Communication Reduced Interaction Protocol between Customer,

Charging Station, and Charging Station Management System

Karl-Heinz Krempels

, Christoph Terwelp

, Stefan W

uller

, Tilman Frosch

and Sevket G

okay

Information Systems & Databases, RWTH Aachen University, Aachen, Germany

HGI, Ruhr-University Bochum, Bochum, Germany

Keywords:

Charging Station, Interaction Protocol, Authentication Protocol, Reduced Communication Effort, QR Code.

Abstract:

The emerging build-ups of charging station infrastructures require sufﬁciently secure and economic authen-

tication protocols. Existing protocols for the purpose of authenticating a customer against a charging station

have the inherent disadvantage that they expect a network connection to the management system, produce a

communication overhead, or might reveal sensitive customer data depending on the protocol. The protocol,

provided by us, enables a multiple-operator customer-to-charging station authentication system. The particu-

larity of the protocol is that it does not require a permanent network connection between charging stations and

a corresponding management system, reduces the communication overhead between the involved entities, and

protects sensitive customer data at a high rate.

1 INTRODUCTION

The availability of a charging infrastructure is one of

the decisive factors for the success of electric vehi-

cles. Thus, one can currently observe signiﬁcant ef-

forts in Germany and many other countries to expand

the existing charging infrastructure. In the course

of this expansion, costs must be kept in check. Be-

sides the acquisition costs, installation and mainte-

nance costs should be kept as low as possible. One

factor within the operational costs is the need for a

continuous communication channel between charging

stations and a corresponding charging station man-

agement system.

The interaction protocol we propose in this pa-

per enables a dynamic control of charging stations

without the necessity of a constant communication

channel to a management system. This is achieved

by leveraging the Internet connection of the cus-

tomer’s smartphone as a communication channel be-

tween charging station and charging station manage-

ment system for authentication purposes.

The remainder of the paper is organized as fol-

lows: In Section 2 we describe authentication pro-

tocols for charging stations that are currently in use.

Section 3 and 4 provide a detailed description of the

our solution and optional protocol extensions. In Sec-

tion 5, we discuss core design decisions and give a

summary of the advantages of our protocol and the

provide the possibility to extend its application ﬁeld

in Section 6.

2 RELATED WORK

In the following we present the state of art of pro-

tocols used to authenticate customers against charg-

ing stations. The major disadvantage of those proto-

cols is the costly communication effort needed before

and after each charging process between the charg-

ing station and the charging station management sys-

tem. The interchanged data basically consists of cus-

tomer’s authentication data, information for the ad-

justment of meter readings, as well as control com-

mands, e.g., the activation of the charging station.

Partially, the protocols enable a local authentication

by utilizing whitelists (OCPP Steering Group, 2012).

However, this jeopardizes the conﬁdential customer

data which are deposited within the charging station.

Furthermore, the following protocols rely on a

network connection, i.e., a network connection is es-

sential for the commissioning of a charging station.

Below, we will show that a network connection is

not absolutely necessary to fulﬁll the requirements of

those systems.

118

Krempels K., Terwelp C., Wüller S., Frosch T. and Gökay S..

Communication Reduced Interaction Protocol between Customer, Charging Station, and Charging Station Management System.

DOI: 10.5220/0004971801180125

In Proceedings of the 3rd International Conference on Smart Grids and Green IT Systems (SMARTGREENS-2014), pages 118-125

ISBN: 978-989-758-025-3

 2014 SCITEPRESS (Science and Technology Publications, Lda.)

2.1 Authentication Via RFID-card

(OCPP Steering Group, 2012)

The customer owns an RFID-card which enables him

to authenticate against a charging station with an in-

tegrated RFID-reader. If the read ID appears on the

charging station’s local whitelist, the user is success-

fully authenticated against the charging station with-

out involving the charging station management sys-

tem into the authentication process. Otherwise, if the

ID does not appear on the whitelist, there has to be

a data exchange between the charging station and the

charging station management system. The customer

logoff works analogously. Meter readings and control

data is communicated using the network connection

to the charging management system.

2.2 Authentication Via Plug and Charge

(ISO, 2013)

The costumers connects his vehicle with a charging

station by a charging cable carried along. The au-

thentication is done in two steps. The charging cable

plug is endued with an RFID-chip. If the plug of the

charging cable is in an appropriate range the ID is read

by the RFID-reader of the charging station. Reading

a valid ID induces the charging station to open it’s

socket. For the second authentication step the com-

munication channel established by the charging cable

is used. A certiﬁcate which is deposited within the ve-

hicle is used in the underlying protocol to authenticate

the customer against the charging station. The charg-

ing process is terminated by unplugging the charging

cable.

