Wireless Sensor Network, 2010, 2, 520-527
doi:10.4236/wsn.2010.27064 Published Online July 2010 (http://www.SciRP.org/journal/wsn)
Developing a Multi-Layer Strategy for Securing Control
Systems of Oil Refineries
Musaria K. Mahmood1, Fawzi M. Al-Naima2
1Department of Electrical Engineering, College of Engineering, Tikrit University, Tikrit, Iraq
2Department of Computer Engineering, College of Engineering, Nahrain University, Baghdad, Iraq
Received May 12, 2010; revised May 21, 2010; accepted May 27, 2010
Abstract
The energy industry and in particular the Oil Refineries are extremely important elements in Iraq’s infra-
structure. A terrorist attack on one oil refinery will have a catastrophic impact on oil production and the
whole economy. It can also cause serious damage to the environment and even losses of human lives. The
security of information systems and industrial control systems such as Supervisory Control and Data Acqui-
sition (SCADA) systems and Distributed Control System (DCS) used in the oil industry is a major part of
infrastructure protection strategy. This paper describes an attempt to use several security procedures to de-
sign a secure, robust system for the SCADA and DCS systems currently in use in the North Oil Refinery in
the city of Baiji located in northern Iraq.
Keywords: DCS, SCADA, Security, Encryption, Internet, Public Key, DMZ, Data Security
1. Introduction
The increased use of computer network in control sys-
tems and the use of internet as communication backbone
have brought benefits in communication and for the pro-
cess control in general. The ability to share information,
making decision concerning production stoking and dis-
tribution has been greatly improved [1]. New informa-
tion technologies have recently been introduced for con-
trol system based on open standards like Visual Basic,
Java, Open Database Connectivity (ODBC), Ethernet
communication and finally the use of internet based con-
trol system. Internet or web based control system uses
some internet protocols in the application layer to moni-
tor and control plants at a distance. Older control systems
were based on closed protocols implemented by vendors
of SCADA, DCS, and Programmable Logic Controller
(PLC) systems. However, the new common technology
and the use of internet introduced many challenges in the
area of security, such as cyber threats [2].
The North Oil Refinery in Baiji is a large industrial
complex located on various sites scattered around the
city of Baiji, north of Baghdad. This complex comprises
four basic big refineries: 1) Salah Aldine-1, 2) Salah Al-
dine-2, 3) North, and 4) Chemical Products. All these
refineries are functioning under the same direction and
are technically sported by unified sections. Each ref-
inery unit has many DCS systems controlling several
chemical processes. These processes are independent of
each other and are situated at different geographical lo-
cations. Each DCS acts as standalone control system
with its private communication network and local control
room (LCR).
DCS systems in operation are based on new informa-
tion technology (IT) with open standards such as TCP/IP
protocols using fiber optic Ethernet for data transmission
between field and LCR. The refineries direction begins a
new project to connect all DCS systems to a Central
Control Room (CCR) via intranet. Local server of each
DCS will be connected to system server located at the
CCR. System server will be connected to the Internet as
part of web based control system of the refineries.
Multi-layered security system is suggested for Baiji
Oil Refineries. This strategy of security will be based on
the “ring of defense” around each local network of each
DCS and also around the overall corporate intranet
(which will be similar to SCADA system) [2].
The suggested security system can also be used for
other SCADA systems in general like those used in Ele-
ctrical Power Plants, Water Purification and many other
Oil Industries.
2. Networking DCS’s
The model to be adopted in this work is to consider each

M. K. MAHMOOD ET AL.
521
DCS system as a node or “station” of a global SCADA
system in the perspective of grouping all LCR of all
DCS's of the North Refineries in the CCR. CCR will be
the central control of the overall SCADA system. Each
DCS will keep its local server, local database, and local
LCR, and will be connected to the CCR via appropriate
communication links.
The connection of local servers to the system server
(or web server) of SCADA system located at the CCR is
made via corporate intranet as depicted in Figure 1.
