Introduction to Cryptography

Unit I: Introduction to Cyber Security & Cryptography

Lecture 7: The Science of Secret Communication

Course: Cyber Security (4353204) | Semester V | Diploma ICT | Author: Milav Dabgar

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What is Cryptography?

🔐 Definition

Cryptography is the science of protecting information by transforming it into an unreadable format for unauthorized users, while allowing authorized users to access the original information.

🎯 Core Objectives

Confidentiality - Keep information secret
Integrity - Ensure data hasn't been altered
Authentication - Verify identity of sender
Non-repudiation - Prevent denial of actions

📚 Etymology

Crypto (Greek) = Hidden, Secret
Graphy (Greek) = Writing
Cryptography = Secret Writing

🔄 Cryptographic Process

graph LR
    A[Plaintext] --> B[Encryption Algorithm]
    B --> C[Ciphertext]
    C --> D[Decryption Algorithm]
    D --> E[Plaintext]
    
    F[Key] --> B
    G[Key] --> D
    
    style A fill:#e8f5e8
    style C fill:#fff3e0
    style E fill:#e8f5e8
    style F fill:#f3e5f5
    style G fill:#f3e5f5

🔑 Key Components

Plaintext - Original readable message
Ciphertext - Encrypted unreadable message
Algorithm - Mathematical process
Key - Secret parameter that controls encryption
Keyspace - Set of all possible keys

Course: Cyber Security (4353204) | Unit I | Lecture 7 | Author: Milav Dabgar

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Historical Cryptography

🏛️ Ancient Cryptography

Caesar Cipher (50 BC)

Plaintext:  HELLO WORLD
Key:        3 (shift by 3)
Ciphertext: KHOOR ZRUOG

A B C D E F G H I J K L M
↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓
D E F G H I J K L M N O P

Atbash Cipher

Hebrew origin - reverse alphabet
A↔Z, B↔Y, C↔X...
Used in Biblical texts

Scytale (Ancient Greece)

Physical device - rod with leather strip
Transposition cipher
Military communications

🕰️ Evolution Timeline

Middle Ages (500-1500 AD)

Polyalphabetic ciphers
Vigenère cipher (1553)
Frequency analysis attacks

Renaissance (1400-1600)

Diplomatic cryptography
Cipher wheels and devices
Code breaking emergence

Modern Era (1900-1950)

Mechanical devices (Enigma)
World War cryptography
Breaking Enigma - Turing's work

Computer Age (1950+)

DES (1976) - Data Encryption Standard
Public key cryptography (1976)
AES (2001) - Advanced Encryption Standard

Course: Cyber Security (4353204) | Unit I | Lecture 7 | Author: Milav Dabgar

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Types of Cryptographic Systems

🔑 Symmetric Cryptography

🎯 Characteristics

Same key for encryption and decryption
Fast and efficient
Shared secret required
Key distribution problem

📊 Process Flow

graph LR
    A[Alice] --> B[Message + Shared Key]
    B --> C[Encryption]
    C --> D[Ciphertext]
    D --> E[Network/Storage]
    E --> F[Ciphertext]
    F --> G[Decryption]
    G --> H[Message + Shared Key]
    H --> I[Bob]
    
    style A fill:#e3f2fd
    style I fill:#e3f2fd
    style C fill:#f3e5f5
    style G fill:#f3e5f5

🔧 Common Algorithms

AES (Advanced Encryption Standard)
3DES (Triple Data Encryption Standard)
ChaCha20 (Stream cipher)
Blowfish (Block cipher)

🔐 Asymmetric Cryptography

🎯 Characteristics

Different keys (public/private pair)
Slower than symmetric
No shared secret needed
Solves key distribution problem

📊 Process Flow

graph LR
    A[Alice] --> B[Message + Bob's Public Key]
    B --> C[Encryption]
    C --> D[Ciphertext]
    D --> E[Network]
    E --> F[Ciphertext]
    F --> G[Decryption]
    G --> H[Message + Bob's Private Key]
    H --> I[Bob]
    
    style A fill:#e3f2fd
    style I fill:#e3f2fd
    style C fill:#e8f5e8
    style G fill:#e8f5e8

