CS458: Introduction to Information Security
Notes 3: Symmetric Cryptography
Yousef M. Elmehdwi
Department of Computer Science
Illinois Institute of Technology
[email protected]
January 29
th
, 2019
Slides: Modified from Computer Security: Principles and Practice, 4th Edition &
Stephen R. Tate
UNC Greensboro
1 / 90

Outline
Threat Model
Symmetric encryption principles
Data Encryption Standard (DES)
Advanced Encryption Standard (AES)
Cipher block modes of operation
Key distribution
2 / 90

Cryptography: Threat Model
What does the adversary know?
Algorithms? Typical user behavior?
What can the adversary access?
Access communication contents?
What can the adversary do?
Passive or active? Computing power?
3 / 90

Threat Model : Adversary Knowledge - Algorithms
Saltzer and Schroeder Design Principles: Open Design
Much older idea than Saltzer and Schroeder...
Kerckhoff’s Principle (1883)
:
The security of a cryptosystem depends
on the
strength
of the algorithm and the
secrecy
of the key.
Trying to keep algorithms secret (“security through obscurity”) almost
never works.
Bottom Line: Better to use a system that experts have tried (and failed)
to break
4 / 90

Threat Model : Adversary Knowledge - Behavior
Some things an attacker might know:
Language of messages (e.g., English)
Common phrases (email headers, signatures, ...)
Likely keys/pass-phrases (names, birthdays, etc.)
5 / 90

Threat Model : Adversary Access and Power
For now:
Access: Attacker can intercept/modify all communication content
Power: “Probabilistic Polynomial-Time Algorithms”
This is really important if you get into crypto - not so much here
How to model crypto use for confidential communication?
6 / 90

Modern Cryptography
Symmetric cryptography
Public key (asymmetric) cryptography.
We will cover it next
7 / 90

Symmetric Encryption
Also referred to as:
Conventional encryption
Secret-key or single-key encryption
Only alternative before public-key encryption in 1970’s
Still most widely used alternative
Two requirements for secure use:
Need a strong encryption algorithm
Sender and receiver must have obtained copies of the secret key in a secure
fashion and must keep the key secure
Has five ingredients:
Plaintext
Encryption algorithm
Secret key
Ciphertext
Decryption algorithm
8 / 90

Simplified Model of Symmetric Encryption
9 / 90

Symmetric Encryption: Stream Cipher
Encrypts a digital data stream one bit or one byte at a time
One time pad is example; but practical limitations
Typical approach for stream cipher:
Key (
K
) used as input to bit-stream generator algorithm
Algorithm generates cryptographic bit stream (
k
i
) used to encrypt
plaintext
Users share a key; use it to generate keystream
10 / 90

Symmetric Encryption: Block Cipher
Most common type of symmetric cipher
Encrypt a block of plaintext as a whole to produce same sized ciphertext
Properties of a block cipher
Must supply a full block of input bits in order to evaluate
Partial block? Use padding
More than one block?

#### You've reached the end of your free preview.

Want to read all 90 pages?

- Fall '09
- Information Security, Cryptography, Computer Security, Advanced Encryption Standard, Block cipher, Data Encryption Standard