Design and Development of Layered Security

23 Pages·2016·3.1 MB·English

sensors
Article
Design and Development of Layered Security:
Future Enhancements and Directions in Transmission
AamirShahzad1,MalreyLee1,*,SuntaeKim2,KangminKim3,*,Jae-YoungChoi4,*,
YounghwaCho4andKeun-KwangLee5
Received:5October2015;Accepted:12December2015;Published:6January2016
AcademicEditor:LeonhardM.Reindl
1 CenterforAdvancedImageandInformationTechnology,SchoolofElectronics&InformationEngineering,
ChonBukNationalUniversity,664-14,1Ga,Deokjin-Dong,Jeonju,Chonbuk561-756,Korea;
[email protected]
2 DepartmentofSoftwareEngineering,ChonBukNationalUniversity,664-14,1Ga,Deokjin-Dong,Jeonju,
Chonbuk561-756,Korea;[email protected]
3 DivisionofBiotechnology,CollegeofEnvironmental&BioresourceSciences,ChonbukNationalUniversity,
Iksan570-752,Korea
4 CollegeofInformationandCommunicationEngineering,SungkyunkwanUniversity,Suwon440-746,Korea;
[email protected]
5 DepartmentofBeautyArtsCare,KoguryeoCollege,Naju520-930,Korea;[email protected]
* Correspondence:[email protected](M.L.);[email protected](K.K.);[email protected](J.-Y.C.);
Tel.:[phone](M.L.);[phone](K.K.);[phone](J.-Y.C.)
Abstract: Today,securityisaprominentissuewhenanytypeofcommunicationisbeingundertaken.
Liketraditionalnetworks,supervisorycontrolanddataacquisition(SCADA)systemssufferfrom
anumberofvulnerabilities. Numerousend-to-endsecuritymechanismshavebeenproposedfor
the resolution of SCADA-system security issues, but due to insecure real-time protocol use and
the reliance upon open protocols during Internet-based communication, these SCADA systems
can still be compromised by security challenges. This study reviews the security challenges and
issues that are commonly raised during SCADA/protocol transmissions and proposes a secure
distributed-network protocol version 3 (DNP3) design, and the implementation of the security
solutionusingacryptographymechanism. DuetotheinsecuritiesfoundwithinSCADAprotocols,
thenewdevelopmentconsistsofaDNP3protocolthathasbeendesignedasapartoftheSCADA
system,andthecryptographicallyderivedsecurityisdeployedwithintheapplicationlayerasapart
oftheDNP3stack.
Keywords: supervisory control and data acquisition; distributed network protocol; dynamic
cryptographybuffer
1. Introduction
Supervisorycontrolanddataacquisition(SCADA)systemshavebeenaccordedaprestigious
statusandplayimportantroleswithinthereal-timeindustrialprocessingandautomationfields[1,2].
Alongwiththemassivechangesinthefieldofinformationtechnology(IT)andtheincreasinguseofIT
byhumansthroughouttheirdailylives,SCADAsystemshavealsochangedfromsimplestand-alone
systems,or“relaylogic”,tonetwork-basedsystemslikeatraditionalcomputernetwork[2,3].
Typically,aSCADAsystemisgovernedbyacentralcontrollerormaincontrollerthatisapartof
itshierarchicalnetworkstructurethatisdesignedandconfiguredtocompriseseveralremotestations
orremoteterminalunits(RTUs);theseremotedevicescollectinformationfromphysicaldevicesand
transfertheinformationbacktothemaincontrollerforscrutinyandcontrolpurposes[3,4].
Sensors2016,16,37;doi:10.3390/s16010037 www.mdpi.com/journal/sensors
Sensors2016,16,37 2of23
SCADA-system protocols were originally designed and used as proprietary protocols; each
protocolwasdevelopedbyaspecificmanufacturerforaspecificindustryasapartofaproprietary
SCADAsystemtofulfillthebasicneedsofthatspecificindustry[3,4]. Withtheincreasingdemands
on SCADA systems and the underlying interoperability problems, the need for non-proprietary
protocols was identified [4]. Today, many open-standard protocols such as distributed network
protocolversion3(DNP3),Modbus,andFieldbushavebeendevelopedbySCADAorganizations,and
haveresolvedthedilemmaofinteroperabilitybyprovidingconnectivityforthedevicesandequipment
ofdifferentmanufacturersandvendors[5]. Giventheevolutionoftheopen-standardprotocolsthat
areusedwithinSCADAcommunication,userscannowpurchaseanduseequipmentsuchasmaster
terminal units (MTUs), RTUs, and other physical devices from a variety of manufacturers. This
largerinterconnectivitybetweenanumberofopen-standardprotocolsandtheproprietaryprotocols,
however,hasresultedintheheightenedvulnerabilityofSCADAsystemstoseveraltypesofsecurity
attacks[6,7].
DNP3isaconventionalprotocolthathasbeenusedinSCADAindustrialprocessingincluding
electricitygenerationanddistributionandwaterdistribution. DNP3isanopen-standardprotocol
thatisbuiltonathree-layered,enhanced-performance-architecture(EPA)model. TheDNP3protocol
contains the following three layers: the application layer, data-link layer, and physical layer. One
additional layer, called the “pseudo-transport layer”, is added to the DNP3 stack to permit the
transmissionofenormousquantitiesofdata[8]. TheDNP3protocolfacilitatesreliablecommunication
betweentheSCADAnodes[8,9]. Theproposedstudyemphasizesnotonlytheapplicationlayerinthe
DNP3protocol-developmentprocess,butalsothevulnerabilitiesthathavearisenatanapplication
level,regardingthemessagetransmissionto/fromtheSCADA/DNP3protocol,andregardingTCP
(transportcontrolprotocol)/IP(internetprotocol)communicationovertheInternet.
2. ProblemStatement
ThedetailsofthevulnerabilitiesandsecurityissuesthataffectSCADAcommunicationswere
reviewedandanalyzed[10–12];basedonthisreview,anappropriate,flexible,andprotectivesecurity
solutionforSCADAcommunicationcouldnotbeidentified. Eachofthepossiblesecuritysolutionhas
limitationsduetoitsdependencyonothersecuritymechanismsandprotocols[13–15].
A SCADA system’s potential security challenges exist because of the larger interconnectivity
issuesbetweenproprietaryandnon-proprietaryprotocolsandtheuseofmodernIT[15–22]. Ageneric
securitysolutionisdesirablefortheresolutionofsecurityissuessuchasconfidentiality,authentication,
integrity,andnon-repudiation[23–27]. EstablishedglobalorganizationsincludingtheU.S.National
Security Agency, U.S. Homeland Security Department, Schweitzer Engineering Laboratories, Inc.,
theAmericanGasAssociation,andtheGasTechnologyInstitute,havebeenvulnerabletosecurity
threats due to the Internet’s open connectivity [28,29]. These organizations and other researchers
havedevelopedseveralsecuritysolutionsandprotocolstoguardagainstcyber-securityissues,and
thecryptography-basedsecuritymechanismshavebeenselectedasthe“best”securityapproaches
for secure SCADA communications [26,30–42]. As a consequence, the DNP3 users group defined
and described the cryptography approaches, such as the asymmetric and symmetric methods, to
enhancethesecurityoftheDNP3protocol;therefore,thedetailsofseveralcryptographyalgorithms
are available along with the information regarding their security parameters [43–57]. The main
parameters such as integrity and authentication reportedly ensure and secure communication at
an application level by employing a challenge-response mechanism and are significant in their
protection against spoofing, modification, and replay attacks [8,58]. Several limitations, however,
areacknowledgedregardingthedesignoftheDNP3secure-authenticationmechanism(mostofthe
workremainsunderdevelopment)includingacapabilitythatonlyensuresend-to-endsecurityrather
thanprotocol-embeddedsecurity[55–57].
