Comparison and Improvement of RC5 and RC6 Algorithms of Atmega128 Microcontroller

introduction

In a wireless LAN, the transmission medium is mainly radio waves and infrared rays. Any eavesdropper with receiving ability may intercept the data in the wireless channel, grasp the content of the transmission, and cause data leakage. Therefore, for wireless LAN, data encryption is one of the key technologies.

AVR high-speed embedded microcontroller is an 8-bit RISC MCU. It only takes one clock cycle to execute most instructions. It is fast (the running speed of 8MHz AVR is approximately equal to that of 200MHz C51). 32 general registers are directly connected to ALU to eliminate the bottleneck of operation. It has built-in Flash and EPPROM that can be serially downloaded or self-programmed, with many functions and multiple operating modes.

According to the 802.11 wireless LAN protocol standard released by IEEE in 1999, the wireless data transmission device was developed using Atmel's Atmega128 high-speed embedded single-chip microcomputer. In order to achieve the security of wireless data transmission and save costs as much as possible, software is used for encryption and decryption. This puts high demands on the simplicity, high speed and adaptability of the algorithm. The two new block encryption algorithms RC5 and RC6 can better meet the above requirements.

1 RC5 and RC6 algorithms

1.1 Parameters of RC5 and RC6

RC5 and RC6 are parameter variable grouping algorithms, which are actually a family of encryption algorithms determined by three parameters. A specific RC5 or RC6 can be expressed as RC5-w/r/b or RC6-w/r/b. The three parameters w, f and b are defined as listed in Table 1.

Table 1 RC5 and RC6 algorithm parameter definitions

1.2 RC5 and RC6 word operation components

Both RC5 and RC6 consist of three parts: the mixed key generation process, the encryption process, and the decryption process. In these two algorithms, a total of six basic operations are used:

① Modulo 2w addition operation, represented by "+";

② Modulo 2w subtraction operation, represented by "-";

③ Bit-by-bit XOR operation, expressed as +;

④ Circular left shift: word a is circularly shifted left by b bits as “a<<

⑤ Circular right shift: word a is circularly shifted right by b bits as “a>>>b”;

⑥ Modulo 2w multiplication, expressed as “×”.

The RC5 algorithm uses the above-mentioned ① to ⑤ calculation parts, and the RC6 algorithm uses all of the above-mentioned calculation parts.

1.3 RC5 Algorithm

(1) Pseudocode representation of the RC5 algorithm hybrid key generation process

S[0]=Pw

for i=1 to t-1 do

S[i]=S[i-1]+Qw

Input user keys K[0] to K[b-1] with a bit size of 8 and a key length of b

Convert K[0] to K[b-1] to an L[] array with length c and number of bits w

i=j=0 x=y=0

do 3×max(t,c)times:

S[i]=(S[i]+x+y)<<<3;X=S[i];i=(i+1)mod t

L[j]=(L[j]+x+y)<<<(x,y);X=L[j];j=(j+1)modC

Where c = [b × 8 / w] square brackets indicate the rounding operation, t = 2r + 2, when w is 16, 32, 64, respectively, the constants Pw, Qw are listed in Table 2.

Table 2 Constants Pw, Qw value table

(2) Pseudocode representation of the RC5 encryption algorithm process

Input(A,B)

A=A+S(0)B=B+S[1]

for i=1 to r do

A=((A+B)<<

B=((B+A)<<

Output(A,B)

The initial A and B are two bits of data w to be encrypted, and the final A and B are two bits of data w encrypted.

(3) Pseudocode representation of the RC5 decryption algorithm process

Input(A,B)

for i=r down to 1 do

B=((BS[2i+1])>>>A)+A

A=((AS[2i])>>>B)+B

A=AS[0] B=BS[1]

Output (A,B)

The data in the initial A and B are the encrypted data with the number of bits being w, and the data in the final A and B are the decrypted data with the number of bits being w.

1.4 RC6 Algorithm

(1) Pseudocode representation of the RC6 algorithm hybrid key generation process

The RC6 hybrid key generation process is the same as RC5, except that the value of t is 2r+4.

(2) Pseudocode representation of the RC6 encryption algorithm process

Input(A,B,C,D)

B=B+S[0]D=D+S[1]

for i=1 to r do

t=(B×(2B+1))<<

u=(D×(2D+1))<<<1og2w

A=((A+t)<<

C=((C+u)<<

(A,B,C,D)=(B,C,D,A)

A=A+S[2i+2]C=C+S[2i+3]

Output(A,B,C,D)

The initial A, B, C, and D are four bits of data w to be encrypted, and the final A, B, C, and D are four bits of data w that have been encrypted.

(3) Pseudocode representation of the RC6 decryption algorithm process

Input(A,B,C,D)

C=CS[2i+3]A=AS[2i+2]

for i=1 to r do

(A,B,C,D)=(D,A,B,C)

u=(D×(2D+1))<<

t=(B×(2B+1))<<

C = ((CS[2(ri)+3])>>>t)+u

A=((AS[2(ri)+2])>>>u)+t

D=DS[1] B=BS[0]

Output(A,B,C,D)

The initial A, B, C, and D are four bits of data with a number of w that have been encrypted, and the final A, B, C, and D are four bits of data with a number of w that have been decrypted.

2 Implementation and Improvement of RC5 and RC6 Algorithms

2.1 RC5 and RC6 algorithm flow of AVR microcontroller

The implementation flow chart of the RC5 and RC6 algorithm encryption process is shown in Figure 1, the implementation flow chart of the decryption process is shown in Figure 2, and the overall process flow chart is shown in Figure 3.

