PRE as a Service within Smart Grid City

Sbai Anass, Drocourt Cyril and Dequen Gilles

MIS Laboratory, University of Picardie Jules Verne, Amiens, France

Keywords:

Smart-Grid, Cloud Computing, Proxy Re-encryption, Privacy, Conﬁdentiality, Bank of Energy.

Abstract:

In the context of Smart Grid Cities, legal obligations require that certain personal data must be stored in the

long term and protected. To deal with conﬁdentiality issues, we use the concept of Proxy Re-Encryption

(PRE) which allows sharing encrypted data. We present the ﬁrst implementation of the Chow’s algorithm,

and propose an optimized instantiation thanks to elliptic curves. This is the ﬁrst unidirectional algorithm with

CCA security that does not rely on pairing, which guarantees its high performance. This allows its use in real

conditions. We have implemented it in JavaScript for direct use in a web browser by the user. In order to

be able to process the data asynchronously, we then deﬁne the notion of PREaaS (Proxy Re-Encryption as a

Service) that also allows use in a service-oriented context.

1 INTRODUCTION

Demands in terms of energy continues to grow. Nev-

ertheless, old sources of production (as nuclear, fossil

...) are less appreciated and tend to be abandoned due

to the pollution that manufacturing causes. The use of

renewable energy sources has become privileged and

paramount. Thus, creating a new energy market be-

comes a great challenge for companies. A very neat

management must be adapted, starting by the smart

metering systems. This involves the installation of a

whole infrastructure and communication networks in-

cluding smart meters. This also requires a great deal

of attention at the security level, whether it is to en-

sure the conﬁdentiality of consumption data, integrity

to avoid fraud or, above all, to be immune from at-

tacks aimed at destabilizing production systems such

as STUXNET (Matrosov et al., 2010) which target

precisely the data related to a speciﬁc type of soft-

ware such as SCADA. Finally this becomes a con-

cern of national security. Recently, (Ronen et al.,

2017) exploited a vulnerability in the ZLL protocol,

allowing to inset a worm spreading quickly and in-

visibly on the network. Such a chain reaction could

rattle a nuclear power station. Other interesting facts,

(Brinkhaus et al., 2011) have shown that smart me-

ters are capable of becoming surveillance devices, as

long as an attacker can access the load curves stored

or transmitted by smart meters. These data, can be

used to identify all the devices connected to the elec-

trical network such as the refrigerator, microwave etc.

Thus, by making predictions about the energy con-

sumed by different devices, see if there is a correla-

tion between predictions and actual data, we can even

guess what movie was viewed.

This paper deals with the secure management of

large amounts of data within the context of the VERT-

POM project.

The latter, currently being developed,

consists in creating the energy bank, which is a tool

dedicated to the territory energy management. It aims

to maintain an optimized balance between the avail-

able energy from production (conventional and re-

newable energy) with regard to usage (consumption

and losses), in connection with the energy storage

means (Boronat, 2017). For this purpose, the en-

ergy bank uses prediction algorithms and simulations

of energy production, consumption and losses on the

various distribution systems. Thus the energy net-

works must be more responsive, ﬂexible, and thus

foster interactions between market players. These

goals are partly achieved by collecting data on net-

works through remotely controllable sensors and de-

vices. As shown in Fig 1, data needed by the energy

bank for its smooth operation are sent back by the

smart meters via the data concentrators to the servers

of the network manager. Some data managed by the

energy providers (EP) and other information coming

from street (light sensors etc ...) are also sent to the

BE. We will ensure that all the data, once produced

will be encrypted and since, only the owner could ac-

This work is supported by the Future Investments Pro-

gram operated by ADEME.

394

Anass, S., Cyril, D. and Gilles, D.

PRE as a Service within Smart Grid City.

DOI: 10.5220/0007838503940401

In Proceedings of the 16th International Joint Conference on e-Business and Telecommunications (ICETE 2019), pages 394-401

ISBN: 978-989-758-378-0

Figure 1: Architectural model for the Bank of energy.

cess the data. On the other hand, the data should be

shared between different actors. Thus we propose a

solution based on proxy re-encryption designed pri-

marily to allow decryption delegation

The paper is organized as follows: The next sec-

tion is about the related works. We present our ap-

proach, implementation details and performances in

section 3. Finally we conclude with discussion on our

solution and present some perspectives.

