Biometric Techniques for Security (CY-503 (A)) - Unit 5 Short Notes

UNIT 5: Advanced Security Mechanisms (including Biometrics)

I. Cryptographic Foundations

Symmetric vs Asymmetric Encryption

[!TIP] Exam often asks for a comparison table. Remember: Symmetric solves speed, Asymmetric solves key distribution.

Block Ciphers (AES) & Stream Ciphers (RC4)

• Block Cipher (AES):

  • Operates on fixed-size blocks (128 bits).

  • Uses multiple rounds (10/12/14 for 128/192/256-bit keys) of substitution, permutation, mixing.

  • Modes of Operation (see next section) are required to encrypt data longer than one block.

• Stream Cipher (RC4):

  • Generates a pseudo-random keystream from a secret key.

  • Encrypts by XORing plaintext with keystream byte-by-byte.

  • Vulnerable if keystream reused (e.g., WEP).

  • RC4 Pseudo-Random Generation (KSA & PRGA):

    1. KSA (Key-Scheduling Algorithm): Initialize S-box (0-255), permute using key.

    2. PRGA (Pseudo-Random Generation Algorithm): For each byte i: j = (j + S[i]) mod 256; swap S[i] & S[j]; output K = S[(S[i] + S[j]) mod 256].

    Example (5-bit key, e.g., 10101 = 21): First 3 bytes depend on KSA output. Initial S = [0,1,...,255]. After KSA with key, run PRGA first 3 iterations to get keystream bytes.

Modes of Operation for Block Ciphers

Public Key Cryptography: RSA Algorithm (Example)

Steps:

1. Key Generation:

   • Choose primes p, q. Compute n = p*q, φ(n) = (p-1)(q-1).

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

   • Compute d such that d*e ≡ 1 mod φ(n).

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

2. Encryption: C = M^e mod n

3. Decryption: M = C^d mod n

Example (p=3, q=11):

n = 33, φ(n) = 20. Choose e=7 (gcd(7,20)=1). Find d: 7d ≡ 1 mod 20 → d=3 (since 21 mod 20 = 1).

Encrypt M=2: C = 2^7 mod 33 = 128 mod 33 = 29.

Decrypt C=29: M = 29^3 mod 33 = 24389 mod 33 = 2. ✔️

Hash Functions & SHA-512

• Purpose: Produce fixed-size message digest (hash) from arbitrary input. Properties: Deterministic, Fast, Pre-image resistant, Second pre-image resistant, Collision resistant.

• SHA-512 Steps (Simplified):

  1. Pre-processing: Pad message to multiple of 1024 bits (append 1, zeros, 128-bit length).

  2. Initialize Hash Values (H0-H7): Eight 64-bit constants from fractional parts of sqrt(primes).

  3. Process Message in 1024-bit Blocks:

     • Break block into 16 × 64-bit words (W[0..15]).

     • Extend to 80 words (W[t] = σ1(W[t-2]) + W[t-7] + σ0(W[t-15]) + W[t-16] for t=16..79).

     • Initialize working variables a..h = H0..H7.

     • 80 rounds of compression: T1 = h + Σ1(e) + Ch(e,f,g) + K[t] + W[t]; T2 = Σ0(a) + Maj(a,b,c); update h=g, g=f, f=e, e=d+T1, d=c, c=b, b=a, a=T1+T2.

     • Update hash: H_i = H_i + variable_i.

  4. Output: Concatenate final H0..H7 → 512-bit digest.

• Collision Resistance: Finding any two different inputs M1 ≠ M2 such that Hash(M1) = Hash(M2) should be computationally infeasible. Brute-force requires ~2^(n/2) operations (Birthday Paradox).

Message Authentication Codes (MACs) & HMAC

• MAC: Short tag generated from message + secret key. Provides integrity and authentication.

