Network Security (CY-503 (B)) - Unit 1 Short Notes

UNIT 1: NETWORK SECURITY – SHORT NOTES

I. CRYPTOGRAPHIC FOUNDATIONS

Symmetric Encryption

• Principle: Uses the same secret key for both encryption and decryption. Provides confidentiality.

• Process:

  1. Sender & Receiver agree on a secret key K via a secure channel.

  2. Sender: C = E(K, P) (Ciphertext = Encryption of Plaintext with Key).

  3. Receiver: P = D(K, C) (Plaintext = Decryption of Ciphertext with Key).

• Challenge: Secure key distribution and management.

Block Ciphers: AES (Advanced Encryption Standard)

• Structure: Iterative Substitution-Permutation Network. Operates on fixed-size blocks (128 bits). Key sizes: 128, 192, 256 bits.

• Core Rounds (per block):

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

  2. ShiftRows: Permutation (row shifting).

  3. MixColumns: Mixing columns (linear transformation).

  4. AddRoundKey: XOR with round key.

• Example (Simplified 128-bit key, 1 round):

  • Plaintext Block: P = 00112233445566778899aabbccddeeff

  • Round Key: RK = 000102030405060708090a0b0c0d0e0f

  • AddRoundKey(P, RK) → Intermediate state → (SubBytes, ShiftRows, MixColumns) → Final ciphertext block.

Stream Ciphers: RC4

• Algorithm: Generates a pseudo-random keystream from a secret key, which is XORed with plaintext.

• Keystream Generation (KSA & PRGA):

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

  2. PRGA (Pseudo-Random Generation Algorithm): Continuously generate keystream bytes by swapping S[i] and S[j], output S[(S[i]+S[j]) % 256].

• Example (5-bit key K=10101):

  • Key length l=5. S-box init S=[0,1,2,3,4].

  • KSA: For i=0 to 4, j = (j + S[i] + K[i mod l]) mod 5, swap S[i] and S[j].

  • PRGA: i=0, j=0, i=(i+1) mod 5, j=(j+S[i]) mod 5, swap, output S[(S[i]+S[j]) mod 5]. Repeat.

  • Keystream bytes generated are XORed with plaintext bits.

Classical Cipher: Caesar Cipher

• Principle: Substitution cipher shifting each letter by a fixed number k (shift).

• Encryption: C = (P + k) mod 26 (P, C are letter positions 0-25).

• Decryption: P = (C - k) mod 26 or P = (C + (26-k)) mod 26.

• Example: Decrypt ZICVTWQNGRZGVTWAVZHCQYGLMGJ with shift k=17.

  • k' = 26 - 17 = 9 (reverse shift).

  • Z(25) -> (25+9) mod 26 = 8 -> I, I(8) -> (8+9) mod 26 = 17 -> R, etc.

  • Plaintext: RETURNHOMEATONCE.

[!TIP] Exam Focus: RSA example (p=3,q=11) and RC4 5-bit keystream generation are highly frequent.

Modes of Operation for Block Ciphers

Block vs. Stream Ciphers

II. HASH FUNCTIONS & MESSAGE AUTHENTICATION

Cryptographic Hash Functions (e.g., SHA-512)

• Properties:

  1. Pre-image resistance: Given h, hard to find M such that hash(M)=h.

  2. Second pre-image resistance: Given M1, hard to find M2≠M1 with hash(M1)=hash(M2).

  3. Collision resistance: Hard to find any M1, M2 with same hash.

• SHA-512 Process:

  1. Padding: Append 1 bit, then 0s, then 128-bit message length. Total bits ≡ 896 mod 1024.

  2. Parse: Split padded message into 1024-bit blocks M^1 ... M^N.

  3. Initialize Hash: 8 initial 64-bit constants (H0^0 ... H7^0).

  4. Process Each Block: For each block, 80 rounds of message schedule & compression function updating the hash state.

  5. Output: Final hash = H0^N || H1^N || ... || H7^N (512 bits).

Message Authentication Codes (MACs)

• Purpose: Provide integrity + authenticity (not confidentiality). Detects message modification.

