Chapter 3 — Physics for computing | Engineering Information security

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Chapter 3 — Physics for computing

Lesson map

Lesson 03 published. Map, example, EX007.

flowchart TD
  N0["Learning outcomes"]
  N1["3.1 Physics and information"]
  N2["3.2 Side channels"]
  N3["3.3 Remanence and disposal"]
  N4["3.4 Environment and availability"]
  N0 --> N1 --> N2 --> N3 --> N4
  N4 --> EX["Worked example"]
  EX --> Q["Exercise EX007"]

Logical drawing

flowchart LR
  N0["Learning outcomes"]
  N1["3.1 Physics and information"]
  N2["3.2 Side channels"]
  N3["3.3 Remanence and disposal"]
  N4["3.4 Environment and availability"]
  N0 --> N1 --> N2 --> N3 --> N4 --> Q["EX007"]

[Learning outcomes]
    |
    v
[3.1 Physics and information]
    |
    v
[3.2 Side channels]
    |
    v
[3.3 Remanence and disposal]
    |
    v
[3.4 Environment and availability]
    |
    v
[Worked example] --> [Exercise EX007]

Learning outcomes

You will (1) relate physical properties to security; (2) explain side channels and media remanence; (3) consider environmental threats to availability.

3.1 Physics and information

Computers store and move information as physical states: charges, magnetic domains and signals on wires or in the air. Lindiwe learns that physical properties create security effects. Wireless signals from a depot access point leak beyond the building, so an attacker in the car park can receive them. Encryption protects confidentiality where physics cannot. She notes that security design must start from what physically reaches an adversary, not from where the network diagram draws its boundary. She measures signal strength at the depot fence with a simple phone app.

3.2 Side channels

Devices emit information through power use, timing, sound and electromagnetic radiation. A cryptographic operation that takes slightly longer for some keys can reveal those keys to a careful observer. Lindiwe reads how such side channels have been exploited in research and why constant-time implementations matter. At Saltmarsh the practical lesson is narrower: use well-reviewed cryptographic libraries rather than home-made code, since library authors handle timing behaviour that application developers rarely consider. Lindiwe checks which libraries Saltmarsh's payment and login services use and confirms that both rely on widely reviewed implementations maintained by large communities.

3.3 Remanence and disposal

Data persist physically after deletion. On magnetic disks, deleted files remain until overwritten; on solid-state drives, wear-levelling may keep old copies in blocks the operating system cannot address. Saltmarsh retires about a hundred laptops a year. Lindiwe recommends full-disk encryption from the start, so that destroying the key renders remaining data unreadable, plus certified destruction for drives that held payroll data. She learns that disposal is a physics problem as much as a policy one. The disposal supplier's certificates are now filed with each asset record.

3.4 Environment and availability

Heat, power and water threaten availability. A server room above its design temperature throttles processors and shortens component life; a power cut without backup stops depots invoicing. Lindiwe surveys Saltmarsh's main server room and finds cooling adequate but the uninterruptible power supply sized years ago for fewer servers. She calculates that its runtime at current load is under ten minutes. Prof. Hale notes that availability failures often begin with physics nobody rechecked. The facilities manager orders a larger unit after reading her calculation, and Lindiwe adds a yearly load check to the maintenance calendar.

Worked example

Lindiwe connects radio leakage, side channels, remanence and power capacity to security controls at Saltmarsh, recommending encryption, reviewed libraries and a power supply review.

Synthetic case. Measurements are synthetic.

Errors and limits

Deleting files does not remove data from storage media.

Exercises

EX007 — Radio leakage. Why does encryption matter for depot wireless? Rubric: 3 = signals reach beyond the building;; 2 = partly correct;; 1 = vague;; 0 = none.
EX008 — SSD remanence. Why can deleted data survive on solid-state drives? Rubric: 3 = wear-levelling keeps old copies;; 2 = partly correct;; 1 = vague;; 0 = none.
EX009 — UPS runtime. What runtime does the power supply give at current load? Rubric: 3 = under ten minutes;; 2 = partly correct;; 1 = wrong;; 0 = none.

Continue studying — lesson 03 (chapter 03 of 50)

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