2.2 Improvement of RC5 and RC6 algorithms for AVR microcontrollers

① When arranging the algorithm flow, considering that the AVR high-speed embedded microcontroller has only 32 8-bit registers, in order to save the use of registers, the data to be encrypted should be assigned to the registers after the hybrid key generation process is executed. In this way, the registers before the hybrid key generation process can be used without affecting the execution result of the entire algorithm.

② When selecting the parameters of the RC5 and RC6 algorithms, considering that the AVR high-speed embedded microcontroller instructions only support 16-bit data addition at most and the simplicity of the program, w is selected as 16 instead of 32 in this program, and the values ​​of r and b are taken as 12 and 16 respectively according to Rivest's suggestion.

③ When executing the left or right loop operation in the algorithm, considering the effect of circular shift, the actual number of bits of circular shift should be the lower 1log2w bits of the number of shifts to be performed. In this program, it is the last four bits of the number of shifts to be performed.

④ When executing the modulo 2w addition, modulo 2w subtraction, and modulo 2w multiplication operations in the algorithm, since the value of 2w is 65536, and the two 8-bit registers (0 to 15 bits) can represent a maximum data value of 65535, if the data is larger, it must be carried to the higher bits. Therefore, when executing the above algorithm in this program, you only need to consider the values ​​expressed by the two 8-bit registers to obtain the final result of the above operation without performing the modulo 2w operation.

⑤ In order to increase the speed of data encryption and decryption, the mixed key generation process can be executed in advance to generate a mixed key table. This table is loaded into the Flash of the Atmega128 high-speed embedded microcontroller at the data sending end and the Atmega128 high-speed embedded microcontroller at the data receiving end, so that the mixed key can be directly used in the subsequent encryption and decryption process. It is worth noting that whenever the user key entered by the user changes, the mixed key generation process must be re-executed and the regenerated mixed key table must be re-loaded into the Flash. In this program, the RC5 mixed key table occupies a total of 52 8-bit register units, and the RC6 mixed key table occupies a total of 56 8-bit storage units.

⑥ In this program, addition and shift operations are used to implement the unsigned operation of 16-bit binary number multiplication by 16-bit binary number. The subroutine of this operation is as follows:

chengfa:clr result2

clr result3

ldi count1,16

lsr chengshu1

ror chengshu0

chengfa0:

brcc chengfa1

add result2,beichengshu0

adc result3,beichengshu1

chengfa1:

ror result3

ror result2

ror result1

ror result0

dec count1

brne chengfa0

ret

3 Experimental results, comparison and analysis of RC5 and RC6 algorithms

The comparison of the effects of the mixed key process, encryption process, decryption process and overall process of the RC5 and RC6 algorithm experiments is listed in Tables 3, 4, 5 and 6.

Table 3 Comparison of the effects of the mixed key process of the RC5 and RC6 algorithms

Table 4 Comparison of the encryption process effects of RC5 and RC6 algorithms

Table 5 Comparison of the decryption process effects of RC5 and RC6 algorithms

Table 6 Comparison of overall process effects of RC5 and RC6 algorithms

From Table 3, we can find that the size of the program for generating a mixed key is the same for the RC6 algorithm and the RC5 algorithm, but the RC6 algorithm saves more time than the RC5 algorithm. This is because the mixed key generation method executes a loop and performs a comparison operation when finally generating a mixed key. When the comparison range t is exceeded, the pointer address must be reset. The value of t in the RC6 algorithm is greater than the value of t in the RC5 algorithm, so the RC6 algorithm performs fewer reset operations. This saves the running cycle, so the RC6 algorithm saves more time than the RC5 algorithm when generating a mixed key.

All the above experimental results are obtained by using Atmel's Atmega128 high-speed embedded microcontroller as the experimental equipment platform on the AVR Studio4.07 simulation software. Select parameters w=16, r=12, b=16, and calculate c=8, t=26 (RC5 algorithm) or t=28 (RC6 algorithm) according to the calculation formula at a simulation speed of 12MHz.

From the experimental results in Tables 3, 4, 5 and 6, the following conclusions can be clearly drawn.

① From the perspective of program execution efficiency, the RC5 algorithm is more efficient than the RC6 algorithm in both encryption and decryption.

Therefore, the RC5 algorithm should be used in situations where program execution efficiency is very important but data security requirements are not very high.

② From the perspective of program execution time, whether in the encryption process or not in the decryption process, the RC5 algorithm saves time than the RC6 algorithm. Therefore, in some cases where the program execution time is very demanding and the data security requirements are not very high, the RC5 algorithm can be used.

③From the perspective of program size, the RC5 algorithm is simpler than the RC6 algorithm in both encryption and decryption. Therefore, the RC5 algorithm can be used in some cases where the program space size is very high and the data security requirements are not very high.

④ From the perspective of security, the RC6 algorithm is based on the RC5 algorithm to address the loopholes in the RC5 algorithm. The main problem is that the displacement of the cyclic shift does not depend on all the bits to be moved. By introducing the multiplication operation to determine the number of cyclic shifts, the RC5 algorithm is improved, thereby greatly improving the security of the RC6 algorithm. Therefore, in some cases where data security is very high, the RC6 algorithm should be used.

Conclusion

RC5 and RC6 algorithms are two new block ciphers, both of which have variable word length, variable number of encryption rounds, and variable key length; at the same time, they only use common elementary operations, which makes them very adaptable, high computing speed, and very suitable for hardware and software implementation. The two algorithms have their own advantages and disadvantages. In engineering applications, the most suitable method should be selected according to actual needs to obtain the best effect.