• HMAC (Hash-based MAC):

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

  • ipad = 0x36, opad = 0x5C (fixed padding).

  • Uses underlying hash (SHA-256, SHA-512). Resistant to length extension attacks.

• CBC-MAC:

  • Encrypt message in CBC mode with a secret key K.

  • MAC = last ciphertext block.

  • Vulnerable if message length not fixed (needs length prepending or CMAC variant).

Collision Resistance & Birthday Attack

• Birthday Paradox: In a set of ~√N random items, probability of a collision is ~50% (N = hash output space size).

• Implication: For an n-bit hash, collision attack complexity is ~2^(n/2), not 2^n.

• Example: SHA-1 (160-bit) theoretically breakable in ~2^80 operations (theoretical), SHA-256 secure against this (~2^128).

III. DIGITAL SIGNATURES

• Mechanism:

  1. Signing: Sender computes Sig = D(Private_Key, Hash(Message)) (or uses specific algorithm like RSA-PSS, ECDSA).

  2. Verification: Receiver computes Hash(Message) and V = E(Public_Key, Sig). Valid if V == Hash(Message).

• Workflow:

  
  Sender: Message → Hash → Sign with Private Key → (Message + Signature)
  
  Receiver: (Message + Signature) → Hash(Message) → Verify Signature with Sender's Public Key → Valid/Invalid
  
  

• Importance:

  • Non-repudiation: Sender cannot deny sending.

  • Integrity: Message not altered.

  • Authentication: Confirms sender identity.

• Applications: Software distribution (code signing), financial transactions (digital cheques), legal documents.

IV. EMAIL SECURITY: PRETTY GOOD PRIVACY (PGP)

• Working Principle: Hybrid cryptosystem.

  1. Confidentiality: Message encrypted with fast symmetric session key.

  2. Session Key encrypted with recipient's public key.

  3. Authentication/Integrity: Digital signature created using sender's private key on message hash.

• PGP Message Format (Layered):

  
  [Session Key Encrypted with Recipient's Public Key]
  
  [Signature (if present): Sender's Key ID, Signature Packet]
  
  [Compressed (optional) & Encrypted Data Packet]
  
  [Literal Data Packet: actual message]
  
  

  *

  DiagramCANVAS: A block diagram showing: "Original Message" → "Compress" → "Sign" (using Sender's Private Key) → "Encrypt" (using Session Key) → "Encrypt Session Key" (with Recipient's Pub Key) → Final PGP Message Packet containing Encrypted Session Key, Signature, and Encrypted Data.

• Authentication & Confidentiality Process:

  1. Alice wants to send confidential, signed email to Bob.

  2. Alice generates random session key K_s.

  3. Alice: Compress(M) → Sign(Compress(M), Alice_Priv) → Encrypt(Compressed+Signature, K_s).

  4. Alice: Encrypt(K_s, Bob_Pub).

  5. Sends: [Enc_Ks] || [Encrypted_Data].

  6. Bob: Decrypts K_s with Bob_Priv. Decrypts data with K_s. Decompresses. Verifies signature with Alice_Pub.

V. NETWORK LAYER SECURITY: IPSEC

IPSec Protocols

• AH (Authentication Header):

  • Provides data origin authentication, integrity, anti-replay.

  • Does NOT provide confidentiality.

  • Transport Mode: AH protects IP payload (TCP/UDP), IP header partially protected (mutable fields excluded).

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

• ESP (Encapsulating Security Payload):

  • Provides confidentiality (encryption), authentication, integrity, anti-replay.

  • Transport Mode: ESP encrypts & authenticates IP payload. Original IP header unprotected.

  • Tunnel Mode: ESP encrypts & authenticates entire original IP packet. New IP header added.

Security Associations (SA)

• Concept: A unidirectional logical connection between two IPsec endpoints providing security services.

• Parameters (SAD - Security Association Database):

  • SPI (Security Parameter Index): 32-bit unique identifier for SA.

  • IP Destination Address.

  • Security Protocol (AH/ESP).

  • Mode (Transport/Tunnel).

  • Cryptographic algorithms & keys (encryption, authentication).

  • Lifetime (time/bytes).

• Management: Manual configuration or via IKE (Internet Key Exchange) protocol (IKEv1/v2).

VI. TRANSPORT LAYER SECURITY: SSL/TLS

SSL Handshake Protocol (Simplified TLS 1.2)

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

2. ServerHello: Server chooses TLS version, cipher suite, random R_S. Sends Server Certificate (contains PubKey).

3. ServerHelloDone: Server signals end of hello messages.

4. ClientKeyExchange: Client generates premaster secret, encrypts with Server's PubKey (from cert), sends.

5. ChangeCipherSpec (Client): Client signals subsequent messages will be encrypted.

6. Finished (Client): Encrypted hash of all handshake messages so far.

7. ChangeCipherSpec (Server) & Finished (Server): Server does same.

8. Secure Channel Established: Both derive master secret from premaster secret, R_C, R_S. Then derive session keys for encryption/MAC.

SSL Record Protocol Services

• Confidentiality: Using symmetric encryption (session key).

• Integrity: Using MAC (HMAC) on record.

• Anti-replay: Sequence numbers.

• Fragmentation & Reassembly: of application data.

SSL Connection vs. SSL Session

VII. NETWORK SECURITY DEVICES

Firewalls

Intrusion Detection Systems (IDS)

• HIDS (Host-based IDS):

  • Components: Audit/log monitor, signature database, detection engine.

  • Monitoring: System calls, file integrity (Tripwire), log files on specific host.

  • Deployment: On critical servers/workstations.

• NIDS (Network-based IDS):

  • Components: Network sensors (promiscuous mode), analysis engine, management console.

  • Monitoring: Network traffic (packet headers/payloads) at strategic points (DMZ, network perimeter).

  • Deployment:

    DiagramSEARCH: NIDS deployment diagram showing sensors at network segments sending data to central analysis console

  • Detection: Parameter Pattern Matching (Signature-based):

    • Compare traffic against database of known attack signatures (byte patterns, sequence of packets).

    • Pros: Low false positives for known attacks.

    • Cons: Cannot detect zero-day/novel attacks. Signature maintenance critical.

[!TIP] IDS vs Firewall: Firewall prevents attacks (proactive, access control). IDS detects attacks (reactive, monitoring).

VIII. VIRTUAL PRIVATE NETWORKS (VPN)

Types of VPN

• Site-to-Site VPN (Gateway-to-Gateway):

  • Connects entire networks (e.g., branch office to HQ).

  • Uses tunnel mode IPSec/SSL.

  • Gateways handle encryption/decryption.

• Remote Access VPN:

  • Connects individual remote users to corporate network.

  • Uses transport mode (often SSL/TLS) or tunnel mode.

  • Client software (Cisco AnyConnect, OpenVPN) on user's device.

VPN vs Trusted Operating Systems

IX. WIRELESS SECURITY

WLAN Security Challenges & Threats

• Eavesdropping: Wireless medium is broadcast.

• Rogue Access Points: Unauthorized APs inside network.

• Denial-of-Service (DoS): Jamming, deauthentication attacks.

• 802.11 Protocol Issues:

  • WEP (Wired Equivalent Privacy): Broken (RC4 weak IVs, no key management).

  • WPA/WPA2 (Wi-Fi Protected Access): Uses TKIP (WPA) or CCMP/AES (WPA2). PSK (Pre-Shared Key) vulnerable to offline dictionary attacks.

  • WPA3: SAE (Simultaneous Authentication of Equals) resists offline dictionary. Forward secrecy.

Wireless Application Protocol (WAP) Security

• Architecture:

  
  Mobile Device → WAP Browser → WAP Stack → **WAP Gateway** (protocol conversion) → Internet/Web Server
  
  

  *

  DiagramSEARCH: WAP architecture diagram showing mobile phone, WAP stack layers (WAE, WSP, WTP, WTLS), WAP Gateway, and Web Server.

• WTLS (Wireless Transport Layer Security):

  • Purpose: Provide security (like TLS) in wireless, constrained environments (low bandwidth, high latency).

  • Mechanisms: Optimized TLS variant. Supports datagram (WDP), short certificates, message fragmentation.

  • Security Issues: Gateway Decryption Model: WTLS terminates at WAP Gateway, which decrypts and re-encrypts with TLS to server. Gateway is a trusted point—if compromised, all wireless traffic exposed. End-to-end security lost.

Access Point Security in Public Networks

• Risks:

  • Evil Twin: Rogue AP with same SSID as legitimate one. Captures credentials.

  • Packet Sniffing: Unencrypted traffic easily captured.

  • Man-in-the-Middle (MitM): Rogue AP intercepts/alters traffic.

• Mitigation:

  • WPA2/WPA3-Personal (PSK): Minimum for user encryption.

  • Captive Portals: Force authentication via web page (but traffic still unencrypted until HTTPS).

  • VPN: Always use corporate VPN over public Wi-Fi.

  • 802.1X (Enterprise): Individual user authentication (EAP-TLS, PEAP).

WLAN Technical Details

• Protocol Stack: IEEE 802.11 (PHY + MAC) → LLC → Network Layer (IP).

• MPDU (MAC Protocol Data Unit) Format:

  
  [Frame Control (2)] [Duration (2)] [Addr1 (6)] [Addr2 (6)] [Addr3 (6)] [Seq Ctrl (2)] [Addr4 (6, optional)] [Frame Body (0-2312)] [FCS (4)]
  
  

  • Frame Control: Type/Subtype, flags (To/From DS, More Fragments).

  • Addresses: Addr1=Receiver, Addr2=Transmitter, Addr3=Filtering (BSSID), Addr4=WDS.

  • FCS: CRC-32 for error detection.

X. SECURE ELECTRONIC TRANSACTION (SET)

• Security Concerns in Online Transactions:

  • Confidentiality: Cardholder data (PAN) must be secret.

  • Integrity: Transaction data must not be altered.

  • Authentication: Cardholder, merchant, bank must authenticate each other.

  • Non-repudiation: Parties cannot deny transaction.

• SET Participants:

  • Cardholder, Merchant, Issuing Bank, Acquiring Bank, Certificate Authority (CA).

• Key Mechanism: Dual Signature

  • Cardholder creates two hashes: H1 = Hash(Order Info), H2 = Hash(Payment Info).

  • Cardholder computes H = Hash(H1 || H2) and signs H with private key → Dual Signature.

  • Merchant receives Order Info & Dual Signature. Can verify H1 matches Order Info and H is valid (using cardholder's cert). Cannot see Payment Info.

  • Bank receives Payment Info & Dual Signature. Can verify H2 matches Payment Info and H is valid. Cannot see Order Info.

  • Privacy: Merchant sees order, bank sees payment, neither sees both.

• Transaction Flow (Simplified):

  1. Cardholder & Merchant exchange certificates.

  2. Cardholder sends dual-signed order/payment to merchant.

  3. Merchant forwards payment info + dual signature to acquiring bank.

  4. Bank validates with issuer, sends authorization to merchant.

  5. Merchant completes order, sends confirmation to cardholder.

• Business Apps & Adoption: Designed for credit card transactions over open networks. Not widely adopted due to complexity, cost, need for all parties to implement, competition from simpler SSL-based solutions.

XI. MALWARE AND SECURITY THREATS

Types of Malicious Software

General Security Considerations & Threats

• Attack Vectors: Phishing, drive-by downloads, malicious attachments, unpatched vulnerabilities, social engineering.

• Defense-in-Depth:

  • Prevention: Firewalls, patching, user training, least privilege.

  • Detection: IDS/IPS, antivirus/EDR, log monitoring.

  • Response: Incident response plan, backups, isolation.

XII. AUTHENTICATION MECHANISMS

Biometric Authentication

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

• Advantages:

  • Hard to lose/forge (unique to individual).

  • Convenient (no token/password to remember).

• Challenges:

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

  • False Acceptance Rate (FAR): Impostor accepted.

  • Privacy: Storage of biometric templates (must be encrypted, not raw image).

  • Spoofing: Fake fingerprints, photos, voice recordings. Liveness detection needed.

  • Irrevocability: Cannot "change" biometric if compromised.

Smart Cards

• Types:

  • Contact: Requires insertion into reader (chip & pins).

  • Contactless (RFID/NFC): Proximity-based (e.g., access cards, payment cards).

• How They Enhance Security (Two-Factor):

  1. Something you have: The physical card.

  2. Something you know: PIN/password.

  • Card contains secure microprocessor storing cryptographic keys, performing operations internally. Private keys never leave card.

  • Resistant to replay, key extraction (if tamper-resistant).

XIII. ADDITIONAL TOPICS

Web Traffic Security Approaches

• SSL/TLS (HTTPS): De facto standard. Secures HTTP at transport layer (TCP). Provides server authentication (certificates), optional client auth, encryption, integrity.

• S-HTTP (Secure HTTP): Obsolete. Secured message-level (application layer). Could mix secured/unsecured pages on same server. Never widely adopted.

• IPsec: Can secure web traffic at network layer (tunnel mode). Used for site-to-site or remote access VPNs carrying web traffic. Transparent to applications.

Trusted Operating Systems

• Concept: OS designed to meet high security requirements (e.g., Orange Book B1/A1).

• Reference Monitor: Abstract machine mediating all subject-object accesses. Must be:

  1. Tamper-proof.

  2. Always invoked (unbypassable).

  3. Small enough to be verifiable.

• Security Kernel: Minimal implementation of reference monitor.

• Examples: SELinux (Flask architecture, Type Enforcement), Trusted Solaris (multilevel security), Multics (early reference monitor).

• Features: Mandatory Access Control (MAC), formal verification, secure boot, auditing.

Parameter Pattern Matching

• As used in IDS/IPS: Signature-based detection.

• Process: Compare network packet headers/payloads or system audit logs against a database of known attack patterns (signatures).

• Signature: Byte sequence, pattern of system calls, specific packet flag combinations.

• Role: Effective for known malware, exploits, attack tools. Cannot detect novel (zero-day) attacks or polymorphic malware that changes signature.

• Used in: Snort (NIDS), OSSEC (HIDS), antivirus software.

Cipher Block Modes of Operation (Detailed)

• ECB: Insecure for patterns. Only for single block encryption (e.g., encrypting a key).

• CBC: Most common for bulk encryption (disk encryption, TLS pre-1.3). Requires random, unique IV (not secret). Sequential.

• CTR: Turns block cipher into stream cipher. Parallelizable (encrypt/decrypt). Requires unique nonce/counter per key. Used in IPSec, TLS 1.3.

• OFB/CFB: Stream cipher modes. Error propagation limited. OFB keystream independent of plaintext; CFB keystream depends on previous ciphertext.

• XTS (XEX-based Tweaked CodeBook mode): For disk encryption (IEEE 1619). Tweaks based on sector number. No authentication.

[!TIP] Exam Focus: Be prepared to draw/explain PGP format, SSL handshake, NIDS deployment, WAP architecture, MPDU format, and compare VPN/Trusted OS, Block/Stream ciphers, Firewall types, AH/ESP modes. Always include examples where specified (RSA p=3,q=11; RC4 5-bit; Caesar shift=17).