schmittlauch/hs-pseudonym-schemes-in-v2x
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Trolli Schmittlauch 2018-06-11 23:36:41 +02:00
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@ -325,15 +325,15 @@ The proposed countermeasure is again the adoption and regular change of pseudony
\subsection{Pseudonym Change Strategies}
A crucial parameter of pseudonym schemes has been left out so far: How and when pseudonyms are actually changed. To show why that is so important, let us imagine a lone car on a street in the countryside: If a single car just changes pseudonyms there, immediately continuing its broadcasts under the new pseudonym, linkage of the both pseudonyms is trivial for an observer. \\
Another example: Let us look at a traffic jam with 10 cars standing within reception range of an observer. Now there are multiple cars around making the mapping of pseudonyms to cars not totally trivial. But if we assume that each car only changes pseudonyms every 24 hours and does this at an arbitrary time, the probably that only 1 vehicle changes pseudonyms within a short time range is very high, making linkage os pseudonyms easy again. \\
A last example so far: Focusing on one vehicle, let us assume it changes its pseudonym in a perfectly ambiguosly way which can't be linked to the old one reliably. But after the pseudonym change, an alredy enqueued packet is sent, containing identifiers linkable to the preious pseudonyms.
Another example: Let us look at a traffic jam with 10 cars standing within reception range of an observer. Now there are multiple cars around making the mapping of pseudonyms to cars not totally trivial. But if we assume that each car only changes pseudonyms every 24 hours and does this at an arbitrary time, the probability that only 1 vehicle changes pseudonyms within a short time range is very high, making linkage of pseudonyms easy again. \\
A last example so far: Focusing on one vehicle, let us assume it changes its pseudonym in a perfectly ambiguously way which can't be linked to the old one reliably. But after the pseudonym change, an already enqueued packet is sent, containing identifiers linkable to the previous pseudonyms.
These examples already show important points to take care of when changing pseudonyms: There needs to be some ambiguity regarding which node changed to which pseudonym – there shall be other nodes present within the reception range, coordination and frequency of change matter, and all identifiers need to be changed simultaneously with buffers being flushed or discarded.
The \ac{ETSI} \ac{ITS} working group released gathers a number of concept for pseudonym change strategies in \cite{europeantelecommunicationsstandardsinstituteetsiETSITR1032018}: The parameters deciding about a pseudonym change (e.g. time period or way length) shall be randomized to prevent linkability by analyzing the periodicity of changes. After changing pseudonyms, random-legth \textit{silent periods} shall be abided in which nodes stop sending any packages. When using a \textit{vehicle-centric} strategy, pseudonym change time, its frequency and duration of silent periods are influenced by the vehicle's mobility and trajectory to make linkage of pseudonyms based on broadcasted movement parameters harder. When using a density-based approach, pseudonyms are changed only if enough other vehicles are around to avoid unnecessary unambiguous pseudonym changes.
The \ac{ETSI} \ac{ITS} working group released gathers a number of concept for pseudonym change strategies in \cite{europeantelecommunicationsstandardsinstituteetsiETSITR1032018}: The parameters deciding about a pseudonym change (e.g. time period or way length) shall be randomized to prevent linkability by analyzing the periodicity of changes. After changing pseudonyms, random-length \textit{silent periods} shall be abided in which nodes stop sending any packages. When using a \textit{vehicle-centric} strategy, pseudonym change time, its frequency and duration of silent periods are influenced by the vehicle's mobility and trajectory to make linkage of pseudonyms based on broadcasted movement parameters harder. When using a density-based approach, pseudonyms are changed only if enough other vehicles are around to avoid unnecessary unambiguous pseudonym changes.