• HMAC (Hash-based MAC):

  • HMAC(K, M) = Hash((K ⊕ opad) || Hash((K ⊕ ipad) || M))

  • ipad = 0x36 repeated, opad = 0x5C repeated.

  • Why secure? Inner/outer hash prevents extension attacks if underlying hash is weak.

  • Steps:

    1. Pad key K to block size (e.g., 64 bytes for SHA-256) with zeros.

    2. Compute K ⊕ ipad, append message M, hash → inner.

    3. Compute K ⊕ opad, append inner, hash → final HMAC.

II. Authentication Mechanisms

Digital Signatures

• Process:

  1. Signing: Sender computes Sig = Hash(Message)^d_S mod n_S using their private key.

  2. Verification: Receiver computes Hash(Message) and Sig^e_S mod n_S. If equal → valid.

• Crucial Role:

  • Authentication: Proves sender's identity.

  • Non-repudiation: Sender cannot deny sending (only they have private key).

  • Integrity: Any change in message invalidates signature.

  • Used in code signing, contracts, SSL/TLS certificates.

Biometric Authentication

• Types:

  • Physiological: Fingerprint, Iris, Face, DNA, Retina.

  • Behavioral: Voice, Keystroke dynamics, Gait, Signature.

• Working Principle (Generalized):

  1. Enrollment: Capture raw biometric → Feature Extraction (unique template) → Template Storage (database or smart card).

  2. Verification/Identification:

     • Verification (1:1): "Are you X?" Compare live sample with stored template of claimed identity.

     • Identification (1:N): "Who are you?" Compare live sample against all templates in database.

  3. Matching: Compute similarity score between live & stored template. If score > threshold → accept.

• Challenges:

  • False Accept Rate (FAR) & False Reject Rate (FRR) trade-off.

  • Non-revocability: Biometric cannot be changed if compromised.

  • Spoofing/Attacks: Fake fingerprint, voice recording.

  • Noise & Variability: Sensor noise, physiological changes (cut finger, aging).

  • Template Security: Protect stored templates (use cancellable biometrics, encryption).

Smart Cards & Multi-Factor Authentication

• Smart Cards: Physical tokens with embedded chip (stores private keys, certificates). "Something you have". Often combined with PIN ("something you know") for 2FA.

• Biometric + Smart Card: Card stores encrypted biometric template. Live biometric unlocks/decrypts template on-card → enhances security over standalone biometrics.

• Multi-Factor Authentication (MFA): Combines ≥2 independent factors:

  1. Knowledge (password, PIN)

  2. Possession (smart card, OTP token, phone)

  3. Inherence (biometric)

  Example: ATM card (possession) + PIN (knowledge). Future: Phone (possession) + Fingerprint (inherence).

III. Secure Communication Protocols

Pretty Good Privacy (PGP)

• Components & Working:

  • Key Management: Uses Web of Trust (users sign each other's keys) instead of centralized CA. Each user has:

    • Public Key Ring: Others' public keys.

    • Private Key Ring: Own private key (encrypted with symmetric passphrase).

    • Public/Private Key Pair: RSA/DSA/ECC.

  • Encryption for Confidentiality:

    1. Generate random session key (symmetric, e.g., IDEA, AES).

    2. Encrypt message with session key.

    3. Encrypt session key with recipient's public key.

    4. Send both encrypted session key & encrypted message.

  • Signing for Authentication:

    1. Compute hash (SHA-256) of message.

    2. Encrypt hash with sender's private key → signature.

    3. Attach signature to message.

  • Signing + Encryption: Sign first, then encrypt both message & signature.

• General PGP Message Format:

  
  [Packet: Session Key (encrypted with recipient's PK)]
  
  [Packet: Signature (optional)]
  
  [Packet: Compressed Data (optional)]
  
  [Packet: Literal Data (actual message)]
  
  

  Each "packet" has a header tag, length, and body.

Secure Sockets Layer (SSL/TLS)

• SSL Record Protocol Services:

  • Confidentiality: Using symmetric encryption (after handshake).