Supremesecurityprotocolsand/orsolutions,suchassecuresockets-layer(SSL)/transport-layer
security (TLS), Internet protocol security (IPSec), secure shell (SSH), and key agreements and
Sensors2016,16,37 3of23
management, among others [43–48], have been deployed within traditional network systems
and/or SCADA systems; however, these solutions have several limitations due to the protocol
dependency, an end-to-end-security focus, and a reliance upon the other cryptography protocols
Sensors 2016, 16, 37 3 of 22
forsecurity[5,14,49,50]. Anactualsecurityenhancementwasmadetoconquerthesecurityissuesthat
usuaSllCyAreDsAid seyisntetmhse; throawnesmveirs, stihoenses soofluthtieonSsC hAaDveA sesvyesrtaelm lim;iintatthioinsse dnuhea ntoc tehme epnrot,tothcoel DdeNpPen3dpernoctyo, colis
consiadne reendd,-aton-denedn-csreycuprtiitoyn faoncudsa, uatnhde na tircealitaionncem uepcohna nthisem osthtehra tcrmypotdoigfyratphheyo prirgoitnoacollfsr afmore soefcuthrietyD NP3
[5,14,49,50]. An actual security enhancement was made to conquer the security issues that usually
protocol are employed. The cyclic-redundancy-check (CRC) bytes are replaced with new security
reside in the transmissions of the SCADA system; in this enhancement, the DNP3 protocol is
fieldsthataredesignatedtocomputethesecurityviathecryptographymechanism. Thescopeofthe
considered, and encryption and authentication mechanisms that modify the original frame of the
development,however,islimitedtoadata-linklayer,andthesecuritydevelopmentisalsolimitedto
DNP3 protocol are employed. The cyclic-redundancy-check (CRC) bytes are replaced with new
onlytheprovisionofastrongerauthenticationandnon-repudiationsecurityparametersfortheDNP3
security fields that are designated to compute the security via the cryptography mechanism. The
protosccoolp[e5 o9f] .the development, however, is limited to a data-link layer, and the security development is
aInlsoc olnimclituesdi otno, oannlyi ntchleu spirvoevisseiocnu roitfy as oslturotniogner isaurtehqeuntiirceadtiorna thanedr tnhoann-reepnudd-itaot-ioenn dsesceucruitryi ty to
resolpvaeramtehteersse for tlhime DitNatPio3 npsrotocaonl d[59]. enhance the security of SCADA/DNP3-protocol
communiIcna tcioonncslu[s5io,1n4, ,2a5n, 3i0n,c3lu2,s4iv5e,5 s1e]c.uAritsye csuolruittiyons oilsu trieoqnuiorerdm roatdheelr hthaasnt heenrde-ftoor-eenbde esencuprriotyp otos edto
resolve these limitations and enhance the security of SCADA/DNP3-protocol communications
supplantend-to-enddeploymentandtoenhancethesecurityoftheDNP3protocolaspartofacritical,
[5,14,25,30,32,45,51]. A security solution or model has therefore been proposed to supplant
infrastructuralcommunication(orSCADA)-system-using,protocol-embeddedsecurity.
end-to-end deployment and to enhance the security of the DNP3 protocol as part of a critical,
infrastructural communication (or SCADA)-system-using, protocol-embedded security.
3. ScopeofStudy
3In. Scthopise osft uSdtuyd,y a DNP3 protocol stack was designed and an inclusive security solution
that uses cryptography algorithms such as AES, RSA, and SHA-2 is solely deployed at the
In this study, a DNP3 protocol stack was designed and an inclusive security solution that uses
DNPc3r-ypprtootgorcaoplh-ya ppallgicoartiithomnsl ayseurc;h hoaws evAeErS,, enRdSAu,s erasndw erSeHaAl-l2o wies dstooledlye pldoeyplaonydedt esatt ththeeo ther
cryptDoNgrPa3p-phryotaolcgool-raipthpmlicsa.tiTonh elaaypepr; lihcoawtieovnerl,a yenedr iussceorsn swidereer eadllotwheedm toos tdespenlosyi tiavned lateyset rthine tohtheeDr NP3
protoccroypltboegcraapuhsey tahlgeorpirthomtosc. oTlhme aepsspaligceatiiosng leanyeerr aiste cdonastidthereeda pthpel imcaotsito nse-nlasyitievre lleavyeelr winh tehne DaNmPe3s sage
is depsrigotnoactoel dbeacasussee nthteo prrorteocceoilv medes,saagned ist hgeenemraetsesda agte thise atphpelnicaptiaosns-eladyetro leavello wwheern lae vmeelssfaogre fius rther
execudteisoingn[a8t,e1d0 ,a5s8 ]s.enTth oisr rsetcuedivyetdh, earnedf othree smheoswsasgeth ise tdheepn lpoaysmseedn ttoo af lsoewcuerr ilteyvealt ftohr efuarpthpelric eaxteiocuntiloenv el,or
[8,10,58]. This study therefore shows the deployment of security at the application level, or the
thedeploymentofapplicationprotocoldataunit(APDU)securityasapartoftheapplicationlayer,
deployment of application protocol data unit (APDU) security as a part of the application layer,
beforetheAPDUbytesaretransmittedtothelowerlayersoftheDNP3protocolasapartoftheSCADA
before the APDU bytes are transmitted to the lower layers of the DNP3 protocol as a part of the
system(security)and/oropenprotocols. FurtherdetailsareshowninFigure1,asfollows.
SCADA system (security) and/or open protocols. Further details are shown in Figure 1, as follows.

Figure 1. Research scope and study gap.
Figure1.Researchscopeandstudygap.
4. Research Objectives
4. ResearchObjectives
The main objectives of this study are as follows:
Themainobjectivesofthisstudyareasfollows:
1. The bytes of the application layer or the APDU bytes are fixed at the application level; the
1. Thedbyyntaemsioc fcrtyhpetoagprapplihcya tbiuofnfelra (yDeCrBo)r ist hueseAd PfoDr Utheb ryetmesaianrinegfi bxyetdes aatfttehr ethaep fpixliincga tpioroncelsesv iesl ; the
complete. The APDU bytes are therefore readily aligned with the pseudo-transport-layer bytes
dynamiccryptographybuffer(DCB)isusedfortheremainingbytesafterthefixingprocessis
or the TPDU bytes during the transmission of the message or the bytes.
Sensors2016,16,37 4of23
complete. TheAPDUbytesarethereforereadilyalignedwiththepseudo-transport-layerbytesor
theTPDUbytesduringthetransmissionofthemessageorthebytes.
