Week 1 - Substitution Cyphers

Question 1

What are the two main techniques used in traditional cyphers?

Answer 1

1. Substitution - Replacing plaintext with ciphertext
2. Permutation (rearranging text of plaintext)

Question 2

What is a Caesar cipher?

Answer 2

Where letters are shifted left or right by a number

e.g.

A B C key = 3

A B C = D E F

Question 3

What is the Vigenère cipher?

Answer 3

1. Pick a keyword = KEY
2. message = “HELLO”
3. Repeat keyword to match message
   KEYKE
   HELLO
4. shift letters by number of places of corresponding letters
   7 10 17
   H + K = R
   E + E = I
   …
5. Result = RIJVS

Question 4

What is Permutation?

Answer 4

Rearranges the order of text.

e.g

take every 2nd letter and move to bottom

HELLO WORLD

top :H L O O L
bottom: E L W R D
result = HLOOLELWRD