2 RELATED WORKS

2.1 Cloud Storage

To date, cloud storage remains restrictive in terms

of conﬁdentiality. In fact, the best-known provider

(e.g GoogleDrive, Dropbox, iCloud, pCloud, OVH

... ) do not ensure total conﬁdentiality of their cus-

tomers data. Either the data are encrypted under a

key known by the cloud or stored in plaintext. Be-

cause these CSPs are considered as an all-trusted

part. Several works exist that aim to protect privacy

such as CryptDB (Popa et al., 2011) but still have

some limitations. For example: it cannot be used

for no-sql databases or ﬁle systems, and key manage-

ment complexity increase with the number of users.

ESPRESSO (Kang et al., 2014) delivers an encryp-

tion service for CSP to maintain conﬁdentiality. It

uses only symmetric encryption and requires to trust

a third party or the CSPs to ensure the encryption

and key management. More closer to our context

Authentication issue is out of the scope of this paper

Table 1: The terms deﬁned below will be used in the rest of

the paper.

Term Deﬁnition

BE Bank of Energy

NEM Network Energy Manager

EP Energy Provider

Alice, a Sender/Delegator

Bob, b Receiver/Delegate

Public Key of a

Private Key of a

a→b

Re-encryption Key from a to b

PRE Proxy Re-Encryption

CSP Cloud Service Provider

DO Data Owner

DP Data Producer

DC Data Consumer

KEM Key Encapsulation Mechanism

DEM Data Encapsulation Mechanism

CCA Chosen Ciphertext Attack

CPA Chosen Plaintext Attack

asym

Asymmetric Encryption using PK

sym

Symmetric Encryption using K

IBE Identity Based Encryption

ES4AM (Hasan and Mouftah, 2015) provides encryp-

tion schemes for smart grid AMI (Advanced Meter-

ing Infrastructure) based on symmetric encryption. In

the latter case the trust remains a concern and none

of these solutions allow data sharing. This issue is

addressed by various solutions in the literature, start-

ing with the broadcast encryption designed by (Fiat

and Naor, 1993). Within this case, each user can ac-

cess data independently from the others. This requires

at the time of encryption the knowledge of who will

PRE as a Service within Smart Grid City

395

have the privilege to access this data. Another similar

approach was introduced by (Sahai and Waters, 2005)

which is Attribute Based Encryption. Inspired by the

work of D Boneh on the Identity Based Encryption

schemes, their idea was to create a new type of IBE

(Boneh and Franklin, 2001) system that they called

fuzzy IBE to combine encryption and access control.

In this case, access privileges are not addressed to a

set of users but only to users with a speciﬁc number

of attributes. Unfortunately, this does not allow for

a selective sharing. As an alternative to these solu-

tions, we choose to use the proxy re-encryption(PRE).

Thanks to this scheme, we will be able to delegate de-

cryption rights to speciﬁc entities. We found out that

it was used as a cloud based solution for ﬁle sharing

called Skycryptor by (Jivanyan et al., 2015). Basi-

cally, the proposed solution uses a unique symmet-

ric key for each ﬁle to be encrypted with AES and

then encrypt the key with the asymmetric public key

of the user generated thanks to the PRE algorithm.

The solution is a dedicated device software and is now

marketed under the name of BeSafe. Each user’s de-

vice has its own key pair and re-encryption is used

to share ﬁles between different devices or users. But

above all, the users must install the BeSafe software

and use it to encrypt the data. The solution could be

adapted to our problematic and used as a solution but

a lot of changes need to be done. Instead, we propose

the PREaaS which dosn’t need any heavy client and

which is more ﬂexible, modular and transparent. We

consider important to give deﬁnition and the state of

art of the PRE so that the reader can better understand

our choice of PRE as well as the rest of the paper.

2.2 Proxy Re-encryption

First appeared in 1998, it was designed by Blaze

Bleum and Strauss (Blaze et al., 1998) based on the

asymetric encryption scheme ElGamal. They show

that it’s possible to incorporate a substitution key

to re-encrypt an already encrypted message without

compromising it. One of the drawbacks of their

method is that the system is bidirectional. That is

to say, if Alice delegate decryption rights to Bob,

the latter would consequently delegate decryption

rights to Alice. (Ivan and Dodis, 2003) formalizes

the design of proxy re-encryption schemes by catego-

rizing these systems in two types: unidirectional and

bidirectional.

