Project 02: Substitution Cipher

Outline

  1. Substitution Ciphers
  2. Project Demo
  3. Getting Started

Substitution Ciphers

Secrecy

Since ancient times, people have relied on secret storing and transmission of sensitive information:

  • Military secrets
    • coordination
    • battle plans
  • Secret diary
  • Secure payments (e.g., debit/credit card transactions)
  • …

Setup

  • start with cleartext message
    • message that contains sensitive info
  • want to generate a ciphertext
    • obscures sensitive info in cleartext
    • cleartext can be recovered from ciphertext
      • but only if you know how!
    • key is secret that allows one to recover cleartext
      • key might be e.g., password

Ciphers and Keys

A cipher is a method of generating ciphertext from cleartext and vice versa

More formally, a cipher has two methods:

  1. encrypt : (cleartext, key) -> ciphertext
  2. decrypt : (ciphertext, key) -> cleartext

We require

  • if cleartext encrypted and decrypted with same key, get the same message back

We hope

  • without key, it is hard recover cleartext from ciphertext

Caesar Cipher

Idea: map each letter to another letter

  • use a single (cylic) shift $k$ (key)

Example: a cyclic shift of $k = +3$:

  • to encrypt: follow arrow from each cleartext letter
  • to decrypt: reverse direction

Encrypt Example

Encrypt RETREAT

Decrypt Example

Decrypt DWWDFN

How Secure Caesar?

A More General Cipher

The substitution cipher

  • choose an arbitrary one-to-one map between alphabets
  • encryption/decryption the same as with Caesar

Substitution Example

Encrypt RETREAT

Project 02

Implement a substitution cipher!

Two differences with previous example:

  1. key should be a long value
    • create Random object with long key as seed
    • use Random to create substitution table
    • same key $\implies$ same table
    • use table to encrypt or decrypt
  2. alphabet consists of all 255 possible char values
    • not just chars A through Z

Program Demo

Today