Encryption / Ciphers

• An attacker can look at the ciphertext and see which letter appears the most, using this to try and infer the key.
• Letter frequency attack assumes the plaintext was written in English, an attacker can use the relative frequencies of letters to infer the key.
• Plaintext is the English text that we can understand, and ciphertext is the text after it has been encrypted.
• The caesar cipher uses a key to translate the plaintext into cipher text.
• If the key in the caesar cipher was 3, then the letter A would be D, as D is 3 letters after A.
• Solutions to strengthen the Caesar cipher include increasing the number of possible keys to make a brute force attack impractical, applying a random shift to each letter/character to prevent letter frequency analysis.
• Key weaknesses of the Caesar cipher include a small number of possible keys, the same shift applied to each character, and the likelihood of using the same shift for each message.
• To decrypt ciphertext that has been encrypted using a caesar cipher, we would go 3 letters back from that letter.
• D would become A if the key in the caesar cipher was 3.
• Assuming that the attacker does not know the key, they could use a brute force approach, where all possible keys are tried on the ciphertext until the plaintext is discovered.
• What are the 3 rules for the one-time pad?

  > It must be truly random

  > Greater than or equal in length to the plaintext

  > Only ever used once
• How do you encode with a Vernam cipher?

  > Convert the message into 8-bit binary

  > Convert the key into 8-bit binary

  > Do XOR on the message and the key

  > The output is the encrypted message
• How do you decode a Vernam cipher?

  > Convert the encrypted message into 8-bit binary

  > Convert the key into 8-bit binary

  > Do XOR on the encrypted message and the key
• The Vernam cipher has perfect security as long as what is true?

  > The key is never reused

  > The key is disposed of securely

  > The key is truly random

  > The key is transferred securely
• What is meant by computational security with ciphers?

  > Ciphers other than the Vernam cipher are based on algorithms and computers

  > Therefore, given enough ciphertext and time, any cipher apart from the Vernam cipher can be broken