Cryptography Overview

1. Symmetric Ciphers - Overview

Symmetric cipher (or secret-key cipher) uses the same key for both encryption and decryption. It ensures confidentiality as long as the shared key remains secret.

Key Characteristics:

• Single shared secret key
• High encryption/decryption speed
• Suitable for bulk data encryption
• Requires secure key distribution

Basic Process:

1. Encryption: Ciphertext = E(K, Plaintext)
2. Decryption: Plaintext = D(K, Ciphertext)
   (Where K is the secret key)

Types of Symmetric Ciphers:

1. Stream Ciphers: Encrypt data bit-by-bit or byte-by-byte using a keystream (e.g., RC4).
2. Block Ciphers: Encrypt data in fixed-size blocks (e.g., AES, DES, Blowfish).

Advantages:

• Fast and efficient
• Less computationally intensive than asymmetric encryption

Disadvantages:

• Key distribution problem
• Scalability issue (requires n(n−1)/2 keys for n users)
• No inherent authentication or non-repudiation

Examples:

• DES (Data Encryption Standard)
• AES (Advanced Encryption Standard)
• 3DES, Blowfish, RC5

---

2. Public Key Encryption, Digital Signatures

Public Key Encryption (Asymmetric Encryption):
Uses a pair of keys: a public key (shared openly) and a private key (kept secret).

Process:

1. Encryption: Ciphertext = E(Public Key, Plaintext)
2. Decryption: Plaintext = D(Private Key, Ciphertext)

Key Properties:

• Different keys for encryption and decryption
• Eliminates key distribution problem
• Slower than symmetric encryption

Examples:

• RSA (Rivest-Shamir-Adleman)
• ECC (Elliptic Curve Cryptography)
• ElGamal

Advantages:

• Secure key exchange
• Enables authentication and non-repudiation

Disadvantages:

• Slower computation
• Larger key sizes required for equivalent security

2.1 Digital Signatures:

Used to verify authenticity, integrity, and non-repudiation of a message.

Process:

1. Sender generates a hash of the message.
2. Sender encrypts the hash using their private key → Digital Signature.
3. Receiver decrypts the signature using sender’s public key and compares hash values.

Properties:

• Authenticity: Confirms sender’s identity
• Integrity: Detects message tampering
• Non-repudiation: Sender cannot deny authorship

Common Algorithms:

• RSA Digital Signature
• DSA (Digital Signature Algorithm)
• ECDSA (Elliptic Curve DSA)

---

3. Authentication Protocols

Authentication protocols are cryptographic procedures used to verify the identity of communicating parties before data exchange.

Objectives:

• Verify user or entity identity
• Prevent impersonation
• Ensure data integrity and confidentiality

Basic Types:

1. Password-Based Authentication: Uses passwords or PINs; simple but vulnerable to replay and brute-force attacks.
2. Challenge-Response Protocols: Server sends a random challenge; client responds with encrypted/hashed result (e.g., using shared key).
3. Mutual Authentication: Both parties authenticate each other (used in SSL/TLS).
4. Token-Based Authentication: Uses tokens or session keys generated during login (e.g., Kerberos).
5. Certificate-Based Authentication: Relies on digital certificates issued by a trusted CA (Certificate Authority).

Common Protocols:

• Kerberos: Uses tickets and symmetric cryptography for mutual authentication in a distributed network.
• Needham-Schroeder Protocol: Uses nonce-based challenge-response with symmetric keys.
• TLS/SSL Handshake: Uses public-key cryptography for mutual authentication and session key establishment.
• OAuth / OpenID Connect: Used for web-based federated authentication.

Threats:

• Replay attacks
• Man-in-the-Middle (MitM) attacks
• Password guessing and credential theft

Countermeasures:

• Use of nonces/timestamps
• Encrypted channels (TLS/SSL)
• Multi-factor authentication

---

4. System Security

System Security involves protecting computer systems and data from unauthorized access, misuse, modification, or denial of service.

Goals (CIA Triad):

1. Confidentiality: Prevent unauthorized disclosure of information.
2. Integrity: Ensure data accuracy and prevent unauthorized modification.
3. Availability: Ensure systems and data are accessible when needed.

Key Components:

1. Authentication: Verifying user identity.
2. Authorization: Granting access rights based on privileges.
3. Accounting (Auditing): Tracking user actions and system events.
4. Access Control: Mechanisms like DAC (Discretionary), MAC (Mandatory), and RBAC (Role-Based).

Security Mechanisms:

• Encryption: Protects data confidentiality.
• Firewalls: Filter network traffic based on rules.
• Intrusion Detection/Prevention Systems (IDS/IPS): Detect or block malicious activity.
• Antivirus/Antimalware: Protect against malware threats.
• Patching & Updates: Fix known vulnerabilities.
• Backup & Recovery: Protect against data loss.

Common Threats:

• Malware (virus, worm, trojan, ransomware)
• Phishing & social engineering
• Denial of Service (DoS/DDoS)
• Insider threats
• Zero-day exploits

Best Practices:

• Implement least privilege principle
• Regular software updates
• Strong password and MFA policies
• Network segmentation and monitoring
• Incident response planning