Data Security (CY-503 (C)) - Unit 2 Short Notes

UNIT 2: Data Security - Comprehensive Short Notes

I. CRYPTOGRAPHIC FOUNDATIONS

A. Core Encryption Principles

Symmetric vs. Asymmetric Cryptography

[!TIP] Exam Focus: Symmetric is fast but key distribution is hard. Asymmetric solves distribution but is slow. Often used together (e.g., TLS uses asymmetric to exchange a symmetric session key).

Block Ciphers vs. Stream Ciphers

Cipher Block Modes of Operation

Used to encrypt data larger than block size, prevent pattern leakage.

[!TIP] Common Pitfall: Never reuse a nonce/counter in CTR mode. It completely breaks confidentiality.

B. Symmetric Key Algorithms

Advanced Encryption Standard (AES)

• Block Size: 128 bits

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

• Structure: Substitution-Permutation Network (SPN)

• Round Operations (per round except last):

  1. SubBytes (non-linear substitution via S-box)

  2. ShiftRows (permutation: shift rows cyclically)

  3. MixColumns (mixing columns via matrix multiplication)

  4. AddRoundKey (XOR with round key)

• Final Round: No MixColumns.

• Key Expansion: Round keys derived from initial key via Rijndael's key schedule.

Example (Conceptual):

Plaintext block = 00112233445566778899aabbccddeeff (128-bit)

Key = 000102030405060708090a0b0c0d0e0f (128-bit)

After 10 rounds of transformations → ciphertext.

RC4 Stream Cipher

• Type: Variable key-size stream cipher (8-bit operations).

• Components:

  • KSA (Key Scheduling Algorithm): Initialize S-box (256 bytes) using key.

  • PRGA (Pseudo-Random Generation Algorithm): Generate keystream bytes from S-box.

• KSA Steps:

  1. Initialize S: S[i] = i for i=0..255.

  2. j = 0

  3. For i = 0 to 255:

     • j = (j + S[i] + key[i mod keylen]) mod 256

     • Swap S[i] and S[j]

• PRGA Steps:

  1. i = (i + 1) mod 256

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

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

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

  5. Keystream byte = S[t]

• Encryption: Ciphertext[i] = Plaintext[i] XOR Keystream[i]

[!TIP] RC4 5-bit Key Example:

Key = [1, 2, 3, 4, 5] (5 bytes, but RC4 uses 8-bit bytes; for 5-bit, we'd mask to 5 bits).

Simplified 5-bit variant (illustrative):

• S-box size = 32 (2^5).

• KSA: S = [0..31], j=0. For i=0..31: j = (j + S[i] + key[i mod 5]) mod 32, swap.

• PRGA: similar with mod 32.

Generate first 3 keystream bytes after KSA.

In exam, show KSA table swaps and first 3 PRGA iterations.

C. Asymmetric Key Algorithms

RSA Algorithm (Rivest–Shamir–Adleman)

• Mathematical Foundation: Difficulty of factoring large integers.

• Key Generation:

  1. Choose large primes p, q.

  2. Compute n = p * q.

  3. Compute φ(n) = (p-1)(q-1).

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

  5. Compute d such that d * e ≡ 1 mod φ(n) (using Extended Euclidean Algorithm).

  6. 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):

1. n = 3 * 11 = 33

2. φ(n) = (3-1)(11-1) = 2*10 = 20

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

4. Compute d: 7d ≡ 1 mod 20 → d=3 (since 7*3=21 ≡1 mod20)

5. Public Key: (7, 33), Private Key: (3, 33)

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

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

Public Key Cryptography Principles

• One-way Function: Easy to compute f(x), hard to invert f⁻¹(y).

• Trapdoor Function: One-way function with secret "trapdoor" (private key) allowing easy inversion.

• Key Distribution: Public keys can be shared openly via certificates (PKI) or web of trust (PGP).

D. Classical Ciphers (Application-Based)

Caesar Cipher

• Type: Substitution cipher.

• Encryption: C = (P + k) mod 26, where P = plaintext letter index (A=0), k = shift.

• Decryption: P = (C - k) mod 26

• Example Decryption:

  Ciphertext: ZICVTWQNGRZGVTWAVZHCQYGLMGJ

  Shift k = 17

  Decrypt each letter:

  Z (25) → (25 - 17) mod 26 = 8 → I

  I (8) → (8 - 17) mod 26 = 17 → R

  ... continue for all letters.

  Result: I HOPE YOU ARE WELL AND GOOD HEALTH (with spaces inserted).

