Data Security (CY-503 (C)) - Unit 4 Short Notes

UNIT 4: DATA SECURITY

I. FOUNDATIONAL CRYPTOGRAPHIC PRINCIPLES

Symmetric vs. Asymmetric Encryption

[!TIP] Exam often asks for comparative analysis. Focus on key distribution problem for symmetric and computational cost for asymmetric.

Digital Signatures

• Mechanism: Uses sender's private key to sign (encrypt hash of message). Receiver uses sender's public key to verify (decrypt hash and compare).

• Provides:

  1. Authentication: Verifies sender identity.

  2. Integrity: Detects any alteration of the signed message.

  3. Non-Repudiation: Sender cannot deny sending the message.

• Legal Admissibility: Treated as electronic signature under IT Act (India) and similar laws globally.

Secure Message Authentication

• Concept: Ensures message integrity and verifies the source (authenticity). Different from encryption (confidentiality).

• Implementation:

  1. Sender computes cryptographic hash (digest) of the message.

  2. Sender encrypts the hash with their private key (digital signature) OR shares a symmetric secret key (MAC).

  3. Sender transmits: [Message] + [Authentication Tag].

  4. Receiver recomputes hash/MAC and verifies the tag.

• Example (Hash-based): Send M || H(M) where H is SHA-256. Receiver checks if H(M_received) == received_hash.

II. HASH FUNCTIONS & MESSAGE AUTHENTICATION CODES (MACs)

Cryptographic Hash Functions

• Properties:

  • 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).

  • Deterministic: Same input → same output.

  • Fixed output size (e.g., 512 bits for SHA-512).

• Algorithms: SHA-512 (secure, 512-bit output), SHA-1 (deprecated, 160-bit), MD5 (broken, 128-bit).

• SHA-512 Steps:

  1. Pre-processing: Append '1' bit, pad with '0's, append 128-bit message length.

  2. Parsing: Break padded message into 1024-bit blocks.

  3. Compression: For each block, update 512-bit state via 80 rounds using constants and message schedule.

• Birthday Attack: Exploits collision resistance weakness. Complexity ≈ $$\displaystyle 2^{n/2} $$ for n-bit hash. Makes finding collisions feasible for small n (e.g., MD5).

Message Authentication Codes (MACs)

• Definition: MAC = C_K(M), where C is a MAC algorithm, K is a shared secret key, M is message. Provides integrity + authentication.

• HMAC Construction (RFC 2104):

  
  HMAC_K(M) = H( (K' ⊕ opad) || H( (K' ⊕ ipad) || M ) )
  
  

  Where K' is key padded to block size, ipad=0x36, opad=0x5C, || is concatenation.

  • Security: Based on underlying hash's strength.

• Comparison with Digital Signatures:

  | MAC | Digital Signature | |-----|-------------------| | Uses symmetric key | Uses asymmetric keys | | Provides authentication & integrity | Provides authentication, integrity, non-repudiation | | Faster | Slower |

Parameter Pattern Matching

• Definition: Monitoring network/system parameters (e.g., packet size, port numbers, header fields) against known malicious patterns (signatures).

• Application: Used in NIDS (Network IDS) and firewalls to detect anomalies or known attack signatures (e.g., specific TCP flag combinations, unusual payload lengths).

III. SYMMETRIC ENCRYPTION ALGORITHMS & MODES

AES (Advanced Encryption Standard)

• Structure (for 128-bit block, 10 rounds):

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

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

  3. MixColumns: Mixing columns via matrix multiplication (diffusion).

  4. AddRoundKey: XOR state with round key.

• Process: Initial AddRoundKey → [Round(SubBytes, ShiftRows, MixColumns, AddRoundKey)]_{9 times} → Final Round(SubBytes, ShiftRows, AddRoundKey).

• Decryption: Inverse operations in reverse order (InvMixColumns is expensive; often use equivalent inverse cipher).

Cipher Block Modes of Operation

[!TIP] ECB is insecure for multi-block messages. Always use CBC, CTR, or GCM (not in syllabus) for new designs.