2. Theassemblageoftheprotocolbytesfromthelowerlayertotheupperlayer,wherebysecurityis
implementedandtestedintermsofintegrity,authentication,confidentiality,andnon-repudiation
attheapplicationlayeroftheDNP3protocol.
3. The proposed security development is validated using formal proofs and the performance
results are evaluated with security tools. The security results are also measured
during the SCADA/DNP3-protocol end-to-end communication for the purposes of a
performancecomparison.
5. ResearchContribution
Thecontributionofthisstudyisthree-fold,asfollows: anewsecuritymechanismwasdesigned
andimplementedwithintheoriginalSCADA/DNP3-protocolstack;thebytesareconstructedateach
layer,flowedwithinthestack,andtransmittedto/fromtheTCP/IPprotocolsatbothsidesoronthe
send/receiveside.
Theapplication-layerbytesarealignedwiththepseudo-transport-layerbyteswithintheDNP3
stack; the remaining 56 bytes are used for the DCB. The DCB was designed and deployed to
keep track of the bytes during message construction and security implementation. The new
security mechanism was implemented and tested using cryptography to enhance the security of
theSCADA/DNP3-protocolcommunications,anditalsoprovidesaplatformtotestandimplement
othercryptographyapproaches(oralgorithms).
The security mechanism was designed and implemented according to the requirements of
SCADA/DNP3unicastingcommunication. Formalmethodsandtoolshavebeenusedtovalidatethe
proposedsecuritydevelopmentandtoevaluateitsperformance. Theoverallperformanceresultswere
measuredandevaluatedaftertheverificationofformalproofs.
6. DevelopmentoftheApplicationLayer
TheapplicationlayeristhehighestortoplayeroftheDNP3-protocolstackthatinteractsdirectly
withuser-applicationprogramsortakesdata/messagesfromtheuser-applicationinterface/program.
The application layer interacts with the user-application program and accepts user-requested
data/messagesdependingontherequirementsoftheuser. Datamaybeinanyformsuchasdigital,
analog,events(ortheactionsperformedforanevent),alarms,reports,anddatafiles. Datasizeisnot
limitedintheDNP3protocol;theDNP3-applicationlayertakesrandomdatafromauser-application
programandmanipulatesthisdata. Theapplicationlayerconvertsthedatafromtheuser-application
programintomanageablysizedblocks. Theseblocksarealsoreferredtoasapplicationservicedata
units(ASDUs),andeachASDUsizeislimitedto2046/2044bytesinthecaseofarequest/response[2,9].
The application layer creates APDUs by combining or adding an application header that is
called“applicationprotocolcontrolinformation”(APCI).TheAPCIfieldsconsistoftwoorfourbytes
dependingonwhetherthedatarequestsareplyback. Intheproposedimplementation,theAPDUsize
islimitedto1992bytes;iftherequested/responsedataislargerthan1992bytes,thenmorethanone
APDUwillbecreated,dependingonthesizeoftheuserdata. IntheoriginalDNP3documentation,
the numbers of ASDU blocks are not limited but each APDU’s size is limited to 2048 bytes. The
APDU-buffersizeissufficienttoprocesstheentiretyoftheinformationinthedatarequested/received
and,afterprocessing,thebufferisdiscardedforthenextAPDUbytes[9].
ThetotalnumberofuserblocksintheASDUiseight. Eachblockhas249bytesofinformation
thathavebeenacceptedfromthelastblockbecausethelastblockhas247bytesinthecaseofarequest
and245bytesinthecaseofaresponse. Theseblocksreadilyfitintothetransport-protocoldataunits
(TPDUs)ofthepseudo-transportlayer. ADCBiscreatedbasedontheremaining56bytesfromeach
ASDU and is used for storage information that is related to cryptography deployment and other
serviceinformationforthedevelopmentofthetestbed.
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ThefixingprocessfortheASDUbytesistwo-fold,asfollows: theAPDUbytesarealignedwith
theTPDUbytesandtheremaining56bytescouldbeusedforcryptographydeploymentwithinthe
appliSceantsoiorsn 20l1a6y, 1e6r, 3o7 ftheDNP3protocol. WhentheAPDUisconstructedintheapplication5l aofy 2e2r ,the
controlispassedtothecryptographysolutionforsecuritydeployment.InFigure2,themasterterminal
terminal unit (MTU) generates the request message and sends it to the remote terminal unit (RTU)
unit(MTU)generatestherequestmessageandsendsittotheremoteterminalunit(RTU)thatthen
that then generates the application-level response to the MTU after it receives the request. In the case
generates the application-level response to the MTU after it receives the request. In the case of a
of a command execution between the MTU and RTU, the header is only transmitted without any
commandexecutionbetweentheMTUandRTU,theheaderisonlytransmittedwithoutanyASDU
ASDU bytes. The following two types of message headers are in the application layer of the DNP3
bytes. ThefollowingtwotypesofmessageheadersareintheapplicationlayeroftheDNP3protocol:
protocol: request and response headers. The response header has an additional two-byte field called
requetshtea inndterrensapl oinndseichateioand e(rIsIN. T) hthearte sinpdoincsaetehs etahde edrihffaesreanncea dbdetiwtioeenna ltthwe or-ebqyuteestfi ealndd ctahlele dretshpeoninset ernal
indicahteiaodne(rIsI iNn )ththe aAtPinCdI.i catesthedifferencebetweentherequestandtheresponseheadersintheAPCI.

FiguFriegu2r.eA 2p. Applipcalitciaotnio-nla-lyaeyrers tsatatete--ttrraannssiittiioonn pprroocceesss swwithit hcrcyrpytopgtoragprhayp hsoylustoiolunt. ion.
