Performance of orthogonal frequency division multiplexing based advanced encryption standard

ection will be the 
discussion of previous work, while the third In the whole of this work, AES-128 is 
section would like to explain how AES-OFDM chosen to implement. 
system work and its diagram. The fourth Step 3: There are four functions 
section analyzes simulation results and assesses implemented sequentially except for the last 
the security and performance of the round. The general AES algorithm is 
AES-OFDM model. The conclusion will be determined as following pseudo-code: 
given in the final section. 
 Algorithm 1. Pseudo-algorithm at transmitter 
2. Proposed method 1 Begin 
 2 Add round key with current state 
2.1. Advanced encryption standard algorithm 3 Expand key; 
 4 For i = 1 to 9 do 
 AES is an algorithm adopted by the 5 Hexadecimal numbers. 
U.S government and widely used to protect data 6 Substitution of state using S-box; 
[15]. AES cipher block of 128-bit or 16-byte 7 Shift left each word in round 
data symmetrically. The basic unit in AES is a Mix Columns state using 
 8 
byte. XOR operation effectuates the addition of arithmetic over GF(28) 
two bytes. The multiplication of two bytes in Add round key [i] with current 
AES is a multiplication in GF(28) which has state by XOR. 
an irreducible polynomial 9 End 
 843
 m xxxxx 1. The secret key of Substitution of state by S-box 
AES has three types of length which are 16, 24 Shift left each word in round 
 Add round key [10] with current 
and 32 bytes (128, 192 and 256 bits). AES-128, 
AES-192, AES-256 is three algorithms state by XOR 
corresponding to the length of the cipher key. 10 End 
60 D-T. Truong et al. / VNU Journal of Science: Comp. Science & Com. Eng., Vol. 36, No. 1 (2020) 57-64 
 ciphering key (CK) and an integrity key (IK). 
 MME sends RAND and AUTN to UE to check 
 authentication and calculate response (RES). 
 RES is sent back to MME to compare with 
 XRES. If RES equals XRES, MME sends None 
 Access Stratum (NAS) Security mode 
 command (cipher algorithm, integrity 
 algorithm, NAS key set ID and Capability - 
 CAP) is sent to UE. After UE calculates CK 
 from KASME and NAS encryption algorithm, 
 the AES algorithm uses CK to encrypt at the 
 transmitter and decrypt at the receiver. CK is 
 secure because there is no threat to steal CK 
 without knowledge about MSISDN, IMEI, and 
 IMSI. Figure 2 illustrates the above process. 
 Figure 1. Shows the overall AES cryptosystem 
 that illustrates the symmetric feature 
 of the AES algorithm. 
2.2. Sharing key process 
 There are three procedures to protect 
information transmitted on mobile systems. 
They are identification, authentication, and 
encryption. Center Equipment Identity Register 
takes Mobile Station International Subscriber 
Directory Number (MSISDN) and International 
Mobile Station Equipment (IMEI) from User 
Equipment (UE) to check for subscriber 
identification. If the subscriber identification is Figure 2. State diagram for authentication 
precise, an authentication protocol is applied to in mobile communication. 
supply to UE some important parameters such 
as cipher key. Figure 3 demonstrates the LTE 
security protocol in mobile communication. 
The authentication between a mobile station 
(MS) and a network is two-way where the 
master secret key K is used. Posterior to that 
user UE sends International Mobile Subscriber 
Identity (IMSI) to Home Network (HN), HN 
sends back an authentication vector (AV) to 
Mobile Management Entity (MME). Each AV 
contains a group of expected response (XRES), 
a random number (RAND), an authentication 
token (AUTN), and a master secret key 
KASME which contains information of a Figure 1. Proposal AES-OFDM model. 
 D-T. Truong et al. / VNU Journal of Science: Comp. Science & Com. Eng., Vol. 36, No. 1 (2020) 57-64 61 
2.3. AES-OFDM model For instance, a sub-stream 
 bbbb   has M bits. Those bits 
 To secure transmit data at the physical llllM 12
 are constellation mapped. If 16-QAM is 
layer, this article proposes a combination of 
 applied, the number of elements each substream 
AES and OFDM, so-called AES-OFDM. The having now is 32: 
main idea is encoding QAM symbols directly in 
the OFDM classical model. Figure 2 illustrates 
the proposed AES-OFDM model. The process stsssllll  1232  
of the proposed system is mostly the same as 
the original OFDM model except for the The elements are encrypted by AES to 
constellation mapping step where the AES become completely new symbols. 