🔧 Common Algorithms

RSA (Rivest-Shamir-Adleman)
ECC (Elliptic Curve Cryptography)
DSA (Digital Signature Algorithm)
Diffie-Hellman (Key exchange)

Course: Cyber Security (4353204) | Unit I | Lecture 7 | Author: Milav Dabgar

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Symmetric Encryption: AES Algorithm

🏆 AES Overview

📋 Specifications

Block size: 128 bits
Key sizes: 128, 192, 256 bits
Rounds: 10, 12, 14 (based on key size)
Standard: NIST FIPS 197

🔄 AES Operations

SubBytes - Byte substitution using S-box
ShiftRows - Cyclic shift of rows
MixColumns - Linear transformation
AddRoundKey - XOR with round key

📊 AES Structure

Round 1-9:  SubBytes → ShiftRows → MixColumns → AddRoundKey
Round 10:   SubBytes → ShiftRows → AddRoundKey

🔧 AES Implementation Example

from cryptography.fernet import Fernet
from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
from cryptography.hazmat.backends import default_backend
import os

# AES encryption example
def aes_encrypt(plaintext, key):
    # Generate random IV
    iv = os.urandom(16)
    
    # Create cipher object
    cipher = Cipher(
        algorithms.AES(key),
        modes.CBC(iv),
        backend=default_backend()
    )
    
    encryptor = cipher.encryptor()
    
    # Pad plaintext to block size
    padded_text = pad(plaintext.encode(), 16)
    
    # Encrypt
    ciphertext = encryptor.update(padded_text) + encryptor.finalize()
    
    return iv + ciphertext

# Key generation
key = os.urandom(32)  # 256-bit key

🛡️ Security Features

Proven security through extensive analysis
Resistant to known attacks
Hardware optimization available
Government approved (FIPS 140-2)

Course: Cyber Security (4353204) | Unit I | Lecture 7 | Author: Milav Dabgar

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Asymmetric Encryption: RSA Algorithm

🔢 RSA Mathematical Foundation

🎯 Key Generation Process

Choose two large prime numbers p and q
Compute n = p × q (modulus)
Compute φ(n) = (p-1)(q-1)
Choose e such that gcd(e, φ(n)) = 1
Compute d such that e × d ≡ 1 (mod φ(n))

🔑 Keys

Public Key: (n, e)
Private Key: (n, d)

📊 RSA Operations

Encryption: C = M^e mod n
Decryption: M = C^d mod n

Where:
- M = Message (plaintext)
- C = Ciphertext
- e = Public exponent
- d = Private exponent
- n = Modulus

🔒 RSA Security & Applications

🛡️ Security Basis

Integer factorization problem
Large prime factorization difficulty
Computational complexity theory

⚡ Performance Considerations

Slow compared to symmetric encryption
Key size affects performance (1024, 2048, 4096 bits)
Hybrid cryptography common approach

🔧 RSA Applications

Digital signatures
Key exchange protocols
Certificate authorities
Secure communications (TLS/SSL)

📈 Hybrid Cryptography Example

1. Generate random AES key
2. Encrypt data with AES (fast)
3. Encrypt AES key with RSA (secure)
4. Send both encrypted data and encrypted key

Course: Cyber Security (4353204) | Unit I | Lecture 7 | Author: Milav Dabgar

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Digital Signatures

✍️ Digital Signature Concept

🎯 Purpose

Authentication - Verify sender identity
Integrity - Detect message tampering
Non-repudiation - Prevent denial of signing

🔄 Signature Process

graph TB
    A[Message] --> B[Hash Function]
    B --> C[Message Digest]
    C --> D[Sign with Private Key]
    D --> E[Digital Signature]
    