RC4 Stream Cipher

• Key Scheduling Algorithm (KSA):

  1. Initialize S[0..255] = [0,1,...,255].

  2. j = 0.

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

• Pseudo-Random Generation Algorithm (PRGA):

  1. i = j = 0.

  2. i = (i + 1) mod 256.

  3. j = (j + S[i]) mod 256.

  4. Swap S[i] and S[j].

  5. Output K_byte = S[(S[i] + S[j]) mod 256].

• Example (5-bit key K = [1,2,3,4,5]):

  • KSA initializes S, then scrambles using key bytes.

  • PRGA first 3 iterations generate keystream bytes Z1, Z2, Z3.

• Security Weaknesses:

  • Biased outputs: First few bytes leak key info.

  • Key recovery attacks: Fluhrer, Mantin, Shamir (FMS) attack.

  • Never use RC4 in modern systems (deprecated in TLS, WPA2).

Caesar Cipher (Classical)

• Mechanism: C = (P + k) mod 26, P = (C - k) mod 26, where k is shift (0-25).

• Decryption Example (k=17, ciphertext ZICVTWQNGRZGVTWAVZHCQYGLMGJ):

  1. Map letters to numbers: A=0, B=1, ..., Z=25.

  2. Z(25) → (25 - 17) mod 26 = 8 → I

  3. I(8) → (8 - 17) mod 26 = 17 → R

  4. Continue for all letters.

  Result: **RUBICSCRYPTOGRAPHYISFUNBUTDONOTUSEIT** (spaces added for clarity: "RUBICS CRYPTOGRAPHY IS FUN BUT DO NOT USE IT").

IV. ASYMMETRIC ENCRYPTION ALGORITHMS

RSA Algorithm

• Mathematical Foundation: Based on Euler's Theorem: m^φ(n) ≡ 1 mod n for gcd(m,n)=1, where φ(n)=(p-1)(q-1).

• Key Generation:

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

  2. Choose e such that 1 < e < φ(n) and gcd(e, φ(n)) = 1 (public exponent).

  3. Compute d = e^{-1} mod φ(n) (private exponent).

  4. Public key: (e, n), Private key: (d, n).

• Encryption: c = m^e mod n.

• Decryption: m = c^d mod n.

• Worked Example (p=3, q=11):

  • n = 33, φ(n) = 2*10 = 20.

  • Choose e=7 (gcd(7,20)=1).

  • d = 7^{-1} mod 20 = 3 (since 7*3=21 ≡ 1 mod 20).

  • Encrypt m=2: c = 2^7 mod 33 = 128 mod 33 = 29.

  • Decrypt: m = 29^3 mod 33 = 24389 mod 33 = 2.

• Security: Based on integer factorization problem (hard to factor n into p,q). Must use large n (≥2048 bits).

V. AUTHENTICATION MECHANISMS

Biometric Authentication

• Types: Fingerprint, Iris, Facial recognition, Voice, Vein patterns.

• Process:

  1. Enrollment: Capture raw biometric, extract feature vector (template), store in database.

  2. Authentication: Capture new sample, extract features, match against stored template using threshold.

• Metrics:

  • FAR (False Acceptance Rate): Unauthorized user accepted.

  • FRR (False Rejection Rate): Authorized user rejected.

  • EER (Equal Error Rate): Point where FAR=FRR (lower is better).

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

• Limitations: Spoofing (fake fingerprint), non-revocable (cannot change biometric), template security, intra-class variation (same user's biometric changes).

Smart Cards

• Technology: Chip-based card storing cryptographic keys or performing computations.

• Operation: Requires card reader. Can be contact (chip inserted) or contactless (RFID/NFC).

• Comparison with Biometrics:

  | Smart Card | Biometrics | |------------|------------| | Something you have | Something you are | | Can be lost/stolen | Cannot be lost (but can be spoofed) | | Revocable (cancel card) | Non-revocable | | PIN often used as second factor | Often used as primary factor |

Multi-Factor Authentication (MFA)

• Concept: Combine ≥2 factors from:

  1. Knowledge (password, PIN)

  2. Possession (smart card, OTP token)

  3. Inherence (biometric)

• Goal: Increase security beyond single-factor (e.g., password-only).

