Show notes

Why cryptography?

  • Confidentiality
  • Data integrity
  • Authentication of a person or message
  • Nonrepudiation

What is a cipher

cipher is an algorithm for performing encryption or decryption (

Quick history

  • transposition/shift ciphers
    • plaintext (which are commonly characters or groups of characters) are shifted according to a regular system
    • (X + K) mod 26 = Y
    • Rot13
  • substitution ciphers
  • Caesar cipher
  • Steganography
    • Hidden messages
    • An early example, from Greece, was a message tattooed on a slave’s shaved head and concealed under the regrown hair


  • Relatively small problem space == brute-forceable
  • Statistical analysis
    • Frequency analysis
    • Homophonic substitution tries to reduce that by letters are mapped together. One letter can map to multiple characters. Can use n-gram analysis against that.
  • Essentially all ciphers remained vulnerable to cryptanalysis using the frequency analysis technique until the development of the polyalphabetic cipher. 14th century
    • Vigenère cipher used multiple Caesar ciphers in sequence with different shift values.
    • In 1863, Friedrich Kasiski was the first to publish a successful general attack on the Vigenère cipher
    • Enigma from WWII was also using a polyalphabetic cipher



One key is used to encrypt and decrypt. Analogous to using a key with a lock. All primitive ciphers are symmetric. Downside is you need to trust the person at the other end, and everyone managing N keys for each person they interact with = N^2 keys

XOR (One-Time-Pad)

With all the perfect conditions, XOR is a really strong solutions. It actually provides provably perfect secrecy (ciphertext is no different from total randomness) if key is random, used only once as is as long as the message.