# Patent

## Title
Encryption apparatus and method in a wireless communications system

## Abstract

An encryption apparatus and method for generating a ciphertext from an input plaintext of the same length as the ciphertext by parallel processing of the input signal. Since a non-delayed signal is synchronized to a delayed signal, an accurate ciphertext is produced. Therefore, the encryption speed is increased, the number of devices for timing synchronization is reduced, an encryption system is stabilized, and production cost is reduced.

## Background
PRIORITY
This application claims priority under 35 U.S.C. § 119 to an application entitled “Encryption Apparatus and Method in a Wireless Communications System” filed in the Korean Industrial Property Office on Oct. 8, 2002 and assigned Serial No. 2002-61179, the contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a wireless communications system, and in particular, to an encryption apparatus and method for implementing confidentiality and integrity algorithms in a wireless communications system.
2. Description of the Related Art
As the first generation analog encryption system has evolved into the second generation digital encryption system, more advanced encryption techniques have been used. The current third generation encryption system provides encryption service for multimedia service i.e., audio and video information. Thus, the importance of encryption has increased in order to provide confidentiality to voice signals, multimedia service, and user data. An integrity algorithm is required to authenticate control signals between mobile terminals in a wireless communication system and a network. The 3rd Generation Project Partnership (3GPP) has selected the KASUMI algorithm as the f8 confidentiality and f9 integrity algorithms for a third generation system based on a Global System for Mobile communication (GSM) core network, and a Universal Mobile Telecommunication System (UMTS).
FIG. 1 is a block diagram illustrating an example of a conventional KASUMI algorithm. Referring to FIG. 1, KASUMI is an 8-round Feistel unit cipher that provides a 64-bit output ciphertext from a 64-bit input plaintext with 8-round encryption. The 64-bit input signal is divided into a 32-bit signal L0 and a 32-bit signal R0. FLi units (1≦i≦8) 110 to 180 and FOi units (1≦i≦8) 210 to 280 encrypt the signals L0 and R0 under corresponding encryption keys KLi (1≦i≦8), KOi (1≦i≦8), and KIi (1≦i≦8) and output the 64-bit ciphertext.
Encryption in accordance with FIG. 1 occurs in the following manner. An FL1 unit 110 encrypts the input 32-bit signal L0 with an encryption key KL1 and outputs a ciphertext L01. An FO1 unit 210 encrypts the 32-bit ciphertext L01 with encryption keys KO1 and KI1 and outputs a ciphertext L02. An Exclusive-OR operation is performed to logically “exclusive OR” the ciphertext L02 and the 32-bit signal R0 to provide a 64-bit ciphertext. This encryption occurs eight times and a final 64-bit ciphertext is generated in the KASUMI.
FIG. 2A is a block diagram illustrating an example of FOi units. Referring to FIG. 2A, FOi denotes an ith FO unit. The FOi unit comprises a plurality of f1i,j sub-ciphers (1≦i≦3, 1≦i≦3) to provide 3-rounds of encryption. Here, the operation of the FO1 unit 210 will be described by way of example. The 32-bit input signal is divided into two 16-bit signals L0 and R0. An Exclusive-OR operation is performed to logically “exclusive OR” the 16-bit signal L0 and a 16-bit sub-encryption key KO1,1, to provide a signal L1. A f11,1 sub-cipher 201 encrypts the signal L1 with a 16-bit sub-encryption key KI1,1 and outputs a signal L1D. Meanwhile, a first delay (D1) 10 delays the 16-bit signal R0, which is equivalent to the signal R1, in order to synchronize the 16-bit signal R0 with the signal L1D and output a delayed signal R1D. For a second-round of encryption, an Exclusive-OR operation is performed to logically “exclusive OR” the 16-bit signal RID and a 16-bit sub-encryption key KO1,2 to provide a signal L2. A f11,2 sub-cipher 203 encrypts the signal L2 with a 16-bit sub-encryption key KI1,2 and outputs a signal L2D. Meanwhile, an Exclusive-OR operation is performed to logically “exclusive OR” the 16-bit signal R1D and the signal L1D, to provide a signal R2. A second delay (D2) 20 delays the signal R2 in order to synchronize the signal R2 with the signal L2D and output a delayed signal R2D. For a third-round of encryption, an Exclusive-OR operation is performed to logically “exclusive OR” the 16-bit signal R2D and a 16-bit sub-encryption key KO1,3, resulting in a signal L3. A f11,3 sub-cipher 205 encrypts the signal L3 with a 16-bit sub-encryption key KI1,3 and outputs a signal L3D. Meanwhile, an Exclusive-OR operation is performed to logically “exclusive OR” the 16-bit signal R2D and the signal L2D to provide a signal R3. A third delay (D3) 30 delays the signal R3 in order to synchronize the signal R3 with the signal L3D and output a delayed signal R3D. An Exclusive-OR operation is performed to logically “exclusive OR” the 16-bit signal R3D and the signal L3D, to provide a signal R4. The 16-bit signal R4 is operated with the 16-bit signal R3D (=L4), resulting in a 32-bit ciphertext L4//R4.
The FO1 unit uses the three delays 10, 20 and 30 to synchronize to the output timings of the sub-ciphers 201, 203 and 205.
FIG. 2B is a block diagram illustrating another example of the FOi units. Referring to FIG. 2B, a FOi unit comprises a plurality of f1i′,j′ sub-ciphers (1≦i′≦3 1≦j′≦3), for 3-rounds of encryption. Here, the FO1 unit 210 will be described by way of example. The 32-bit input signal is divided into two 16-bit signals L0′ and R0′. An Exclusive-OR operation is performed to logically “exclusive OR” the 16-bit signal L0′ and a 16-bit sub-encryption key KO1,1, to provide a signal L1′. A f11′,1′ sub-cipher 211 encrypts the signal L1′ with the 16-bit sub-encryption key KI1,1 and outputs a signal L1D′. Meanwhile, a fourth delay (D4) 40 delays the 16-bit signal R0′ (=R1′) and outputs a delayed signal R1D′. An Exclusive-OR operation is performed to logically “exclusive OR” the signals L1D′ and R1D′ to provide a signal L2′. Simultaneously, an Exclusive-OR operation is performed to logically “exclusive OR” the 16-bit signal R0′ and a 16-bit sub-encryption key KO1,2, to provide a signal R2′. A f11′,2′ sub-cipher 213 encrypts the signal R2′ with a 16-bit sub-encryption key KI1,2 and outputs a signal R2D′. An Exclusive-OR operation is performed to logically “exclusive OR” the signals L2′ and R2D′ to provide a signal R3′. Another Exclusive-OR operation is performed to logically “exclusive OR” the signal L2′ and a 16-bit sub-encryption key KO1,3, to provide a signal L3′. A f11′,3′ sub-cipher 215 encrypts the signal L3′ with a 16-bit sub-encryption key KI1,3 and outputs a signal L3D′. Meanwhile, a fifth delay (D5) 50 delays the signal R3′ and outputs a delayed signal R3D′. An Exclusive-OR operation is performed to logically “exclusive OR” the signals L3D′ and R3D′ to provide a 16-bit signal L4′. The 16-bit signal L4′ is operated with the 16-bit signal R3D′ (=R4′), resulting in a 32-bit ciphertext L4′//R4′.
The above advanced FOi unit uses the two delays 40 and 50 to synchronize to the output timings of the F1 sub-ciphers 211 and 215. However, due to the use of the delays, a large chip capacity is required.
FIG. 3 is a block diagram illustrating an example of the f1i,j sub-ciphers illustrated in FIGS. 2A and 2B. By way of example, the f11,1 sub-cipher 201 will be described below. Referring to FIG. 3, the 16-bit input signal is divided into a 9-bit signal RL0 and a 7-bit signal RR0. An SBox91 (S91) operator 310 generates a 9-bit signal y0, y1, . . . , y8 from the input signal RL0 using