    F[Message + Signature] --> G[Hash Function]
    G --> H[Message Digest]
    E --> I[Verify with Public Key]
    I --> J[Original Digest]
    H --> K{Compare}
    J --> K
    K --> L[Valid/Invalid]
    
    style A fill:#e8f5e8
    style E fill:#f3e5f5
    style L fill:#fff3e0

🔧 Digital Signature Implementation

📋 Common Algorithms

RSA signatures
DSA (Digital Signature Algorithm)
ECDSA (Elliptic Curve DSA)
EdDSA (Edwards-curve DSA)

💻 Example: RSA Signature

from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives.asymmetric import rsa, padding
from cryptography.hazmat.primitives.serialization import Encoding, PublicFormat

# Generate key pair
private_key = rsa.generate_private_key(
    public_exponent=65537,
    key_size=2048
)
public_key = private_key.public_key()

# Sign message
message = b"Important document content"
signature = private_key.sign(
    message,
    padding.PSS(
        mgf=padding.MGF1(hashes.SHA256()),
        salt_length=padding.PSS.MAX_LENGTH
    ),
    hashes.SHA256()
)

# Verify signature
try:
    public_key.verify(signature, message, ...)
    print("Signature valid")
except:
    print("Signature invalid")

Course: Cyber Security (4353204) | Unit I | Lecture 7 | Author: Milav Dabgar

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Key Management

🗝️ Key Management Lifecycle

🔄 Key Lifecycle Phases

Generation - Create cryptographic keys
Distribution - Securely share keys
Storage - Protect keys at rest
Usage - Apply keys for cryptographic operations
Rotation - Replace keys periodically
Revocation - Invalidate compromised keys
Destruction - Securely delete old keys

⚡ Key Generation Requirements

True randomness (entropy)
Sufficient length
Proper algorithms
Secure random generators

🔒 Key Storage Options

Hardware Security Modules (HSM)
Key management systems
Secure software storage
Cloud key management

🛡️ Key Management Best Practices

📋 Security Principles

Separation of duties in key handling
Least privilege access
Key escrow for recovery
Audit trails for key operations
Regular key rotation

🔧 Key Distribution Methods

Public Key Infrastructure (PKI)
Key exchange protocols
Symmetric key pre-distribution
Key derivation functions

📊 Key Management Challenges

Scalability - Managing thousands of keys
Compliance - Meeting regulatory requirements
Integration - Working with existing systems
Cost - Hardware and operational expenses
Availability - Ensuring key access when needed

💡 Best Practice Example

Key Management Policy:
  Generation: FIPS 140-2 Level 3 HSM
  Storage: Encrypted key vault with RBAC
  Rotation: Every 90 days for signing keys
  Backup: Encrypted offsite storage
  Destruction: Crypto-shredding approved methods

Course: Cyber Security (4353204) | Unit I | Lecture 7 | Author: Milav Dabgar

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Cryptographic Applications

🌐 Modern Applications

💳 E-Commerce Security

Payment card encryption (PCI DSS)
Transaction integrity
Customer data protection
Secure communications (TLS)

📱 Mobile Security

Device encryption (full disk)
App data protection
Secure messaging
Mobile payments (Apple Pay, Google Pay)

☁️ Cloud Security

Data at rest encryption
Data in transit protection
Key management as a service
Homomorphic encryption (future)

🏦 Financial Services

Banking transactions
ATM communications
Trading systems
Regulatory compliance

🔮 Emerging Cryptography

🧮 Quantum Cryptography

Quantum key distribution (QKD)
Post-quantum cryptography
Quantum-resistant algorithms
NIST standardization process

🔐 Advanced Techniques

Zero-knowledge proofs
Homomorphic encryption
Multi-party computation
Blockchain cryptography

🛡️ Cryptography Challenges

Quantum computing threat
Implementation vulnerabilities
Side-channel attacks
Performance vs. security trade-offs

📈 Future Trends

Quantum-safe migration
Lightweight cryptography for IoT
Privacy-preserving technologies
Automated key management

