Network Security

7.1 Symmetric Encryption

Symmetric encryption uses the same secret key for both encryption and decryption. Both parties Must share the key securely before communication.

Block cipher modes of operation:

Theorem 7.1. ECB mode is insecure for messages longer than one block because identical plaintext Blocks produce identical ciphertext blocks, revealing patterns.

Proof. If the plaintext contains repeated blocks $P_i = P_j$Then under ECB, $C_i = E_K(P_i) = E_K(P_j) = C_j$. An attacker observing identical ciphertext blocks knows the corresponding Plaintext blocks are identical, regardless of the key. $\blacksquare$

Key distribution problem. Symmetric encryption requires a secure channel to exchange keys. For $n$ parties, $n(n-1)/2$ keys are needed. This motivates asymmetric encryption and key exchange Protocols.

7.2 Asymmetric Encryption

Asymmetric encryption uses a key pair: a public key (for encryption/verification) and a private Key (for decryption/signing). The public key can be freely distributed.

RSA. Based on the difficulty of factoring large integers.

1. Choose two large primes $p$ and $q$. Compute $n = pq$ and $\phi(n) = (p-1)(q-1)$.
2. Choose $e$ such that $1 \lt e \lt \phi(n)$ and $\gcd(e, \phi(n)) = 1$.
3. Compute $d$ such that $e \cdot d \equiv 1 \pmod{\phi(n)}$.
4. Public key: $(n, e)$. Private key: $(n, d)$.
5. Encrypt: $c = m^e \bmod n$. Decrypt: $m = c^d \bmod n$.

Diffie-Hellman key exchange. Allows two parties to establish a shared secret over an insecure Channel without prior shared key.

1. Public parameters: prime $p$ and generator $g$.
2. Alice picks secret $a$Sends $A = g^a \bmod p$.
3. Bob picks secret $b$Sends $B = g^b \bmod p$.
4. Shared secret: $s = B^a \bmod p = g^{ab} \bmod p = A^b \bmod p$.

An eavesdropper who sees $g$, $p$, $A$, $B$ cannot compute $g^{ab}$ without solving the discrete Logarithm problem.

Digital signatures. The sender signs a message hash with their private key. Anyone can verify Using the sender”s public key. Provides authentication, integrity, and non-repudiation.

7.3 TLS in Depth

TLS 1.3 (RFC 8446) provides a clean, secure design:

• 1-RTT handshake: Client and server exchange key shares in the first round trip, enabling immediate encrypted communication.
• 0-RTT resumption: On subsequent connections, the client can send application data immediately using a pre-shared key from a previous session. Vulnerable to replay attacks.
• Forward secrecy: Every session uses ephemeral ECDHE keys, so compromise of the long-term private key does not allow decryption of past sessions.
• Cipher suites: Only AEAD ciphers are supported (AES-GCM, ChaCha20-Poly1305). No CBC or RC4.

Certificate chain validation:

1. Server presents its certificate (signed by intermediate CA).
2. Client verifies the signature chain up to a trusted root CA.
3. Client checks: validity dates, hostname match (SAN), revocation (CRL/OCSP).
4. Client verifies the server’s …/1-number-and-algebra/3_proof-and-logic-of-possession for the private key.

7.4 Firewalls

A firewall controls network traffic based on security rules.

Types:

Stateful inspection. Unlike simple packet filtering, a stateful firewall maintains a connection Table. It can distinguish between a new TCP SYN (allowed) and an unsolicited SYN+ACK (blocked), And it tracks UDP “connections” by observing request-response patterns.

DMZ (Demilitarised Zone). A separate network segment for publicly accessible services (web Servers, mail servers). The firewall allows external access to the DMZ but restricts DMZ-to-internal Access.

7.5 VPNs

A Virtual Private Network creates an encrypted tunnel over a public network.

IPsec architecture (RFC 4301):

• Security Association (SA): A one-way logical connection with parameters: SPI (Security Parameters Index), destination IP, protocol (AH or ESP), encryption algorithm, key, lifetime.
• IKE (Internet Key Exchange): Protocol for establishing SAs. IKEv2 is the current standard. Uses Diffie-Hellman for key exchange and digital signatures for authentication.
• AH (Authentication Header): Provides integrity and authentication but NOT confidentiality. Protects the entire IP packet (immutable fields). Protocol number 51.
• ESP (Encapsulating Security Payload): Provides confidentiality, integrity, and authentication. Protocol number 50.

WireGuard. A modern VPN protocol (Linux kernel 5.6+):

• Uses ChaCha20 for encryption, Poly1305 for authentication, Curve25519 for key exchange.
• No static keys: each peer has a public/private key pair.
• Under 4000 lines of code (vs ~100,000 for OpenVPN + OpenSSL).
• Roaming: peers can change IP without reconfiguration.

7.6 Packet Filtering

Rule syntax (generic):

Key principles:

• Rules are evaluated top-to-bottom; first match wins.
• Default deny policy: the last rule should deny everything not explicitly allowed.
• Specific rules must precede general rules.
• Stateful firewalls automatically allow return traffic for established connections.

:::caution Common Pitfall Encryption does not imply authentication. A message encrypted with a public key guarantees Confidentiality but does not prove who sent it. Digital signatures (signing with a private key) Provide authentication and non-repudiation. TLS combines both via the certificate chain.

7.7 Common Network Attacks

Denial of Service (DoS) and Distributed DoS (DDoS). Overwhelm a target with traffic, preventing Legitimate access. Amplification attacks (DNS, NTP) use small requests that generate large responses Directed at the victim.

Man-in-the-middle (MITM). An attacker intercepts communication between two parties. Defences: TLS with certificate pinning, mutual authentication, VPNs.

ARP spoofing. See Section 4.4. An attacker sends forged ARP messages to redirect traffic through Their machine.

DNS spoofing (cache poisoning). Injecting forged DNS records into a resolver’s cache, redirecting Users to malicious sites. Defences: DNSSEC (cryptographic signatures on DNS records), source port Randomisation, TSIG.

SQL injection. An attacker inserts malicious SQL into application input fields. Not strictly a Network attack, but often delivered over HTTP. Defences: parameterised queries, input validation.

TCP SYN flood. An attacker sends many SYN packets without completing the handshake, exhausting The server’s connection table. Defences: SYN cookies (encode state in the initial sequence number), Rate limiting, connection throttling.

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