In the application layer, the MTU is responsible for sending request messages to the RTU and
In the application layer, the MTU is responsible for sending request messages to the RTU
the RTU is responsible for sending the response according to the MTU request; however, the
and tDhNePR3T-aUppislicraetsiopno nlasyiebrl ehafos ra ssepnedciianl gcltahsse orfe srepsopnosnesea tchcaotr disi nusgedto btyh teheM RTTUU rfeoqr uthees tp;uhropwoseev oefr , the
DNPs3e-nadpipnlgi ciantfioornmalatiyoenr toh aitss aprsepdeecfiinaeldc lmasasstoerf srteastipoonn. Tsehitsh sapteicsiaul scleadssb oyf rtehsepoRnTsUe isf oarlstoh cealpleudr pano se of
sendi“nugnsionlfiocirtmeda trieosnpotnosiet”s apnrded ies fitnypedicamllya sbtaesresdt aotnio nth.eT chriisticsaple ecviaelnctsl aosrs porforceessps ocnhsaengisesa ltshoatc aalrlee dan
“unsodliectietcetdedr ewsipthoinns ae ”coamndmuisntiycaptiicoanl.l ybasedonthecriticaleventsorprocesschangesthataredetected
withinacIonm Fimguurnei 2c,a tthioe nh.ighlighted part shows the confirmation set (bit) that is transmitted between the
master terminal unit (MTU) and remote terminal unit (RTU) and/or vice versa. The application
InFigure2,thehighlightedpartshowstheconfirmationset(bit)thatistransmittedbetweenthe
control (AC) is accounted as a main field of application layer header which used to control the
masterterminalunit(MTU)andremoteterminalunit(RTU)and/orviceversa. Theapplicationcontrol
communication between the nodes. AC contained four subfields, such as first fragment (FIR), final
(AC)isaccountedasamainfieldofapplicationlayerheaderwhichusedtocontrolthecommunication
fragment (FIN), confirmation (CON), and sequence number and in the below communications 1 and
betweenthenodes. ACcontainedfoursubfields,suchasfirstfragment(FIR),finalfragment(FIN),
2 these subfields are represented in the form of [A0, A1, A2, A3] and [B0, B1, B2, B3], with
confirdmistaitnigounis(hCeOd Nse)t, baint d(i.see.,q CuOeNnc oern Au2m/Bb2e =r a0 nodr 1in). tMhoerbe edloetwailc iosm prmovuindiecda tiino tnhse 1suabnsdeq2utehnet speasrtu obffi elds
are rethpirse psaepnetre,d asi nfoltlhowesf:o rm of [A0, A1, A2, A3] and [B0, B1, B2, B3], with distinguished set bit
(i.e.,CONorA2/B2=0or1). Moredetailisprovidedinthesubsequentpartofthispaper,asfollows:
MTU/RTU Communication 1 {
MTUI/fR RTeUquCesot:m MmTUun_AicCa t[iAo0n, 1A1{, A2 = 0, A3] && Response: RTU_AC [A0, A1, A2 = 1, A3]
Then Confirmation: MTU_AC [A0, A1, A2 = 0, A3]
IfRequest: MTU_AC[A0,A1,A2=0,A3]&&Response: RTU_AC[A0,A1,A2=1,A3]
} // Master station has initialized the request without confirmation and upon receiving the RTU
ThenConfirmation: MTU_AC[A0,A1,A2=0,A3]
response with a confirmation set (bit), the MTU also sends the confirmation.
} // Master station has initialized the request without confirmation and upon receiving the RTU
respoMnsTeUw/RitThUa Ccoomnfimrumnaictaiotinons e2t {( bit),theMTUalsosendstheconfirmation.
If Request: MTU_AC [B0, B1, B2 = 1, B3] then Confirmation: RTU_AC [B0, B1, B2 = 0, B3]
MTU/RTUCommunication2{
&& Response: RTU_AC [B0, B1, B2 = 1, B3]
IfRequest: MTU_AC[B0,B1,B2=1,B3]thenConfirmation: RTU_AC[B0,B1,B2=0,B3]
Then Confirmation: MTU_AC [B0, B1, B2 = 0, B3]
&&Response: RTU_AC[B0,B1,B2=1,B3]
Sensors2016,16,37 6of23
ThenConfirmation: MTU_AC[B0,B1,B2=0,B3]
}//Masterstationinitializedtherequestwiththeconfirmationset(bit)andtheRTUalsosendsthe
confirmationmessage. Afterward,theRTUwillsendaresponsewithaconfirmationset(bit)upon
receiptandtheMTUalsosendstheconfirmationtotheRTU.
WhentheMTUsendsarequestmessagetotheRTUwithaconfirmationbit,uponthereceipt
Sensors 2016, 16, 37 6 of 22
ofthemessage,theRTUalsosendsanacknowledgment(message)totheMTU.TheRTUassembles
Sensors 2016, 16, 37 6 of 22
} // Master station initialized the request with the confirmation set (bit) and the RTU also sends the
the necessary information related to the requested message; once the message is ready, then it is
confirmation message. Afterward, the RTU will send a response with a confirmation set (bit)
senttothe}M // TMUas.teTrh setatMionT iUnitaiallsizoeds ethned rseqauecsot nwfiitrhm thaet cioonnfir(mmaetisosna sgeet )(biift)t ahned cthoen RfiTrUm aalstoio snenbdsit thise setinthe
upon receipt and the MTU also sends the confirmation to the RTU.
confirmation message. Afterward, the RTU will send a response with a confirmation set (bit)
response(message).
uWphoenn r tehcee iMptT aUn ds etnhde sM aT rUeq aulessot smenesdssa tghee t coo tnhfeir RmTaUti owni ttho ath ceo RnfTirUm. ation bit, upon the receipt of
InaSCADAsystem,communicationbetweenfielddevicesiseitherintheformofcommands
the message, the RTU also sends an acknowledgment (message) to the MTU. The RTU assembles the
When the MTU sends a request message to the RTU with a confirmation bit, upon the receipt of
or data. Unseuceaslslayr,y tihnfeorRmTaUtionw rielllatgede ntoe trhaet ereqthueestdeda tmaersesasgpeo; onnscee tahcec moersdsiangge ist oretahdye, MtheTnU it ics osemntm toa nds. The
the message, the RTU also sends an acknowledgment (message) to the MTU. The RTU assembles the
message onthreecd eMsasTtaaUry. aTinlhwfeo raMmyTasUtio canols nroet lsaaetinenddss t oas pcthoeenc friierfiqmcuaetdsiotaendt a( mmoeessbssjaaeggceet);s oifn atchnee dt choefn umfinremscstaaitogioen nis bc rioet adisde ysse, tt thihne nath tiet airser sespenoatnd tsode ed at the
(message).
start of atchoe mMTmUu. Tnhiec aMtiToUn a-lmsoe ssesnadgs ea croenqfiurmesattioonr (mreessspagoen) sife t.he TcohnefirSmCatAioDn bAit/ isD seNt iPn3 thpe rreostpoocnosel provides
In a SCADA system, communication between field devices is either in the form of commands or
reliablecom(mmesusangiec).a tionbetweendifferentvendor’sdevicesandmakesdatameaningfulacrossseveral
data. Usually, the RTU will generate the data response according to the MTU commands. The
In a SCADA system, communication between field devices is either in the form of commands or
platforms.mTehsseagfoe lolor wdaitna galwseacytsi oconnstaainres stpheecicfioc rdeataap opbljeicctast aiondn -fluanycetironf ucondcetsi othnast tahrea atdhdaevde atb teheen stuarst edduring
data. Usually, the RTU will generate the data response according to the MTU commands. The
messagecomonfe sasts racuogmec tomiro udnna,itcaaa ntaildownta-hmysee cstsoyanpgtaee isnreso qsfupeiensctif fooicrr mdraeatsapt iooobnnjseecs.t usT cahhned aS fsCuAtnhcDteiAof/nuD cnNocdPte3ios pnthroacttoo adcroeel aapdnrdodevdidd aeatst tahreeol isbatbjaelrect tsthatare
communication between different vendor’s devices and makes data meaningful across several
involvedinof cao cmommmuunniiccaattiioonn-m. essage request or response. The SCADA/DNP3 protocol provides reliable
platforms. The following sections are the core application-layer functions that have been used
communication between different vendor’s devices and makes data meaningful across several
during message construction, and the types of information such as the function code and data
6.1. ApplicpatlaiotfnorPmrso. tTochoel fCololonwtrionlg Insefcotriomnas tiaorne the core application-layer functions that have been used
objects that are involved in communication.
