Network Security (CY-503 (B)) - Unit 3 Short Notes

1.0 CRYPTOGRAPHIC FOUNDATIONS

1.1 Symmetric Encryption

Definition: Encryption where the same secret key is used for both encryption and decryption.

1.1.1 Block vs. Stream Ciphers

1.1.3 Modern Block Ciphers: AES (Rijndael)

• Block Size: 128 bits (16 bytes)

• Key Sizes: 128, 192, 256 bits → 10, 12, 14 rounds respectively.

• Core Operations per Round (for 128-bit key):

  1. SubBytes: Non-linear substitution using S-box.

  2. ShiftRows: Cyclic shift of rows in state matrix.

  3. MixColumns: Mixing columns using matrix multiplication in GF(2⁸).

  4. AddRoundKey: XOR state with round key.

• Final Round: Omits MixColumns.

• Key Expansion: Round keys generated from cipher key via RotWord, SubWord, Rcon.

1.1.4 Modes of Operation

[!TIP] Exam Focus: Be prepared to draw the encryption/decryption block diagrams for CBC and CTR modes. Understand why ECB is insecure.

1.1.5 RC4 Stream Cipher

• Key Scheduling Algorithm (KSA): Initializes S-box (256 bytes) using secret key K[].

  
  for i=0 to 255: S[i] = i
  
  j = 0
  
  for i=0 to 255:
  
      j = (j + S[i] + K[i mod keylen]) mod 256
  
      swap(S[i], S[j])
  
  

• Pseudo-Random Generation Algorithm (PRGA): Generates keystream bytes.

  
  i = j = 0
  
  while generating:
  
      i = (i + 1) mod 256
  
      j = (j + S[i]) mod 256
  
      swap(S[i], S[j])
  
      t = (S[i] + S[j]) mod 256
  
      K = S[t]
  
      output K
  
  

1.1.6 RC4 Practical Example (5-bit key)

Given 5-bit key K = [1, 2, 3] (keylen=3). S-box size = 2⁵ = 32.

• KSA: Initialize S[0..31] = [0,1,2,...,31]. Iterate i=0..31, compute j, swap.

• PRGA: After KSA, generate first 3 keystream bytes Z1, Z2, Z3 using PRGA steps.

[!TIP] Common Pitfall: In RC4, the first few output bytes of PRGA are biased. Discard first 256 bytes in practice.

1.2 Asymmetric (Public-Key) Cryptography

Core Principle: Uses mathematically linked key pairs: Public Key (shared) for encryption/verification, Private Key (secret) for decryption/signing. Based on one-way functions (easy to compute, hard to invert).

1.2.2 RSA Algorithm

• Key Generation:

  1. Choose large primes p, q.

  2. Compute n = p * q, φ(n) = (p-1)(q-1).

  3. Choose e such that 1 < e < φ(n) and gcd(e, φ(n)) = 1.

  4. Compute d = e⁻¹ mod φ(n).

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

• Encryption: C = Pᵉ mod n

• Decryption: P = Cᵈ mod n

Worked Example (p=3, q=11):

1. n = 3 * 11 = 33

2. φ(n) = (3-1)*(11-1) = 2*10 = 20

3. Choose e=7 (gcd(7,20)=1).

4. d = 7⁻¹ mod 20 = 3 (since 7*3=21 ≡ 1 mod 20).

5. Encrypt P=2: C = 2⁷ mod 33 = 128 mod 33 = 29.

6. Decrypt C=29: P = 29³ mod 33 = 24389 mod 33 = 2.

[!TIP] Exam Must-Know: Derive d using Extended Euclidean Algorithm. Show all modular arithmetic steps.

1.3 Cryptographic Hash Functions & MACs

1.3.1 Properties of Hash Functions H(m):

• Pre-image Resistance: Given h, hard to find m such that H(m)=h.

• Second Pre-image Resistance: Given m1, hard to find m2≠m1 with H(m1)=H(m2).

• Collision Resistance: Hard to find any pair (m1, m2) with H(m1)=H(m2).