Communication links between LCR sites and CCR can
be accomplished by fiber optics, coaxial cable or wire-
less links regarding several factors related to topology,
geography, and required bandwidth. Reliability, avail-
ability and redundancy are major factors for the design of
network topology. System database and all local data-
bases use the same standard SQL database. The system
database is located near the system server at the interface
zone between corporate intranet and the internet. This
location is chosen at the border of the system to mini-
mize the risk of unauthorized intruder. The topology
definition and the reliability of the communication net-
work are not a part of this work.
3. Security Strategy
The similarity of the new SCADA system and the ordi-
nary IT system leads to the adoption of the same security
approach used in computer communication network. The
principal of networks separation into “rusted network”
and “non-trusted network” fit perfectly for securing
SCADA systems [3]. The ring of defense (multi-layer
security system) will be implemented using various
techniques, which are cited as the principal countermea-
sure for securing SCADA [4]. The goal of such model
will be the repetition of countermeasures to enhance the
global system security as shown in Figure 2.
Figure 1. Networking DCS’s.
Each layer will acts against one or more of possible
security challenges. There exist many cyber threats fac-
ing control systems as cited in Table 1 [5]. Each security
layer will be part of the defense on depth against threats.
The possible attack will be considered from the exter-
nal non trusted network (the Internet) in the direction of
the internal trusted network (the corporate intranet).
The Internet
Non-trusted net.
Medium trust
DCS network
Trusted net.
Refinery
intranet
(1) Enerypted authentication
(2) Data eneryption
(3) Security policles
(4) Demilitarized zone (DMZ)
(5) Proxy server
(6) Applications
(7) DCS DMZ
(8) Data eneryption intern to DCS
Atrack direction
Figure 2. Ring of defense.
Table 1. Correlation of security threats and properties.
Properties Cyber threats Importance
Eavesdropping Low
Message
confidentiality Traffic analysis Low
Message modification High
Message
integrity False message injection High
Message
freshness Message replay High
Denial-of-service Middle
Availability
Malicious codes Middle
Masquerade High
Source
authentication Unauthorized access Middle

M. K. MAHMOOD ET AL.
522
The compatibility of countermeasures like encryption
standards, firewall, antivirus, intrusion detection techni-
ques will be accomplished by the use of standardized
techniques. Each DCS system can be isolated immedi-
ately from the SCADA network and then be controlled
locally if needed. The connection or the disabling of
connection of one DCS can be made locally or from the
CCR by some fixed authorization protocol. The connec-
tion of DCS system to the CCR server and then to the
internet will be subject to several layers of security de-
fense rings as illustrated in Figure 3. Between each DCS
system and the corporate intranet network a demilitarized
zone (DMZ) is used where the local server and database
of DCS reside. The global SCADA system is connected
to the internet via another DMZ. To login from browser
to the SCADA intranet an original process of encrypted
authentication and digital signature is implemented.
3.1. Encrypted Authentication
It is the first security stage for an external client. An origi-
nal authentication process is developed based on asym-
metric key encryption algorithm as shown in Figure 4.
The RSA algorithm is used to encrypt the password two
times for the implementation of authentication and the
secrecy of password [3,6].
Local
darabase
DCS
Inner firewall
DCS
DCS
server Outer firewall
DCS
router
To Internet
router
Outer firewall
SCADA
Inner firewall
SCADA
System server
System
database
Figure 3. Security layers from DCS to internet.
Browser side
Eneoding the password
The browser writes the password and username
Eneiphering the result with the browser private key
and then again with public key of the server
Sending the message
(Authentication + confidentiality)
The server checks the username, finds the
corresponding public key of the browser
Server side
Internet
The server receives the encrypted password
Decryption with server private key and again
Decryption the result with browser public key
Compare the password with that in the serve
r
Sene an acknowledgment to the browser as accept
for communication
Figure 4. Authentication process.
The implementation of public key algorithm was made
with two key groups (e1,d1), and (e2,d2), where e1 is the
public key of the first key group, d1 is the private of
browser (personal private key), e2 is the second public
key, and d2 is the private key of server.