algorithm is embedded. After converting from 
serial to parallel, each sub-channel contains 128 '''' 
 sl t s l1 s l 2 s l 32 
bits, thus 1000 sub-channels constitute 128000 
bits. The data transmission rate is the same at Table 2. QAM states and corresponding 
all individual channels because of orthogonality hexadecimal number 
and the same bandwidth. AES algorithm 
 Carrier Carrier Hexadecimal 
operates with a byte as the data unit which is Input 
represented as a couple of hexadecimal phase amplitude mapping 
 o 
numbers. Consequently, 16-QAM modulation 0000 225 0.33 0 
 o 
is appropriate to cooperate with the AES 0001 255 0.75 1 
 o 
algorithm due to that a byte can convey two 16- 0010 195 0.75 3 
 o 
QAM states also. This way not only improves 0011 225 1.0 2 
 o 
the security of pure OFDM but also makes the 0100 135 0.33 4 
 o 
attacker hard to decrypt the information. The 0101 105 0.75 5 
 o 
reason is that the encryption is performed with 0110 165 0.75 7 
 o 
16-QAM symbols while the normal security 0111 135 1.0 6 
 o 
methods apply AES on the bit-stream. Thus, the 1000 315 0.33 C 
 o 
attempt of attacker to decrypt the bit-stream or 1001 285 0.75 D 
 o 
to decrypt at the application layer will fail. In 1010 345 0.75 F 
 o 
detail, the mapping of 16-QAM states and 1011 315 1.0 E 
 o 
hexadecimal numbers are shown in Table 2. 1100 45 0.33 8 
 After the encryption process, the ciphertext 1101 75o 0.75 9 
will be remapping to QAM again and perform 1110 15o 0.75 B 
similar steps as the traditional OFDM model. 1111 45o 1.0 A 
Whole system operation can be represented by 
mathematical as follow: Therefore, the transmitted data will be 
 Firstly, the original data is paralleled by N totally different from the original data. This 
substreams which contain 128 bits each as step ensures the transmission security. 
 After that, IFFT is used to divide signals 
 b11 b 12 b 1128
 b b b into several frequency stacks. The final 
shown: 21 22 2128 transmitted AES-OFDM is given as below: 
 b b b N
 NNN1 2 128 N 128 '
 m ts tf t cos 2 l 
 l 0
62 D-T. Truong et al. / VNU Journal of Science: Comp. Science & Com. Eng., Vol. 36, No. 1 (2020) 57-64 
 Pseudo-algorithm at the transmitter is Execute time is an important parameter to 
considered as follow: consider a system being available or not with a 
 temporary technology. The required 
 Algorithm 2. Pseudo-algorithm at transmitter transmission time interval in a 4G system must 
 1 Begin below 1 millisecond. In the journal of Schneier 
 2 For each frame do et al. [16], AES - Rijndael encryption and 
 3 Modulated Data = 16-QAM decryption setup take respectively 300 and 
 modulation of original data; 1370 clock cycles on 32-bit CPUs. On the other 
 4 Plain text = mapping 16-QAM hand, each OFDM symbol needs 7142 clocks 
 modulated symbols to hexadecimal 
 numbers. cycles to be processed entirely [17]. Definitely, 
 5 Ciphered text = implement AES total required clocks for AES-OFDM 
 with plaintext and key; processing is maximum at around 23600 cycles 
 6 Ciphered symbols = Remapping that takes 9.83 microseconds on 2.4 GHz CPUs. 
 ciphertext to 16-QAM symbols; That executive time is much less than the 
 7 IFFT ciphered symbols; required transmission time interval in 4G. Thus, 
 8 Add cyclic prefix; 
 9 End the proposed AES-OFDM system can be 
 10 End possible to deal with 4G technology. 
 At the receiver, symmetric blocks are used 
to demodulate sent signals. Due to the effect of 3. Simulation result 
the channel, the received message differs from 
the transmitted signal. Thus, received symbols In this section, simulation results focus on 
is fluctuated with fixed values in table 1, so it two criteria, security, and error rate of AES-
requires a balancing method in blocks of AES OFDM. The scenario is there will be 32000 16-
decryption. By applying boundaries, every QAM symbols randomly created to transmit by 
symbol is assigned to a fixed value in Table 2. AES-OFDM. The simulation results are 
This approach improves the symbol error rate investigated on the AWGN channel. 
which is mentioned in the next section. The To determine the security of AES-OFDM, 
pseudo algorithm at the receiver is shown the 16-QAM symbols before and after AES are 
as below: observed. It is notable that there is no clue to 
 Algorithm 3. Pseudo-algorithm at receiver detect the key when the attackers have both 
 original and encrypted symbols without 
 1 Begin knowledge of the cipher algorithm. In a random 
 2 For each frame do 
 3 Remove cyclic prefix; test case as an instance, there are three symbols 
 4 FFT received symbols; represented as 3.0000 + 1.0000i in thirty-two 
 5 Estimate received symbols to 16 original symbols. However, the three 
 values of 16-QAM; corresponding symbols after applying AES are 
 6 Ciphertext = Mapping received totally nonrelative, -1.0000 - 1.0000i -3.0000 
 symbols to hexadecimal numbers + 3.0000i -1.0000 + 3.0000i. Therefore the 
 7 Plaintext = AES decryption of 
 ciphertext and key security of the OFDM signal is ensured. 
 8 Modulated Symbols = Remapping However, the security in this work relies on the 
 plaintext to 16-QAM symbols secret key mostly. If the key is not reveal, the 
 9 Output data = demodulate attacker cannot decrypt the encrypted signals. 
 modulated symbols Since the secret key is generated randomly, the 
 10 End protection of the AES-OFDM is certain. 
 11 End 
 D-T. Truong et al. / VNU Journal of Science: Comp. Science & Com. Eng., Vol. 36, No. 1 (2020) 57-64 63 
 longer than the time of OFDM. So, the 
 execution time of AES-OFDM is still less than 
 1 millisecond which is appropriate for applying 
 in 4G communications. For further work, AES-
 OFDM needs to improve the data rate by 
 increasing the level of modulation, which is 16-
 QAM in the current model. 
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