Lecture 2

Question 1

What makes a cipher “Perfectly Secure”?

Answer 1

When the distribution of characters/letters does not change when different plaintexts are used.

That is, when a ciphertext does not have any possibility of revealing further information about the plaintext.

Question 2

What is the “One-Time Pad”?

Answer 2

The one-time pad (OTP) is an encryption method that relies on a unique, random key as long as the plaintext, combining characters using XOR to produce ciphertext, ensuring perfect secrecy when keys are never reused.

Question 3

What are the advantages of the “One-Time Pad”?

Answer 3

The ciphertext will not reveal any information about the plaintext.

Perfectley secure when used correctly.

Computationally cheap.

Question 4

What are the disadvantages of the “One-Time Pad”?

Answer 4

1. The key always has to be as long as the plaintext.
2. The key has to be truly random.
3. Insecure against the “two-time pad” (if the same key is used twice)