1. Cryptography Introduction
CS 465
Cryptography Introduction
Last Updated: Sep 7, 2017

2. Cryptography Literal definition means “hidden writing”
Until modern times, cryptography was synonymous with encryption, but the field has expanded
This lecture reviews a high-level description of four cryptographic primitives we will learn about this semester
Symmetric Encryption (AES)
Public-Key Cryptography (RSA)
Secure One-Way Hash (SHA-256)
Message Authentication Code (MAC)

3. What is Encryption?
Transforming information so that its true meaning is hidden
Requires “special knowledge” to retrieve
Modern encryption algorithms use transposition and substitution in complex ways that are hard to reverse
Examples from history that are easy to break
ROT-13 (aka Caesar Cipher) is easy to break, simple substitution cipher
Vigenere cipher – polyalphabetic substitution cipher
Examples of strong encryption
AES
3DES
RC4
RSA

4. Types of Encryption Schemes
Cipher
Classical
Substitution
Steganography
Transposition
Rotor Machines
Modern
Public Key
Secret Key
GOOD DOG
PLLX XLP
PLSX TWF
GOOD DOG
DGOGDOO
Hierarchy & Examples based on:

5. Perfect Encryption Scheme?
One-Time Pad (XOR message with key)
Example*:
Message: ONETIMEPAD
Key: TBFRGFARFM
Ciphertext: IPKLPSFHGQ
The key TBFRGFARFM decrypts the message to ONETIMEPAD
The key POYYAEAAZX decrypts the message to SALMONEGGS
The key BXFGBMTMXM decrypts the message to GREENFLUID
The Russians used pen and pencil one time pad systems, and they were broken because they broke the rule and re-used the pad.
Be suspicious of companies that claim they have an encryption system that is a one time pad.
*From Applied Cryptography (Schneier)

6. Symmetric Encryption Also known as Conventional encryption
Secret-key encryption
Single-key encryption

7. Symmetric Encryption Model
Alice
Bob
Key
Key
Plaintext
Ciphertext
Plaintext
Encryption Algorithm
Decryption Algorithm

8. Use Cases Web browsing using HTTPS
Encrypted chat (WhatsApp and Signal)
Encrypted (PGP)

9. Cryptographic Hash Function
Source:

10. Hashing Use Cases Digital signature
File integrity verification (TripWire)
Password hashing
Rootkit detection

11. Message Authentication Code (MAC)
This is usually the primitive that students are least familiar with, and they struggle with this the most on exams.
Source: Network Security Essentials (Stallings)

12. HMAC Use Cases Web browser message integrity (HTTPS)
Integrity of messages in authentication protocols
Cookie integrity
Web application remote procedure calls

13. Model for Encryption with Public Key Cryptography
Alice
Bob
Bob’s Public Key
Bob’s Private Key
Plaintext
Ciphertext
Plaintext
Encryption Algorithm
Decryption Algorithm

14. Model for Digital Signature with Public Key Cryptography
Alice
Bob
Alice’s Private Key
Alice’s Public Key
Signing Algorithm
Verification Algorithm
I encourage students to learn and understand the high-level diagrams. They are an oversimplification, and we will go deeper into the actual algorithms later in the course. ?
Plaintext
Plaintext

15. Symmetric Encryption

16. Requirements
Strong algorithm (cipher) Attacker is unable to decrypt ciphertext or discover the key even if attacker has samples of ciphertext/plaintext created using the secret key
Fast
Assumption: Sender and receiver must securely obtain and store the secret key

17. Kerckhoffs’ Principle
The security of the symmetric encryption depends on the secrecy of the key, not the secrecy of the algorithm
You should be very suspicious of a company that advertises a super secure new algorithm and they don’t tell you what the algorithm is
Dr. Auguste Kerckhoffs ( )
Dutch linguist and cryptographer

18. Types of Ciphers Block cipher (3DES, AES) Stream cipher (RC4)
Plaintext is broken up into fixed-size blocks
Typical block size (64, 128 bits)
Stream cipher (RC4)
Process plaintext continuously
Usually one byte at a time

19. What can go wrong? Algorithm Key
Relying on the secrecy of the algorithm
Example: Substitution ciphers
Using an algorithm incorrectly
Example: WEP used RC4 incorrectly
Key
Too big
Slow
Storage
Too small
Vulnerable to brute force attack – try all possible keys

20. 123 Big Numbers Cryptography uses REALLY big numbers
1 in 261 odds of winning the lotto and being hit by lightning on the same day
292 atoms in the average human body
2128 possible keys in a 128-bit key
2170 atoms in the planet
2190 atoms in the sun
2233 atoms in the galaxy
2256 possible keys in a 256-bit key

21. Thermodynamic Limitations*
123
Physics: To set or clear a bit requires no less than kT
k is the Boltzman constant (1.38*10-16 erg/ºK)
T is the absolute temperature of the system
Assuming T = 3.2ºK (ambient temperature of universe)
kT = 4.4*10-16 ergs
Annual energy output of the sun 1.21*1041 ergs
Enough to cycle through a 187-bit counter
Build a Dyson sphere around the sun and collect all energy for 32 years
Enough energy to cycle through a 192-bit counter.
Supernova produces in the neighborhood of 1051 ergs
Enough to cycle through a 219-bit counter
Cover this slide briefly. The point is to realize that key sizes are chosen to be impossible to brute force in practice.
*From Applied Cryptography (Schneier)

22. Public Key Cryptography
Terminology
Public Key
Private Key
Digital Signature
Confidentiality
You encrypt with a public key, and you decrypt with a private key
Integrity/Authentication
You sign with a private key, and you verify the signature with the corresponding public key
Examples
Diffie-Hellman
RSA
Elliptic Curve Cryptography (ECC)
Identity-based Encryption (IBE)
For integrity and authentication, I always avoid saying encrypt with the private key and decrypt with the public key to avoid confusion with the confidentiality case.

23. Assignment
Review this slide deck regularly to learn the high level abstractions for these primitives. I’ll expect you to describe them to me on an exam without any notes.
You don’t really know something until you can teach it to someone else
Study the AES NIST spec and watch the flash demo