[!TIP] Exam Step: Always convert letters to 0-25 indices, apply formula, convert back. Show at least 3-4 letter conversions.

II. AUTHENTICATION & INTEGRITY

A. Hash Functions & Message Digests

Cryptographic Hash Functions Properties

1. Pre-image Resistance: Given hash h, hard to find any m such that hash(m)=h.

2. Second Pre-image Resistance: Given m1, hard to find m2≠m1 with same hash.

3. Collision Resistance: Hard to find any two distinct messages m1, m2 with hash(m1)=hash(m2).

4. Deterministic: Same input → same output.

5. Fixed Output Size: e.g., SHA-512 → 512-bit digest.

6. Avalanche Effect: Small change in input → large change in output.

SHA-512 Algorithm Steps

1. Preprocessing:

   • Append 1 bit, then 0 bits, then 128-bit message length (in bits).

   • Result length ≡ 1024 mod 1024 (multiple of 1024-bit block).

2. Parsing: Break message into 1024-bit blocks M¹, M², ....

3. Initialize Hash Values (H⁰): Eight 64-bit constants from square roots of first 8 primes.

4. For each block:

   • Prepare 80-word message schedule W[0..79] from block.

   • Initialize working variables a..h with current hash value.

   • For t=0..79:

     
     T1 = h + Σ1(e) + Ch(e,f,g) + K[t] + W[t]
     
     T2 = Σ0(a) + Maj(a,b,c)
     
     h = g; g = f; f = e; e = d + T1; d = c; c = b; b = a; a = T1 + T2
     
     

   • Update hash: H[i] = H[i-1] + variable[i] for i=0..7.

5. Output: Concatenate final H⁷ as 512-bit digest.

[!TIP] SHA-512 Order for Same Digest: Finding collisions is computationally infeasible (birthday attack requires ~2²⁵⁶ operations). Pre-image attack requires ~2⁵¹² operations.

B. Message Authentication Codes (MACs)

MAC Definition: A short tag generated from a message and a secret key, providing integrity and authentication.

HMAC (Hash-based MAC) Construction

• Uses a cryptographic hash function H (e.g., SHA-256).

• Formula:

  HMAC(K, m) = H( (K⁺ ⊕ opad) ∥ H( (K⁺ ⊕ ipad) ∥ m ) )

  where:

  • K⁺ = key padded/truncated to hash block size.

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

  • ∥ = concatenation.

• Steps:

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

  2. Compute inner hash: inner = H( (K⁺ ⊕ ipad) ∥ m ).

  3. Compute outer hash: HMAC = H( (K⁺ ⊕ opad) ∥ inner ).

Comparison Table

C. Digital Signatures

Mechanism using Public-Key Cryptography

1. Signing:

   • Sender computes hash of message: h = hash(m).

   • Encrypt hash with private key: sig = h^d mod n (RSA) or generate signature via ECDSA algorithm.

2. Verification:

   • Receiver decrypts signature with public key: h' = sig^e mod n.

   • Compute hash of received message: h = hash(m).

   • If h' == h, signature valid.

Importance in Secure Communication

• Authentication: Confirms sender identity.

• Integrity: Any change in message invalidates signature.

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

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

Digital Signature Standards (DSS)

• NIST standard: DSA (Digital Signature Algorithm) based on discrete logarithm problem.

• Later standards: RSA-PSS, ECDSA (Elliptic Curve DSA).

III. KEY MANAGEMENT & PROTOCOLS

A. PGP (Pretty Good Privacy)

Architecture & Services

• Confidentiality: Hybrid encryption (session key encrypted with recipient's public key).

• Authentication: Digital signatures using sender's private key.

• Compression: Applied before encryption (ZIP).

• Email Integration: Works with MIME.

PGP Message Format

[Signature] (optional)

[Session Key] (encrypted with recipient's public key)

[Encrypted Data] (compressed message encrypted with session key)

DiagramCANVAS: PGP message block structure showing signature, encrypted session key, and encrypted data sections

Operations Workflow

1. Sending:

   • Generate random session key.

   • Compress message.

   • Encrypt compressed message with session key (symmetric, e.g., AES or IDEA).

   • Encrypt session key with recipient's public key.

   • Optionally sign message hash with sender's private key.

   • Combine into PGP message.

2. Receiving:

   • Decrypt session key with own private key.

   • Decrypt message with session key.

   • Decompress.

   • Verify signature if present using sender's public key.