y0=(x0x2)⊕x3⊕(x2x5)⊕(x5x6)⊕(x0x7)⊕(x1x7)⊕(x2x7)⊕(x4x8)⊕(x5x3)⊕(x7x8)⊕‘1’;

y1=x1⊕(x0x1)⊕(x2x3)⊕(x0x4)⊕(x1x4)⊕(x0x5)⊕(x3x5)⊕x6⊕(x1x7)⊕(x2x7)⊕(x5x8)⊕‘1’;

y2=x1⊕(x0x3)⊕(x3x4)⊕(x0x5)⊕(x2x6)⊕(x3x6)⊕(x5x6)⊕(x4x7)⊕(x5x7)⊕(x6x7)⊕x8⊕(x0x8)⊕‘1’;

y3=x0⊕(x1x2)⊕(x0x3)⊕(x2x4)⊕x5⊕(x0x6)⊕(x1x6)⊕(x4x7)⊕(x0x8)⊕(x1x8)⊕(x7x8);

y4=(x0x1)⊕(x1x3)⊕x4⊕(x0x5)⊕(x3x6)⊕(x0x7)⊕(x6x7)⊕(x1x0)⊕(x2x3)⊕(x3x0);

y5=x2⊕(x1x4)⊕(x4x5)⊕(x0x6)⊕(x0x6)⊕(x3x7)⊕(x4x7)⊕(x6x7)⊕(x5x3)⊕(x6x8)⊕(x7x8)⊕‘1’;

y6=x0⊕(x2x3)⊕(x1x5)⊕(x2x5)⊕(x4x5)⊕(x3x6)⊕(x4x6)⊕(x5x6)⊕x7⊕(x1x8)⊕(x3x8)⊕(x5x8)⊕(x7x8);

y7=(x0x1)⊕(x0x2)⊕(x1x2)⊕x3⊕(x0x3)⊕(x2x3)⊕(x4x5)⊕(x2x6)⊕(x3x5)⊕(x2x7)⊕(x5x7)⊕(x8⊕‘1’;

y8=(x0x1)⊕x2⊕(x1x2)⊕(x3x4)⊕(x1x5)⊕(x2x5)⊕(x1x6)⊕(x4x6)⊕x7⊕(x2x8)⊕(x3x8);   (1)