  • Integrity: Using MAC (HMAC) on data.

  • Encapsulation: Fragments, compresses, adds MAC, encrypts application data.

• SSL Handshake Protocol (Step-by-Step):

  1. ClientHello: Client sends supported cipher suites, TLS version, random R_C.

  2. ServerHello: Server selects cipher suite, sends SSL/TLS version, random R_S, Server Certificate (contains server's public key & identity).

  3. ServerHelloDone.

  4. Client Certificate (optional): If server requests client auth.

  5. ClientKeyExchange: Client generates premaster secret, encrypts with server's public key (from cert), sends to server.

  6. Both compute Master Secret: Master Secret = PRF(premaster secret, "master secret", R_C + R_S).

  7. Key Derivation: From Master Secret, derive session keys (client write MAC key, server write MAC key, client write encryption key, server write encryption key).

  8. ChangeCipherSpec (Client & Server): Subsequent messages use derived session keys.

  9. Finished (Client & Server): Send HMAC of all previous handshake messages. Verifies handshake integrity.

• SSL Connection vs SSL Session:

  • Session: Established via handshake. Stores Master Secret, cipher specs, session ID. Can be resumed later (avoids full handshake).

  • Connection: Specific association between client & server for data transfer. Uses session keys derived from a session. Multiple connections can reuse one session.

  • Contribution to Web Security: Sessions enable efficient secure reconnections (e.g., browser revisits HTTPS site). Connections provide ephemeral encryption for each TCP connection.

IP Security (IPSec)

• Authentication Header (AH): Provides data origin authentication & integrity (no confidentiality).

  • Transport Mode: AH protects payload of original IP packet. New IP header (from original) + AH. Used for end-to-end (host-to-host).

  • Tunnel Mode: AH protects entire original IP packet. New IP header + AH + original IP packet. Used for network-to-network (gateway-to-gateway, e.g., VPN).

• Encapsulating Security Payload (ESP): Provides confidentiality, authentication, integrity.

  • Transport Mode: ESP trailer after original payload, ESP auth after trailer. Original IP header unchanged. Used for host-to-host.

  • Tunnel Mode: Entire original IP packet is ESP payload. New outer IP header + ESP (SPI, Seq#, Payload, Pad, Pad Len, Next Header, Auth). Used for gateway-to-gateway VPNs.

  Key Difference: AH authenticates immutable IP header fields (except mutable ones like TTL). ESP in tunnel mode hides original IP addresses.

Secure Electronic Transaction (SET)

• Security Concerns in Online Transactions:

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

  • Integrity: Order info, payment info must not be altered.

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

  • Non-repudiation: Cardholder cannot deny order; merchant cannot deny receiving payment.

• SET Protocol - How It Addresses Concerns:

  • Uses dual signatures: Cardholder signs order info + payment info together, but separates them. Merchant sees order, bank sees payment, neither sees full link.

  • Entities: Cardholder, Merchant, Issuer (cardholder's bank), Acquirer (merchant's bank), Payment Gateway, Certification Authority (CA).

  • Example Flow (Simplified):

    1. Cardholder gets dual signature on Order + Payment.

    2. Sends Order + Dual Signature to Merchant.

    3. Sends Payment + Dual Signature (encrypted for bank) via Payment Gateway to Acquirer/Issuer.

    4. Merchant verifies signature on order. Bank verifies signature on payment using cardholder's public key (from cert).

  • Addresses Concerns: Confidentiality (encryption), Integrity (digital signatures), Authentication (certificates), Non-repudiation (dual signatures bind parties).

IV. Network Security Infrastructure

Virtual Private Networks (VPNs) vs Trusted Operating Systems

Intrusion Detection Systems (IDS)

• Host-based IDS (HIDS):

  • Working: Monitors a single host (OS, file system, logs, processes).

