Digital Certificates (Cambridge (CIE) A Level Computer Science): Revision Note

Digital certificates

What is a digital certificate?

• A digital certificate is an electronic file that confirms someone’s identity and proves that a public key belongs to them

• It is issued by a trusted third party called a Certificate Authority (CA)

• A digital certificate includes:

  • The owner’s public key

  • The owner’s identity details (e.g. name, email, company)

  • The expiry date of the certificate

  • The Certificate Authority’s digital signature

Hash function

• A hash function is a one-way algorithm that takes an input (e.g. a message) and produces a fixed-length output, called a hash value or message-digest

• Key features:

  • The output is always the same length, regardless of input size

  • It is one-way — you cannot reverse it to get the original input

  • Even a small change in input produces a completely different output

  • Commonly used in digital signatures and password storage

• Think of it as a fingerprint for data

Message-digest

• A message-digest is the output (the hash value) produced when a message is processed through a hash function

• It is:

  • A fixed-length summary of the original message

  • Unique to the message (ideally – collisions are rare)

  • Used to check whether a message has been altered

• Think of it as the unique ID or checksum of a message

How is a digital certificate acquired?

1. Leila wants to be able to sign documents digitally

2. She generates a key pair – one private key and one public key

3. Leila sends a Certificate Signing Request (CSR) to a Certificate Authority (CA)

   1. This includes her public key and identity details

4. The CA verifies Leila’s identity using documents or other checks

5. If approved, the CA digitally signs a certificate and sends it back to Leila

   1. This certificate contains Leila’s public key, identity, and the CA’s signature

How is a digital certificate used to produce a digital signature?

1. Leila writes a message she wants to send to Jonas

2. She applies a hash function to the message to create a message-digest

3. Leila then encrypts the message-digest using her private key

   1. This becomes her digital signature

4. She sends Jonas:

   • The original message

   • Her digital signature

   • Her digital certificate

5. Jonas:

   • Uses Leila’s public key (from the certificate) to verify the digital signature

   • Uses the CA’s public key to verify that the certificate is genuine and hasn’t been forged

Summary