Cryptography

1
Cryptography

• Are your communications secure?

2

• Private Key Encryption (Synchronous)
• Public Key Encryption (Asynchronous)

3
Private key Substitution cipher

• Message "Hello"
• Apply key
• A - gt D
• B - gt E
• C - gt F
• . . .
• X - gt A
• Y - gt B
• Z - gt C
• ? ciphertext "Khoor"

4
Decrypt
Send ciphertext Apply same key
A - gt D B - gt E
C - gt F
. . . X - gt A
Y - gt B Z
- gt C ? Message Hello This is usually done
on a computer on bits
5
Difficulty using the private key methods

• Need to secretly bring key to your partner
• Must make a key-pair for everyone you need to
  communicate with
• Why?

6
Public Key encryption

• Imagine a safe with two one-way keys after a key
  locks the safe it cannot unlock it.
• -but the its partner key can unlock it

7
Public Key Method cont.

• Call one key private and one public
• Give public to all your friends
• Everyone can now encrypt messages and send them
  to you. You can receive messages.
• Why does this work?
• What about you sending messages?

8
Public method slow

• Public Key method raises the message and
  ciphertext to large powers which is 10,000 times
  slower than encrypting using the simpler
  single-key method.

9
PGP

• At the sending end
• Compresses the message
• Creates a session key that is used only once
  during this session. Created from randomly
  selected mouse movements and keystrokes.
• Session key is used to conventionally (single key
  Synchronous encryption) encrypt the message.
• The receiver's public key is used to encrypt the
  session key.
• The encrypted message and encrypted session key
  are sent to the receiver.
• At the receiving end
• Receiver uses private key to decrypt session key
• The session key is used to decode the message
  text.
• The text is decompressed.
• Session key is discarded.

10
Session Key

• What is a session Key?
• How is it used in PGP
• How is it used in SSL (secure Socket Layer)?
• Questions?