## 9 Challenges
### Privacy  
The information that is collected from the car's sensors has to be protected and should only be used after anonymizing it. The concern is that your car should warn you if you are over the speed limit  
\begin{table}
\begin{tabular}{c c c c} \hline
**S/No** & **Schemes** & **Features** & **Disadvantages** \\ \hline \multirow{4}{*}{1} & & _(a)_ User privacy is achieved by using & _(a)_ Thousands of certificates to be \\  & & pseudonym coupled with PKI; & downloaded to OBU; (b) Certificates need to \\  & Pseudonym & (b) Certificates (public, private key pairs) are & be replenished periodically; _(c)_ CRL has to be \\  & _cum_ PKI & downloaded from trusted authority; & maintained and it keeps on changing and is \\  & & (c) Pseudonyms are changed continuously for & time- and resource-hungry; (d) The CA can \\  & & preserving privacy. & link pseudonyms with vehicles, and therefore \\ \hline \multirow{4}{*}{2} & & _(a)_ Privacy achieved by forming groups; & _(a)_ TA can reveal the real identity of user \\  & Group & (b) Reduced transmission by ways of & _(b)_ CRL has to be maintained and checked \\  & Signatures & periodic broadcasts by a single member & and it increases with the size of the group. \\ \hline \multirow{4}{*}{3} & & Information accepted based on trust. & _(a)_ No initial trust information available as \\  & Trust-Based & Trust established based on previous record in & centralized system would be too slow/too \\  & Schemes & a centralized authority or based on current & huge; (b) Protection against inside attackers is \\  & and previous interactions with the user in the & difficult; (c) Trust score has to be maintained \\  & & same session. & and checked. \\ \hline \multirow{4}{*}{4} & & _(a)_ Messages are disseminated by ways of & Flooding is used to disseminate messages \\  & & flooding it to neighbours; (b) Ensures privacy & which if not effective, can overwhelm the \\  & & as long as size of group is adequate. & network and eat up bandwidth for the same \\  & & & message. Efficient and practical \\  & & & algorithm missing. \\ \hline \multirow{4}{*}{5} & Identity-Based & _(a)_ User’s public key is derived from his & _(a)_ Secure channel needed for private key \\  & Encryption & eliminates the need for public key distribution & distribution; (b) PKG has to be highly secure; \\  & (IBE) With & (b) No need for certificate downloads and & (c) TRA can reveal the real identity of the \\  & Pseudonyms & storage in OBUs (c) Additional information & user, therefore must be secure; (d) Revocation \\  & & such as a time stamp can also be added. & is still an open problem. \\ \hline \end{tabular}
\end{table}
Table 2: Qualitative Comparison of Security Schemes in VANETs.  
but should not turn against you, _i.e._, reporting to the authorities that you went over the speed limit. However, there are many instances when sharing location information with some service providers might actually be beneficial for the user, e.g., insurance companies might offer to reduce the insurance premiums if such details are shared. There can be similar incentives from other service providers to use vehicle sensor data in return for some reward that can actually encourage participation from the users.