A ZE1 unit 320 receives the signal RR0, adds two zeroes to the Most Significant Bit (MSB) of the signal RR0, and outputs a 9-bit signal. An Exclusive-OR operation is performed to logically “exclusive OR” the outputs of the S91 operator 310 and the ZE1 unit 320 to provide a 9-bit signal RL1. Another Exclusive-OR operation is performed to logically “exclusive OR” the signal RL1 and a 9-bit sub-encryption key KI1,1,2, to provide a 9-bit signal RL2.
A TR1 unit 330 removes two zero bits from the MSBs of the 9-bit signal RL1. An SBox71 (S71) operator 340 generates a 7-bit signal y0, y1, . . . , y6 from the input signal RR0 (=RR1) by

y0=(x1x3)⊕x4⊕(x0x1x1)⊕x5⊕(x2x5)⊕(x3x4x5)⊕x6⊕(x0x6)⊕(x1x6)⊕(x3x6)⊕(x2x4x6)⊕(x1x5x6)⊕(x4x5x6);

y1=(x0x1)⊕(x1x4)⊕(x2x4)⊕x5⊕(x1x2x5)⊕(x0x3x5)⊕x5⊕(x0x2x3)⊕(x3x6)⊕(x4x5x6)‘1’;

y2=x0⊕(x0x3)⊕(x2x3)⊕(x1x2x4)⊕(x0x3x4)⊕(x1x5)⊕(x0x2x5)⊕(x0x5)⊕(x0x1x6)⊕(x2x6)⊕(x1x6)⊕‘1’;

y3=x1⊕(x0x1x2)⊕(x1x4)⊕(x3x4)⊕(x0x5)⊕(x0x1x5)⊕(x2x3x5)⊕(x1x4x5)⊕(x2x6)⊕(x1x3x6);

y4=(x0x2)⊕x3⊕(x1x3)⊕(x1x4)⊕(x0x1x4)⊕(x2x3x4)⊕(x0x5)⊕(x1x3x5)⊕(x0x4x5)⊕(x1x6)⊕(x3x6)⊕(x0x3x6)⊕(x5x6)⊕‘1’;

y5=x2⊕(x0x2)⊕(x0x3)⊕(x1x2x3)⊕(x0x1x4)⊕(x0x5)⊕(x2x5)⊕(x4x5)⊕(x1x6)⊕(x1x2x6)⊕(x0x3x6)⊕(x3x4x6)⊕(x2x5x6)⊕‘1’;

y6=(x1x2)⊕(x1x1x3)⊕(x0x4)⊕(x1x5)⊕(x3x5)⊕x6⊕(x0x1x6)⊕(x2x3x6)⊕(x1x4x5)⊕(x0x5x6);  (2)

An Exclusive-OR operation is performed to logically “exclusive OR” the outputs of the TR1 330 and the S71 operator 340 via a sub-encryption key KI1,1,1, to provide a 7-bit signal RR2.
A SBox92 (S92) Operator 350 generates a 9-bit signal y0, y1, . . . , y8 from the signal RL2 by Eq. (1). A ZE2 unit 360 receives the signal RR2, adds two zeroes to the MSB of the signal RR2, and outputs a 9-bit signal. An Exclusive-OR operation is performed to logically “exclusive OR” the outputs of the S92 operator 350 and the ZE2 unit 360 to provide a 9-bit signal RL3. A TR2 unit 370 removes two zero bits from the MSBs of the 9-bit signal RL3. A SBox72 (S72) operator 380 generates a 7-bit signal y0, y1, . . . , y6 from the input signal RR2 (=RR3) using Eq. (2). Another Exclusive-OR operation is performed to logically “exclusive OR” the outputs of the TR2 370 and the S72 operator 380 to provide a 7-bit signal RR4.
The 9-bit signal RL3 (=RL4) and the 7-bit signal RR4 are operated, resulting in a 16-bit ciphertext RL4//RR4.
As described above, the S91 operator 310 and the S92 operator 350 each sequentially perform an AND operation to perform a logical “AND” and an exclusive-OR operation to perform a logical “Exclusive-OR” using Eq. (1), to thereby generate an output signal y0, y1, . . . , y8. Similarly, the S71 operator 340 and the S72 operator 380 sequentially perform an AND operation to perform a logical “AND” and an exclusive-OR operation to perform a logical “Exclusive-OR” using Eq. (2), to thereby generate an output signal y0, y1, . . . , y6. Consequently, the encryption speed is decreased. Moreover, a gate delay involved in the operations of the S91, S92, S71 and S72 operators 310, 350, 340, and 360 gradually increases glitch.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide an encryption method for generating a ciphertext bit stream of length 2n from a plaintext bit stream of length 2n. 
It is another object of the present invention to provide an encryption apparatus for generating a ciphertext bit stream of length 2n from a plaintext bit stream of length 2n. 
To achieve the above objects, in an encryption method for dividing a first plaintext bit stream of length 2n into first and second sub-bit streams of length n, dividing a second plaintext bit stream of length 2n into third and fourth sub-bit streams of length n, and generating a ciphertext bit stream of length 2n from the first, second, third and fourth sub-bit streams using 2-rounds of encryption, first and second ciphertext bit streams of length n are generated by encrypting the first and second sub-bit streams with predetermined first encryption codes KO1,1, KO1,2, KO1,3, KI1,1, KI1,2, and KI1,3, the second ciphertext bit stream being output with a predetermined time delay from the first ciphertext bit stream, in a first-round encryption. A first operated ciphertext bit stream is generated by performing a logical exclusive-OR-operation on the first ciphertext bit stream and the third sub-bit stream, and a second operated ciphertext bit stream is operated by performing a logical exclusive-OR-operation on the second ciphertext bit stream and the fourth sub-bit stream. In a second-round of encryption, third and fourth ciphertext bit streams of length n are generated by encrypting the first operated ciphertext bit stream and the second operated ciphertext bit stream with predetermined second encryption codes KO2,1, KO2,2, K2,3, KI2,1, KI2,2, and KI2,3 and the third and fourth ciphertext bit streams a concurrently output.
In an encryption apparatus for dividing a first plaintext bit stream of length 2n into first and second sub-bit streams of length n, dividing a second plaintext bit stream of length 2n into third and fourth sub-bit streams of length n, and generating a ciphertext bit stream of length 2n from the first, second, third and fourth sub-bit streams using 2-rounds of encryption, a first ciphering unit receives the first and second sub-bit streams, and generates first and second ciphertext bit streams of length n by encrypting the first and second sub-bit streams with predetermined first encryption codes KO1,1, KO1,2, KO1,3, KI1,1, KI1,2, and KI1,3. Here, the second ciphertext bit stream is output with a predetermined time delay from the first ciphertext bit stream. An operating unit generates a first operated ciphertext bit stream by performing a logical exclusive-OR operation on the first ciphertext bit stream and the third sub-bit stream, and generates a second operated ciphertext bit stream by performing a logical exclusive-OR-operation on the second ciphertext bit stream with the fourth sub-bit stream. A second ciphering unit receives the first operated ciphertext bit stream and the second operated ciphertext bit stream having the predetermined time delay, generates third and fourth ciphertext bit streams of length n by encrypting the first operated ciphertext bit stream and the second operated ciphertext bit stream with predetermined second encryption codes KO2,1, KO2,2, KO2,3, KI2,1, KI2,2, and KI2,3 and concurrently outputs the third and fourth ciphertext bit streams.



BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:
FIG. 1 is a block diagram illustrating an example of a conventional KASUMI algorithm;
FIG. 2A is a block diagram illustrating an example of FOi units illustrated in FIG. 1;
FIG. 2B is a block diagram illustrating another example of the FOi units illustrated in FIG. 1;
FIG. 3 is a block diagram illustrating an example of f1i, j sub-ciphers illustrated in FIGS. 2A and 2B;
FIG. 4 is a block diagram illustrating an example of a KASUMI algorithm according to the present invention;
FIG. 5 is a block diagram illustrating an example of SLIMFOi units illustrated in FIG. 4 according to the present invention; and
FIG. 6 is a block diagram illustrating an example of f1i, j sub-ciphers illustrated in FIG. 5 according to the present invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
An embodiment of the present invention will be described herein below with reference to the accompanying drawings. Also, a description of known functions and configurations have been omitted for conciseness.
A KASUMI algorithm according to the present invention is a ciphering algorithm used as the f8 confidentiality and f9 integrity algorithms. The f8 confidentiality algorithm encrypts a plaintext signal having a predetermined number of bits by exclusive-OR-operating the plaintext with an encryption key and decrypts a ciphertext by exclusive-OR-operating the ciphertext with the encryption key. The f9 integrity algorithm derives a message authentication code from a received signal. The KASUMI algorithm, as previously discussed, has emerged as a significant issue to confidentiality and integrity.
FIG. 4 is a block diagram illustrating an example of a KASUMI algorithm according to the present invention. Referring to FIG. 4, a KASUMI of the present invention provides a 64-bit output ciphertext from a 64-bit input plaintext using first, second and third encryption keys. The 64-bit input signal is divided into a 32-bit signal L0 and another 32-bit signal R0. FLi units (1≦i≦8) 410 to 480 and SLIMFOi units (1≦i≦4) 510 to 540 are used to encrypt the signals L0 and R0 using corresponding encryption keys KOi (1≦i≦8) and KIi (1≦i≦8) to output a 64-bit ciphertext.
Describing FIG. 4 in more detail, an FL1 unit 410 encrypts the input 32-bit signal L0 with an encryption key KL1 and outputs a ciphertext L1. An SLIMFO1 unit 510 encrypts the 32-bit ciphertext L1 with encryption keys KO1 and KI1, outputs a signal SR1 by encrypting the signal L1 with the 32-bit signal R0, and then outputs a signal R1 by encrypting the signal SR1 with encryption keys KO2 and KI2. An FL2 unit 420 encrypts the signal R1 with an encryption key KL2 and outputs a ciphertext R2. An Exclusive-OR operation is performed to logically “exclusive OR” the signals L0 and R2 to provide a signal L2 (=SL1).
An FL3 unit 430 encrypts the signal L2 with an encryption key KL3 and outputs a ciphertext L3. An SLIMFO2 unit 520 encrypts the signal L3 with encryption keys KO3 and KI3, outputs a signal SR2 by operating the encrypted signal L3 with the signal SR1, and then outputs a signal R3 by encrypting the signal SR2 with encryption keys KO4 and KI4. An FL4 unit 440 encrypts the signal R3 with an encryption key KL4 and outputs a ciphertext R4. An Exclusive-OR operation is performed to logically “exclusive OR” the signals L2 (=SL1) and R4 to provide a signal L4 (=SL2).
An FL5 unit 450 encrypts the signal L4 with an encryption key KL5 and outputs a ciphertext L5. An SLIMFO3 unit 530 encrypts the signal L5 with encryption keys KO5 and KI5, outputs a signal SR3 by operating the encrypted signal L3 with the signal SR2, and then outputs a signal R5 by encrypting the signal SR3 with encryption keys KO6 and KI6. An FL6 unit 460 encrypts the signal R5 with an encryption key KO6 and outputs a ciphertext R6. An Exclusive-OR operation is performed to logically “exclusive OR” the signals L4 (=SL2) and R6 to provide a signal L6 (=SL3).
An FL7 unit 470 encrypts the signal L6 with an encryption key KL7 and outputs a ciphertext L7. An SLIMFO4 unit 540 encrypts the signal L7 with encryption keys KO7 and KI7, outputs a signal SR4 by operating the encrypted signal L7 with the signal SR3, and then outputs a signal R7 by encrypting the signal SR4 with encryption keys KO8 and KI8. An FL8 unit 480 encrypts the signal R7 with an encryption key KL8 and outputs a ciphertext R8. The signals L6 (=SL3) and R8 are exclusive-OR-operated, resulting in a signal L8 (=SL4). Consequently, the eight FLi units (1≦i≦8) 410 to 480 and the four SLIMFOi units (1≦i≦4) 510 to 540 encrypt the 64-bit plaintext and output the 64-bit ciphertext, that is, the 32-bit signal SL4//the 32-bit SR4.
