Introduction to Cryptography

• This starts on page 86. Section 2.3
  • This section covers mostly theory.
  • The mathematical section is chapter 12.
• Encryption or Cryptography means "secret writing"
  • It has been around for a long time.
  • 
• The authors claim "it might be the strongest defense in the arsenal of computer security protection."
• However "weak or flawed encryption creates only the illusion of protection"
• A fraemework
  • consider sending a secret message.
    • A sender S
    • A recipient R
    • A transmission media T
    • An intruder or inteceptor O
  • The intent is that S sends a mesage to R via T
  • O would like to do something bad like
    • Block the message and keep R from reading it. break availability
    • Intercept the message and read it, break confidentiality
    • Modify the message: break integrity
    • send a fake message: break integrity
• A few terms:
  • Encryption is the process of encoding a message so that the meaning is not obvious
  • Decryption is the reverse of this process, transforming a message back into the normal, original form.
  • We could also use encode, decode, encipher, decipher.
  • The combination forms a cryptosystem
  • The plaintext is the original message.
  • The ciphertext is the encrypted message.
• And the start of notation
  • Let P be the plaintext message
  • Let C be the ciphertext message
  • Let E() be the encryption algorithm
  • Let D() be the decryption algorithm
  • Then:
    • C = E(P)
    • P = D(C)
    • P = D(E(P))
  • A very simple encryption scheme
    • Given a plaintext string P, swap ajacent letters to form C

    • Plaintext:

      Crypttext:

  • Note it is not required that D(x) = P(x) , ie they might not be the same algorithm.
  • Caesar Cypher
    • To encrypt, shift each letter by 3 A->D
    • To decrypt, shift each letter by -3 D->A

    • Input:

      Output: