Intro to Cryptography

• Encryption or Cryptography : secret writing.
  • Have you ever encrypted/decrypted messages?
  • Have you ever written in secret code?
  • Why would you do this?
  • Can you name any secret codes?
• In computer systems we encrypt data
  • To conceal it's value
    • like passwords.
    • Or messages on the internet.
  • To prove identity (authorization)
  • To secure transactions.
• One primary use is in communications.
  • Bob, the sender, wants to send a message to Alice, the recipient.
  • Bob will transmit a message over a medium
    • Say it.
    • Put it on paper and hand it to Alice
    • Put send it over a telegraph wire
    • Broadcast it by radio wave
    • Send it over a network.
  • Each of these systems has rules for transmission.
  • However these rules are well known.
  • Eve would like to mess with Bob's message to Alice.
    • Intercept it and read it.
    • Block it.
    • Modify it.
    • Fake it.
  • In general we would like to maintain the authenticity and security of a message in a insecure environment.
• Some terms
  • Encryption is the process of encoding a message so its meaning is not obvious.
  • Decryption is the reverse process. Transforming an encrypted message back into its normal form.
  • Generally the message is in clear text, or plain text.
  • The encrypted message is cipher text.
  • Other terms : encode, decode, encipher, decipher.
  • A cryptosystem is the system for encryption and decryption.
• We use functional notation to indicate this.
  • C = E(P) means to encode the plaintext message P to produce the ciphertext message C
  • P = D(C) is the opposite.
  • We would like a system where P = D(C(P)).
  • If a key is involved P = D($K_D$, C($K_E$,P))
    • If $K_D$ = $K_E$, this is a symmetric or single key cipher.
    • If $K_D$ ≠ $K_E$, this is an asymmetric cipher.
  • If no key is needed, it is a keyless cipher.
  • Look at the pictures on page 88, 89