FIG. 5 is a block diagram illustrating an example of the SLIMFOi units illustrated in FIG. 4 according to an embodiment of the invention. Referring to FIG. 5, a SLIMFOi unit is an ith SLIMFO unit and implemented using parallel operations of signals in two FOi units. The SLIMFO1 unit 510 of FIG. 4 will be described by way of example. The SLIMFO1 unit 510 comprises an FO1 cipher 501 and an FO2 cipher 502. Each FO cipher includes f1i,j sub-ciphers (1≦i≦2, 1≦j≦3), for 3-round encryption.
The signal resulting from encrypting the 32-bit signal L0 with the encryption key KL1 in FIG. 4 is divide into a 16-bit signal L0 (=L1) and a 16-bit signal R0 (=R1) in the FO1 cipher 501. A signal L2 is generated by performing a logical exclusive-OR operation on the signal L1 with a sub-encryption key KO1,1. An f11,1 sub-cipher 511 generates a signal L2D by encrypting the signal L2 with a sub-encryption key KI1,1. A delay (D6) 600 delays the signal R1 and outputs a delayed signal R1D. A signal L3 is generated by performing a logical exclusive-OR operation on the signals R1D and L2D. Meanwhile, a signal R2 is generated by performing a logical exclusive-OR operation on the signal R1 with a sub-encryption key KO1,2. An FL1,2 sub-cipher 512 generates a signal R2D by encrypting the signal R2 with a sub-encryption key KI1,2. A signal R3 is generated by performing a logical exclusive-OR operation on the signals R2D and L3. A signal L4 is generated by performing a logical exclusive-OR operation on the signal L3 with a sub-encryption key KO1,3. An f11,3 sub-cipher 513 generates a signal L4D by encrypting the signal L4 with a sub-encryption key KI1,3. A delay (D7) 620 delays the signal R3 and outputs a delayed signal R3D. A 16-bit signal L5 is generated by performing a logical exclusive-OR operation on the signals R3D and L4D.
The 32-bit signal R0 which was divided from the 64-bit signal in FIG. 4 is further divided into a 16-bit signal L0′ and a 16-bit signal R0′ in the FO2 cipher 502. A signal L6 is generated by performing a logical exclusive-OR operation on the signal L0′ using the 16-bit signal L5. Meanwhile, a signal R4 is generated by performing a logical exclusive-OR operation on the signal R0′ using the 16-bit signal R3. A signal R5 is generated by performing a logical exclusive-OR operation on the signal R4 using a sub-encryption key KO2,1. An f12,1 sub-cipher 514 generates a signal R5D by encrypting the signal R5 with a sub-encryption key KI2,1. A signal R6 is generated by performing a logical exclusive-OR operation on the signals R5D and L6. That is, the f11,3 sub-cipher 513 and the f12,1 sub-cipher 514 synchronize the signal L6 to the signal R6 without using delays. A signal L7 is generated by performing a logical exclusive-OR operation on the signal L6 with a 16-bit sub-encryption key KO2,2. An f12,2 sub-cipher 515 generates a signal L7D by encrypting the signal L7 with a 16-bit sub-encryption key KI2,2. A delay (D8) 640 delays the signal R6 and outputs a delayed signal R6D. A signal L8 is generated by performing a logical exclusive-OR operation on the signals L7D and R6D. A signal R7 is generated by performing a logical exclusive-OR operation on the signal R6 with a 16-bit sub-encryption key KO2,3. An f12,3 sub-cipher 516 generates a signal R7D by encrypting the signal R7 with a 16-bit sub-encryption key KI2,3. A signal R8 is generated by performing a logical exclusive-OR operation on the signals R7D and L8. Consequently, a 32-bit ciphertext L8∥R8 is generated by operating the 16-bit signal L8 with the 16-bit signal R8.
As described above, the SLIMFO1 unit encrypts the input plaintext by processing the 16-bit signals L0 and R0 in parallel in the FO1 cipher 501 and processing the 16-bit signals L0′ and R0′ in parallel in the FO2 cipher 502. The parallel processing of the 32-bit signals L0 and R0 which were divided from the 64-bit input signal in the SLIMFOi units remarkably increases encryption speed and reduces the number of delays used to synchronize a delayed signal to a non-delayed signal.
FIG. 6 is a block diagram illustrating an example of the f1i,j sub-ciphers illustrated in FIG. 5 according to an embodiment of the invention. By way of example, the f11,1 sub-cipher 511 will be described below.
Referring to FIG. 6, the f11,1 sub-cipher 511 includes a first ciphering unit and a second ciphering unit. In the first ciphering unit, a 16-bit input signal is divided into a 9-bit signal RL0 and a 7-bit signal RR0. An S91 operator 710 generates a 9-bit signal y0, y1, . . . , y8 from the input signal RL0 by

