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

• Download and run code
• Caesar cipher already implemented
• Use code to decrypt a message
  • secret-message.txt
  • the key is the year of Julius Caesar’s assassination
  • paste the decrypted text here