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Engineering · Chapter 3 · English

Chapter 3 — Physics for computing

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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 Storage persistence"]
  N2["3.2 Memory and power"]
  N3["3.3 Physical disturbance"]
  N4["3.4 Heat and reliability"]
  N5["3.5 Physical access"]
  N6["Encryption and physical theft"]
  N7["Synthesis"]
  N0 --> N1 --> N2 --> N3 --> N4 --> N5 --> N6 --> N7
  N7 --> EX["Worked example"]
  EX --> Q["Exercise EX007"]

Logical drawing

flowchart LR
  N0["Learning outcomes"]
  N1["3.1 Storage persistence"]
  N2["3.2 Memory and power"]
  N3["3.3 Physical disturbance"]
  N4["3.4 Heat and reliability"]
  N5["3.5 Physical access"]
  N6["Encryption and physical theft"]
  N7["Synthesis"]
  N0 --> N1 --> N2 --> N3 --> N4 --> N5 --> N6 --> N7 --> Q["EX007"]

[Learning outcomes]
    |
    v
[3.1 Storage persistence]
    |
    v
[3.2 Memory and power]
    |
    v
[3.3 Physical disturbance]
    |
    v
[3.4 Heat and reliability]
    |
    v
[3.5 Physical access]
    |
    v
[Encryption and physical theft]
    |
    v
[Synthesis]
    |
    v
[Worked example] --> [Exercise EX007]

Learning outcomes

You will (1) relate physical properties of hardware to security; (2) explain storage persistence and power; (3) understand why hardware can fail or leak.

3.1 Storage persistence

Data on solid-state drives and hard disks persist without power. Deleting a file usually removes only its index, not the data. This is why a patron's "deleted" CV may still be recoverable, and why Fernhill needs full restores and disk encryption rather than relying on users to delete files.

Mateo demonstrated this to Marta on a lab PC with synthetic files. After an ordinary delete, a recovery tool found the file's contents still on the disk. After a restore from the clean image, the patron area held nothing from before. The demonstration convinced her more than any explanation did.

3.2 Memory and power

Memory loses its contents when power is removed, but not instantly; this is one reason why encryption keys are protected while devices are running and why devices should lock or shut down when unattended.

For Fernhill this means a public PC left logged in with an open session is a risk even if its disk is encrypted, because the key is in use while it runs. Mateo's requirements therefore include automatic logoff after inactivity and a restart at closing time. Shutting down clears memory and returns the device to a state where the encryption protects the disk again.

3.3 Physical disturbance

Kim et al. (2014) showed that repeatedly accessing rows in DRAM can flip bits in neighbouring rows, a physical effect of densely packed memory cells. It shows that hardware behaviour can create security problems that software alone did not expect.

Fernhill cannot fix such effects itself; they are addressed by hardware makers and firmware updates. Mateo notes the lesson for his design: keep firmware updated, buy hardware with published mitigations and do not assume that software controls alone describe what the machine can do. He records firmware versions in the inventory so updates can be tracked. A software audit that ignores the hardware it runs on gives only part of the picture.

3.4 Heat and reliability

Heat shortens component life. Public PCs in small enclosures fail more often, which matters when planning reliability in chapter 32.

Mateo checked the failure records and found that PCs in the cramped study carrels at Millbrook failed more often than those on open desks. Better airflow, or moving the units, may cost less than replacing them. Heat also affects how long nightly restores take, since overheating drives slow down. These are practical details, but a system that resets every night depends on hardware that keeps working every night.

3.5 Physical access

Anyone at a public PC has physical access: they can attach devices, reboot or open firmware menus. Physical security and firmware settings matter as much as software.

Mateo's design therefore locks firmware settings with an administrator password, disables booting from external devices and places cases where they can be seen by staff. None of these makes tampering impossible. Together they make it harder and more noticeable, which suits a public space where staff are present but cannot watch every screen.

Encryption and physical theft

Because physical access is unavoidable at a public PC, disk encryption matters if a machine is stolen. Encryption turns the stored data into unreadable form without the key, which the TPM releases only to the expected boot chain. This links physics, architecture and cryptography, topics later chapters develop.

Worked example

Marta asks why staff could not just delete leftover files each evening. Mateo explains that deletion leaves data recoverable, whereas restoring the disk from a clean image overwrites the patron area, and encryption protects data if a disk is removed.

Synthetic case. Synthetic.

Errors and limits

Physical effects are complex; engineers rely on published research rather than speculation.

Exercises

Synthesis

Hardware physics shapes what software controls can promise.

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