during message construction, and the types of information such as the function code and data
Theaopbpjelcitcsa tthiaot narhe einavdoelvreids ina lcsoomcmaullneicdattihone. applicationprotocolcontrolinformation(APCI).APCI
6.1. Application Protocol Control Information
hastwo-bytefieldsforarequestheaderandafour-bytelengthfieldforaresponseheader,whereby
6.1. ATphpeli caaptpiolnic Patriootnoc hole Cadonertr iosl aInlsfoor cmaallteiodn t he application protocol control information (APCI). APCI
therequestandresponseheadersdifferbytwobytesintheapplicationlayer. Theresponseheader
has two-byte fields for a request header and a four-byte length field for a response header, whereby
The application header is also called the application protocol control information (APCI). APCI
hasanaddthitei orenqaulestwt aond-b ryestepofineseld h,edadeesrisg dnifafeter dbya tswtoh beyItIeNs i,nw thhe iacphpdliciasttiionng luayiserh. eTsheb eretswpoenesne htheaedreer questand
has two-byte fields for a request header and a four-byte length field for a response header, whereby
responsehheaas dane rasddinitiotnhael tAwPo-CbyI.teF fiieglud,r dee3sigsnhaotewd sast thhee IAINP, CwIhiochr dthisetincgausieshoefs bAetPwCeeInb tyhtee rseqbueeisnt agndse nt,while
the request and response headers differ by two bytes in the application layer. The response header
response headers in the APCI. Figure 3 shows the APCI or the case of APCI bytes being sent, while
Figure4shhoasw asn tahddeitAioPnCal Itwoor-bthytee cfiaeslde, doefsaignnaAtePdC asI -tbhey ItIeNr, ewshpicohn dsies.tinguishes between the request and
Figure 4 shows the APCI or the case of an APCI-byte response.
response headers in the APCI. Figure 3 shows the APCI or the case of APCI bytes being sent, while
Figure 4 shows the APCI or the case of an APCI-byte response.

Figure 3. Request header structure.
Figure3.Requestheaderstructure.
Figure 3. Request header structure.

Figure 4. Response-header structure.
Figure 4. Response-header structure.
The first byte of the APCFI irgequurees4t .oRr reessppoonnssee -ihs edaedsiegrnastterdu catsu trhee. application control (AC). The
communication between the MTU and RTU is controlled by the AC field. The AC field carries one
The first byte of the APCI request or response is designated as the application control (AC). The
byte of information, having three bits or flags, such as FIR, FIN, and CON, and a five-bit sequence
communication between the MTU and RTU is controlled by the AC field. The AC field carries one
Thefinrusmtbbeyrt ethoatf ist hueseAd tPoC mIorveeq furaegsmteonrtsr (eosrp AoPnDsUesi)s ind ae sseiqguneantteiadl oarsdetrh. eapplicationcontrol(AC).The
byte of information, having three bits or flags, such as FIR, FIN, and CON, and a five-bit sequence
communicnautmiobInne rM btheTatUtw i/sRe uTesUned tc htoome mmMouvTnei Ucfraatagionmnd,e niRft stT h(oUer MAisPTDcUoU snse)tt risn ot alhl ese edcqoubnefynirtmitahla etoiroAdne Crb. ifit aesl dC.OTNh e= A1 Cwitfiheinl dthcea rriesone
requested message, then the RTU will also be responsible for sending a confirmation within the
byteofinformIna tMioTnU,/hRaTvUi ncogmtmhureneicabtiiotsn, oifr tflhae gMs,TUsu scehts atsheF cIoRn,fiFrmINat,ioann bdit CasO CNO,Na n=d 1 awfiithvien- bthiet sequence
response message with the same sequence as the MTU-requested sequence number. In a case where
numberthraetquisesutesde dmetsosamgeo, vtheefnr athgem ReTnUt sw(ioll raAlsoP DbeU ress)pionnsaibslee qfoure snetnidailnog rad ecor.nfirmation within the
multiple messages are received by the MTU/RTU with the same sequence number, then the second
response message with the same sequence as the MTU-requested sequence number. In a case where
In MTnuUm/bRerT wUillc boem igmnourendi caandti othne, fiirfstt ohnee MwilTl Ube ascectespttehde; ifc noontfihirnmg iast rieocneivbeidt bays thCeO MNTU=/RT1Uw, ithin the
multiple messages are received by the MTU/RTU with the same sequence number, then the second
requestednmumebsesra wgeil,l bthe eignnotrhede aRnTd Uthew fiirlslt oanlseo wbille ber eascpceopntesdi;b ilfe noftohrinsge ins dreicnegivead cboy nthfier MmTaUti/RoTnUw, ithin the

responsemessagewiththesamesequenceastheMTU-requestedsequencenumber. Inacasewhere
multiplem essagesarereceivedbytheMTU/RTUwiththesamesequencenumber,thenthesecond
Sensors2016,16,37 7of23
numberwillbeignoredandthefirstonewillbeaccepted;ifnothingisreceivedbytheMTU/RTU,then
therequestwillbetransmittedagain.ThesequencenumberisincrementedwiththeAPDUincrements,
andtheresponse-APDUsequencenumberisthesameastherequested-APDUsequencenumber.
ThefirstAPCIfieldisusedtomanagethecommunicationflowandthesecondfieldisdesignated
afunctioncode(FC);theFCcarrysonebyteofinformationthatisusedtodefinetheactualmeaning
of the message/data to be sent/received between the MTU and RTU. Data is the most important
partofSCADA/DNP3communication. Aspreviouslymentioned,theDNP3protocolisdesignedto
manipulatedifferenttypesofdata,suchasdigital,analog,eventoranactionperformedforanevent,
alarms,reports,anddatafiles. TheDNP3protocolnotonlyprovidesreliablecommunicationbetween
fielddevicesthatareconnectedinaSCADAnetwork,butitalsomakesthedatameaningfulacross
differentnetworkstounderstandthemeaningandpurposeofthemessage.
InthesecondAPCIfield,thefunctioncodesareusedtodefineoridentifyspecificfunctionsfor
thedatathatarebeingoperatedorthefunctionthatisusedtoperformwiththedata. Somefunction
codesarelimitedtothespecificdataonwhichtheyoperate;therefore,data-objectreferencesarealso
includedwithintheASDU.
TheIINfieldthatisusedintheresponseheader(orAPCI)containstwobytesofinformation
followedbyafunctioncode,anditisusedforsendingaresponsetoadesiredstation. Eachbitinside
theIINfieldhasaspecificmeaningforthepurposesofresponse-messagegeneration,usuallyfromthe
RTUtotheMTU.TheRTU-responsemessagedefinestheflagsthatcorrespondtotheIINfieldthatare
copiedinthemessageeachtimethemessageissentfromthesubstationtothemasterstation. Using
theIINfieldintheresponseheader,thesubstationwillthenreporttothemasterstation.
In a SCADA system, the sent/received message carries commands and sometimes data as a
responsefromtheRTU,wherebythedataiscomposedoffunctioncodesanddataobjects. TheFC
definesthemeaningandusesofthebytes(ormessage)intheDNP3protocolandthedataobjects
definethestructureandthedatainterpretationprocess.
6.2. ApplicationServiceDataUnit
Asdiscussed,therearetwotypesofdata/messagecommunicationswithinaSCADAsystemsuch
asarequestfromtheMTU,aresponsefromtheRTU,and/oranunsolicitedresponse.Eachtime,oneor
moreAPDUsarecreated,dependingonthesizeoftheuserdatathatisbeingrequested/respondedto.
