From MCA553 (Principles of Cryptography and Cyber Security).
This is one of the most important units for exams.
---
Introduction to Cryptography
Cryptography is the science of protecting information by converting it into a secret form.
Goals of Cryptography
1. Confidentiality
2. Integrity
3. Authentication
4. Non-Repudiation
---
Plaintext and Ciphertext
Plaintext
Original readable message.
Example:
HELLO
Ciphertext
Encrypted unreadable message.
Example:
XKJ92A
Encryption
Converts plaintext into ciphertext.
Decryption
Converts ciphertext back into plaintext.
---
Symmetric Key Cryptography
Uses the same key for encryption and decryption.
Plain Text
↓
Encryption Key
↓
Cipher Text
↓
Decryption Key (Same Key)
↓
Plain Text
Advantages
Fast
Efficient
Suitable for large data
Disadvantages
Key distribution problem
Less secure for communication over open networks
Examples
DES
AES
Triple DES
---
Asymmetric Key Cryptography
Uses two different keys:
1. Public Key
2. Private Key
Public Key → Encrypt
Private Key → Decrypt
Advantages
Better security
Solves key distribution problem
Disadvantages
Slower than symmetric encryption
Examples
RSA
Diffie-Hellman
ECC
---
Difference Between Symmetric and Asymmetric Cryptography
Symmetric Asymmetric
One key Two keys
Faster Slower
Less secure key sharing More secure
DES, AES RSA, ECC
---
Message Authentication
Ensures that the message is genuine and has not been modified.
Methods:
Hash Functions
Digital Signatures
MAC (Message Authentication Code)
---
Public Key Cryptosystem Principles
Requirements:
1. Easy to generate key pair
2. Easy to encrypt
3. Easy to decrypt
4. Difficult to derive private key from public key
5. Difficult to recover plaintext without key
---
Diffie-Hellman Key Exchange
Used for securely sharing a secret key over an insecure network.
Steps
Suppose:
Prime number P = 23
Generator G = 5
Alice chooses:
a = 6
Bob chooses:
b = 15
Alice computes:
A = G^a mod P
Bob computes:
B = G^b mod P
They exchange A and B publicly.
Both calculate:
Secret Key = B^a mod P
and
Secret Key = A^b mod P
Result: Same secret key generated on both sides.
---
RSA Algorithm
Most important topic for exams.
RSA is based on:
> Difficulty of factoring large prime numbers.
---
RSA Key Generation
Step 1
Choose two prime numbers.
p = 3
q = 11
Step 2
Calculate:
n = p × q
n = 33
---
Step 3
Calculate:
φ(n) = (p−1)(q−1)
\phi(n)=(p-1)(q-1)
For this example:
φ(n) = 20
---
Step 4
Choose e such that:
1 < e < φ(n)
Choose:
e = 3
---
Step 5
Find d:
d × e ≡ 1 mod φ(n)
Result:
d = 7
---
Public Key
(e,n)
=
(3,33)
Private Key
(d,n)
=
(7,33)
---
Key Management
Process of:
Creating keys
Distributing keys
Storing keys
Revoking keys
Poor key management can break even strong encryption.
---
Symmetric Cipher Modes
Used to encrypt large amounts of data.
ECB
Electronic Code Book
Simple
Less secure
CBC
Cipher Block Chaining
More secure
Most commonly used
CFB
Cipher Feedback
OFB
Output Feedback
---
Substitution Technique
Replace characters with other characters.
Example:
A → D
B → E
C → F
Used in Caesar Cipher.
---
Transposition Technique
Characters remain the same but positions change.
Example:
HELLO
→
LHEOL
---
Block Cipher
Encrypts data block by block.
Example:
64-bit block
128-bit block
Popular Algorithms:
DES
AES
---
Data Encryption Standard (DES)
Developed by IBM.
Characteristics:
Symmetric algorithm
64-bit block size
56-bit key
Advantages
Fast
Disadvantages
Small key size
Vulnerable to brute force attack
---
Strength of DES
Originally strong.
Today:
Not secure enough
Can be cracked using modern computers
---
Differential Cryptanalysis
Studies differences in ciphertext to discover keys.
Purpose:
Break encryption algorithms
---
Linear Cryptanalysis
Uses linear relationships between plaintext and ciphertext.
Another method used to attack DES.
---
Block Cipher Design Principles
Good block cipher should have:
1. Confusion
2. Diffusion
3. Strong key management
4. Resistance to attacks
---
Important Exam Questions
Short Questions
1. Define Cryptography.
2. Difference between Symmetric and Asymmetric Encryption.
3. What is RSA?
4. What is Diffie-Hellman?
5. Define DES.
6. What is Ciphertext?
7. What is Key Management?
8. What is a Block Cipher?
---
Long Questions
1. Explain RSA algorithm with example.
2. Explain Diffie-Hellman key exchange.
3. Compare Symmetric and Asymmetric Cryptography.
4. Explain DES and its strengths.
5. Explain Differential and Linear Cryptanalysis.
6. Discuss block cipher design principles.
---
Quick Revision
Cryptography = Protecting information.
Symmetric = One key.
Asymmetric = Public + Private key.
RSA = Public key cryptography.
Diffie-Hellman = Secure key exchange.
DES = Symmetric block cipher.
Ciphertext = Encrypted message.
Key Management = Handling cryptographic keys.
Next Unit 4 covers AES, Triple DES, RC4, Hash Functions, MAC, and Message Authentication, which is also very important for university exams.