The veriﬁcation of the authentication data and the

exchange of meter readings is ensued via the network

connection to the charging station management sys-

tem.

2.3 Authentication Via Hotline

The customer dials the operator’s hotline which is

ﬁxed on the charging station and communicates his

user ID, password and the charging station ID. A

successful veriﬁcation induces the activation of the

charging station for the authenticated customer. The

customer logoff works analogously.

Meter readings and control data is communicated

using the network connection to the charging manage-

ment system.

2.4 Authentication Via Internet

The authentication via Internet proceeds analogously

to the authentication via hotline. The customer com-

municates user ID, password, and charging station ID

by ﬁlling the appropriated form on the website of the

charging station operator or using a provided smart-

phone application for this purpose.

3 THE BASIS PROTOCOL

Before we describe the details of our protocol, we

give a concise overview with Section 3.1. The pro-

tocol ﬂow is given by the sequence diagrams depicted

in Figure 1 and 2. During an initialization phase

each charging station (CS) is endued with a pub-

lic/private key pair (pk

, sk

) and the public key(s)

of the respective charging station management sys-

tem(s) (pk

CSMS

), respectively. The charging station

management system (CSMS) retains a matching pri-

vate key and the public key of each charging station.

Keys are stored in a non-removable secure storage,

from which they cannot be extracted by unauthorized

entities. When a new user signs up with a provider,

he receives a set of unique credentials to authenticate

at the CSMS.

3.1 Protocol Flow

On the push of a button integrated into the CS, it gen-

erates a QR code and shows it on its display. The cus-

tomer scans the QR code using his smartphone and

communicates its content in combination with his au-

thentication data to the CSMS. Optionally, the user

can provide additional position information using the

provided smartphone application, e.g., his GPS coor-

dinates. The CSMS provides the user with authen-

tication information for this CS, which the applica-

tion on the user’s smartphone displays as a QR code.

The CS scans this QR code from the phone’s dis-

play. If the authentication information is valid, it un-

locks / enables the power socket. Charging ends when

the electric vehicle’s battery management signals suf-

ﬁcient charge, the customer presses a button at the

CS and presents the QR code again, or the customer

manually releases the power cable using a mechan-

ical release device integrated in his vehicle. When

charging ends, the CS creates a data tuple to be used

for billing and stores it in temper-evident memory.

The CSMS periodically tries to connect to every CS.

When successful, it recovers all billing data tuples re-

tained within the CS.

CommunicationReducedInteractionProtocolbetweenCustomer,ChargingStation,andChargingStationManagement

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119

charging station

management system

charging station

randomly generate

auth. token T of bit

length 256

customer scans QR code using his

smartphone application

c, customer ID, password

verify authentication

data

m = dec

CSMS

(c)

verify signature and

check t

smartphone

(customer)

charging station scans QR code

from customer's smartphone display

verify signature, t

and T

start charging

customer connects charging station

and vehicle through charging cable

lock plug

customer authentication against the

charging station management system,

handing out the PIN

(secured via SSL)

generation of the QR code

authentication of customer against charging station,

charging initiated

((pk

, sk

), pk

CSMS

)

(customer ID, password)

((pk

CSMS

,sk

CSMS

, pk

, …)

customer pushes the button

m =

, T, sig

(ti,T))

c = enc

CSMS

(m)

generate a QR

code from c and

display it

m' =

, T, ID

sig

CSMS

, T, ID

))

generate QR code

from m' and display it

open socket

Figure 1: Basis Authentication Protocol (Part 1).

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customer presents QR code again

to the charging station

stop charging

leave parking lot

customer removes charging cable

close socket

sign out and termination of the charging

process

verify signature

billing data

exchange

request billing data

tuple

tuple =

start

, t

end

, meterID,

start

, Z

end

, ID

sig

start

, t

end

meterID, Z

start

end

, ID

))

charging station

(pk

, sk

, pk

CSMS

)

smartphone

(customer)

(customer ID, password)

charging station

management system

((pk

CSMS

,sk

CSMS

, pk

, …)

Figure 2: Basis Authentication Protocol (Part 2).