Usually a PRE scheme can be deﬁned as a tuple

ζ : {Setup, KeyGen, ReKeyGen, ξ

asym

, ReEnc, Dec}.

Where :

• Setup(1

) = params : takes as input a security

parameter k and generates the scheme parameters

that deﬁne in general the recommended message

and keys length.

• KeyGen(params) = (Pk, Sk) : is the function that

generate the pair public/private key.

• RekeyGen(Sk

, Pk

) = Rk

a→b

in the case of unidi-

rectional PRE, it takes as input a’s private key and

b’s public key to generate the re-encryption key.

• ξ

asym

(M) = C

is the encryption function.

• ReEnc(C

, Rk

a→b

) = C

is the re-encryption func-

tion.

• Dec(C, Sk) = M is the decryption function.

In some cases we can ﬁnd two more functions used

for encryption and decryption in such a way that the

cipher cannot be re-encrypted and only the owner of

the private key can decrypt.

(Ateniese et al., 2006) gives a more formal deﬁnition

for PRE and deﬁnes concretely the properties such

that :

• Unidirectional: Delegation of decryption rights

from Alice to Bob does not allow Alice to decrypt

Bob’s cipher.

• Non-interactive: The re-encryption key can be

generated by Alice without interacting with Bob,

and thus using only Bob’s public key.

• Transparent: Or invisible, meaning that the del-

egate can not distinguish between an encrypted

message and a re-encrypted message.

• Key-optimal: The size of Bob’s secret storage

must remain unchanged, no matter how many del-

egations he accepts.

• Original access: The sender can decrypt any re-

encrypted message which he was originally the

owner.

• Collusion-safe: If the proxy and Bob collude, they

shouldn’t get Alice’s secret key.

• Non-transitive: The proxy can not re-delegate re-

encryption rights. (e.g from Rk

a→b

and Rk

b→c

the

proxy can not calculate Rk

a→c

)

• Non-transferable: The proxy and delegates can

not redeﬁne decryption rights. (e.g from Rk

a→b

and Pk

and Sk

we can not calculate Rk

a→c

)

• Temporary: Bob can decipher the messages re-

ceived from Alice only at a certain point in time.

2.3 PRE Security

Several works have been published concerning PRE

security. The ﬁrst functional system of an IBE-based

SECRYPT 2019 - 16th International Conference on Security and Cryptography

396

PRE using pairing was proposed in (Green and Ate-

niese, 2007) by Ateniese. These works have been im-

plemented and patented by the authors. (Canetti and

Hohenberger, 2007) proposes the ﬁrst bidirectional

PRE CCA Secure, where he proves the security of

his scheme using the UCS method (Universally Com-

posable Security (Canetti, 2001)) . In (Deng et al.,

2008), the authors tackle the open problem presented

by Canetti concerning the construction of a CCA Se-

cure PRE without pairing. The cost of calculation

decreases and their construction also makes it possi-

ble to reduce the size of the secret at the level of the

re-encryption, which was not the case in their earlier

scheme. (Ateniese et al., 2009) formalizes the notion

of key privacy. It means that using the re-encryption

key we cannot recover the identity of both the dele-

gate and the delegator. Ateniese shows why the previ-

ous systems were not key-private and proposes a new

re-encryption system considered as the ﬁrst unidirec-

tional PRE that is key private. Their construction is

single-use CPA and not CCA Secure. (Chow et al.,

2010) has demonstrated the possibility of conducting

a CCA attack on the Shao system (Shao and Cao,

2009) and shows how to ﬁx the issue. This leads to

reduce the performance of the scheme. They then pro-

posed their own scheme without using pairing and re-

lying only on elGamal and the Shnorr signature. The

system is unidirectional CCA Secure in the random

oracle model.

3 OUR CONTRIBUTION

3.1 Architecture

If we review the global system, there are three main

actors in addition to the Cloud Storage Provider

(CSP

) as illustrated in Fig.2 (according to (Nu

nez

et al., 2017)):

• Data Producers: In our case it contains all the

sensors, meters and devices generating data which

has to be encrypted before storage.

• Data Owners: Party who is entitled to access their

data and also those who can grant access to data.

