Encryption, Digital Signatures and Certificates

Overview

Recall questions on symmetric and asymmetric encryption and key exchange, how a digital signature is created and verified, which key does what, and what digital certificates contain and why they are needed.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Explain symmetric encryption.

   Model answer

   In symmetric encryption the same single key is used both to encrypt the plaintext and to decrypt the ciphertext, so both parties must hold the same secret key. It is computationally fast, which makes it suitable for encrypting large volumes of data, but it has a key distribution problem: the key must somehow reach the other party without being intercepted, and every pair of communicating parties needs its own key, so the number of keys required grows very quickly with the number of participants.

2. 2

   Explain asymmetric encryption.

   Model answer

   Asymmetric, or public key, encryption uses a mathematically related pair of keys: a public key, which may be published to anyone, and a private key, which the owner keeps secret and never shares. What one key of the pair encrypts, only the other key can decrypt. Knowing the public key does not let anyone work out the private key in any practical amount of time. This removes the key distribution problem, because the key that must be sent is the one that does not need to be secret, but it is far slower than symmetric encryption.

3. 3

   If Ali wants to send Beth a confidential message using asymmetric encryption, whose key does he use, and which one?

   Model answer

   Ali encrypts the message with Beth's public key. Only Beth's private key can decrypt what her public key has encrypted, and only Beth holds it, so only Beth can read the message. Ali's own keys play no part in keeping this message confidential.

4. 4

   Why can Ali not simply encrypt a confidential message with his own private key?

   Model answer

   Because anything encrypted with his private key can be decrypted with his public key, and his public key is available to everyone. That would provide no confidentiality at all. Encrypting with the private key proves who sent the message; encrypting with the recipient's public key is what keeps it secret.

5. 5

   Explain how asymmetric encryption is used to solve the key exchange problem.

   Model answer

   The two techniques are combined. The sender generates a symmetric key at random for this one session, then encrypts that symmetric key with the recipient's public key and sends it across. Only the recipient's private key can recover it, so the symmetric key arrives secretly even over a channel anyone can read. Both parties now hold the same symmetric key and use fast symmetric encryption for the bulk of the conversation. This gives the security of asymmetric key distribution with the speed of symmetric encryption for the actual data.

6. 6

   What is a digital signature and what does it prove?

   Model answer

   A digital signature is a value attached to a message that proves who sent it and that it has not been altered. It is produced by passing the message through a hashing algorithm to produce a fixed-length digest, then encrypting that digest with the sender's private key. It proves authenticity, because only the holder of that private key could have produced it; integrity, because any change to the message would produce a different digest; and non-repudiation, because the sender cannot later deny having sent a message only their private key could have signed.

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