y0=(x0x2)⊕x3⊕(x2x5)⊕(x5x6)⊕(x0x7)⊕(x1x7)⊕(x2x7)⊕(x4x8)⊕(x5x3)⊕(x7x8)⊕‘1’;

y1=x1⊕(x0x1)⊕(x2x3)⊕(x0x4)⊕(x1x4)⊕(x0x5)⊕(x3x5)⊕x6⊕(x1x7)⊕(x2x7)⊕(x5x8)⊕‘1’;

y2=x1⊕(x0x3)⊕(x3x4)⊕(x0x5)⊕(x2x6)⊕(x3x6)⊕(x5x6)⊕(x4x7)⊕(x5x7)⊕(x6x7)⊕x8⊕(x0x8)⊕‘1’;

y3=x0⊕(x1x2)⊕(x0x3)⊕(x2x4)⊕x5⊕(x0x6)⊕(x1x6)⊕(x4x7)⊕(x0x8)⊕(x1x8)⊕(x7x8);

y4=(x0x1)⊕(x1x3)⊕x4⊕(x0x5)⊕(x3x6)⊕(x0x7)⊕(x6x7)⊕(x1x8)⊕(x2x3)⊕(x3x8);

y5=x2⊕(x1x4)⊕(x4x5)⊕(x0x6)⊕(x1x6)⊕(x3x7)⊕(x4x7)⊕(x6x7)⊕(x5x3)⊕(x6x8)⊕(x7x8)⊕‘1’;

y6=x0⊕(x2x3)⊕(x1x5)⊕(x2x5)⊕(x4x5)⊕(x3x6)⊕(x4x6)⊕(x5x6)⊕x7⊕(x1x8)⊕(x3x8)⊕(x5x8)⊕(x7x8);

y7=(x0x1)⊕(x0x2)⊕(x1x2)⊕x3⊕(x0x3)⊕(x2x3)⊕(x4x5)⊕(x2x6)⊕(x3x5)⊕(x2x7)⊕(x5x7)⊕x8⊕‘1’;

y8=(x0x1)⊕x2⊕(x1x2)⊕(x3x4)⊕(x1x5)⊕(x2x5)⊕(x1x6)⊕(x4x6)⊕x7⊕(x2x8)⊕(x3x8);  (3)

That is, the S91 operator 710 generates the 9-bit signal y0, y2, . . . , y8 by performing parallel logical AND operations and then performing a logical exclusive-OR operation of a 9-bit signal x0, x1, . . . , x8 in parallel. A ZE1 unit 720 receives the signal RR0, adds two zeroes to the MSB of the signal RR0, and outputs a 9-bit signal. An Exclusive-OR operation is performed to logically “exclusive OR” the outputs of the S91 operator 710 and the ZE1 unit 720 to provide a 9-bit signal RL1. Another Exclusive-OR operation is performed to logically “exclusive OR” the signal RL1 and a 9-bit sub-encryption key KI1,1,2, to provide a 9-bit signal RL2. The signal RL2 is temporarily stored in a first register (register 1) 800.
Simultaneously, an S71 operator 740 generates a 7-bit signal y0, y1, . . . , y6 from the input signal RR0 (=RR1) by

y0=(x1x3)⊕x4⊕(x0x1x4)⊕x5⊕(x2x5)⊕(x3x4x5)⊕x6⊕(x0x6)⊕(x1x6)⊕(x3x6)⊕(x2x4x6)⊕(x1x5x6)⊕(x4x5x6);

y1=(x0x1)⊕(x1x4)⊕(x2x4)⊕x5⊕(x1x2x5)⊕(x0x3x5)⊕x6⊕(x0x2x3)⊕(x3x6)⊕(x4x5x6)⊕‘1’;

y2=x0⊕(x0x3)⊕(x2x3)⊕(x1x2x4)⊕(x0x3x4)⊕(x1x5)⊕(x0x2x5)⊕(x0x5)⊕(x0x4x6)⊕(x2x6)⊕(x4x6)⊕‘1’;

y3=x1⊕(x0x1x2)⊕(x1x4)⊕(x3x4)⊕(x0x5)⊕(x0x1x5)⊕(x2x3x5)⊕(x1x4x5)⊕(x2x6)⊕(x1x3x6);

y4=(x0x2)⊕x3⊕(x1x3)⊕(x1x4)⊕(x0x1x4)⊕(x2x3x4)⊕(x0x5)⊕(x1x3x5)⊕(x0x4x5)⊕(x1x6)⊕(x3x6)⊕(x0x3x6)⊕(x5x6)⊕‘1’;

y5=x2⊕(x0x2)⊕(x0x3)⊕(x1x2x3)⊕(x0x1x4)⊕(x0x5)⊕(x2x5)⊕(x4x5)⊕(x1x6)⊕(x1x2x6)⊕(x0x3x6)⊕(x3x4x6)⊕(x2x5x6)⊕‘1’;

y6=(x1x2)⊕(x1x1x3)⊕(x0x4)⊕(x1x5)⊕(x3x5)⊕x6⊕(x0x1x6)⊕(x2x3x6)⊕(x1x4x5)⊕(x0x5x6);  (4)

