Encryption

Overview

Recall questions on the Caesar and Vernam ciphers, frequency analysis, why the one-time pad alone is unbreakable, and symmetric against asymmetric keys.

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

   Define encryption, and define the terms cipher, plaintext, ciphertext and key.

   Model answer

   Encryption is the process of transforming data so that it cannot be understood by anyone who intercepts it without the means to reverse the transformation. The plaintext is the original readable message; the cipher is the algorithm used to transform it; the key is the secret value that controls the transformation; and the ciphertext is the scrambled output. Decryption is the reverse process, turning ciphertext back into plaintext using the key. The data can still be intercepted — encryption makes it unintelligible, not unobtainable.

2. 2

   Describe how the Caesar cipher works.

   Model answer

   Every letter of the plaintext is replaced by the letter a fixed number of places further along the alphabet, wrapping round from Z back to A. That fixed number of places is the key, and it is the same for the whole message. Decryption shifts every letter back by the same amount.

3. 3

   Encrypt the plaintext HELLO using a Caesar cipher with a shift of 3.

   Model answer

   KHOOR. H becomes K, E becomes H, each L becomes O, and O becomes R — each letter moved three places forward through the alphabet.

4. 4

   Decrypt the ciphertext WKLV, which was encrypted with a Caesar cipher with a shift of 3.

   Model answer

   THIS. Shift each letter three places back: W to T, K to H, L to I and V to S.

5. 5

   Encrypt the plaintext ZEBRA using a Caesar cipher with a shift of 4, taking care with the wrap-round.

   Model answer

   DIFVE. Z sits at the end of the alphabet, so a shift of four carries it four places past the end, so it wraps round to D; E becomes I, B becomes F, R becomes V and A becomes E.

6. 6

   How many possible keys does a Caesar cipher have?

   Model answer

   Only 25 useful ones. There are 26 shifts, but a shift of 0, or equivalently of 26, leaves the message unchanged, so only 25 actually encrypt anything.

Other ways to work this

The same content, four other ways.

What is inside Levo

Six ways to work the same material.