B. SSL/TLS Protocol Suite

SSL Record Protocol Services

• Confidentiality: Symmetric encryption (AES, 3DES).

• Integrity: MAC (HMAC) or AEAD.

• Authentication: Optional via certificates (X.509).

SSL Handshake Protocol (Step-by-Step)

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

2. Server Hello: Server chooses cipher suite, sends TLS version, random R_S, certificate.

3. Key Exchange:

   • If RSA: Client generates premaster secret, encrypts with server's public key, sends.

   • If Diffie-Hellman: Server sends DH parameters, signed.

4. Finished:

   • Both compute master secret from premaster + R_C + R_S.

   • Derive session keys (client/server write keys, MAC keys).

   • Exchange Finished messages encrypted with session keys.

SSL Connection vs. SSL Session

[!TIP] Exam Distinction: Session = negotiated state (can be reused). Connection = active data flow (uses session keys).

C. IPSec (IP Security)

Goals & Services

• Confidentiality: ESP encryption.

• Data Integrity: AH/ESP authentication.

• Authentication: AH/ESP.

• Anti-replay: Sequence numbers + sliding window.

Security Associations (SA)

• Definition: Unidirectional logical connection providing security services.

• Parameters: SPI (Security Parameter Index), IP destination address, security protocol (AH/ESP), mode, keys.

• Management: Manual or via IKE (Internet Key Exchange).

Authentication Header (AH)

• Provides integrity and authentication (no encryption).

• Transport Mode: AH protects IP payload (and selected header fields) but not IP header. Used end-to-end.

• Tunnel Mode: AH protects entire original IP packet + new IP header. Used for gateways.

Encapsulating Security Payload (ESP)

• Provides confidentiality, integrity, authentication.

• Transport Mode: ESP trailer encrypts payload, authenticates payload + ESP header (but not outer IP header).

• Tunnel Mode: Entire original IP packet encrypted and authenticated, encapsulated in new IP packet.

DiagramCANVAS: IPSec transport vs tunnel mode showing original packet, ESP/AH trailer, new IP header

D. SET (Secure Electronic Transaction)

Protocol Goals

• Confidentiality: Payment info encrypted (only bank sees).

• Integrity: All messages authenticated.

• Cardholder Authentication: Via digital signature.

• Merchant Authentication: Via certificate.

• No merchant sees card number (critical for trust).

Participants & Roles

SET Transaction Flow with Dual Signature

1. Cardholder obtains certificates (from CA, Issuer).

2. Order Info (OI) + Payment Info (PI) created.

3. Dual Signature: Sig = Sign(Priv_C, hash(OI) ∥ hash(PI)). Links OI and PI without revealing each to other parties.

4. Cardholder sends:

   • OI (encrypted with merchant's public key)

   • PI (encrypted with payment gateway's public key)

   • Dual signature (both can verify)

5. Merchant sees OI, verifies dual signature, forwards PI to gateway.

6. Gateway decrypts PI, verifies dual signature, processes payment.

DiagramCANVAS: SET transaction showing dual signature linking OI and PI, separate encryption paths

IV. NETWORK SECURITY INFRASTRUCTURE

A. Firewalls

Classification & Types

Firewall Merits & Demerits

• Merits: First line of defense, access control, logging, network segmentation.

• Demerits: Cannot stop internal threats, encrypted traffic bypass, misconfiguration risks, single point of failure.

Firewall vs. Trusted Operating System

B. Intrusion Detection Systems (IDS)

Intrusion Definition: Unauthorized access or misuse of system/resources.

Network-Based IDS (NIDS) Architecture

DiagramSEARCH: network-based IDS architecture diagram showing sensors, management server, database

• Sensors: Capture packets (promiscuous mode).

• Management Server: Analyzes data, generates alerts.

• Database: Stores events, signatures.

• Operation: Sensors → Management → Alert/Log.

Host-Based IDS (HIDS)

• Installed on individual hosts.

• Monitors system logs, file integrity (Tripwire), process behavior.

• Example: OSSEC, Wazuh.

IDS vs. Firewalls

Parameter Pattern Matching

• Detection technique: Compare packet headers/payloads against signature database.

• Example: Snort rules matching specific byte patterns.

• Limitation: Only detects known attacks; evadable via polymorphism.

C. Malware Defense

Types of Malicious Software

Role of IDS and Firewalls

• Firewalls: Block malicious traffic (ports, IPs), prevent worm propagation.