3.2 QR Code Generation

On the push of a button the CS randomly chooses an

authentication token T of 256 bit length. It only cre-

ates n tokens per time t. n, t are chosen as a compro-

mise between usability and security. Here, usability

primarily concerns the fact that each customer willing

to use the system should be able to receive an unused

token. Under a security perspective one must consider

that the source of randomness must not be exhausted

or the system overloaded by very frequent pressing

of the button, where each button press triggers a set

of cryptographic operations that consume system re-

sources.

The CS creates a message m = (timestamp =

, token = T, signature = sig

, T )), creates c =

enc

CSMS

(m), and creates a QR code containing c

that it displays to the customer. t

is a timestamp pro-

duced at message creation, in a non-ambiguous time

representation. The customer scans this QR code us-

ing his smartphone application.

3.3 Customer Authentication

The smartphone application connects to the CSMS

using a secure variant of Transport Layer Security

(TLS), a cryptographic protocol frequently used to se-

cure a cornucopia of communications on the Internet,

e. g., for online banking. We recommend to use TLS-

DHE, which has recently been proven to be secure by

Jager et al. (Jager et al., 2012).

The user authenticates himself at the CSMS and

establishes a secure session. Within this session the

application transmits the message c contained in the

CS’ QR code which the CSMS decrypts and veri-

ﬁes. After successful veriﬁcation the CSMS checks

CommunicationReducedInteractionProtocolbetweenCustomer,ChargingStation,andChargingStationManagement

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121

whether t

is within a deﬁned time period to be con-

sidered fresh. Optionally, the user can provide addi-

tional position information using the provided smart-

phone application, e.g., his GPS coordinates. We

discuss this extension in Section 4. The CSMS

transmits a message m

= (timestamp = t

, token =

T, customerID = ID

, sig

CSMS

, T, ID

)) back to the

application. ID

is a unique identiﬁer for the authen-

ticated customer. The application creates a QR code

from m

, which the user presents to the CS. The CS

scans the QR code and veriﬁes m

and T . If T and

the signature over T, customerID is valid, and t

within a deﬁned time period, the power socket is un-

locked / enabled.

3.4 Storage and Transmission of Billing

Data

Upon the end of the charging process, the CS creates a

data tuple for billing and ensures its authenticity dur-

ing transport and storage by means of a digital signa-

ture. The tuple is formed as

tuple =(t

start

, t

end

, meterID, Z

start

, Z

end

, ID

sig

start

, t

end

, meterID, Z

start

, Z

end

customerID)),

where t

start

is a timestamp recorded upon starting

to charge, t

end

is a timestamp indicating the end of

charging, meterID is the unique identiﬁcation num-

ber of the energy meter (if required by local regu-

lation), Z

start

and Z

end

are the energy meter values

at beginning an end of charging, respectively. We

chose digital signatures over symmetric methods such

as Message Authentication Codes, as both can ensure

message integrity and authenticity, but the former can

also provide non-repudiability. Non-repudiability is

essential to be able to prove that only the respective

CS could have created the signed tuple. However, this

objective cannot be reached using means of symmet-

ric cryptography.

The CS stores the data tuple in temper-evident

memory, such that any attempt to erase a billing tu-

ple can be discovered. The CSMS periodically tries

to connect to each CS. Upon successfully establishing

a secure connection, it recovers all billing data tuples

and stores them centrally, such that they are available

for the billing process. When the CSMS has success-

fully stored and veriﬁed a tuple, this tuple is remove

from the CS’ memory.

4 PROTOCOL EXTENSIONS

In the following we describe further optional exten-

sions to the basis protocol which affect the security

of the protocol and the cooperation between different

charging station operators.

4.1 QR Code Relay Attack Prevention

We consider the following theoretical attack scenario:

An attacker positions a manipulated charging station

with the ability to receive QR code data generated by

the attacker and to display it to the customer.If an au-

thentication token is read by the manipulated charging

station, it has to be transmitted immediately to the at-

tacker whereas an error message is displayed to the

customer. An existing connection to the charging net-

work is not necessary to accomplish this task. Utiliz-

ing the manipulated charging station to his own au-

thorization process the attacker is enabled to charge

his vehicle for free or at the cost of the victim.

The attacker positions his vehicle in front of an

authorized charging station, reads the displayed QR

code, sends the QR code data to the manipulated

charging station (e.g. using an implemented smart-

phone application for this purpose), and waits until

the valid QR code, provided by the customer at the

manipulated charging station, is received. If the QR

code loses validity during the transmission, the at-

tacker repeats the proceeding with a fresh QR code.

In this scenario, the charging station management sys-

tem provides an authentication token for an autho-

rized customer and connects the billing information to

his account. Once the customers presents the QR code

to the manipulated charging station, an error message

is displayed, informing about a defective and refer-

encing to another charging station. Since the cus-

tomer cannot distinguish between the authorized and

the manipulated charging station, he is not aware of

the fact that an attacker positioned at another charg-

ing station uses his QR code to charge a vehicle at

his cost. Under certain assumptions, this attack can

be thwarted by integrating spatial data into the au-

thentication protocol: The mobile application deter-

mines the customer’s GPS coordinates during authen-

tications. This position is then concatenated to the

message c the customer transmits to the charging sta-

tion management system.

With this protocol extension, the charging sta-

tion management systems checks if the customer’s

GPS coordinates match with corresponding charging

station’s coordinates considering a certain tolerance.

Only if this additional condition is satisﬁed the man-

agement systems issues m

. The assumption for this

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security feature is that the manipulated and the autho-

rized charging station the attacker uses are sufﬁciently

wide apart from each other exceeding the maximal ac-

cepted tolerance.

4.2 Multi-operator Authentication

A simple extension to the basis protocol of Section

3 enables a multi-operator authentication without the

necessity of sharing customer data between operators.

Each operator obtains an operator ID. Instead of just

pressing a button, the customer chooses the operator

at the CS. The CS encrypts the message m using the

respective operator’s public key pk

CSMS

and then in-

corporates the appropriate operator ID within the QR

code in plaintext. Basing on the operator ID, the pro-

vided smartphone application decides which operator

the authentication message is forwarded to.

Manipulating the operator ID can not harm the system

because the assigned operator is not able to decrypt

the data which is necessary to return an authentica-

tion token to the customer.

4.3 Privacy-preserving Solution

Locational privacy can be deﬁned as ’the ability of

an individual to move in public space with the expec-

tation that under normal circumstances their location

will not be systematically and secretly recorded for

later use’ (Andrew J. Blumberg and Peter Eckersley,

2009) and has become increasingly relevant with rise

of long-term data retention – itself made facilitated by

increasingly cheap data storage.

While the solution we presented so far does not

preserve a customers location privacy – it allows

for the creation of a movement proﬁle based on the

spatio-temporal location of each charging process –

a limited set of changes can mitigate this threat: In-

stead of sporadically connecting to the Internet to

allow for the CSMS to retrieve billing data, each

CS uses this temporary Internet link to connect to

the Tor network (Mathewson et al., 2004), such that

it is addressable as a location-hidden service. This

means, it is known to the CSMS via its .onion ad-

dress, but not via its IP address or physical loca-

tion. While this may sound far-fetched and imprac-

tical in reality, the Tor network is a highly redundant,

distributed system that can provide connectivity with

sufﬁcient throughput and latency for the application

at hand (Frosch et al., 2013). The authenticity and

non-repudiation of messages from CS to CSMS does

not longer depend on classical signature algorithms.

Instead, messages are signed using a group signa-

ture scheme, like XSGS (Delerabl

ee and Pointcheval,

2006). Message m will be formed as (timestamp =

, token = T, address = onionaddress

, signature =

gsig(t

, T, onionaddress

)), while c will still be cre-

ated as enc

CSMS

(m). Instead of signing the billing

data tuple with a conventional signature, the CS uses

the a modiﬁed eXtremely Small Group Signature as

proposed by Frosch et al. (Frosch et al., 2013).

5 DISCUSSION

In the following we discuss the potential of a com-

pletely ofﬂine solution, the advantages of using a ran-

dom binary token over a numeric Personal Identiﬁca-

tion Number (PIN), as well as, the issue of a trustwor-

thy time source.

5.1 Ofﬂine Solution

In the unlikely case that a charging station is lo-

cated such that it can never access the Internet,

small changes can be made to the protocol leverage

the user’s communication with the CSMS to trans-

port most billing relevant data within. Including

the current energy meter value Z

in message m,

such that m = (meter = Z

, timestamp = t

, token =

T, signature = sig

, t

, T )). The charging pro-

cess can only be terminated by performing the authen-

tication procedure again, such that a message m

(meter = Z

, timestamp = t

, token = T

, signature =

sig

, t

, T

)) is transmitted to the backend.