• Data Consumers: Access with the permission of

data owners to the shared information through the

CSP. Mainly the BE, and other entities like the

NEM.

We assume that a shared secret exists between

Data Owner (DO) and Data Producer (DP) which

The CSP in our use case could be a NEM client, NEM

city or an EP.

Figure 2: Main actors in a data sharing scenario.

encrypt data. Our system uses the KEM/DEM tech-

nique (ξ

asym

(K)kξ

sym

(M)) where the shared secret

(eg. the session key ’K’) is used to encrypt data with

symmetric encryption and then encapsulated within

the cipher of the session key generated by the DP

using asymmetric encryption of PRE. The result is

sent to be stored in the cloud. When the data needs to

be shared with Data Consumers (DC), the DO creates

a re-encryption key and transfers it to the PRE. This

one will re-encrypt them using the corresponding

re-encryption keys and forward it to the recipient.

Finally, the data can be consumed by DC.

3.1.1 Nave Approach (Fig. 3)

Figure 3: Proxy Re-Encryption as a Service.

Our ﬁrst intuition is to put the PRE as a proxy server

in the BE side. BE will send requests with its pub-

lic key to the different entities passing by the PRE

to retrieve the needed data. The corresponding enti-

ties should generate re-encryption keys if it belongs

to owner’s domain, or on the other hand should send

the public key of BE to the DOs to generate them.

Thus the PRE could re-encrypt the data for the BE

to be treated properly, the corresponding keys (public

key and its re-encryption key) will then be stored for

forthcoming operations. The re-encryption procedure

can be very expensive. In addition to key manage-

ment issues, having only one proxy to do it can gen-

erate delays. It wouldn’t be possible as well to share

data between EPs and NEMs.

PRE as a Service within Smart Grid City

397

3.1.2 Second Approach (Fig. 4)

Figure 4: Second approach: PRE used by each entity.

Our second intuition is that the use of Proxy in each

cloud could be more appropriate than the ﬁrst ap-

proach. It allows the NEM and EP to share data with

each other and relieve the bank from doing all the

work. Thus, each entity will manage their own keys,

re-encrypt the needed data and transfer it to the recip-

ient as illustrated in Fig.3. Nevertheless, it still takes

a long time for re-encryption due to its nature. Also,

in a more general case there can be NEMs and EPs

at a time. Then, it will be more complicated if every

entity is forced to have their own PRE and manage a

large data ﬂow.

3.1.3 PRE as a Service Approach (Fig.5)

Figure 5: Proxy Re-Encryption as a Service.

Our ﬁnal intuition is to propose the PRE as a service

for data sharing in multi-cloud systems. Data ﬂows

already exist, and instead of NEM providing informa-

tions to the EPs, it will be enough to refer to PREaaS

rather than to EPs. The potential of cloud computing

would take charge of re-encryption time consump-

tion. Our solution can be considered as a part of Secu-

rity As A Service (SECaaS). Our PREaaS, has the ad-

vantage to manage only encrypted data and key man-

agement handle only public keys and Re-encryption

keys. It does not affect under no circumstances the

conﬁdentiality, even if CSPs or users are corrupted

and collude with the PREaaS. This is guaranteed by

careful choice on the algorithms used by the service.

3.2 Implementation

In this section we present the algorithm used for PRE,

implementation environment and the resulting perfor-

mance. The algorithm used is the newest version of

Chow’s algorithm (Chow et al., 2010). We chose it

owing to its efﬁciency which is due to the fact that it

does not need pairings. Thanks to this survey (Qin

et al., 2016), we can see the computational compar-

ison of PRE schemes. By keeping only the schemes

that are CCA secure we can conclude that Chow’s al-

gorithm is the more efﬁcient among them. In (Selvi

et al., 2017) the authors ﬁnd a ﬂaw in the security

proof of Chow’s construction and propose to ﬁx it

by incorporating additional information to the exist-

ing Encrypt algorithm along with ciphertext validity

checks in both the Re-Encrypt and the Decrypt algo-

rithm. Most importantly the scheme is unidirectional,

collision resistant and CCA secure in the random ora-

cle model. Two implementations were made. First we

use a generic group with prime order length 3072-bit,

and the second one using NIST Standard ECC p-256

(Gueron and Krasnov, 2015) thanks to SJCL (Stand-

ford Javascript Crypto Library) (Stark et al., 2013).