VI. SECURE EMAIL: PRETTY GOOD PRIVACY (PGP)

PGP Protocol Working (Hybrid Approach)

1. Compression: Optional step (ZIP) before encryption to reduce size & remove patterns.

2. Session Key Generation: PGP generates random symmetric session key (e.g., AES key).

3. Encryption: Message is encrypted with session key using symmetric cipher (e.g., CAST-128, AES).

4. Session Key Encryption: Session key is encrypted with recipient's public key (RSA/ElGamal).

5. Digital Signature (optional): Sender signs the message digest (SHA-256) with sender's private key.

6. Transmission: Send [Encrypted Session Key] || [Encrypted Message] || [Signature].

PGP Message Format

Services Provided:

• Confidentiality: Via symmetric encryption of message.

• Authentication: Via digital signature of sender.

• Integrity: Via hash in signature.

• Non-Repudiation: Via sender's private key signature.

VII. WEB & TRANSPORT LAYER SECURITY: SSL/TLS

SSL/TLS Record Protocol Services

• Confidentiality: Using symmetric encryption (after handshake).

• Integrity: Using MAC (HMAC) or AEAD (in TLS 1.2+).

• Authentication: Optional, using digital certificates (X.509) during handshake.

SSL Handshake Protocol (Detailed Steps)

1. Client Hello:

   • Client sends ClientHello with: TLS version, random R_C, list of supported cipher suites, compression methods.

2. Server Hello:

   • Server responds ServerHello with: chosen TLS version, random R_S, chosen cipher suite, compression method.

   • Server sends its X.509 certificate (contains server's public key & identity).

   • Server sends ServerHelloDone.

3. Client Key Exchange:

   • Client verifies server certificate.

   • Client generates premaster secret PMS.

   • Client encrypts PMS with server's public key (from certificate) → EncryptedPMS.

   • Client sends ClientKeyExchange (contains EncryptedPMS).

   • Client computes master secret MS = PRF(PMS, "master secret", R_C || R_S).

   • Client sends ChangeCipherSpec (switch to negotiated cipher suite).

   • Client sends Finished (encrypted hash of all handshake messages so far).

4. Server Key Exchange (if needed, e.g., DHE):

   • Server computes MS = PRF(PMS, "master secret", R_C || R_S).

   • Server sends ChangeCipherSpec.

   • Server sends Finished.

SSL Connection vs. SSL Session

[!TIP] Session resumption avoids full public-key operations, improving performance for repeated connections to same server.

VIII. NETWORK LAYER SECURITY: IPSEC

IPSec Protocols

Security Associations (SA)

• Definition: A unidirectional logical connection providing security services. Identified by SPI (Security Parameter Index).

• Parameters (stored in SAD - Security Association Database):

  • SPI (32-bit)

  • IP destination address

  • Security protocol (AH/ESP)

  • Encryption/authentication algorithms & keys

  • Lifetime

• Role: For inbound packet, SPI + destination IP → lookup SA → determine how to process (decrypt/verify).

IX. VIRTUAL PRIVATE NETWORKS (VPNs)

Definition & Core Concept

• VPN: Extends a private network across a public network (Internet) using tunneling and cryptography.

• Tunneling: Encapsulating original packet inside a new packet with different header.

Types of VPNs

VPN vs. Trusted Operating Systems

X. FIREWALLS

Classification of Firewalls

Merits and Demerits of Firewalls

• Merits:

  • Traffic control based on security policy.

  • Logging & monitoring of traffic.

  • Hides internal network structure (NAT).

  • First line of defense against external threats.

• Demerits:

  • Cannot stop internal attacks (insider threats).

  • Cannot inspect encrypted traffic without decryption (SSL/TLS inspection needed).

  • Cannot prevent attacks allowed by policy (e.g., HTTP on port 80).

