17: Advanced Ciphers

Question 1

How does Vigenère cipher work ?

Answer 1

1) We find the distance from key at index i to ‘A’ (eg, 25)
2) We take k=distance (eg k=25)
3) We offset the input text’s letter at index i and offset it by k to the left, ala Caesar (eg A->B)
4) Repeat for next i

Question 2

How does frequency analysis affect Vigenère ?

What weakness does it have ?

Answer 2

It doesn’t break it because it has different values of k everytime.

By finding repeated blocks, we can find the distance and determine the key length possibilities.

Eg, dist=16, then k length can only be 1,2,4,8, or 16.

Question 3

What is Kasiki test for breaking ?

Answer 3

Break cipher into 4 string (for eg). Then use for frequency analysis.

Works if the secret key is short.

Question 4

What happens if the secret key is as long as the input text ?

Answer 4

It is vulnerable to frequency analysis.

As we have to pass the secret key in secret, it makes encryption less practical.

Question 5

What is a one-time pad ?

What’s special about it?

Answer 5

When the secret key is chosen at random, making all replacement letters random.

It is theoretically unbreakable.

Question 6

What are the 2 types of encryption algorithms ?

Answer 6

• Substitution

- Transposition

Question 7

Give 5 examples of transposition ciphers ?

Answer 7

• Zigzag/ Rail fence cipher
• Route cipher
• Columnar transposition
• Double transposition
• Disrupted transposition