In this section, the proposed homomorphic authentication scheme for practical VANETs is illustrated in detail. The automotive healthcare monitoring and detection strategies for all passengers of passing vehicles can be achieved. Sensitive personal medical data are locally validated and then uploaded to remote for further analysis and historical retrieving. Subsequently, the efficient infection tracking mechanism on suspected cases can be done, where the precise time-oriented travelling route of individual passenger could be retrieved. Therefore, the current practical healthcare monitoring requirements for COVID-19 pandemic control can be met. Intuitively, our design emphasizes the automotive authentication and medical data sharing in high-mobility VANET scenarios. The pairing-free certificateless cryptography is employed for key escrow resilience. User anonymity for all participating vehicles, as well as the involving passengers, are well preserved. Meanwhile, random identity updating design for various communication session is provided. Motivated by the blockchain design, the hash value for each vehicle is maintained by each RSU upon validation. Moreover, the successive RSUs could efficiently verify the correctness of the chain information by taking use of the data sharing characteristic of edge RSU clusters.
4.1. Device Initilization Phase
The device initialization phase is designed for system initialization and vehicle registration prior to authentication. Notably, the
is defined as the validated and trustworthy entity during the whole communication session. Therefore, the crucial VANET system parameters and master key are issued and distributed by
. Initially,
define
as the cyclic group generated by the large prime order
q, where
P denotes the generator of the cyclic group. Additionally, the utilized one-way hash functions
are, respectively, performed as
In this case, the VANET system parameters set will be published in the form of
.
As for individual RSU,
assigns the original identity
to each legitimate RSU during offline registration. The corresponded RSU secret key
is randomly generated and distributed to RSU as well. Therefore, the confidential RSU identity set
is safely stored in both
and RSU itself. Similarly, the initial registration process of vehicle should be conducted in advance. That is, the distinctive vehicle original identity
and the corresponded vehicle secret key
are issued by
during offline registration. The confidential vehicle identity set is defined as
. Note that the secure data exchange for RSU and vehicle initialization is assumed. At this point,
maintains the records of all the registered RSUs and vehicles in its database. Notably, the private vehicular information, such as user name, address, social security identifier, and phone number, are stored.
Table 2 shows the data structure of the vehicular records in
.
With the purpose of illegal tracing prevention and privacy protection, the RSU anonymous identity is created by each legitimate RSU. That is, the registered RSU randomly generates its partial secret key
and periodically extracts the time-oriented anonymous identity
, as
where the above
is referred to as the current timestamp, so that the freshness of identity can be assured. The session identity
is effective only within certain time period and will expire in the subsequent time. The RSU partial secret key set
is preserved in its storage, while
is kept secret to
.
According to the confidential information, the homomorphic encryption infrastructure can be built for each registered RSU. Initially, RSU selects two large prime
and
, so that
holds. Subsequently, RSU randomly chooses
where
. Hence, the computation on
and
can be conducted according to
where
. At this point, the RSU homomorphic encryption key set is extracted in the form of
. Afterwards, RSU carries out the following calculations:
where
denotes the latest timestamp. Therefore, RSU broadcasts the parameters set
periodically to all devices within its range.
4.2. Blockchain-Based Key Agreement Phase
In this section, the authentication and key management for vehicle is introduced. Initially, while assuming the vehicle with
is approaching the effective domain of the aforementioned RSU with anonymous identity
, the vehicle itself generates the random partial secret key
. In this case, the partial secret key set
is stored in vehicle side. For anonymity protection, the vehicle temporary identity is applied as
As mentioned above, vehicle is acknowledged of the broadcast RSU public information set
. Firstly, freshness validation on the received timestamp
is first performed by comparing whether
holds, where
refers to the current timestamp. Subsequently, correctness of the certificate
is verified, so as to guarantee the message integrity. Upon verification, the RSU homomorphic encryption key pair
can be extracted by vehicle. Meanwhile, similar homomorphic encryption design for vehicle can be constructed as well. That is, the vehicle with identity
selects two large prime
and
so that
holds. Subsequently, vehicle randomly chooses
, where
. Hence, the computation on
and
can be conducted according to
where
. At this point, the vehicle homomorphic encryption key set is extracted in the form of
.
