.

  • Only the company holds the private key, so a signature that verifies with the company’s certified public key proves the update came from the company. ✓
  • Any change to the update (e.g. inserted malware) changes the digest, so tampering is detected. ✓
  • The signature is an encrypted digest, not encryption of the update itself, so the update can still be read; confidentiality needs separate encryption. ✓

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.