Both correspond to the same security level that is 128-

bit. We give a formal version of the Chow’s algorithm

with ECC below :

• Setup(1

): for 128-bit, choose a prime p of

256-bit length and an elliptic curve E

such

that a = p − 3. G = (x

, y

), a point on the

curve, known as the base point with order n.

The elliptic curve equation is then : y

= x

−

3x + b mod (p). (You can ﬁnd the exact val-

ues of NIST p-256 curve parameters in (Gueron

and Krasnov, 2015)). Choose ﬁve hash func-

tions: H

: {0, 1}

× {0, 1}

→ Z

∗

, H

: E

→

{0, 1}

, H

: {0, 1}∗ → Z

∗

, H

: E

→

∗

, H

: E

× {0, 1}

→ E

. The message

space M is {0, 1}

, where l

= l

= k. params =

(p, E

, G, H

, H

, l

• KeyGen(params):

– Pick x

a,1

, x

a,2

←− Z

∗

– Compute Pk

a,1

= x

a,1

.G , Pk

a,2

= x

a,2

– Return Sk

=(x

a,1

, x

a,2

) & Pk

=(Pk

a,1

, Pk

a,2

• RekeyGen(Sk

, Pk

– Pick h

←− {0, 1}

, π

←− {0, 1}

– Compute v = H

(h, π), V = v.Pk

b,2

and W =

(v.G) ⊕ (h||π).

– Deﬁne rk =

a,1

(Pk

a,2

) + x

a,2

– Return Rk

a→b

= (rk, V, W ).

SECRYPT 2019 - 16th International Conference on Security and Cryptography

398

• ξ

asym

(m):

– Pick u

←− Z

∗

, w

←− {0, 1}

– Compute D = u.[(H

(Pk

b,2

).Pk

a,1

)∗Pk

a,2

] , r =

(m, w) & E = r.[(H

(Pk

b,2

).Pk

a,1

) ∗ Pk

a,2

– Compute F = H

(r.G) ⊕ (m||w).

– Compute D = u.H

(Pk

a,1

, Pk

a,2

, D, E, F) &

E = r.H

(Pk

a,1

, Pk

a,2

, D, E, F).

– Compute s = u + r × H

(D, E, F) mod(p).

– Return C

= (D, E, F, s).

• ReEnc(C

, Pk

, Rk

a→b

– Compute D and D:

D = s.[(H

(Pk

b,2

).Pk

a,1

)*Pk

a,2

(E, E, F).E)

−1

D = (s.H

(Pk

a,1

, Pk

a,2

, D, E, F))*

(E, E, F).E)

−1

u.[H

(Pk

a,1

, Pk

a,2

, D, E, F)]

– Check if:

s.[(H

(Pk

b,2

).Pk

a,1

)*Pk

a,2

]=D*(H

(E, E, F).E)

(1)

s.[H

(Pk

a,1

, Pk

a,2

, D, E, F)]=D*(H

(E, E, F).E)

(2)

– If the above check fail return +. Else, compute

= E

= (r × h).G

– Return C

= (E

, F, V, W ).

• Dec(C

, Pk

, Sk

(Original CipherText)

– Compute D and D. Then check if (1) & (2)

holds.

– If the condition hold, compute (m||w) = F ⊕

(

a,1

(Pk

a,2

+ x

a,2

)

.E)

– If : E = H

(m, w).[(H

(Pk

b,2

).Pk

a,1

)*Pk

a,2

]

and E = H

(m, w).[H

(Pk

a,1

, Pk

a,2

, D, E, F)]

return m , else return +.

(Transformed CipherText)

– Compute (h||π) = W ⊕ H

(

a,2

.V ).

– Extract (m||w) = F ⊕ H

(

– If : V =H

(h, π).Pk

a,2

and E

=(h × H

(m, w)).G

return m , else return +.

We chose to use JavaScript as a core technology.

And thus to be executed in the client side directly by

navigator or even mobile devices without any soft-

ware and also in the server side, thanks to Nodejs.

For the tests we used a 2,5 GHz intel core i7, with 16

GB RAM.

Table 2. shows the time resources consumed by

the different functions of Chow’s algorithm for both

Table 2: Computational efﬁciency of Chow algorithm in

(ms).