1.3.3 Message Authentication Codes (MACs)

• Definition: Short tag generated from message M and secret key K: T = MAC_K(M).

• Purpose: Provides integrity and authentication (only parties with K can generate/verify valid T).

• Construction: Can be based on block ciphers (CBC-MAC) or hash functions (HMAC).

1.3.4 HMAC Algorithm

Construction: HMAC_K(M) = H((K⁺ ⊕ opad) || H((K⁺ ⊕ ipad) || M))

• K⁺: Key padded to block size.

• ipad = 0x36, opad = 0x5C.

• Security: Proven secure if underlying hash is secure.

1.3.5 Secure Message Authentication Example

Using SHA-256 as H:

1. Sender: Compute T = HMAC_K(M). Send (M, T).

2. Receiver: Recompute T' = HMAC_K(M). Accept if T' == T.

[!TIP] Key Distinction: Hash function → integrity only (no key). MAC/HMAC → integrity + authentication (uses secret key).

2.0 SECURITY PROTOCOLS & APPLICATIONS

2.1 Digital Signatures

Working Principle:

1. Sender computes digest = H(message).

2. Sender encrypts digest with their private key: Sig = E_{Priv_Sender}(digest).

3. Receiver decrypts Sig with sender's public key to get digest'.

4. Receiver computes H(message) independently. If equal → signature valid.

Role:

• Authentication: Proves sender's identity.

• Integrity: Any change in message invalidates signature.

• Non-Repudiation: Sender cannot deny sending.

2.2 Email Security: Pretty Good Privacy (PGP)

2.2.1 Operational Model (Cryptographic Components)

• Digital Signature: For authentication & integrity (using sender's private key).

• Symmetric Encryption: For message confidentiality (session key).

• Public-Key Encryption: For secure session key transmission (using recipient's public key).

• Compression: Before encryption.

• Radix-64 Conversion: For email compatibility.

2.2.2 PGP Message Format

DiagramPGP Message Format

+---------------------+

|   Session Key Enc   |  (encrypted with recipient's public key)

+---------------------+

|   Signature Packet  |  (signed with sender's private key)

+---------------------+

|   Compressed Data   |  (actual message, compressed)

+---------------------+

|   Radix-64 Encoded  |

+---------------------+

Note: Actual PGP uses a "signed, then encrypted" or "encrypted, then signed" approach. The diagram shows the logical layering.

2.2.3 Providing Authentication & Confidentiality

1. Confidentiality: Sender generates random session key Ks. Encrypts message with Ks (symmetric cipher, e.g., CAST-128). Encrypts Ks with recipient's public key.

2. Authentication: Sender creates digital signature of message (or its hash) using their private key.

3. Both Ks (encrypted) and signature are attached to message.

2.3 Web & Transport Layer Security: SSL/TLS

2.3.1 SSL Record Protocol Services

• Confidentiality: Using symmetric encryption (session key).

• Integrity: Using MAC (e.g., HMAC).

• Authentication: Optional, using digital certificates.

• Fragmentation & Reassembly: Breaks data into manageable blocks.

2.3.2 SSL Handshake Protocol (Simplified)

ClientHello --> (CipherSuites, Random_C)

ServerHello --> (CipherSuite, Random_S, SessionID)

Certificate --> (Server's Cert)

ServerHelloDone -->

[Client verifies cert, generates PreMasterSecret]

ClientKeyExchange --> (PreMasterSecret encrypted with Server's PubKey)

[Both derive MasterSecret from Random_C, Random_S, PreMasterSecret]

ChangeCipherSpec -->

Finished (encrypted with new keys) -->

ChangeCipherSpec -->

Finished (encrypted with new keys) -->

• Result: Agreed session key, secure channel established.

2.3.3 SSL Connection vs. SSL Session

2.3.4 Contribution to Web Traffic Security

• Provides end-to-end encryption between browser and server.

• Server Authentication via X.509 certificates.

• Optional Client Authentication.

• Underpins HTTPS (https://).

2.4 IP Security (IPSec)

2.4.1 Security Associations (SA)

• Definition: Unidirectional logical connection providing security services.