Ifmorethan1992byteswithaheaderaresentfromtheMTU,thenmultipleAPDUsaregeneratedand
transmittedinasequentialorder. EachAPDUisrepresentedasafragmentwithfieldssuchasAPCI
andASDU.TheASDUismadeupofdataobjectsandeachdataobjectisfollowedbyanobjectheader.
IntheASDUfield,theproposedresearchlogicallyfixedthenumberofdatablocksat8andeachblock
is249bytes,withtheexceptionofthelastblock;furthermore,theproposedresearchlogicallyfixedthe
ASDUsizeupto1990bytesinthecaseofarequestand1988bytesinthecaseofaresponse.
Here,eachdatablock(orblock)ofanASDUcontainsanobjectheaderanddataobjects. Through
logicalfixingoftheASDU,thedatablockseasilyfitinto(oralignwith)thetransportprotocoldata
units (TPDUs) in the pseudo-transport layer of the DNP3 protocol; for example, the 1992 bytes of
APDUaretransmittedandaretreatedasuserbytesinthepseudo-transportlayer. Thebytesofthe
transportservicedataunit(TSDU)areassembledandthenconvertedinto8datablocks,and1byte
ofheaderinformationisaddedtoeachdatablockresultingintheformationof8TPDUs. Because
thelowerlayer(ordata-linklayer)onlycarriesupto250bytesfromthepseudo-transportlayer,the
logicalfixingprocessoftheASDUistwo-fold,asfollows: 1)TheAPDUbytes(or1992APDUbytes)
arealignedwiththeTPDUbytesinthepseudo-transportlayer. 2)Theremaining56bytes(fromthe
totalof2048APDU)areusedfortheDCB.IntheoriginalDNP3documentation,theuser-datablocks
arenotfixed.
In the ASDU, each data block is composed of the following two subfields: object header and
dataobject. EachASDUfieldiscomposedofoneormoredatablocks, buttheblocksarelogically
limited to 8 (blocks) and the maximum ASDU size is up to 1990 bytes/1988 bytes in the case of a
Sensors2016,16,37 8of23
request/response. Figure5showstherequest/responseASDUstructure(ornew,fixedASDUblocks)
Sensors 2016, 16, 37 8 of 22
withsubfieldssuchas“objectheader”and“dataobject”.
Theobjectrheqeuaedst/errespsopnesec.i Ffiigeusret h5 sehodwast tah-eo rebqjueecstt/rteysppoensae nAdSDtUh setriunctsutrae n(ocr eneowf, fdixaedta ASthDUat bliosckbse) ingreferenced
with subfields such as “object header” and “data object”.
bythemessage. ThTeheo obbjjeecctt hheaedaedr sepreciisfiems thaed deatua-pobojecft tthypee oanbdj ethcet ,inqstuanaclei fiofe dra,taa tnhdat irsa bneignge rfiefeerlednscebd etween3bytes
and11bytesofblye nthge tmhe.ssage. The object header is made up of the object, qualifier, and range fields between 3
bytes and 11 bytes of length.

Figure 5. New logical fixing of ASDU.
Figure5.NewlogicalfixingofASDU.
In the ASDU, “object header”, “qualifier”, and “range” fields are used to identify the specific
data points of each object group, and the variations that are being referred to as data are used on
IntheASDboUth, t“heo sbenjedcert ahnde aredceeivre”r ,si“dqesu. Ian lai fireequre”s,t manesdsag“er, oannlyg tehe” dfiatea liddesntaifirceatiuons ewdill btoe reiqdueirnedti fythespecific
datapointsofeaancdh doabtaj eocbtjecgtsr oaurep i,ncalunddedt hine rvesaproinaseti omnesssatgheas tfraomre tbhee isnubgstraetifoenr. rTehde qtouaalifsiedr afiteald aisr eusedonboth
composed of the qualifier code or “Q-code”, the index size or “I-size”, and provides the
thesenderandqrueacleifiievr-ecordsei vdaeluse.. TInhe aquraeliqfiuere fsietldm weitshs taheg rea,ngoen flieyldt whoerkds atot afuildly eidnetnitfiifyc athtei odnataw obiljelctbs erequiredand
dataobjectsaretihnatc floulldowe deacinh orbejescpt hoenadseer min ethses AaSgDeUs. fInro thme otbhjeect shueabdsetr,a tthieo onb.jeTct hfieeldq ius asulbifidievridfiede ilndtoi scomposedof
two fields such as the object group that defines the general type of data and the object variation that
thequalifiercodeor“Q-code”,theindexsizeor“I-size”,andprovidesthequalifier-codevalue. The
defines a particular variation of data and up to 2 bytes of length.
qualifierfieldwiththerangefieldworkstofullyidentifythedataobjectsthatfolloweachobjectheader
6.3. System Model: Application Layer Development
intheASDU.Intheobjectheader,theobjectfieldissubdividedintotwofieldssuchastheobjectgroup
This study used the open library of the DNP3 protocol and the explicit codes were deployed,
thatdefinesthegeneraltypeofdataandtheobjectvariationthatdefinesaparticularvariationofdata
thereby finalizing the proposed system’s design and development. To validate the proposed
andupto2byteapspolicfaltieonn-glatyher. (message) design and development, Postulate 1 to Postulate 4 were employed, and
these are further used for the verification of the algorithm and during the security-development
phase. The basic notations that are used in this section are summarized in Table 1.
6.3. SystemModel: ApplicationLayerDevelopment
Table 1. Terminologies for system design and development.
ThisstudyusedtheopenlibraryoftheDNP3protocolandtheexplicitcodesweredeployed,
Notations Descriptions
thereby finalizing the proposed systReamndo’ms bydtees fsroimg unsera-anppdlicatdione lvayeerl oorp ramndoemn intp.ut byTteos frovma lidate the proposed
(cid:1850)
user-application layer.
application-layer (message) design and development, Postulate 1 to Postulate 4 were employed,
(cid:1858)(cid:3091) Function that defines bytes X
andthesearefurtherused(cid:1866)f ortheverifi(cid:1866)c=a 1t,i2o,3n,…o,(cid:1863)f,t(cid:1875)hℎ(cid:1857)e(cid:1870)(cid:1857)a(cid:1863)l(cid:1861)g(cid:1871)o(cid:1853)r(cid:1864)(cid:1861)i(cid:1865)t(cid:1861)h(cid:1872)(cid:1857)m(cid:1856)(cid:1861)(cid:1866)a(cid:1872)(cid:1857)n(cid:1859)(cid:1857)d(cid:1870) duringthesecurity-development
(cid:1858)(cid:3002)(cid:3017)(cid:3004)(cid:3010)((cid:1850)) Function that computes the application-layer header bytes or APCI bytes.
phase. Thebasicnotation(cid:1858)(cid:3002)(cid:3020)s(cid:3005)(cid:3022)t(h(cid:1850)a) tareuseFdunicntiont hthiast cosmepcutteiso thne aapprleicastiuonm-laymer adartai zbyeteds oir nASTDUa bbyltees. 1.
((cid:1850)(cid:3035),(cid:1850)(cid:3031)) Computed header (h) bytes (cid:1850)(cid:3035) and data (d) bytes(cid:1850)(cid:3031).
