Secure Ranging: Delay, Distance, and Policy

Secure Ranging: Delay, Distance, and Policy

Ada converts the chapter’s ranging times into distances and checks each access-policy case against the unlock zone

ADA · CALCULATION AUDIT

Secure Ranging: Delay, Distance, and Policy

Ada: Secure ranging is only useful when the timing number and the policy threshold agree. The chapter already gives the numbers needed to audit the physics and the access decision without inventing a new measurement.

A secure cabinet sets a policy threshold of 1.2 m: a phone presents a valid credential and the cabinet receives a UWB result of 0.74 m with a fresh timestamp, yet that same reading must be denied if it comes from the wrong side of the cabinet or if the ranging record is 2.5 seconds old when policy allows only 500 ms. IEEE 802.15.4z’s Scrambled Timestamp Sequence is meant to stop an attacker from forging an early pulse, and the chapter shows a 20 ns extra round-trip delay reading as about 3 m of extra apparent path. This audit asks the question those policy cases invite: does the chapter’s own timing-to-distance arithmetic actually support each accept, deny, and fallback verdict it claims?

Companion to the chapter UWB Applications and Security — every number here comes from that chapter.

See the relationship before changing it

The figure reads from left to right. The blue card is extra round-trip delay. The middle card applies this page's rule. The green card is apparent extra path. Walk the arrows once: set the input, apply the rule, then read the result with its unit.

The retained audit below checks several chapter fixtures. This model keeps those stated values fixed and changes only extra round-trip delay, so the numeric fixture does not switch without explanation.

Walk the arrows. Added round-trip delay looks like extra distance, so timing integrity belongs inside the access policy.

Derive the baseline in four named moves

1. 1

   Name the input. The chapter baseline is 20 ns.

2. 2

   Name the relationship. extra path = round-trip delay x 0.30 m/ns / 2

3. 3

   Substitute with units. 20 ns x 0.30 m/ns / 2 = 3.0 m

4. 4

   Read the result. Keep the unit beside the value. Use it only inside the technical boundary on this page.

Predict, then change extra round-trip delay

Try Predict the direction of extra path = round-trip delay x 0.30 m/ns / 2. Test another extra round-trip delay, then compare apparent extra path.

Extra round-trip delay

20 ns

Chapter baseline

Apparent extra path

Observe Added round-trip delay looks like extra distance, so timing integrity belongs inside the access policy. Reset extra round-trip delay to 20 and compare apparent extra path.

Explain Added round-trip delay looks like extra distance, so timing integrity belongs inside the access policy.

Check yourself

What should you do before trusting a moved-control result?

Answer: Predict its direction, apply the shown relationship, keep the units, and reset to the worked baseline.

What does this small model leave out?

Answer: Only extra round-trip delay moves here. Field effects named in the technical boundary stay fixed.

Honesty note. This teaching model changes only extra round-trip delay. It is not a field prediction.

Try

Convert an added 20 ns round-trip delay at 0.30 m/ns, then compare the 0.83 m fresh record and 3.8 m delayed path with the 1.2 m unlock zone.

Observe

The relay adds 3.0 m of apparent one-way path; the fresh reading has 0.37 m margin, whereas 3.8 m lies 2.6 m beyond the zone.

Explain

Time-of-flight converts added round-trip delay into distance after division by two, but authorization remains a policy conjunction of range, freshness, confidence, credential, and side-of-door evidence.

Convert the timing evidence and check each policy case

• Convert timing to distance: 1 ns x 0.30 m/ns = 0.30 m of one-way travel.
• Check the relay-delay example: a 20 ns extra round-trip delay becomes 0.30 m/ns x 20 ns / 2 = 3.0 m of extra apparent one-way path.
• Compare the unlock record to the zone: 1.2 m - 0.83 m = 0.37 m of distance margin before freshness, side-of-door, and confidence checks are applied.
• Check the stale-record case: 1.6 s = 1600 ms, which is 1600 ms - 180 ms = 1420 ms older than the fresh example record, so the policy should not treat the old distance as current evidence.
• Check the delayed-path denial: 3.8 m - 1.2 m = 2.6 m beyond the unlock zone, so the distance evidence should deny or fall back even if the credential itself is authorized.

The audit conclusion is bounded: the arithmetic supports the chapter’s policy examples, but the action still depends on credential authorization, confidence, side-of-door evidence, timestamp freshness, and fallback behavior.

Every number above is taken from the chapter’s own ranging and policy examples and re-derived step by step.

Technical boundaries: This propagation-and-threshold audit omits clock and antenna-delay calibration, NLOS multipath, channel-impulse-response scoring, coordinated relay attacks, body shadowing, timestamp uncertainty, and actuator latency.

Ready: work the ledger before checking it.