Course: Cyber Security (4353204) | Unit I | Lecture 7 | Author: Milav Dabgar

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Practical Exercise: Cryptographic Analysis

🎯 Individual Activity (20 minutes)

Scenario: Secure Communication System Design

You're designing a secure communication system for a healthcare organization that needs to:

Exchange patient records between hospitals
Store sensitive medical data
Authenticate healthcare providers
Ensure compliance with HIPAA regulations
Support mobile devices for doctors

Task: Design Cryptographic Solution

Address these requirements:

Data Protection:
- What encryption algorithms would you use?
- How would you protect data at rest vs. in transit?
- What key sizes are appropriate?
Authentication:
- How would you verify healthcare provider identities?
- What digital signature approach would you use?
- How would you handle user certificates?
Key Management:
- How would you generate and distribute keys?
- What key rotation schedule would you implement?
- How would you handle key recovery?
Implementation:
- Symmetric vs. asymmetric encryption usage
- Performance considerations
- Mobile device constraints

Deliverables:

Cryptographic architecture diagram
Algorithm selection justification
Key management plan
Security analysis

Course: Cyber Security (4353204) | Unit I | Lecture 7 | Author: Milav Dabgar

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Cryptography Best Practices

✅ Do's

🔒 Algorithm Selection

Use proven algorithms (AES, RSA, ECC)
Follow current standards (NIST, FIPS)
Adequate key sizes (AES-256, RSA-2048+)
Regular algorithm reviews

🔑 Key Management

Strong key generation
Secure key storage
Regular key rotation
Proper key disposal

🛡️ Implementation

Use established libraries
Follow secure coding practices
Regular security updates
Proper error handling

❌ Don'ts

🚫 Common Mistakes

Don't create custom algorithms
Don't use deprecated algorithms (MD5, SHA-1, DES)
Don't hardcode keys in code
Don't ignore side-channel attacks
Don't implement crypto from scratch

⚠️ Security Anti-patterns

Insufficient key lengths
Poor random number generation
Improper padding schemes
Inadequate key storage
Missing integrity checks

💡 Success Factors

Security by design
Regular security audits
Compliance monitoring
Incident response planning
Continuous learning

Course: Cyber Security (4353204) | Unit I | Lecture 7 | Author: Milav Dabgar

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Next Lecture Preview

🔜 Lecture 8: Hash Algorithms

🎯 Focus Topics:

Hash function properties and applications
SHA family algorithms (SHA-1, SHA-2, SHA-3)
Message authentication codes (MAC)
Digital fingerprints and integrity checking
Hash-based applications
Collision attacks and resistance

📝 Preparation Tasks:

Review hash function mathematical properties
Research recent hash algorithm developments
Think about data integrity use cases
Consider hash function security requirements

🎓 Key Takeaways Today

Cryptography Fundamentals

Cryptography is essential for modern security
Symmetric encryption is fast and efficient
Asymmetric encryption solves key distribution
Digital signatures provide authentication and non-repudiation

Critical Concepts

Key management is crucial for security
Algorithm selection impacts overall security
Implementation quality matters as much as algorithms
Regular updates and rotation are essential

Course: Cyber Security (4353204) | Unit I | Lecture 7 | Author: Milav Dabgar

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Questions & Discussion

🤔 Discussion Points:

What are the main challenges in implementing cryptography?
How do you balance security and performance in encryption?
What role will quantum computing play in future cryptography?

💡 Exercise Review

Share your cryptographic system designs for the healthcare scenario

Course: Cyber Security (4353204) | Unit I | Lecture 7 | Author: Milav Dabgar

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Thank You!

Next Lecture: Hash Algorithms

Digital Fingerprints and Data Integrity

Cyber Security (4353204) - Lecture 7 Complete

The art of keeping secrets! 🔐✨

Course: Cyber Security (4353204) | Unit I | Lecture 7 | Author: Milav Dabgar