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

 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