• IDS: Detect malware communication (C&C traffic), file integrity changes (HIDS), anomaly patterns.

• Combined: Firewall blocks known bad; IDS alerts on suspicious activity that bypasses firewall.

V. WIRELESS & MOBILE SECURITY

A. Wireless LAN (WLAN) Security

Security Challenges

• Open medium → eavesdropping easy.

• Rogue access points.

• Weak default configurations.

• Jamming/DoS.

• Client misassociation.

WLAN Protocol Stack & MPDU Format

DiagramSEARCH: 802.11 MAC frame structure MPDU showing frame control, duration, addresses, sequence control, payload, FCS

• MPDU (MAC Protocol Data Unit):

  • Frame Control (type, subtype, flags)

  • Duration

  • Addresses (up to 4)

  • Sequence Control

  • Payload (LLC data)

  • FCS (CRC)

Access Point Security in Public Networks

• Risks: Evil twin attacks, eavesdropping, captive portal bypass.

• Mitigations:

  • WPA2-Enterprise: 802.1X with EAP (e.g., PEAP, EAP-TLS). Individual user auth.

  • Captive Portals: Web-based auth, but traffic not encrypted (use HTTPS).

  • Client Isolation: Prevent inter-client communication.

  • Rogue AP Detection: Wireless IDS/IPS.

B. Wireless Application Protocol (WAP) Security

WAP Architecture

DiagramSEARCH: WAP architecture diagram showing mobile device, WAP gateway, web server

• Client: WAP-enabled phone.

• WAP Gateway: Protocol conversion (WSP/WTLS → HTTP/SSL), content adaptation.

• Web Server: Origin server.

WTLS (Wireless Transport Layer Security)

• Analogy to SSL/TLS but optimized for wireless:

  • Smaller packet sizes (avoid fragmentation).

  • Faster handshake (optional client cert).

  • Supports datagram (UDP-like) via record layer.

• Role: Provides confidentiality, integrity, authentication between mobile device and gateway.

• Limitations:

  • Weak cryptography (early versions allowed export-grade RSA).

  • Gateway terminates WTLS → gap in end-to-end security (gateway sees plaintext).

  • Short certificates (for small devices).

Security Issues in WTLS

• End-to-End Gap: Gateway decrypts, may re-encrypt with SSL → server sees gateway as client.

• Weak Ciphers: Early WTLS allowed 40-bit RC2.

• Certificate Handling: Limited storage on devices → often gateway stores certs.

• Version Vulnerabilities: WTLS 1.0 had flaws (like SSL 2.0).

VI. APPLICATION & ADVANCED TOPICS

A. Virtual Private Networks (VPN)

Definition & Core Concept

• VPN: Extends private network across public infrastructure via tunneling and encryption.

• Tunneling: Encapsulating packets within other packets.

• Encryption: Confidentiality (IPsec, SSL).

Types of VPNs

VPN Technologies

• PPTP: Point-to-Point Tunneling Protocol. Weak encryption (MPPE), vulnerable.

• L2TP/IPsec: L2TP for tunneling, IPsec for encryption/authentication. More secure than PPTP.

• SSL VPN: Uses HTTPS (TCP 443). No client software (browser-based). Granular access control.

B. Biometric & Smart Card Authentication

Biometric Authentication

• Methods: Fingerprint, iris, face, voice, vein.

• Process:

  1. Enrollment: Capture biometric, extract features → template (stored).

  2. Verification/Identification: Capture live sample, compare to template via matching algorithm.

• Advantages: Hard to forget/lose, non-transferable.

• Challenges: False accepts/rejects, template theft, privacy concerns, cost.

Smart Cards

• Physical: Plastic card with embedded chip (contact or contactless).

• Logical Security: Stores certificates, private keys (often in secure element).

• Authentication Mechanism:

  • Something you have (card) + something you know (PIN) → two-factor.

  • Card performs cryptographic operations (private key never leaves card).

• Types: Memory cards, microprocessor cards (e.g., EMV, government ID).

C. Web Traffic Security Approaches

Contextual Application:

• SSL/TLS: When you need end-to-end security between client and server (e.g., online banking).

• IPsec: When securing network-to-network traffic (e.g., connecting branch offices) or all traffic from a remote user.

• Application Gateway: When you need deep inspection (e.g., blocking malicious web content, enforcing authentication).

[!TIP] Exam Comparison: SSL/TLS secures specific application (port 443). IPsec secures all IP traffic (transparent to apps). Gateway acts as intermediary for specific protocols.