Introduction to Cryptography

Notes

• We will take a basic look at cryptography
• Starting with 2.3 of the book.
• We have done a little work with hash functions, and may extend that
• But we are really interested in encryption
  • The book says: Cryptography conceals data against unauthorized access
• When sending encrypted messages
  • The sender (S) is an entity that wants to transmit a message
  • The receiver(R) or recipient is the person that message is intended for.
  • An interceptor (O) or intruder is someone who wishes to read/change the message.
• The intruder may wish to
  • Block the message
  • intercept and read the message
  • modify the message
  • create a fake message
• Strong encryption can mitigate or prevent all of these.
• Some terms:
  • A message originates in plaintext
  • the sender will encrypt or encode a plaintext message to produce a ciphertext message.
  • The receiver will decrypt or decode the ciphertext message to recover the plaintext message
  • This will be done with an encryption algorithm
  • This will frequently require one or more encryption keys.
• Notation
  • For a plaintext message M
  • An encryption algorithm E()
  • An decryption algorithm D()
  • Note it is possible that D() == E()
  • A keys K_e, K_d
  • And a ciphertext message C
  • C = E(K_e, M)
  • P = D(K_d, C)
  • M = D(K_d,E(K_e,M))
• If K_d = K_E then the system is called
  • symmetric key
  • or single-key
  • or even secret key.
  • Because the key must be secret.
• On the other hand, if K_d ≠ K_e then the system is called
  • asymmetric key
  • public key
  • Frequently you will hear public-key/private-key because one of the keys can be made public
• If a system does not use a key, it is keyless
• There are more terms, but that will do for now.