That is, the S71 operator 740 generates the 7-bit signal y0, y2, . . . , y6 by performing parallel logical AND operations and then performing a logical exclusive-OR operation of a 7-bit signal x0, x1, . . . , x6 in parallel. A TR1 unit 730 removes two zeroes from the MSBs of the 9-bit signal RL1 and outputs the resulting 7-bit signal. A 7-bit signal RR2 is generated by performing a logical exclusive-OR operation on the outputs of the TR1 730 and the S71 operator 740 with a sub-encryption key KI1,1,1,. The signal RR2 is temporarily stored in the first register 800. Upon receipt of a first clock signal CLK1 from a controller (not shown), the register 800 simultaneously outputs the 9-bit signal RL2 and the 7-bit signal RR2. Thus the register 800 functions to synchronize the output timings of signals according to delay involved with encryption in the S91 operator 710, the ZE1 unit 720, the TR1 unit 730, and the S71 operator 740.
In the second ciphering unit, an S92 operator 750 generates a 9-bit signal y0, y1, . . . , y8 from the 9-bit signal RL2 received from the register 800 using Eq. (3). A ZE2 unit 760 adds two zeroes to the MSB of the signal RR2 received from the register 800 and outputs a 9-bit signal. An Exclusive-OR operation is performed to logically “exclusive OR” the outputs of the S92 operator 750 and the ZE2 unit 760 to provide a 9-bit signal RL3. The signal RL3 is temporarily stored in a second register (register 2) 820.
Simultaneously, an S72 operator 780 generates a 7-bit signal y0, y1, . . . , y6 from the 7-bit signal RR2 (=RR3) using Eq. (4). A TR2 unit 770 removes two zeroes from the MSBs of the 9-bit signal RL3 and outputs the resulting 7-bit signal. A 7-bit signal RR4 is generated by performing a logical exclusive-OR-operation on the outputs of the TR2 770 and the S72 operator 780. The signal RR4 is temporarily stored in the second register 820.
Upon receipt of a second clock signal CLK2 from the controller; the register 820 simultaneously outputs the 9-bit signal RL4 and the 7-bit signal RR4. Thus the register 820 functions to synchronize the output timings of signals according to the delay involved with the encryption in the S92 operator 750, the ZE2 unit 760, the TR2 unit 770, and the S72 operator 780.
As described above, the S91 operator 710 and the S92 operator 750 each output a 9-bit signal y0, y1, . . . , y8 by performing parallel logical AND operations and then performing a logical exclusive-OR operation according to Eq. (3). The S71 operator 740 and the S72 operator 780 each output a 7-bit signal y0, y1, . . . , y6 by parallel AND operations and then exclusive-OR operation according to Eq. (4). Therefore, encryption speed is remarkably increased. Furthermore, the use of the registers 800 and 820 for signal timing synchronization enables output of an accurate ciphertext.
In accordance with the present invention, (1) parallel computation of input signals increases signal processing speed; (2) due to synchronization of the output timings of a delayed signal and a non-delayed signal, an accurate ciphertext is achieved and thus an encryption system is further stabilized; and (3) the decrease in devices used for synchronization reduces required chip capacity and production cost.
While the invention has been shown and described with reference to a certain embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

## Claims

1. An encryption method for dividing a first plaintext bit stream of length 2n into first and second sub-bit streams of length n, dividing a second plaintext bit stream of length 2n into third and fourth sub-bit streams of length n, and generating a ciphertext bit stream of length 2n from the first, second, third and fourth sub-bit streams using 2-rounds of encryption, the method comprising the steps of:
performing the first-round of encryption by encrypting the received the first and second sub-bit streams with predetermined first encryption codes an odd number of times and outputting first and second ciphertext bit streams, wherein the first ciphertext bit stream is encrypted a first number of times and the second ciphertext bit stream is encrypted a second number of times, the second number of times exceeding the first number of times;
generating a first operated ciphertext bit stream by performing a logical exclusive-OR-operation on the first ciphertext bit stream and the third sub-bit stream;
generating a second operated ciphertext bit stream by performing a logical exclusive-OR operation on the second ciphertext bit stream and the fourth sub-bit stream; and
performing the second-round of encryption by encrypting the received first operated ciphertext bit stream and the second operated ciphertext bit stream with predetermined second encryption codes an odd number of times and concurrently outputting third and fourth ciphertext bit streams of length n, wherein the third ciphertext bit stream is encrypted a third number of times and the fourth ciphertext bit stream is encrypted the fourth number of times, the fourth number of times exceeding the third number of times,
wherein the last encryption for the second ciphertext bit stream in the first-round is performed at the same time of performing the first encryption of the first operated ciphertext bit stream in the second round of encryption.