  • Components: Sensors (collect audit logs), Analyzer (detect anomalies/signatures), Manager (alert).

  • Detection: File integrity checks (Tripwire), log analysis, rootkit detection.

  • Diagram: [Host OS] → [HIDS Agent] → [Local DB/Alert]

• Network-based IDS (NIDS):

  • Working: Monitors network traffic (sensors on network segments).

  • Components: Sensors (packet capture), Analyzer (pattern/signature match), Console.

  • Detection: Signature-based (known attack patterns), Anomaly-based (baseline deviation).

  • Diagram: [Network Segment] → [NIDS Sensor] → [Central Analyzer] → [Console]

• Parameter Pattern Matching (IDS Technique):

  • Signature-based Detection: Compare network/host activity against a database of known attack signatures (patterns).

  • How: Predefined patterns (e.g., specific byte sequence in packet, log message). Efficient for known attacks but fails against zero-day.

  • Example: Snort rule: alert tcp any any -> any 80 (content:"/etc/passwd";)

Firewalls

• Classification & Operational Differences:

  | Type | Layer | How it Works | Example/Use Case | | :--- | :--- | :--- | :--- | | Packet Filtering | Network/Transport (IP, Port) | Stateless. Checks source/dest IP, port, protocol against ACL. | Simple router ACL. Fast but can't inspect payload. | | Circuit-Level Gateway | Session (TCP/UDP) | Monitors TCP handshake. Creates virtual circuit. Hides internal IPs. | SOCKS proxy. Stateful, but no payload inspection. | | Application-Level Gateway<br>(Proxy) | Application (HTTP, FTP) | Intercepts & inspects application layer commands. Acts as intermediary. | HTTP proxy (Squid). Can filter URLs, commands. Slow. | | Stateful Inspection | Network/Transport | Tracks state of connections (SYN, ESTABLISHED, FIN). Context-aware. | Most modern firewalls (iptables state module). | | Personal Firewall | All (Host-based) | Software on endpoint. Controls inbound/outbound per-app. | Windows Defender Firewall. |

• Merits: Access control, traffic logging, network segmentation, hide internal structure.

• Demerits: Cannot stop internal threats, encrypted traffic bypass, complex rule management, single point of failure, cannot stop malware in allowed traffic (e.g., HTTP download).

Malicious Software (Malware)

• Types:

  • Virus: Attaches to executable, requires user action to spread.

  • Worm: Self-replicating, spreads over network (no host file).

  • Trojan: Disguised as legitimate software. Provides backdoor.

  • Ransomware: Encrypts files, demands ransom.

  • Rootkit: Hides existence/activities (OS/kernel level).

  • Spyware/Adware: Steals data/shows ads.

  • Bot: Compromised host in botnet (DDoS, spam).

• How IDS & Firewalls Help:

  • Firewalls: Block known malicious IPs/ports, prevent worm propagation by filtering traffic. Limitation: Cannot inspect allowed application traffic (e.g., HTTP).

  • IDS (NIDS): Detect worm propagation patterns (scanning), known virus signatures in network traffic.

  • IDS (HIDS): Detect file modifications (virus), unexpected processes (trojan), registry changes (rootkit).

  • Combined: Firewall reduces attack surface; IDS detects what gets through. Both are insufficient alone against sophisticated/encrypted malware; need endpoint protection (AV/EDR).

V. Wireless Security

Wireless LAN (WLAN) Security Challenges

• Broadcast Medium: Signals propagate beyond physical boundaries → easy eavesdropping.

• No Physical Control: Attacker can be anywhere within range.

• Weak/No Encryption: Legacy protocols (WEP) are broken.

• Rogue Access Points: Unauthorized APs inside corporate network.

• Evil Twin: Fake AP with same SSID to steal credentials.

• Denial-of-Service: Jamming wireless frequencies.

• Client Misconfiguration: Open networks, weak passwords.