Preliminarily, with the purpose of managing the historical driving information, the block chain is built in the form of
, where
represents the previous hash value generated by the last encountered RSU. The entire block chain is distributively stored in
, while the vehicle itself stores its successive two hash values of the chain, which contains the authentication timestamp and the information of the last RSU, such as location and verification number. Notably, the vehicle does not preserve all of the chain data in storage for the consideration of inherent resource limitation, while the previous two hash values as well as the related timestamp
for signature are enough for further validation. For better description, the two stored hash values are simplified as
and
, which are generated by the previous RSU with identity
as
Upon extracting the RSU homomorphic encryption key pair
, the vehicle intends to construct the authentication process with RSU. Moreover, the previous blockchain data should also be validated and updated. Hence, the following calculations are conducted:
where the homomorphic encryption
is performed as
At this point, the vehicle requesting packet with its vehicle homomorphic encryption key set
are issued as
where the blockchain information is also included.
Upon receipt of the requesting packet, freshness verification is conducted by checking whether
holds, where
refers to the current timestamp. If validated, RSU is able to decrypt the received
by computing
where the RSU homomorphic decryption
is performed in the way of
The mathematical correctness for decryption can be illustrated as
Hence, is successfully extracted from by RSU. The message confidentiality can be guaranteed by verifying with the acquired and the previously broadcast from RSU. If validated, RSU stores the vehicle homomorphic encryption key set .
Moreover, the extensive validation procedure on blockchain should be carried out. In our assumption, upon successful authentication with certain RSU, vehicle will request RSU to verify and update its current blockchain values
. Dynamic information sharing among nearby RSUs is enabled, according to the aforementioned cloud-assisted VANET system model with edge RSU cluster. That is, the identity information
of the previous RSU will be broadcast in the way of
. Hence, with the received
from RSU cluster, and the current blockchain
from vehicle,
checks
so as to confirm the correctness of chain value. Subsequently,
computes
according to
where the
denotes the current timestamp, and
is the identity information of current
. Meanwhile, with the extracted
and
, RSU conducts the following calculation on
as
At this point, RSU uploads to for the cloud verification. Notably, the vehicle identity information are stored in server. Therefore, is able to confirm the vehicle identity with the transmitted from RSU. If matches, the requesting vehicle is the legitimate registered device. The vehicle access to VANET system will be granted. As for chain value updating, refreshes the stored blockchain values with the uploaded of as well. Hence, the record is updated. The information is securely preserved as additional contents for further vehicle tracking. In our assumption, every time that the vehicle communicates with a new RSU, will receive confirmation message along with the crucial contents for chain updating. With all acquired information, is able to synchronize the decentralized blockchain values with vehicle itself, where the chain updating for vehicle is performed by the involved RSU.
Subsequently,
distributes the acknowledgement message
to RSU, where
Upon receiving the acknowledgement, the vehicle identity can be updated as
which includes the RSU partial key set
. In our design, the anonymous vehicle identity is safely updated as soon as the successful that verifies session is conducted. In this case, the message unlinkability for various communication sessions, and untraceability for specific vehicle, can be achieved.
Next, RSU is able to deliver the essential information
to vehicle following the vehicle homomorphic encryption process with the previous vehicle key set
and its own
as
Note that the homomorphic encryption
can be performed as
Hence, the packet
is then delivered to the destinated vehicle.
Upon receiving
, freshness confirmation is first carried out by checking whether
holds, where
refers to the current timestamp. Subsequently, the received
can be decrypted as
where the vehicle homomorphic decryption
is performed, as
Note that the mathematical correctness for the vehicle homomorphic decryption can be briefly illustrated as
At this point,
can be successfully extracted from
. Confidentiality of the delivered packet can be confirmed by checking
. If validated, the vehicle conducts the final authentication, as
.