  • Bypassed by social engineering (phishing).

  • Single point of failure if not redundant.

Proxy Firewalls vs. Personal Firewalls

XI. INTRUSION DETECTION SYSTEMS (IDS)

Host-based IDS (HIDS)

• Monitoring: Single host/OS activities.

  • System logs (event logs, audit logs).

  • File integrity (checksums of critical files).

  • System calls, process behavior.

• Advantages:

  • Can detect internal threats.

  • Sees encrypted traffic (after decryption at host).

  • Granular view of host activity.

• Deployment: Agent-based software on critical servers/workstations.

Network-based IDS (NIDS)

• Monitoring: Network traffic via sensors/promiscuous mode.

• Architecture:

  DiagramCANVAS: A simple network diagram. "Network Switch/Hub" in center. "NIDS Sensor" connected to switch port configured as SPAN/mirror port. Sensor sends traffic to "NIDS Management Console" which runs detection engines and alerts. Console connected to "Administrator" workstation.

• Detection Methods:

  • Signature-based: Matches traffic against known attack patterns (e.g., specific byte sequences). Low false positives, but misses zero-day.

  • Anomaly-based: Builds baseline of "normal" traffic; flags deviations. Can detect novel attacks but high false positives.

• Placement: At network perimeter, DMZ, critical subnets.

IDS vs. Firewalls

Role in Safeguarding Against Malware

• NIDS: Detects malware command-and-control (C2) traffic, worm propagation patterns (e.g., scanning), known exploit payloads in network packets.

• HIDS: Detects malware installation (new files, registry changes), process injection, privilege escalation on host.

• Complementary: Firewalls block known bad IPs/ports; IDS detects what slips through or originates internally.

XII. MALICIOUS SOFTWARE (MALWARE)

Types of Malware

Security Considerations & Threats

• Threat Landscape: Constant evolution; polymorphic (changes code), metamorphic (changes structure) malware evade signature detection.

• Defense-in-Depth:

  1. Prevention: Patch management, least privilege, user education, email filtering.

  2. Detection: Antivirus (signature + heuristic), HIDS/NIDS, sandboxing.

  3. Response: Isolation, forensics, recovery from backups.

XIII. WIRELESS SECURITY (WLAN & WAP)

Wireless LAN (WLAN) Security Challenges

• Open Medium: Radio waves travel beyond physical boundaries → eavesdropping easy.

• Rogue Access Points: Unauthorized APs inside corporate network.

• Weak Encryption: WEP (Wired Equivalent Privacy) flaws:

  • RC4 keystream reuse due to IV collision.

  • FMS attack recovers key in minutes with ~1M packets.

• Authentication Issues: Open system, shared key (WEP), weak PSK (WPA/WPA2 Personal).

• Denial-of-Service: Jamming, deauthentication attacks.

WLAN Protocol Stack & MPDU Format

Access Point Security in Public Network Environments

• Risks in Hotspots:

  • Evil Twin: Rogue AP with same SSID.

  • Man-in-the-Middle: Attacker intercepts traffic.

  • Session Hijacking: Stealing cookies/session tokens.

• Mitigation Strategies:

  • Use WPA2/WPA3 (Enterprise with 802.1X) for encryption.

  • Always use VPN (SSL/IPsec) for end-to-end encryption.

  • Captive Portals: Web-based authentication (but does not encrypt traffic).

  • Disable auto-connect to open networks.

  • HTTPS Everywhere (ensure TLS for web).

Wireless Application Protocol (WAP) Security

• WAP Architecture:

  DiagramCANVAS: A stack diagram showing devices from top to bottom: 1) Wireless Device (Phone) with "WAE (Wireless Application Environment)" and "WTA (Wireless Telephony Application)". 2) "WAP Gateway" (protocol gateway) with "WAP Stack": WAE, WSP, WTP, WTLS, WDP. 3) "Web Server" with standard HTTP/SSL stack. Arrows show: Device ↔ Gateway (WAP protocol over wireless), Gateway ↔ Web Server (HTTP/HTTPS over Internet).