Function F

ECC

KeyGen 515 68

ReKeyGen 502 48

Encrypt 1000 95

ReEncrypt 967 89

Decrypt 979 78

implementations. We can see that encryption and re-

encryption functions consume the most compared to

keys generation and decryption. Practically, encrypt

and key generation functions wont be often called.

Generating re-encryption keys, depends on the num-

ber of delegations needed. But, still not constraining

in terms of time consumption. On the other side, re-

encryption function is called for each new user, new

delegation and changed session key. Having an in-

dependent service that does the re-encryption work is

lightening.

Figure 6: Tasks distribution between different environ-

ments.

An API will be developed which will act as the

gateway to our service functionalities. This API does

not manage any sensible data. The only concern is

to prevent from DDos attacks, then a simple API key

mechanism can be used as API authentication to iden-

tify the API user and the API consumers (Tang et al.,

2015).

3.3 Data Flow

Figure 7: Interactions between DO, CSP and DP.

Figure 7 is about, in the ﬁrst part the initialization pro-

cess between a DO, DP and a CSP. Which consists in:

PRE as a Service within Smart Grid City

399

•

 The registration of the data owner with the

CSP.

•

 The CSP proposing the sharing option to DOs

in order to use a dedicated algorithm.

•

 Generating a pair public/private key pair by the

DO, encrypting the private key with a symmet-

ric encryption scheme using a key derived from a

password (the use of SC RY PT for the purpose is

recommended) and sending the public key and the

encrypted private key to the CSP.

•

 The CSP storing the encrypted private key and

transmitin the public key.

•

 Encrypting the data with a session key that will

be encrypted with the corresponding public key.

•

 Authenticating DO to the CSP which will grant

access in response to the corresponding data.

•

 The decryption of the session key that will be

deciphered by the DO with his private key.

Figure 8: Interactions between DO, CSP and DC.

Figure 8, describes the sharing process between the

DO and DC. The dotted arrows mean that the requests

are transmitted indirectly passing by the PREaaS. The

latter will at ﬁrst demand access to the needed data.

The CSP will forward the demand to the DO with

the DC public key. As a response he creates a re-

encryption Key and transmits it to the CSP. This one

calls the PREaaS to re-encrypt the corresponding ci-

pher of the session key and send the needed encrypted

data.

As an illustration we provide a concrete example

of data ﬂow in Fig. 9. It includes one NEM, one

client, the BE and the PREaaS. If we take into con-

sideration all the process from registration until data

consuming, then there will be 8 steps:

• 1: Registration of the NEM (who plays the role of

the CSP) with PREaaS.

• 2: Notify the BE that a new NEM has been added.

• 3: Sending a sharing request to access the needed

Data by the BE .

• 4, 5: PREaaS receive and forward the sharing re-

quest by sending the BE’s public key to the NEM

Figure 9: Data Flow.

which will send it to the client with an access re-

quest.

• 6: The client creates and transfers the re-

encryption key to the NEM.

• 7: NEM authenticate with the PREaaS and return

the encrypted data.

• 8: PREaaS re-encrypts the corresponding data

with the key received and transfers it to the BE

to be consumed.

In the case of an ”automatic or predeﬁned” shar-

ing between the NEM and the BE, the PREaaS re-

ceives by default the new data produced from the CSP

which will involve only two steps (7,8) and as soon as

there is a new customer it will be asked by the NEM

to create a re-encryption key that will be returned to

PREaaS and added to its directory (5,6,7,8).

4 CONCLUSIONS

In this paper we present an original approach, which

is PRE as a Service with it application to smartgrid

cities. The PREaaS could be used in a more general

context and applied to different scenarios. We imple-

mented a prototype that use the Chow algorithm and

evaluate the performances. We will see if it would be

more appropriate to instantiate them in different man-

ners e.g with HMAC. Which was proved to be more

secure in different schemes. The PREaaS will allow

the use of different PRE algorithm in future such as

BBS with ephemeral keys, Ateniese scheme... And

thus for scalability and ﬂexibility purpose. As a part

of the VERTPOM’s project, we will work on authenti-

cation issues in multi cloud systems which we haven’t

treated in this work. Our future research will be fo-

cused on the security analysis of PRE, mainly on the

instantiation of random oracle and its effect on the

PRE schemes.

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