Chapter 3 — Physics for computing
Lesson map
Lesson 03 published. Map, example, EX007.
flowchart TD
N0["Learning outcomes"]
N1["3.1 Authenticators as physical objects"]
N2["3.2 Radio and proximity"]
N3["3.3 Biometric sensing"]
N4["3.4 Environment and hardware life"]
N5["Synthesis"]
N0 --> N1 --> N2 --> N3 --> N4 --> N5
N5 --> EX["Worked example"]
EX --> Q["Exercise EX007"]
Logical drawing
flowchart LR
N0["Learning outcomes"]
N1["3.1 Authenticators as physical objects"]
N2["3.2 Radio and proximity"]
N3["3.3 Biometric sensing"]
N4["3.4 Environment and hardware life"]
N5["Synthesis"]
N0 --> N1 --> N2 --> N3 --> N4 --> N5 --> Q["EX007"]
[Learning outcomes]
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v
[3.1 Authenticators as physical objects]
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v
[3.2 Radio and proximity]
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v
[3.3 Biometric sensing]
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v
[3.4 Environment and hardware life]
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v
[Synthesis]
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v
[Worked example] --> [Exercise EX007]
Learning outcomes
You will (1) relate physical properties to authenticators; (2) explain why device-bound keys resist copying; (3) consider environmental limits on hardware.
3.1 Authenticators as physical objects
Many authenticators are physical: smart cards, security keys and phones with secure hardware. Their security rests partly on physics. A private key stored in a secure element is designed so that it cannot be read out through normal interfaces, and extracting it requires specialised equipment and time. NIST SP 800-63B-4 distinguishes authenticators by such properties, including whether a key can be exported. For Sana, the physical nature of authenticators means they can be lost, damaged or left at home, which the council's processes must handle as carefully as the cryptography itself.
3.2 Radio and proximity
Some authenticators communicate over short-range radio, such as near-field communication on staff badges or Bluetooth between a phone and a laptop. Signal strength falls rapidly with distance, which is why these methods require the device to be close. Proximity is useful: it suggests the person holding the device is physically present. But it is not proof, so designs should not treat a nearby signal as a substitute for user verification such as a PIN or fingerprint. Sana notes which council devices use radio and what each assumes about proximity.
3.3 Biometric sensing
Fingerprint and face sensors convert physical features into measurements and compare them with a stored template. Measurements vary with dirt, light, injury and age, so sensors accept matches within a tolerance. That tolerance creates two error rates: wrongly rejecting the right person and wrongly accepting someone else. In passkeys and device unlock, the biometric only unlocks a local key and never leaves the device. Sana explains this to council staff who worry that their fingerprints would be stored centrally, which is not how such systems are designed.
3.4 Environment and hardware life
Physical devices age and fail. Smart card readers wear out, security keys break when dropped and phone batteries die at the end of a shift. In cold depots, some touch sensors respond poorly to gloved hands. Sana lists the council's working environments, from offices to waste depots, and checks that each has an authenticator that works there, with a fallback that is still strong. Planning for physical conditions prevents staff from abandoning secure methods because they simply do not work where they need to sign in.
Worked example
Sana maps four council environments to suitable authenticators: offices to passkeys on laptops, depots to security keys usable with gloves, reception to smart cards, and mobile staff to phone-based passkeys.
Synthetic case. Environments and devices are synthetic.
Errors and limits
Treating a nearby radio signal as proof of presence confuses proximity with verification.
Exercises
- EX007 — Secure element value. Why does a secure element make key copying hard? Rubric: 3 = key cannot be read out through normal interfaces;; 2 = not exportable;; 1 = encryption;; 0 = none.
- EX008 — Biometric errors. Which two error rates does a biometric tolerance create? Rubric: 3 = false rejection and false acceptance;; 2 = two;; 1 = one;; 0 = none.
- EX009 — Depot authenticator. Which authenticator suits gloved depot staff in Sana's mapping? Rubric: 3 = security keys usable with gloves;; 2 = keys;; 1 = fingerprint;; 0 = none.
Synthesis
Physical properties shape how authenticators protect keys, how they fail and where they can be used.
Continue studying — chapter 03 of 50
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