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: