Cryptography Lecture 3: Chantilly Academy

1
Public Key Cryptography

• Cryptography Lecture 3 Chantilly Academy
• Poorvi Vora
• Department of Computer Science
• George Washington University

2
Get the key to Amazon

• Suppose you want to buy something from amazon.com
  online
• Pay with your credit card
• You dont want anyone to be able to snoop the
  number on the line, so you encrypt it, using AES
  (hey, its a new secure standard)
• How do you get the key to amazon?

3
Public Key Cryptography

• In the late 70s, a famous paper by Diffie and
  Hellman
• Suppose two keys
• One made public the other kept private
• Anything encrypted with one can be decrypted by
  the other, and with no other key
• It is not possible determine the private key
  through encrypted messages
• Take amazons public key to encrypt your credit
  card number
• amazon will figure the number out by using its
  private key

4
RSA (Rivest Shamir Adleman)

• RSA is the most popular public-key encryption
  scheme
• Developed in mid-70s
• Turns out it was developed two years earlier by a
  British intelligence agent who could not make it
  public.
• This was revealed in the early 2000s

5
Public Key Cryptography for non-repudiation

• Suppose you want an electronic IOU from a friend
• You need the friend to sign it so he cannot
  later deny he agreed to it
• How about if he encrypts it with his private key?
  
• Nobody else has his private key, so nobody else
  could do it
• YOU can decrypt it with his public key
• Instead of encrypting the whole message, he can
  encrypt a message digest

6
Message Digest

• Shortened version of message such that
• it is difficult to get a message, or any part of
  it, back from its digest
• It is difficult to get another message that has
  the same digest
• It is difficult to find two messages with the
  same digest
• Instead of encrypting a big long IOU, your friend
  can encrypt a digest of the IOU
• SHA-1 provides such a digest