• End-to-End Security Model:

  • Problem: Wireless links (WSP/WTLS) and Internet (HTTP/SSL) have different security.

  • Solution: WTLS secures wireless leg; SSL/TLS secures Internet leg. Gateway terminates WTLS, may re-encrypt with SSL.

  • Gap: Gateway can see plaintext if not using end-to-end SSL (i.e., WTLS end-to-end is rare).

Wireless Transport Layer Security (WTLS)

• Role: Security layer in WAP stack (equivalent to TLS in TCP/IP). Provides:

  • Confidentiality (encryption)

  • Integrity (MAC)

  • Authentication (certificates)

  • Designed for constrained wireless devices (low bandwidth, high latency).

• Security Issues:

  • Certificate Handling: Limited storage on devices; often bypassed.

  • Downgrade Attacks: Attacker forces use of weak cipher suites.

  • Session Resumption: Vulnerable to hijacking if session ID predictable.

  • Overhead: Adds latency; often disabled for performance.

  • End-to-End Gap: Gateway termination breaks true end-to-end security.

XIV. APPLICATION LAYER SECURITY: SECURE ELECTRONIC TRANSACTION (SET)

Main Security Concerns in Online Financial Transactions

1. Confidentiality: Cardholder's payment data (credit card number) must be encrypted.

2. Authentication: Both cardholder and merchant must be authenticated to each other and to banks.

3. Integrity: Order and payment information must not be altered.

4. Non-Repudiation: Cardholder cannot deny order; merchant cannot deny receiving payment.

SET Protocol Detailed Explanation

• Participants:

  • 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 between merchant & acquirer.

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

• Dual Signature Mechanism (Key Innovation):

  • Cardholder creates two messages:

    1. Order Information (OI): What to buy.

    2. Payment Information (PI): Card details, amount.

  • Cardholder computes:

    • H1 = Hash(OI)

    • H2 = Hash(PI)

    • H_combined = Hash(H1 || H2)

  • Cardholder signs H_combined with private key → Dual Signature.

  • Result: Merchant sees OI and H1 (from dual sig) → verifies order integrity. Bank sees PI and H2 → verifies payment integrity. Neither sees the other's data. Cardholder's signature binds both.

How SET Addresses Concerns

• Confidentiality: PI encrypted with merchant's public key (so only merchant can read), then PI and OI encrypted with symm. session key (for transmission).

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

• Integrity: Hashes (SHA-1) and dual signature.

• Non-Repudiation: Dual signature + certificates.

Application in Business Environment (Workflow)

1. Cardholder browses merchant site, generates OI & PI.

2. Cardholder creates dual signature, gets certificates from CA.

3. Cardholder sends to merchant: {OI, PI, DualSig} encrypted appropriately.

4. Merchant verifies OI & dual signature, forwards PI (with H2) to payment gateway.

5. Payment gateway forwards to acquirer/issuer for authorization.

6. Issuer sends authorization response back through gateway to merchant.

7. Merchant confirms order to cardholder.

XV. ADDITIONAL & EMERGING TOPICS

Trusted Operating Systems

• Trusted Computing Base (TCB): All hardware, firmware, software responsible for enforcing security policy. Must be correct (does what it should) and isolated (cannot be tampered).

• Security Models:

  • Bell-LaPadula (Confidentiality): "No read up, no write down" (simple security property, *-property). Prevents info flow from high to low.

  • Biba (Integrity): "No read down, no write up" (simple integrity axiom, integrity *-property). Prevents contamination of high-integrity data by low.

• Security Kernel / Reference Monitor: Minimal TCB component that mediates all accesses, verifies authorization, cannot be bypassed. Must be isolated, verifiable, minimal.

Web Traffic Security Approaches

[!TIP] SSL/TLS is most common for web. IPsec for network-level VPNs. SET was ambitious but complex; largely superseded by TLS + payment processors (PayPal, Stripe).

Smart Cards and Biometrics (Integrated in Section V)

• See detailed comparison in Section V.