The method of defining these keys, and the encryption
algorithm is as follows:
Choose two large prime numbers (p and q), let:
n = p × q (1)
The function β(n) is the number of numbers less than
(n) with no factors in common with (n).

M. K. MAHMOOD ET AL.
523
Choose an integer number e, (e < n) relatively prime
to β(n). Find second integer (d) such that
{e × d mod β(n) = 1}
Then, the Public Key = (e, n)
The Private Key = (d, n)
The encryption function of RSA for a plain text (M) is
(C) such that:
C = M d mod n (2)
Then, the recovered plain text (M) is:
M = C e mod n (3)
If (e) is used instead of (d) for the encryption of equa-
tion (2), then (d) will be used for the decryption in Equa-
tion (3).
The password will be encoded using ASCII code or
other encoding system. Each byte will be encrypted two
times: the first with (d1) and the second with (e2) This
will ensure both secrecy and authentication of the pass-
word. At the server side, the reverse process is executed.
The received cipher password will be first decrypted
using (d2) and then (e1) in order to get the original plain
text password.
The result will be compared to usernames table in the
server. If matching is achieved then the server will open
a communication channel and begin responding to brow-
ser requests.
3.2. Data Encryption
To protect SCADA system already connected to the in-
ternet and its data from unauthorized accesses many al-
gorithms for data protection exist. In this work two en-
cryption algorithms are used to encrypt data.
1) Encryption-1(security layer 2): This encryption al-
gorithm used to encrypt data over the Internet. We pro-
pose to use the Secure And Fast Encryption Routine
(SAFER+), which is one of the known symmetric key
algorithms that accomplishes requirements for real time
data encryption. The SAFER+ was designed by James
Messey for Cylink Corporation in 1998 as the new algo-
rithm of SAFER family (safer-64, safer-128...) [7]. It was
one of the candidates of the Advanced Encryption Stan-
dard (AES) chosen for its good hardware-software tra-
deoff orientation, simplicity, high throughput compared
to other algorithms, and low memory requirement [8],
[9]. Also, this algorithm brought attention recently by the
use in as security measure in Bluetooth and wireless
communication [10,11].
A detailed analysis of maximum bit rate for one DCS
proves that SAFER+ fits well for the encryption of data
over Internet to get the required real data transmission.
SAFER+ is published with three options, 128 bits, 192,
bits and, 256 bits key lengths. All three options are used
to encrypt 16-byte plaintext. The plaintext block then
passes through R rounds of encryption where R is deter-
mined by the key length chosen for encryption in the
following manner:
● If key length = 128 bits then R = 8 rounds.
● If key length = 192 bits then R = 12 rounds.
● If key length = 256 bits then R = 16 rounds.
Our choice was the adoption of SAFER+ with 128 bits
key length as other key lengths were found with certain
key weaknesses [12]. Also, by increasing key length
more computation time is needed related to rounds num-
ber which will affect the assumption of real data trans-
mission in control system.
This algorithm will be implemented as hardware in the
server side and portable software in the client side. If
there is wireless link between two sites interior to the
corporate intranet, SAFER+ must be used with hardware
encryption decryption processes. SAFER+ algorithm is
divided into three blocks: the key scheduling, encryption
process, and decryption process. Programming is accom-
plished using Turbo Pascal.
2) Encryption-2 (security layer 8): This algorithm is
used to encrypt data over the network of one DCS. This
algorithm is not a part of this work but we can use the
American Gas Association (AGA) standard which is
proven to be a good choice for data over DCS network
when dealing with protocols such as MODBUS or DNP3
[13]. In this work we found that securing data over the
DCS network has no significant impact to enhance the
security of overall system because all DCS system are
well physically protected in isolated areas and we take
care about data security when data circulate outside DCS
network by other countermeasures. This encryption pro-
cedure is mentioned here merely for other case studies
with other compromises and challenges related to other
situations.