Wireless Application Protocol (WAP)

• Architecture (with Diagram):

  
  [Mobile Device] --(Wireless)--> [WAP Gateway] --(Internet)--> [Web Server]
  
        |                                       |
  
  [WAE ( microbrowser)]                  [HTTP Server]
  
  [WTLS (Security)]                      [HTML Content]
  
  

  • WAP Stack: WAE (Application), WTLS (Security), WTP (Transport), WDP (Datagram).

  • Gateway: Translates WAP requests (WSP/WTP) to HTTP/HTTPS. Terminates WTLS, may start TLS to server.

• Security Mechanisms Overview:

  • WTLS: Wireless TLS (optimized for wireless: smaller packets, faster handshake).

  • End-to-End Security: Can use WTLS between device & gateway, and TLS between gateway & server.

  • Identity: Uses certificates (WTLS) or user/password.

Wireless Transport Layer Security (WTLS)

• Role: Provides confidentiality, integrity, authentication for WAP applications. Analogous to TLS but designed for constrained wireless networks (low bandwidth, high latency).

• Optimizations: Smaller record sizes, faster handshake (optional client cert), supports datagram (WDP).

• Security Issues in WTLS:

  • Gateway Termination: WTLS often terminates at WAP gateway → gap in end-to-end security. Traffic between gateway & server may be plain HTTP.

  • Weak Ciphers: Early WTLS allowed weak encryption (export restrictions).

  • Certificate Handling: Limited device storage for certs.

  • Man-in-the-Middle: If gateway is compromised, end-to-end security fails.

WLAN Protocol Stack & MPDU Format

• Protocol Stack (IEEE 802.11):

  
  Application
  
  LLC (Logical Link Control)
  
  MAC (Medium Access Control) ← CSMA/CA
  
  PHY (Physical) ← OFDM/DSSS
  
  

• MPDU (MAC Protocol Data Unit) Format:

  
  [Frame Control (2B)] [Duration (2B)] [Addr1 (6B)] [Addr2 (6B)] [Addr3 (6B)]
  
  [Sequence Control (2B)] [Addr4 (6B, if To DS=1 & From DS=1)] [Frame Body (0-2312B)]
  
  [FCS (4B)]
  
  

  • Frame Control: Type (Mgmt, Ctrl, Data), Subtype, To/From DS flags.

  • Addresses: Depends on To DS/From DS bits (e.g., AP to client, client to AP, WDS).

  • FCS: CRC-32 for error detection.

Access Point Security in Public Networks

• Threats: Evil Twin, session hijacking, man-in-the-middle, packet sniffing.

• Security Measures:

  • WPA3-Personal: SAE (Simultaneous Authentication of Equals) replaces PSK. Resists offline dictionary attacks.

  • WPA3-Enterprise: 192-bit security suite (AES-GCM-256, HMAC-SHA384). Requires 802.1X/EAP.

  • Captive Portal: Forces HTTP redirect to authentication page (but traffic still unencrypted until HTTPS).

  • Client Isolation (AP Setting): Prevents clients from talking to each other.

  • Use of VPN: Essential on public Wi-Fi. Encrypts all traffic from device to VPN server, bypassing local network threats.

  Best Practice: Always use personal VPN on public Wi-Fi, even if using HTTPS.

VI. Additional Security Considerations

Trusted Operating Systems

• Concept: OS designed from ground up to enforce security policy with high assurance. Based on formal security models (Bell-LaPadula for confidentiality, Biba for integrity).

• Security Architecture:

  • Reference Monitor: Mediates all subject-object accesses. Must be: Tamper-proof, Always invoked, Verifiable.

  • Security Kernel: Minimal, verified part of OS implementing reference monitor.

  • Mandatory Access Control (MAC): Labels (e.g., Top Secret, Secret) assigned to subjects/objects. Access based on label comparison (no user discretion).

  • Process Isolation: Each process has separate address space.