Mix-zones are geographical areas where no messages of location-aware services are exchanged. This concept is supposed to make linkage of ingoing and outgoing vehicles from the zone difficult. These zones are especially effective in high-density and high-fluctuation areas like intersections or parking spots. \\
Within these zones, vehicles could collaboratively change pseudonyms by first announcing it via broadcast messages and then changing sychronously. The efficiency of that apporach depends heavily on the density of the situation. \\
Mix-zones are geographical areas where no messages of location-aware services are exchanged. This concept is supposed to make linkage of in-going and outgoing vehicles from the zone difficult. These zones are especially effective in high-density and high-fluctuation areas like intersections or parking spots. \\
Within these zones, vehicles could collaboratively change pseudonyms by first announcing it via broadcast messages and then changing synchronously. The efficiency of that approach depends heavily on the density of the situation. \\
A special variant are \textit{cryptographic mix-zones}: Within these zones with a size limited to the radio coverage of \iac{RSU}, no identifying data is sent in plaintext but everything is encrypted with the same symmetric key provided by the \ac{RSU}. This allows the usage of location-aware collision detection messages while preventing an outsider from eavesdropping, without having to switch off important safety features.\todo{insiders, infrastructure tracking}
An alternative to just changing from one pseudonym to the next one from a node's internal storage is swapping pseudonyms randomly between nearby vehicles. This approach is limited though by the inclusion of vehicle-specific data into messages and legal requirements demanding the possibility of an identity resolution for law enforcement.
@ -342,7 +342,7 @@ The \ac{ETSI} survey \cite{europeantelecommunicationsstandardsinstituteetsiETSIT
The SCOOP@F project proposes a timeslot-based round-robin pseudonym selection. The interesting thing about this is that reuse of pseudonyms from the local pool is explicitly allowed as the selection mechanism makes sure they are not always re-used in the same order. This is a useful approach against the problem of pseudonym refill (acquiring new pseudonyms) not always being possible. \\
The strategy proposed by the Car-2-Car Communication Consortium is dividing each trip into at least 3 segments: The first one from the start of the trip to a middle segment, the middle segment being common to a number of people and unassociated to certain origins and destinations, and the last segment to the intended destination of the trip. This shall achieve that locations significant to a user can neither be linked together nor to the user and thus preventing individual movement profiles. The values for changing pseudonyms have been statistically obtained with the outcome of changing pseudonyms at the beginning of a trip, then randomly after 0.8-1.5 km, and from then on randomly at least every 0.8 km or 2-6 minutes.
Some safety requirements of the \ac{ETSI} standard affect pseudonym change: In critical situations when a receiving station would need to take immediate action in response to received safety information, pseudonyms have to be locked. The reason behind that is that cooperational collision avoidance depends on all vehicles broadcasting their location and trajectory. Vehicles in a silent period due to a pseudonym change wouldn't be taken into account, and vehicles changing pseudonyms without silent period could appear as duplicate or ghosting vehicles hindering collison evasion. \todo{who can trigger this locking?} Recognizing such critical situations and initiating the pseudonym locking is done by the receiving \ac{ITS} vehicle, which decreases the risk of an attacker trying to deliberately lock pseudonyms without a critical situation being present.
Some safety requirements of the \ac{ETSI} standard affect pseudonym change: In critical situations when a receiving station would need to take immediate action in response to received safety information, pseudonyms have to be locked. The reason behind that is that cooperational collision avoidance depends on all vehicles broadcasting their location and trajectory. Vehicles in a silent period due to a pseudonym change wouldn't be taken into account, and vehicles changing pseudonyms without silent period could appear as duplicate or ghosting vehicles hindering collision evasion. \todo{who can trigger this locking?} Recognizing such critical situations and initiating the pseudonym locking is done by the receiving \ac{ITS} vehicle, which decreases the risk of an attacker trying to deliberately lock pseudonyms without a critical situation being present.
\subsection{Further Pseudonym Scheme Techniques}
@ -384,7 +384,7 @@ As an advantage of these schemes, nodes don't have to deal with generating, issu
Revocation is more complicated in group signature schemes: As all group nodes are indistinguishable by their exposed pseudonym identifiers, it's not possible to distribute revocation lists. A re-setup of the group by changing system parameters can exclude certain nodes, but has a big overhead as all group members are required to change their keys. A proposed solution for that circumvents the problem by remote-controlling the \ac{HSM} to remove the keys from its memory.
A special kind of group signature schemes not requiring setup and being more dynamic are \textit{ring signture schemes}. Their usage is only briefly covered in \cite{petitPseudonymSchemesVehicular2015}.
A special kind of group signature schemes not requiring setup and being more dynamic are \textit{ring signature schemes}. Their usage is only briefly covered in \cite{petitPseudonymSchemesVehicular2015}.
\subsubsection{Pseudonyms using Symmetric Cryptography}