3.3. SAFER+ Encryption/Decryption
Giving the 16-byte key (128 bits), SAFER+ begin by
calculating a set of 17 keys each with same length 16
bytes. The calculation uses sample arithmetic and logic
functions like bit rotation, bit-by-bit exclusive-or of
bytes, modulo 256 addition of bytes, and selection byte
process.
The 17 sets of keys are used in the encryption rounds.
Two keys are used for each round. Round (i) uses key
K2i-1 and K2i. At the end of 8 rounds key K17 is used for
the output operation which is the output transformation.
The output transformation uses bit-by-bit exclusive-or of
bytes and modulo 256 byte addition as shown in Figure 5.
At the reception the reverse process is used for decrypt-
ing the cipher text. Beginning by the input transforma-
tion, Key K17 is used. The input transformation uses
same functions of the output transformation but with
modulo 256 subtraction of bytes instead of addition of
bytes. Each encryption round (i) begin by the bit-by-bit
Xor of bytes, and modulo 256 addition of bytes for the

M. K. MAHMOOD ET AL.
524
key K2i-1 to the input 16-byte of the round Figure 6. The
16-bytes result are then fed to a layer of nonlinear func-
tion. The value x of byte j is converted to 45x mod 257
for j = 1, 4, 5, 8, 9, 12, 13, and 16 (with the convention
that when x = 128, then 45128 mod 257 = 256 is repre-
sented by a 0). The value x of byte j is converted to
log45(x) for j = 2, 3, 6, 7, 10, 11, 14, and 15 (with con-
vention that when x = 0, then log450 = 128). The output
of the nonlinear layer is then subject to same addition
and Xor operation similar to the first block with key K2i.
At the end of round (i), a block of matrix multiplication
is used. The 16 bytes are multiplied by matrix T in mod
256 arithmetic. T is a 16 × 16 predefined matrix.
The operations in the decryption round are simply
conducted in reverse order to the operations from the
encryption round.
The first operation in the decryption round (i), is to
post multiply the 16-byte round input by matrix T–1,
which is the modulo 256 inverse of T to give the 16-byte
result (S). The first round sub key K16-2i+2, is then “sub-
tracted” from (S) in the manner that the round sub key
bytes 1, 4, 5, 8, 9, 12, 13, and 16 are subtracted modulo
256 from the corresponding bytes of (S), while round sub
key bytes 2, 3, 6, 7, 10, 11, 14, and 15 are added
bit-by-bit modulo 2 to the corresponding bytes of (S).
The 16-byte result is then processed nonlinearly in the
manner that the value x of byte j is converted to log45(x)
for bytes j = 1, 4, 5, 8, 9, 12, 13, and 16 (with the con-
vention that when x = 0, log450 = 128). For j = 2, 3, 6, 7,
10, 11, 14, and 15, the value x is converted to 45x mod
16-Byte input plain text
SAFER+
single round
Round (i)
Output transformation
Mixed xor/byte addition
16-Byte output cipher text
K
2i-1
and K
2i
K
17
Key
Key
sheduling
Figure 5. SAFER+ encryption process.
Round (i) input 16-bytes
B1 B2 B3 B4 B5 B6 B7 B8
B9 B10 B11 B12 B13 B14 B15 B16
K
2i-1
xor add add xor xor add add xo
r
xor
add add xor xor add add xor
B1 B2 B3 B4 B5 B6 B7 B8
B9 B10 B11 B12 B13 B14 B15 B16
exp log log exp exp log log exp
exp log log exp exp log log exp
B1 B2 B3 B4 B5 B6 B7 B8
B9 B10 B11 B12 B13 B14 B15 B16
xor add add xor xor add add xor
xor
add add xor xor add add xor
B1 B2 B3 B4 B5 B6 B7 B8
B9 B10 B11 B12 B13 B14 B15 B16
Multiplication of 16-byte by T (16*16
Matrix) in mod 256 arithmetie
B1 B2 B3 B4 B5 B6 B7 B8
B9 B10 B11 B12 B13 B14 B15 B16
Round (i) output 16-
b
ytes
B1 B2 B3 B4 B5 B6 B7 B8
B9 B10 B11 B12 B13 B14 B15 B16
K
2i
Figure 6. Encryption round (i).