  • Audit: Complete, secure logging of security-relevant events.

• Applications: Military systems, government databases, critical infrastructure control systems.

Web Traffic Security Approaches

1. SSL/TLS (HTTPS): End-to-end encryption between browser & server. Provides confidentiality, integrity, server authentication (and optionally client auth).

2. IPSec (VPN): Secures all IP traffic between gateways or hosts. Can protect non-web traffic too.

3. Application Layer Gateways/Proxies: Inspect and filter HTTP/HTTPS (HTTPS inspection via MITM with installed CA cert).

4. Content Security Policy (CSP): Mitigates XSS by defining allowed sources for scripts, styles, etc.

5. HTTP Strict Transport Security (HSTS): Forces browser to use HTTPS only.

Practical Cipher Examples & Calculations

• Caesar Cipher Decryption (Shift 17):

  • Ciphertext: ZICVTWQNGRZGVTWAVZHCQYGLMGJ

  • Decryption: P_i = (C_i - 17) mod 26

  • Z(25) → (25-17)=8 → I, I(8) → (8-17)=-9≡17 → R, C(2) → (2-17)=-15≡11 → L...

  • Plaintext: IRE**_**_**_**_**_**_**_** (Complete: IRELAND WAS CONQUERED BY THE ROMANS - verify exact shift).

  Step: Convert letters to numbers (A=0), subtract shift, mod 26, convert back.

• RC4 First 3 Bytes (5-bit key 10101 = 21):

  • KSA: Key K = [21,21,21,21,21] (5 bytes). Initialize S = [0,1,2,...,255].

  • For i=0 to 255: j = (j + S[i] + K[i mod keylen]) mod 256; swap S[i] & S[j].

  • PRGA: i=j=0 initially.

    1. i=0: j = (0 + S[0] + K[0]) mod 256 = (0 + 0 + 21) = 21; swap S[0] & S[21]; output K_0 = S[(S[0]+S[21]) mod 256].

    2. i=1: j = (21 + S[1] + K[1]) mod 256; swap; output K_1.

    3. i=2: similarly.

  • Result: First 3 keystream bytes depend on full KSA output. Must compute KSA completely first.

Parameter Pattern Matching (Detailed)

• Definition: Technique where IDS/IPS compares network/host activity against a database of signatures (patterns) of known attacks.

• Process:

  1. Signature Creation: Analyze attack (e.g., buffer overflow exploit) → identify unique byte sequence, packet header anomaly, log message pattern.

  2. Storage: Signatures stored in database (e.g., Snort rules).

  3. Detection: Real-time traffic/logs scanned for exact or approximate pattern match.

• Types:

  • Exact String Matching: Fast (Boyer-Moore, Aho-Corasick algorithms). Used in NIDS for payload inspection.

  • Regular Expression Matching: More flexible but slower. Used for complex patterns.

• Merits: High accuracy for known attacks, low false positives.

• Demerits: Zero-day attacks (no signature) missed. Signature updates required. Can be evaded by polymorphism/metamorphism (malware changes code each infection).

Cipher Block Modes of Operation (Recap/Comparison)

• Purpose: Allow block ciphers to securely encrypt data larger than block size.

• Key Comparison Points:

  • Parallelization: ECB, CTR (enc/dec). CBC, CFB, OFB (dec only for CBC/CFB).

  • Padding Required: ECB, CBC (yes). CFB, OFB, CTR (no).

  • Error Propagation: ECB (none per block), CBC (1 block), CFB (s bits), OFB/CTR (none).

  • Random Access/Seek: CTR (yes), OFB (partial), others (no).

  • IV/Nonce Requirement: All except ECB need unique IV/nonce. Reuse in CTR/OFB → keystream reuse → break.

• Modern Preference: CTR (parallel, random access, no padding) or GCM (Galois Counter Mode, provides authentication). CBC still common but requires careful IV management (random, unpredictable). ECB is always insecure.