• Parameters: SPI (Security Parameter Index), IP Destination Address, Security Protocol (AH/ESP), Mode, Keys, etc.

• Direction: One SA per direction. For bidirectional traffic, need two SAs.

2.4.2 Authentication Header (AH)

• Services: Data origin authentication, connectionless integrity, anti-replay (sequence number).

• Does NOT provide confidentiality.

• Transport Mode: AH protects payload of IP packet (TCP/UDP). IP header unchanged except Next Header & HLen.

• Tunnel Mode: AH protects entire original IP packet (header + payload). New IP header added.

2.4.3 Encapsulating Security Payload (ESP)

• Services: Confidentiality (encryption), data origin authentication, integrity, anti-replay.

• Transport Mode: ESP trailer after payload, ESP header before payload. Original IP header untouched.

• Tunnel Mode: Original IP packet encapsulated within ESP payload. New outer IP header.

[!TIP] Exam Distinction: AH authenticates entire packet (including immutable IP header fields). ESP does not necessarily authenticate outer IP header in tunnel mode.

2.5 Wireless Security Protocols

2.5.1 WAP Architecture & Stack

DiagramWAP Stack

Application Layer (WAE - Wireless Application Environment)

Session Layer (WSP - Wireless Session Protocol)

Transaction Layer (WTP - Wireless Transaction Protocol)

Security Layer (WTLS - Wireless Transport Layer Security)

Transport Layer (WDP - Wireless Datagram Protocol)

Bearer Layer (GSM, CDMA, etc.)

• WAP Gateway: Bridges wireless network (WBMP, WML) and Internet (HTML, HTTP). Performs protocol conversion.

2.5.2 WTLS (Wireless Transport Layer Security)

• Based on TLS but optimized for wireless (low bandwidth, high latency).

• Provides: Confidentiality, integrity, authentication (optional).

• Handshake: Similar to TLS but with optimizations (e.g., session resumption, smaller certificates).

2.5.3 Security Issues in WTLS

• Weak Cryptography: Early versions allowed export-grade (40-bit) keys.

• Certificate Handling: Limited processing power on devices.

• End-to-End Gap: WTLS terminated at WAP gateway → plaintext exposed inside carrier network. Requires additional security (e.g., end-to-end TLS) for true privacy.

• Protocol Conversion Vulnerabilities: Gateway is attack target.

2.6 Secure Electronic Transaction (SET)

2.6.1 Main Security Concerns in Online Transactions

• Confidentiality: Cardholder data (account number) must be secret.

• Integrity: Transaction data must not be altered.

• Authentication: Cardholder, merchant, bank must be authenticated.

• Non-Repudiation: None can deny transaction.

2.6.2 SET Participants & Roles

• Cardholder: Customer with payment card.

• Merchant: Seller of goods/services.

• Issuer: Bank that issued cardholder's card.

• Acquirer: Merchant's bank.

• Payment Gateway: Processes payment messages.

• Certification Authority (CA): Issues digital certificates to all participants.

2.6.3 How SET Addresses Concerns

• Dual Signatures: Cardholder signs two messages (order info + payment info) with one signature, linking them without revealing payment info to merchant.

• Confidentiality: Payment info encrypted with merchant's public key & payment gateway's public key (only gateway can read).

• Authentication: All parties use X.509 certificates issued by CA.

• Non-Repudiation: Digital signatures on all critical messages.

2.6.4 SET Transaction Flow (Simplified)

1. Cardholder sends dual-signed PurchaseRequest (OrderInfo + PaymentInfo) to merchant.

2. Merchant verifies cardholder's signature, forwards PaymentInfo (encrypted) to payment gateway.

3. Gateway decrypts, verifies, forwards to issuer for authorization.

4. Issuer sends authorization response back through gateway → merchant.

5. Merchant confirms order to cardholder.

3.0 NETWORK SECURITY DEVICES & MECHANISMS

3.1 Firewalls

3.1.1 Definition & Goals

• Definition: Network security device (hardware/software) that monitors and controls incoming/outgoing network traffic based on predetermined security rules.