(cid:1850)(cid:3031)(cid:3052) Computed dynamic (dy) bytes (cid:1850)(cid:3031)(cid:3052) via function (cid:1858)(cid:3031)(cid:3052).
Table1.TerminoFloungctiieons thfoatr cosmypsuttees mthe dapepslicigatniona lanyder fdraegmveenlto opr AmPDeUn bty.tes. Such
(cid:1858)(cid:3002)(cid:3017)(cid:3005)(cid:3022)((cid:1850)) that (cid:1858)(cid:3002)(cid:3017)(cid:3005)(cid:3022)((cid:1850))=(cid:1858)(cid:3002)(cid:3017)(cid:3004)(cid:3010)((cid:1850))+(cid:1858)(cid:3002)(cid:3020)(cid:3005)(cid:3022)((cid:1850)).
(cid:2009)∈((cid:1845);(cid:1844)) Distinct identifier(s) for sender(S) or responding (R) message (M)/bytes (b)
Notations ((cid:1858)(cid:3002)(cid:3004),(cid:1858)(cid:3007)(cid:3004),(cid:1858)(cid:3010)(cid:3010)(cid:3015)) c(cid:1858)(cid:3002)a(cid:3017)s(cid:3004)e(cid:3010) o((cid:1850)f s)eonrd AinPgC(SI )b/ryetesps othnadti nagr(eRD c)o. em spcutreidp byt ifuonnctsions: (cid:1858)(cid:3002)(cid:3004),(cid:1858)(cid:3007)(cid:3004),(cid:1858)(cid:3010)(cid:3010)(cid:3015), in the
((cid:1876)(cid:2869),(cid:1876)(cid:2870),(cid:1876)(cid:2871)(cid:1876)(cid:2872),(cid:1876)((cid:3036)(cid:3040)R,(cid:3032)a(cid:3051))n,(cid:1876)d(cid:3036)(cid:3036)o(cid:3041)m) bytPeasrafmroetmers uofs feurn-catiopnp (cid:1858)l(cid:3002)i(cid:3017)c(cid:3004)a(cid:3010)(t(cid:1850)io),n(cid:1871). lSauyche rthaotr, (cid:1858)r(cid:3002)a(cid:3004)n,(cid:1858)(cid:3007)d(cid:3004)o,(cid:1858)m(cid:3010)(cid:3010)(cid:3015)i∈np(cid:1858)(cid:3002)u(cid:3017)(cid:3004)t(cid:3010).b ytesfrom
X ((cid:1858)(cid:3016)(cid:3009),(cid:1858)(cid:3016)(cid:3005)u)s er-applica (cid:1858)t((cid:3002)(cid:1858)i(cid:3020)(cid:3016)o(cid:3005)(cid:3009)n(cid:3022),((cid:1858)(cid:1850)l(cid:3016)a(cid:3005))oy)r,e iAnrS. tDheU c absyete os ft hseant dairneg c(So)m/rpeustpeodn bdyin tgh(eR f)o. llowing functions:
fu ((cid:1876)(cid:2873),(cid:1876)(cid:2874),(cid:1876)(cid:2875),F(cid:1876)(cid:2876)u)n ctionthPaatrdameefitenres sof bfuyntcetisonX (cid:1858)(cid:3002)(cid:3020)(cid:3005)(cid:3022)((cid:1850)), such that (cid:1858)(cid:3016)(cid:3009),(cid:1858)(cid:3016)(cid:3005)∈(cid:1858)(cid:3002)(cid:3020)(cid:3005)(cid:3022).
n ((cid:1876)(cid:2873).(cid:2869),(cid:1876)(cid:2873).(cid:2870)),((cid:1876)(cid:2874)n.(cid:2869),“(cid:1876)(cid:2874).(cid:2870)1) ,2,3,.o((cid:1876)b.(cid:2873)j.e.(cid:2869)c,,t(cid:1876)k q(cid:2873),u.(cid:2870)wa)liahfrieeer ri m(e(cid:1867)kp(cid:1869)li)is eda lfoimr oibtjeecdt ifunntcetigone(r(cid:1867)(cid:1858)) and ((cid:1876)(cid:2874).(cid:2869),(cid:1876)(cid:2874).(cid:2870)) are implied for
fAPCIpXq (cid:1858)(cid:3013)((cid:1850)(cid:3031))F unctionthFautnccotiomn pthuatt leosgictahlely afipxepd ltihcea AtSiDonU -blyateyse irntoh deaatda ebrlocbkys t(DesB)o. rAPCIbytes.
f pXq Functionthatcomputestheapplication-layerdatabytesorASDUbytes.
ASDU
pX ,X q Computedheader(h)bytesX anddata(d)bytesX .
h d h d
X Computeddynamic(dy)bytesX viafunction f .
dy dy dy
FunctionthatcomputestheapplicationlayerfragmentorAPDUbytes.Suchthat
f pXq
APDU f pXq“ f pXq` f pXq.
APDU APCI ASDU
αPpS;Rq Distinctidentifier(s)forsender(S)orresponding(R)message(M)/bytes(b)
f pXqorAPCIbytesthatarecomputedbyfunctions: f ,f ,f ,inthe
pf ,f ,f q APCI AC FC IIN
´ AC FC IIN ¯ caseofsending(S)/responding(R).
x1,x2,x3x4,xpim,exq,xiin Parametersoffunction fAPCIpXq,s.Suchthat, fAC,fFC,fIIN P fAPCI.
f pXqorASDUbytesthatarecomputedbythefollowingfunctions:
pf ,f q ASDU
OH OD pf ,f q,inthecaseofsending(S)/responding(R).
OH OD
px ,x ,x ,x q Parametersoffunction f pXq,suchthat f ,f P f .
5 6 7 8 ASDU OH OD ASDU
px ,x qareimpliedforobjectfunction(of)andpx ,x qareimpliedforobject
px ,x q,px ,x q 5.1 5.2 6.1 6.2
5.1 5.2 6.1 6.2 qualifier(oq)
f pX q FunctionthatlogicallyfixedtheASDUbytesintodatablocks(DB).
L d
Sensors2016,16,37 9of23
Postulate1. ThenumberofbytesXisdefinedandreceivedfromtheuser-applicationlayerthroughthe
useoffunction fµ. Thefunction fAPDU P fµexistsandisperformedatbothsidesofthetransmission
withtheusageofthedistinctidentifiersαPpS;Rq,andsimultaneouslyinteractswithandisupdated
inthedynamicstorageasapartoftheprotocolstack.
TherandombytesXareretrievedfromtheuser-applicationlayerbyperformingthefunction fµ
thatisdesignatedtocheckthebytesXthatarelimitedbytheproposeddevelopment. Inthecaseof
thesending(S)/responding(R)APDUbytes,thefunctions f and f areemployedtocompute
ASDU APCI
theASDUbytes(X ),theAPCIbytes(X ),andpf , f qP f . Inthecasewhereanumber
d h ASDU APCI APDU
offragmentsarerequiredinthetransmission,then f P f andn=1,2,3,... ,k,wherekis
APDU APDUn
thelimitedinteger. Moreover,thecomputedbytesaresimultaneouslyupdatedbytheemploymentof
function f andthedesignationofX . ThecomputedfragmentpFragqwiththeconcatenation(||q of
dy dy
dynamicbytescanbedemonstratedas
ˇˇ
ˇˇ
@X ñFrag“ f pXq“ f pXq` f pXqˇˇX ,
αPpS;Rq APDU APCI ASDU dy
f pXqñ X ^ f pXqñ X ^pX ,X qď Limit
APCI h ASDU d h d
wherelimitisamaximumvaluedependingonthesizeofthebytesthatarespecifiedforafragment.