However, as many electric vehicles come with a man-

ual unlock mechanism for the power connector, even

a honest, but curious, customer can evade the trans-

mission of m

and thus charge without paying. Ad-

ditionally, t

, t

are created when the customer presses

a button and not at the exact time the charging starts.

Depending on local legislation, this may not be pre-

cise enough.

5.2 Random Token vs. Random PIN

Although numeric PINs are frequently used to authen-

ticated customers, e. g., at automatic teller machines

(ATMs), the keyspace of usable-length PINs is very

limited. PIN lengths up to 6 digits can be considered

acceptable to the customer, as they are used in com-

mercial applications. However, as the character reper-

toire is limited to [0..9], the keyspace is limited to 10

The probability that an attacker guesses a valid pass-

word is thus

, i. e., on average an attacker needs

c guesses. As this limitation is well known, many

PIN-based authentication systems require not only the

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knowledge of the PIN but also the possession of phys-

ical token, e. g., a bank card. Additionally, usability is

further reduced by the fact that a customer may only

mistype a PIN n times, before the physical token is

automatically invalidated or conﬁscated. n is often

chosen as 3.

By choosing a QR code reader as input method to

the charging station, instead of a PIN pad, we avoid

the usability issue altogether. The user is not forced to

enter an arbitrary set of numbers correctly, but simply

presents his smartphone to the reader. Thus, we can

choose to a much longer knowledge-based authenti-

cation token. We choose a binary token of 256 bit

length, which results in a keyspace of 2

256

. This

allows for a signiﬁcantly more secure authentication

process and also improves usability as there is no need

enforce arbitrary limitation on how often a user may

try to enter a credential. Vandalism-proof QR-code

readers are widely in use today, e.g., at airport board-

ing terminals.

5.3 Time Source

We use a timestamp as a freshness parameter in our

protocol. However, this implies that we have a trust-

worthy time source at our disposal. This is also a ba-

sic assumption for post-paid systems, as many regula-

tions require to inform the customer when a service or

a good has been delivered. When an attacker can trick

a CS into assuming a time in the past as being current,

he can thus replay an old message m

old

, which will

be accepted if the CS’s time and the message times-

tamp are within a tolerated interval. Even existing

customers can use erroneous system times to their ad-

vantage, as the energy provider will have a hard time

arguing how it can bill, e.g., for a charging process

with at timestamp dating from before the customer

ever signed up with this provider.

However, a trusted time source is not easy to come

by. DCF-77, the Network Time Protocol (NTP), and

GPS are popular time sources. However, none of

these protocols provide information on the authentic-

ity of the content and can thus be manipulated. DCF-

77 transmitters can be built for very limited costs

even using standard soundcards

. NTP packets can be

manipulated in the path of communication. Alterna-

tively, DNS entries can be hijacked by an out-of-path

attacker (Leyden, 2013), pointing to an NTP server

the attacker controls. While Tippenhauer et al. (Tip-

penhauer et al., 2011) have shown that the spooﬁng

of a GPS signal is feasible, this attack induces signiﬁ-

http://endorphino.de/projects/electronics/timemanipula

tor/index en.html

http://0x7.ch/text/dcf77.pdf

cantly higher costs for the attacker than the aforemen-

tioned ones. For the time being, GPS should be the

preferred time source for outdoor CS installations. In

indoor areas, such as parking garages, the GPS time

signal can be forwarded via internal network. Alter-

natively, tlsdate (Applebaum, 2013) may provide

a coarser, but possibly more secure alternative time

source when used as consensus source.

6 CONCLUSION

We presented a protocol that enables providers to op-

erate charging stations without a continuous commu-

nication channel connecting them to the charging sta-

tion management system. This allows a more eco-

nomical operation and a higher security of the charg-

ing station infrastructure, as the charging station has

no privileged access to the backend infrastructure. In-

volving QR codes enables a multi-operator authen-

tication of the customer against the charging station

without any communication between the charging sta-

tion and the management system and without deposit-

ing sensitive data on the charging station.

While we focus on the application of recharging

electric vehicles, our solution is ﬂexible and can also

be adapted to a wide variety of application ﬁelds, such

as time-limited access of rental cars, pedelecs, and bi-

cycles.

ACKNOWLEDGMENTS

This work was supported by the German Federal Min-

istry of Economics and Technology

(Grant 01ME12052 econnect Germany).

This work was supported by the German Federal Min-

istry of Economics and Technology:

(Grant 01ME12025 SecMobil).

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