• Primary Goals: Access Control, Traffic Filtering, Network Segmentation.

3.1.2 Classification of Firewalls

3.1.4 Merits & Demerits

• Merits: First line of defense, network segmentation, logging/auditing.

• Demerits: Cannot stop internal attacks, encrypted traffic inspection difficult, configuration complexity, single point of failure.

3.2 Intrusion Detection Systems (IDS)

3.2.1 Definition of Intrusion

Attempt to compromise confidentiality, integrity, availability of system/resources. Includes attacks, misuse, policy violations.

3.2.2 Classification

• Network-based IDS (NIDS):

  • Architecture: Sensors placed at strategic network points (SPAN ports).

  • Components: Sensor (packet capture), Analyzer (detection engine), Management console.

  • Operation: Monitors network traffic for suspicious patterns.

  • DiagramNIDS Architecture

    
    [Network] --> [Sensor] --> [Analyzer] --> [Alert/Log]
    
                     |
    
                [Management Console]
    
    

• Host-based IDS (HIDS):

  • Architecture: Agent installed on individual hosts.

  • Components: Log file monitor, file integrity checker, rootkit detector.

  • Operation: Monitors system calls, file changes, log files on host.

3.2.3 Detection Techniques

[!TIP] Exam Link: "Parameter Pattern Matching" is essentially signature-based detection. Mention its reliance on up-to-date signature database.

3.3 Virtual Private Networks (VPN)

3.3.1 Definition & Core Concept

• Definition: Extends private network across public infrastructure (Internet) using tunneling and encryption.

• Core Concept: Creates a "virtual" point-to-point connection, making remote resources appear on local network.

3.3.2 Types of VPNs

3.3.3 VPN Security Architecture

1. Tunneling: Encapsulates original packet within new packet (outer header).

2. Encryption: Protects payload confidentiality (AES, 3DES).

3. Authentication: Pre- Shared Keys (PSK) or digital certificates (X.509).

4. Key Management: IKE (Internet Key Exchange) for SA negotiation.

3.3.4 VPN vs. Trusted Operating Systems

4.0 WIRELESS & MOBILE SECURITY

4.1 Wireless LAN (WLAN) Security

4.1.1 Security Challenges & Threats

• Open Medium: Radio waves accessible to anyone within range.

• Weak/Missing Encryption: Early WEP broken in minutes.

• Authentication Weaknesses: Shared keys, no user identification.

• Rogue Access Points: Unauthorized APs inside corporate network.

• Evil Twin: Malicious AP with same SSID as legitimate one.

• Jamming/DoS: Flooding with noise or deauthentication packets.

• War Driving: Locating and mapping WLANs.

4.1.3 Access Point Security in Public Networks

• Captive Portal: Forces user to authenticate/agree to terms before granting access.

• WPA2/WPA3-Enterprise: Uses 802.1X (EAP) for individual user authentication against RADIUS server.

• Isolation: Client-to-client blocking to prevent lateral movement.

• Monitoring: Rogue AP detection, spectrum analysis.

4.2 WAP Security Deep Dive

4.2.1 WAP Architecture with Security Layers

DiagramWAP Security Architecture

[Mobile Device] --(WTLS)--> [WAP Gateway] --(SSL/TLS)--> [Web Server]

       |                              |

   WTLS Layer                    SSL/TLS Layer

   (Optimized for wireless)      (Standard Internet TLS)

• Gap: Gateway terminates WTLS and initiates new SSL/TLS connection → plaintext inside carrier network.

4.2.2 WTLS in Detail

• Handshake: Similar to TLS but optimized (fewer round trips, smaller certificates).

• Record Protocol: Provides confidentiality (encryption) and integrity (MAC) for WTLS records.

• Supports: Session resumption, client authentication.

4.2.3 Gap Between WTLS and SSL/TLS

• Protocol Conversion: At WAP gateway, WTLS session ends. New SSL/TLS session begins to origin server.

• Security Implication: Data between gateway and origin server is in plaintext unless origin server also uses WTLS (rare). Gateway is a trusted point and potential attack target.

