Digital communications and hardware security live next to each other, and the same math that makes communications work makes side-channels work.
- Side-channel as channel. A power-analysis attack treats a CPU's instantaneous current draw as a noisy communication channel whose secret message is the key. The attacker uses cross-correlation (Chapter 3, Chapter 12, the same operation) to extract bits.
- DPA vs Viterbi accelerators. Decoder hardware that branches on key-dependent metrics leaks key bits in its current. Modern designs add metric balancing and traceback randomization.
- CRC is not a MAC. WEP's misuse of CRC-32 inside RC4 broke Wi-Fi privacy in 2001. Use HMAC, AES-GMAC, Poly1305, or other keyed authentication primitives.
- Replay attacks. A correctly-formed and CRC-valid frame replayed unchanged is by definition still valid. CRC catches transmission errors, not malice. Replay protection requires sequence numbers, nonces, or timestamps.
- Spread spectrum and spoofing. Civil GPS codes are public knowledge, so a spoofer with a software-defined radio can synthesize them. Encrypted military codes resist spoofing because their PN sequences are unknown to the attacker. The same spread-spectrum trick that hides the signal from jammers lets defenders authenticate signal source.
- Fault injection vs ECC. Attackers try to glitch chips to flip multiple bits per ECC word, exceeding the code's correction capability. Stronger codes (BCH-T5, multi-bit Hamming, modern LDPC in flash) raise the bar. For ultra-high assurance, designs use lock-step redundant cores with comparator logic.
- TEMPEST and modulation. Old printers, monitors, and even some modern HDMI cables unintentionally radiate the data they handle as faint AM/FM modulations of nearby clock harmonics. A properly tuned receiver can demodulate them at distance, recovering whatever was on screen. This is exactly the same math as AM/FM demodulation; the channel just was not meant to exist.
- Encryption above modulation. Modulation and channel coding are designed for noise tolerance, not confidentiality. Always assume anyone can demodulate and decode your bits; rely on AES, ChaCha20, or post-quantum primitives to keep the contents private.