257 (with the convention that when x = 128, 45128 mod
257 = 256 round sub key K16-2i+1, is then “subtracted”
from the 16-byte result in the manner that the round sub
key bytes 1, 4, 5, 8, 9, 12, 13, and 16 are added bit-by-bit
modulo 2 to the corresponding input bytes. Sub key
bytes 2, 3, 6, 7, 10, 11, 14, and 15 are subtracted modulo
256 from the corresponding input bytes to produce the
16-byte output of the round.
3.4. Demilitarized Zone (DMZ)
DMZ is a good technique for securing communication
network based on the principal of “networks separation
strategy”, between trusted network (like DCS LAN) and

M. K. MAHMOOD ET AL.
525
another network with less level of trust [1,3]. In the
DMZ the server and database reside in safe place, see
Figure 3. Proxy server and application action like anti-
virus are also deployed in the DMZ. In this work a dou-
ble protection is found by the use of two DMZs. The first
is as a security interface between SCADA system and the
external non-trusted Internet (security layer 4), while the
second is between the DCS trusted network and the less
trusted corporate intranet for SCADA (security layer 7).
DMZ is a zone between an inner firewall and an outer
firewall. This firewall, properly configured, can protect
passwords, IP addresses, and files. Firewall acts as a fil-
ter that permits the data to enter from certain ports and
blocks others. At the DMZ output a router is used as
border router to route information to correct destination.
3.5. Security Policies
Security policies must be fixed by the security committee
of the enterprise. Correct procedure of effective policies
can reduce violation of security rules. Training personnel
at the vocation of security can increase the defense in
depth strategy. Developing documentation and well de-
fining the access authorization will be a very active point.
The choice of password must be as random as possible
with at least ten characters, including symbols and num-
bers.
In this work all the required information of the group
of DCS in the North Oil Refinery will be available in the
system server in the master DMZ (between corporate
network and the internet).
4. Simulation Results
RSA and SAFER+ encryption algorithms are implemen-
ted in Turbo Pascal language. As example to show how
the encrypted authentication will be calculated. Let the
password be chosen as the random word ‘playmyaudio’.
Encoding this password with simple code from 01 for (a)
to 26 for (z), will give:
M = 16, 12, 1, 25, 13, 25, 1, 21, 4, 9, 15.
C will be composed by the same number of 11 bytes.
Consider for our example the following values for the
keys are taken:
e1 = 13, d1 = 53, e2 = 37, d2 = 17, n = 77
Using equation (2) twice with browser private key d1
and then with server public key e2 the cipher text will be:
(1653 mod 77)37 mod 77 = 60
(1253 mod 77)37 mod 77 = 45
For the same for other bytes, we have:
C = 60, 45, 1, 58, 13, 58, 1, 21, 37, 53, 15.
At the server, the received cipher text will be con-
verted back to the original password using Equation (3)
twice:
(6013 mod 77)17 mod 77 = 16
(4513 mod 77)17 mod 77 = 12
So on for other bytes to get the original password:
M = 16, 12, 1, 25, 13, 25, 1, 21, 4, 9, 15.
The use of double RSA encryption procedure will
gives both authentication and secrecy.
SAFER+ algorithm uses matrix to represent the 17-
sub keys (17 × 16 matrix).given the secrete key (K1) as
series of 16 bytes, the 16 other keys, each of 16 bytes
will be generated. The calculation of each round is made
using iterative process and many preprogrammed proce-
dure (for Xor, Exp, log..).
As an example for the encryption/decryption process,
let the 16 byte user selected input key (secrete key) be:
41, 35, 190, 132, 225, 108, 214, 174, 82, 144, 73, 241,
241, 187, 233, 235.