5.0 AUTHENTICATION & ACCESS CONTROL

5.1 Authentication Methods

5.1.1 Biometric Authentication

• Principle: Uses unique physiological/behavioral characteristics.

• Types: Fingerprint, Iris, Retina, Face, Voice, Keystroke dynamics.

• Process: Enrollment (template storage) → Verification (1:1 match) or Identification (1:N match).

• Advantages: Hard to forge/lose, user-friendly.

• Disadvantages: Cost, false rejects/accepts, privacy concerns, template theft risk.

5.1.2 Smart Cards

• Principle: Plastic card with embedded microprocessor/memory chip. Stores cryptographic keys or certificates.

• Usage: Requires card reader + PIN (two-factor: something you have + something you know).

• Applications: ATM cards, SIM cards, corporate ID badges.

5.1.3 Comparison: Biometrics vs. Smart Cards

[!TIP] Exam Angle: Often asked to compare. Emphasize revocability as key disadvantage of biometrics.

6.0 MALWARE & THREAT MANAGEMENT

6.1 Malicious Software (Malware)

6.1.1 Types & Characteristics

6.1.2 Infiltration Methods

• Phishing/Social Engineering: Tricking users into executing malware.

• Drive-by Downloads: Compromised websites exploiting browser vulnerabilities.

• Removable Media: Infected USB drives.

• Network Propagation: Exploiting unpatched services (SMB, RPC).

• Email Attachments/Links: Malicious macros, executable attachments.

6.2 Defense Against Malware

6.2.1 Role of Firewalls

• Packet Filtering: Block known malicious IPs/ports (e.g., block inbound SMB port 445 from Internet).

• Stateful Inspection: Prevent unauthorized incoming connections (only allow responses to internal requests).

• Application-Level Gateway: Filter specific commands (e.g., block HTTP POST to unknown external servers).

• Limitation: Cannot inspect encrypted traffic; cannot stop malware already inside via allowed channels (e.g., web browsing).

6.2.2 Role of IDS

• Signature-Based IDS: Detect known malware network activity (e.g., specific beaconing to C&C server, exploit traffic patterns).

• Anomaly-Based IDS: Detect behavioral anomalies (e.g., host suddenly scanning internal network, unusual outbound traffic volume).

• HIDS: Detect file changes (rootkits), registry modifications, suspicious processes.

• Limitation: Post-infection detection; requires tuning to avoid false positives.

7.0 SPECIALIZED TOPICS & SYNTHESIS

7.1 Web Traffic Security Approaches (Beyond SSL/TLS)

• HTTP Strict Transport Security (HSTS): Forces browsers to use HTTPS only.

• Content Security Policy (CSP): Mitigates XSS by specifying allowed script sources.

• Public Key Pinning (HPKP): Binds site to specific certificate/public key (deprecated but conceptually important).

• Secure Cookies: Secure and HttpOnly flags.

• Web Application Firewalls (WAF): Filters HTTP traffic for application-layer attacks (SQLi, XSS).

7.2 Security Considerations and Threats (General Framework)

• CIA Triad: Confidentiality, Integrity, Availability.

• Threat Modeling: Identify assets, threats, vulnerabilities, and countermeasures.

• Common Threats: Eavesdropping, modification, masquerading, replay, denial-of-service.

• Defense-in-Depth: Layered security (physical, network, host, application, data).

7.3 Parameter Pattern Matching

• Definition: Technique of scanning data (packet payload, log files) for specific byte patterns or sequences that indicate malicious activity.

• Application: Core of signature-based IDS/IPS and deep packet inspection firewalls.

• Process:

  1. Maintain database of signatures (e.g., byte pattern |90 90 90 90| for NOP sled, string "cmd.exe").

  2. Scan incoming/outgoing data streams.

  3. Trigger alert/action upon match.

• Challenges: Evasion via polymorphism/metamorphism (malware changes its pattern), encryption, fragmentation, performance overhead for deep inspection.

[!TIP] Exam Synthesis: Connect parameter pattern matching to signature-based IDS and application-layer firewalls. Mention its limitation against novel/obfuscated attacks.