1. Security and Encryption

2. What is Encryption?
• Information is often stored and transmitted in a
file
• A plaintext file uses a common encoding
format such as ASCII or Unicode to represent
the characters of the file.
• Encryption is a translation of a file into a format
that hides the content from selected parties; a
secret code; ciphertext.

3. Uses of Encryption
• Privacy/Secrecy • Confidentiality
• – email messages
• – personnel records
• – wireless phones
• – medical records
• Security
• – passwords • – transaction
• – wireless networks records
• Intellectual • Authentication
Property • – digital signatures
• – copy protection • – login

4. What Is Cryptography?
• Cryptography -- from the Greek for “secret
writing” -- is the mathematical “scrambling” of
data so that only someone with the necessary
key can “unscramble” it.
• Cryptography allows secure transmission of
private information over insecure channels (for
example packet-switched networks).
• Cryptography also allows secure storage of
sensitive data on any computer.

5. Technology and Risk
• Technological advances created new risks
▫ electronic communication makes information
more easily available to more parties
▫ long-term storage of sensitive data requires
stronger encryption techniques
• Some technologies actually reduce risk
▫ fiber optic cable
▫ strong encryption

6. German Enigma Cipher Machine
•In 1918, Arthur Scherbius filed for a
patent for Enigma Cipher Machine
and offered it to the German Navy.
• In 1926, German navy begins using
Enigma Machine, lightly modified
from a commercial model.
•In 1930, German armed forces
introduced a
significantly modified military model.

7. Simple Cryptography

8. Two basic principles
• Substitution
• THIS IS A SECRET (key n=3)
• WKLV LV D VHFUHW
• Transposition
• THIS IS A SECRET (key 4213)
• IHSTSI S EAERTC

9. Caesar Cipher

10. Rotating Key Cipher

11. Goals of Cipher:
• Diffussion:
▫ 􀂉 Small change in plaintext, changes lots of
ciphertext
▫ 􀂉 Statistical properties of plaintext hidden in
ciphertext
• 􀂉 Confusion:
▫ 􀂉 Statistical relationship between key and
ciphertext as complex as possible
▫ 􀂉 So, need to design functions that produce
output that is diffuse and confused

12. General Principles
• Longer keys make better ciphers
• 􀂉 Random keys make better ciphers
• 􀂉 Good ciphers produce “random” ciphertext
• 􀂉 Best keys are used once and thrown away

13. Public Key Cryptography
• Your turn…..
▫ What is it?
▫ How does it work?
▫ Include an analogy
▫ Any weaknesses?
▫ If you come across digital signatures, what are
they?
▫ Use MS Word