At this point, mutual authentication between RSU and requesting vehicle is completed. In our design, the semi-trusted RSUs can perform the authentication and updating procedures without accessing the confidential vehicle secrets. Meanwhile, is used as the shared session key established between remote and participating vehicle. In this case, the constructed homomorphic cryptographic scheme of and could guarantee secure and reliable data exchange. Moreover, the vehicle could also extract the updated blockchain values and related timestamp from . Hence, the previous value can be replaced with the updated . In the next authentication session with successive RSU, the newly generated will be issued in the same way. The blockchain record is maintained by VC and vehicle itself, while the validation processes on successive values of the chain are operated by all of the involved RSUs. With the precise signing information of the encountered RSU on the road, the driving routes of particular vehicle could be securely recorded in a decentralized way. All of these strategies enable the following healthcare monitoring and infection tracking design.
4.3. Healthcare Monitoring Strategy
With the preliminary operations introduced in the previous two phases, the healthcare monitoring strategy can be achieved, along with the infection tracking algorithm for COVID-19 pandemic control. The RSUs can be classified into regular RSUs and the checkpoint RSU, as shown in
Figure 2. Regular RSU is in charge of vehicular data exchange of conventional VANETs, while the checkpoint RSUs take the responsibility of traffic surveillance and healthcare monitoring particularly. As for practical scenarios of pandemic control in transportation system, all of the regular RSUs can be selected as the checkpoint if necessary. Extensive modification on RSU hardware is not required, thus any regular RSUs can switch to checkpoint RSU easily. Therefore, effective and reliable healthcare monitoring functionality could be provided to any road sections under emergency situations. Intuitively, the above key management and mutual authentication operations are illustrated in terms of regular RSU, while the healthcare monitoring strategy in this section will be described with the assistance of checkpoint RSU. That is, real time physical status of the passengers in the passing vehicles are monitored, collected, and uploaded to
at final. Additionally, the driving route information on vehicles will be attached to wearable device of individual passenger. Hence, infection tracking towards suspected persons is available.
We assume that the aforementioned vehicle is approaching the checkpoint RSU in the next (). At this point, the vehicle possesses the essential chain values sent by the previous . The blockchain-based key agreement phase is the same as above until the generation of packet . In the assumption of checkpoint RSU, a simple request is attached to the packet and then sent to destinated vehicle in the form of . After validation, the vehicle is then aware of the request for healthcare monitoring towards its passengers.
In our assumption, the passengers in vehicles are considered to be the essential parties for healthcare monitoring in VANETs. Preliminarily, each passenger should register to in advance. Hence, the confidential identities set of the registered passengers are issued as , where the distributed is the unique original identity. The identity set for all the legitimate passengers is safely stored in server. As for individual passenger, the wearable device, such as smart watch or smart bracelet, is mandatory for medical data measurement and aggregation. Moreover, with the assistance of the intra body area network (intra-BAN) and the connected medical sensors, precise and seamlessly physical data collection can be provided. Notably, each passenger and their corresponded wearable device is assumed to be the same entity with identity .
As mentioned above, in the range of the checkpoint
, the parameters set
is periodically to all devices. Importantly, all of the wearable devices could also acquire the RSU parameters set. Hence, with the same validation and decryption process, the RSU homomorphic encryption key set
is then acquired by passenger with
, provided that there are
n passengers within one vehicle. Note that, for the
n devices, the temporary identity
is generated as
. Hence, each wearable device delivers the sensitive physical data regarding pandemic control to vehicle in the form of
. The vehicle then gathers all
n packets from different passengers and forwards it to RSU in the form of
RSU can then decrypt the identities and medical data from the passengers. If unique patterns are detected, then RSU sends the warning report to and request for retransmission. Eventually, the gathered healthcare data, along with the current RSU information , are uploaded to and stored for further usage. In the further time, if certain passenger is infected, its historical healthcare record and route information can be retrieved in the database, the infection tracking method is accordingly available, which is of great significance for pandemic control.