Encryption

Overview

Recall questions on symmetric and asymmetric encryption, the key distribution problem, which key encrypts, signs and verifies, combining both in secure connections, digital signatures and certificates, and why encryption alone does not guarantee integrity.

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

   What is encryption, and what problem does it solve?

   Model answer

   Encryption is the process of scrambling plaintext into ciphertext using an algorithm and a key, so that anyone intercepting it cannot understand it. Only someone holding the correct key can decrypt the ciphertext back into readable plaintext. It solves the problem that data sent over a network, or stored on a device that might be stolen, can be read by people it was not intended for; encryption means intercepted data is useless to them.

2. 2

   Describe how symmetric encryption works.

   Model answer

   In symmetric encryption a single shared key is used both to encrypt the plaintext and to decrypt the ciphertext. The sender and the recipient must both hold that same secret key. The sender encrypts with it, transmits the ciphertext, and the recipient applies the same key to recover the plaintext.

3. 3

   What is the key distribution problem in symmetric encryption?

   Model answer

   Both parties need the same secret key before they can communicate securely, but sending that key over the same insecure network risks it being intercepted, and if it is intercepted all the encrypted traffic can be read. Handing it over in person is secure but impractical between strangers or across the internet. The problem worsens with scale, because a group of communicators needs a separate key for every pair, so the number of keys to distribute and protect grows very rapidly.

4. 4

   Describe how asymmetric encryption works.

   Model answer

   Asymmetric encryption uses a mathematically related pair of keys: a public key, which the owner publishes freely to anyone, and a private key, which the owner keeps secret and never shares. Data processed with one key of the pair can only be processed back with the other. Knowing the public key does not let an attacker work out the private key in any practical time.

5. 5

   In asymmetric encryption, which key encrypts and which key decrypts when the goal is confidentiality? Explain why that way round.

   Model answer

   The recipient's public key encrypts the message and the recipient's private key decrypts it. It must be that way round because only the recipient holds the private key, so only the recipient can read the message. If the private key encrypted, anyone with the freely published public key could decrypt it and there would be no secrecy at all.

6. 6

   In asymmetric encryption used for authentication or a digital signature, which key signs and which key verifies?

   Model answer

   The sender's private key signs, and the sender's public key verifies. Because only the sender holds the private key, a signature that the sender's public key successfully verifies could only have been produced by that sender, which proves who sent the message. This is the opposite arrangement to confidentiality, where the recipient's public key encrypts and the recipient's private key decrypts.

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