Draft:Early Detect Late Commit

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Early Detect Late Commit (EDLC, ED/LC) is a physical-layer distance-reducing attack affecting wireless ranging systems such as UWB ranging or Chirp Spread Spectrum (CSS). These kinds of systems are used in vehicles for keyless entry, localisation in consumer (e.g., Apple AirTag) and industrial applications. By using the ED/LC attack, an attacker can artificially reduce the measured distance between two wireless devices, effectively circumventing an application's requirement of physical viscinity (e.g., only unlock car if keyfob is sufficiently close).

Ranging Principle

By shortening the processing delay from $t_p$ to $t_p^{\prime }$, an attacker at distance $d^{\prime }$ can pretend to be at location $d$

Time of flight-based ranging systems leveraging Ultra-wide band or Chirp Spread Spectrum (CSS) measure distance by estimating the time it takes a signal to propagate through a medium (usually air) at a known speed (approximately $c$, the speed of light, in air). The total round-trip time $t_{ToF}$ between a verifier (e.g., car) and a prover (e.g., keyfob) a distance $d$ apart equals the sum of the total propagation delay $2d/c$ and a processing delay $t_p$. This processing delay is fixed and known to the verifier, such that it that can be substracted from $t_{ToF}$ to calculate the actual propagation delay and physical distance $d$.

To reduce the apparent distance as measured by the verifier, an attacker has to reduce the round-trip time $t_{ToF}$. As it is not possible to shorten the actual propagation delay of the radio wave (as it is already propagating at the speed of light), an attacker has to reduce the processing time $t_p$. For the attack to be relevant, an attacker has to shorten $t_p$ to such an extent that it completely compensates the additional distance the attacker wants to introduce.

Attack

An attacker can shorten the processing time $t_p$ and therefore the apparent distance by prematurely deciding on the received signal (early detect) and prematurely sending a response whose value is decided later (late commit).

A reduction of the total time can be achieved if an attacker does not need to fully receive a symbol before they can determine the symbol value. This is possible because a symbol has non-zero length and might carry redundant information, e.g., multiple pulses encoding a single bit. Specifically, in the case of chirp signals, an attacker does not have to receive the complete up- or down-chirp lasting $t_{\mathrm{c}\mathrm{h}\mathrm{i}\mathrm{r}\mathrm{p}}$, instead they can early-detect the type of chirp (up or down) prematurely after time $t_{ed}$. Before the attacker learns the actual value of the symbol, they already start to transmit an arbitrary signal. Only when the value of the symbol is known to the attacker after $t_{lc}$, they can switch from the arbitrary signal to the actual symbol value (they late-commit to the actual value). Even if the symbol was arbitrary up to $t_{lc}$, the receiver ideally still correctly decodes the symbol, due to intentional redundance when sending the symbol for the full $t_{\mathrm{c}\mathrm{h}\mathrm{i}\mathrm{r}\mathrm{p}}$ and error tolerance built into the receiver.

Defenses

It is possible to defend against ED/LC attacks in Ultra-wideband-based systems by randomly reordering pulses. As only the sender and receiver (i.e., prover and verifier) know the correct sequence to (de)scramble the pulses, the bits are completely unpredictable for an attacker. Hence, an attacker is unable to detect a symbol value early.