Computer Science · Cheatsheet

Topic 3 · Networks

Chapter 3 · Wireless & security

📋 Reference · always available

Bluetooth

~10 m, low power, 1–50 Mbps. PAN. Earbuds, peripherals. 2.4 GHz.

Wi-Fi

~50–100 m, medium power, 100 Mbps–9 Gbps. LAN. Home/office. 2.4/5/6 GHz.

Cellular (4G/5G)

km via towers, high power, 100 Mbps–Gbps. WAN. Mobile. Hand-off between cells.

Frequency trade-off

Higher freq = more bandwidth BUT shorter range + worse wall penetration.

CIA triad

Confidentiality (no peeking) · Integrity (no tampering) · Availability (reachable).

Symmetric encryption

Same key both sides. Fast. AES, ChaCha20. Key-distribution problem.

Asymmetric encryption

Public key encrypts; private key decrypts. RSA, ECC. ~1000× slower. Solves key distribution.

HTTPS hybrid

Asymmetric for the handshake (share session key) → symmetric AES for bulk data. Best of both.

Digital signature

Encrypt hash with PRIVATE key; verify with PUBLIC key. Proves origin + integrity.

TLS certificate

Site's public key signed by a Certificate Authority (CA). Browser verifies → green padlock.

MFA

Multi-factor: KNOW (password) + HAVE (phone) + ARE (fingerprint). Defeats stolen passwords.

Hashing vs encryption

Hashing = one-way (for passwords, with salt + bcrypt/Argon2). Encryption = reversible.

Threats

MITM · phishing · DoS/DDoS · malware · SQL injection · XSS · brute force.

Padlock ≠ safe site

HTTPS encrypts CONNECTION. Phishing sites can have valid HTTPS too. Always check URL.