Postulate2. Theexistentfunctions f , f ,and f ,areemployedtocomputetheprotocol-header
AC FC IIN
bytes, if, and only if, the parameters x ,x ,x x ,x ,andx are satisfied, such that f , f ,
1 2 3 4 pim,exq iin AC FC
f P f .
IIN APCI
Inthecaseofsendingfragmen´t, ¯
DX ôpx ,x ,x x q P f ^ x P f ^pf , f qP f .
h 1 2 3 4 AC pim,exq FC AC FC APCI
Inthecaseofaresponsefragment,
´ ¯
DX ôpx ,x ,x x q P f ^ x P f ^px qP f ^pf , f , f qP f
h 1 2 3 4 AC pim,exq FC iin IIN AC FC IIN APCI
To compute the header (h) bytes, the AC function f and its parameters are as follows:
AC
rx ,x ,x x s ““ rSequenceNumber,Confirmation,FIN,FIRs. The function f with its
1 2 3 4 FC
parameter-request code (im) and/or response code (ex) are performed in the case of sending (S)
a fragment, while the difference of the two-byte IIN is counted via the function f during the
IIN
responding(R)fragment.
Postulate3. Theprotocol-databytesarecomputedusingtheemploymentofthefunctions f and
OH
f P f ,if,andonlyif,theparameterspx ,x ,x ,andx qaresatisfied;adifferenceoftwobytes
OD ASDU 5 6 7 8
existsintheresponsecase.
DX ôrpx Q x ,x q,px Q x ,x q,x sPf ^px qP f ^p f , f qP f
d 5 5.1 5.2 6 6.1 6.2 7 OH 8 OD OH DO ASDU
For the ASDU bytes, the object header (OHq function f with its parametersrx ,x ,x s ““
OH 5 6 7
rObjectFunctionpofq,ObjectQualifierpoqq,ObjectRangeporqs,andthedataobject(DO)function f with
DO
its parameter [x ] == [user bytes (ub)] are computed in the case of sending (S)/responding (R),
8
but a two-byte difference is accounted for in the responding (R) case that ensures the total size.
Here,rx ,x s““rObjectGrouppogq,ObjectVariationpovqsareimpliedfortheobjectfunction(of)and
5.1 5.2
rx ,x s““rQualifiercodepqcq,IndexSizepisqsareimpliedfortheobjectqualifier(oq).
6.1 6.2
Postulate4. Thefunction f isanexplicit,duallogical-distributionfunctionifitisoperatedonbytesX,
L
andXareretrievedfromfunction fµ,asfollows:
ÿ
fµ ñ@XαPpS;Rq ñ fASDUpXqñ Xd ñ fLpXdqñ fLpXdq« ri“1pDBqi,i“1,2,3,...r
here,theASDUbytesarelogicallypLqfixedintothedatablocks(DB)butthebytesvaryinthecases
ofsendingandresponding,andrisrepresentedforthelogical8blocksinthecaseofafullfragment
(i.e.,1992bytesand f P f ).
ASDU APDU
Sensors2016,16,37 10of23
7. Algorithm: Pseudo-CodeComputingofFragmentwithSecurity
Algorithm1FragmentSecurity
Input: UserlayerbytesX,X ,αPpS;Rq;
α
Output: (X ,X q,Security(EncryptionEny,DecryptionDny,Messag(M),X
h d dy
1. Defineandman!agethenumˇ berofuserlayerbytes,X,Xα,αPpS;R)q;
ˇ
2. CalculateX “ pAC, FCqˇpx ,x ,x x ,x q ďlimit, limitPS ;
! h ˇ´ 1 2 3 4 pim,exq¯ )
ˇ
X “ pAC, FC,IINqˇ x ,x ,x x ,x ,x ďlimit, limitPR ;
h 1 2 3 4 pim,exq iin
3. Calculate X “tpOH, ODq|p x ,x ,x x qďlimit, limitP(S;R)u;«pDBq ,1ďiď8;
d 5 6 7 8 i
4. Byusageofsteps2and3,
ř ř
APDU“tpX ,X q| pX ,X q,limită pX ,X qďlimit},αPpS;Rq;
h d h d h d
X “dy.Updatepq;
dy
5. Whilethereisapplicationprotocoldataunit(APDU)computedbytes(X ,X q,αPpS;Rq;
h d
6. Securityiscomputedbyemployingofcryptographyfunctions,Symmetric
(Symqfunction,Asymmetric(Aymqfunction,Hashing(H)function;
7. EnypAPDUq““MessagpMq““EnyrSympAPDUq,AymtHashpAPDUqus;
X “dy.Updatep);
dy
8. IfEnypAPDUqbytesarereceivedattargetside.Then
9. DnyrMs““DnyrEnytSympAPDUq,AymtHashpAPDUquus;X “dy.Updatepq;
dy
10. Endif
11. IfDnyrMsisperformed.Then
12. (X ,X qarereassembletobytesX;
h d
13. Endif
14. Endwhile
8. Implementation
TheaimofthisworkistodesigntheDNP3stackandtodeploythesecurityalgorithm(s)within
the protocol; therefore, the DNP3 stack has been designed by the deployment of explicit (or real)
functionsintheC#programmingplatform,whilethehistorian(ordatabase)wasdeployedbyusing
the MySQL tool. Several approaches [1,3,5–7,14,15] were analyzed against the SCADA/DNP3 in
termsofsecuritybuttheyarebasedonend-to-enddevelopments. DNP3isaproprietaryprotocoland
issituatedatthetopoftheTCP/IPprotocolsforInternet-basedcommunication[5,8,9,14,15,57–60].
SecurityisamajorissuethathasbeenfacedbytheDNP3protocolsinceInternet-basedcommunication
hasoccurredthroughopenprotocol(s). Thisworkthereforeproposesanewapproachthatdeploysthe
securitywithintheDNP3protocolbeforethetransmissionofbytestonon-proprietaryprotocolssuch
asTCP/IPandUDP.
Intheimplementationphase, thedesignoftheDNP3stackisalsoamajorchallenge, andthe
deploymentofsecurityusingastrongmechanismthathasthepotentialtosecuretheSCADA/DNP3
transmissionagainstattacksisrequired. ThefirstchallengewasresolvedbythedesignoftheDNP3
stack and its relevant functions in C# using implicit/explicit libraries and functions. Prior to the
security-implementationphase,anothermajorchallengeisthestorage,updating,andmaintenanceof
thesecuritytrackswiththecorrespondingstackinformationduringthereal-timedeliveryofbytes.
Toresolvethischallenge,theDCBisdeployedandemployedduringtheentiredevelopmentstage.
TheDCBcontainsthenumbersofthedynamicfunctionalfieldsthatsuccessfullycontrolthesecurity
operationsandstackrelevantinformation.