After the execution of key schedule procedure of Fig-
ure 7, for 128 bit key we have the (17 × 16) matrix
which represents the 17 sub keys each of length 16 bytes
(128 bits). Each key is represented by a row, where row
(1) represents K1 (the secrete key), to row (17) which
represents K17, this will give the matrix (K) shown in
Figure 8. The resulting 17 sub keys and the plain text are
used to generate the cipher text as presented in Figure 5.
The plain text block input (16 byte plain text) = 179,
166, 219, 60, 135, 12, 62, 153, 36, 94, 13, 28, 6, 183, 71,
222.
The resulting cipher text will be = 224, 31, 182, 10, 12,
255, 84, 70, 127, 13, 89, 249, 9, 57, 165, 220.
The predefined bias matrix (B) is given as (16 × 16)
matrix as input to generate sub key matrix [7]. This pro-
cedure will be used to encrypt input data by block of 16
bytes at a time. The schedule procedure will be executed
next time when the secrete key is changed to find the
new matrix (K).
5. Conclusions
The use of open standard improves the control operations
by improving the possibility of interconnecting many
systems from different vendors together without restrict-
ions in term of standards. Action for the security of SC-
ADA system must be up to date regarding the continued
advances in information technology. Care must be taken
for the use of latest version of antivirus, and intrusion
detection programs. Symmetric encryption algorithms
can be used to encrypt data over internet for SCADA
system if the required bit rate is accomplished. SAFER+
algorithm can accomplish the real data transmission re-
quirement. Using SAFER+ give us the possibility to en-
crypt data by software or hardware implementations.

M. K. MAHMOOD ET AL.
526
Enter the key K
1
: By1,By2 …… By16
By1 By2 By3 By4 By5 By6 By7 By8 By9 By10 By11 By12 By13 By14
By15 By16
Composition of By17 bit by bit
By17 = sum (By1
…
By16) bit by bit mod 2
Rotate each byte left by 3bits position
By(1) By(2) By(3) By(4) By(5) By(6) By(7) By(8)become
By(4) By(5) By(6) By(7) By(8) By(1) By(2) By(3) for all 17 bytes
Rotate each byte left by 3 bits position
By1 By2 By3 By4 By5 By6 By7 By8 By9 By10 By11 By12 By13 By14
By15 By16 By(17)
Rotate each byte left by 3 bits position
By1 By2 By3 By4 By5 By6 By7 By8 By9 By10 By11 By12 By13 By14
By15 By16 By(17)
Select key
K
1
= 1,2,3
……
16
Byte: 2,3
…
17
Byte: 3,4
…
17,1
Byte: 17,1,2
…
15
K
1
K
2
K
3
K
17
Mod
256
Mod
256
Mod
256
B2
B3
B17
Figure 7. Sub keys generation process.
4135 190 132225 108214 1748214473241241187233135
951402132016109 133 15673 12966

K
8855119 1135
155 204 342252864236497422114 922242142135
147 13417654199141
8721938 16298 167109138 186230
12329 2559250122240 2186512492575943149127
96 204 15931221892452432445221976177210163209
56190201321224815710916881214 221 102105 5381
152646250 110124 13722274 1351213418149 185
207 6125122417966 183 96253 603778211152229
682159456944935230120133111 1959768203173
78
156 1901811302226159385953238123180138107
221238152 211241232248255101167 37 36 134 238 244 243
55 11116566105237214179862331421453115 165201
3465 73 224185205 107140123117 55 254417982236
212162 9117411755625116323813249505418074
51 1592151817420225315191101 8916798148104
127111 186111621323523018423 19925218675 227149
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 8. Sub key matrix (K).

M. K. MAHMOOD ET AL.
527
The use of computer network protection can be very
useful in SCADA like DMZ, and network segmentation
from the trust point of view. Layered strategy is a good
method to secure SCADA, DCS and control systems in
general. The use of different layered actions against cy-
ber threats makes the defense in depth active. Authenti-
cation is one of most used and active countermeasure for
security in control systems. The use of RSA public key
algorithm for encrypt password will ensure security and
authenticity.
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