2. The encryption apparatus of claim 1, wherein the predetermined first encryption codes comprises at least one of KO1,1, KO1,2, KO1,3, KI1,1, KI1,2, and KI1,3.

3. The encryption apparatus of claim 1, wherein the predetermined second encryption codes comprises at least one of KO2,1, KO2,2, KO2,3, KI2,1, KI2,2, and KI2,3.

4. The encryption method of claim 2, wherein the first-round encryption step comprises the steps of:
generating a first signal by performing a logical exclusive-OR operation on the first sub-bit stream and the first encryption code KO1,1 to provide a first exclusive-OR operated bitstream, encrypting the first exclusive-OR-operated bit stream with the first encryption code KI1,1 to provide a first encrypted signal, and performing a logical exclusive-OR operation on the first encrypted signal and the second sub-bit stream delayed by time required for the encryption;
generating the first operated ciphertext bit stream by performing a logical exclusive-OR-operation on the second sub-bit stream and the first encryption code KO1,2, to provide a second exclusive-OR operated bitstream encrypting the second exclusive-OR-operated bit stream with the first encryption code KI1,2 to provide a second encrypted signal, and performing a logical exclusive-OR-operation on the second encrypted signal and the first signal;
generating the second operated ciphertext bit stream by performing a logical exclusive-OR-operation on the first signal and the first encryption code KO1,3 to provide a third exclusive-OR operated bitstream, encrypting the third exclusive-OR-operated bit stream with the first encryption code KI1,3, and performing a logical exclusive-OR-operation on the encrypted signal with the first sub-bit stream delayed by time required for the encryption.

5. The encryption method of claim 3, wherein the second-round encryption step comprises the steps of:
generating a second signal by performing a logical exclusive-OR-operation on the first operated ciphertext bit stream and the second encryption code KO2,1 to provide a fourth exclusive-OR operated bitstream, encrypting the fourth exclusive-OR-operated bit stream with the second encryption code KI2,1 to provide a third encrypted signal, performing a logical exclusive-OR-operation on the third encrypted signal and the second operated ciphertext bit stream to provide a fifth exclusive-OR operated bitstream;
generating the third operated ciphertext bit stream by performing a logical exclusive-OR-operation on the second operated ciphertext bit stream and the second encryption code KO2,2, encrypting the fifth exclusive-OR-operated bit stream with the second encryption code KI2,2 to provide a fourth encrypted signal, and performing a logical exclusive-OR-operation on the fifth encrypted signal and the second signal delayed by time required for the encryption; and
generating the fourth ciphertext bit stream by performing a logical exclusive-OR-operation on the second signal and the second encryption code KO2,3 encrypting the sixth exclusive-OR-operated bit stream with the second encryption code KI2,3, and performing a logical exclusive-OR-operation on the encrypted signal with the third operated ciphertext bit stream.

6. The encryption method of claim 5, wherein each of the encryptions includes first and second sub-encryptions, and outputs from the first and second sub-encryptions are stored and simultaneously retrieved according to an external clock signal.

7. The encryption method of claim 5, wherein a 16-bit input bit stream is divided into a 9-bit stream and a 7-bit stream, a 9-bit ciphertext bit stream is generated from the 9-bit stream using a first equation, and a 7-bit ciphertext bit stream is generated from the 7-bit stream using a second equation in each of the sub-encryptions, wherein said first equation comprises


y0=(x0x2)⊕x3⊕(x2x5)⊕(x5x6)⊕(x0x7)⊕(x1x7)⊕(x2x7)⊕(x4x8)⊕(x5x3)⊕(x7x8)⊕‘1’;



y1=x1⊕(x0x1)⊕(x2x3)⊕(x0x4)⊕(x1x4)⊕(x0x5)⊕(x3x5)⊕x6⊕(x1x7)⊕(x2x7)⊕(x5x8)⊕‘1’;



y2=x1⊕(x0x3)⊕(x3x4)⊕(x0x5)⊕(x2x6)⊕(x3x6)⊕(x5x6)⊕(x4x7)⊕(x5x7)⊕(x6x7)⊕x8⊕(x0x8)⊕‘1’;



y3=x0⊕(x1x2)⊕(x0x3)⊕(x2x4)⊕x5⊕(x0x6)⊕(x1x6)⊕(x4x7)⊕(x0x8)⊕(x1x8)⊕(x7x8);



y4=(x0x1)⊕(x1x3)⊕x4⊕(x0x5)⊕(x3x6)⊕(x0x7)⊕(x6x7)⊕(x1x0)⊕(x2x3)⊕(x3x0);



y5=x2⊕(x1x4)⊕(x4x5)⊕(x0x6)⊕(x0x6)⊕(x3x7)⊕(x4x7)⊕(x6x7)⊕(x5x3)⊕(x6x8)⊕(x7x8)⊕‘1’;



y6=x0⊕(x2x3)⊕(x1x5)⊕(x2x5)⊕(x4x5)⊕(x3x6)⊕(x4x6)⊕(x5x6)⊕x7⊕(x1x8)⊕(x3x8)⊕(x5x8)⊕(x7x8);



y7=(x0x1)⊕(x0x2)⊕(x1x2)⊕x3⊕(x0x3)⊕(x2x3)⊕(x4x5)⊕(x2x6)⊕(x3x5)⊕(x2x7)⊕(x5x7)⊕(x8⊕‘1’;



y8=(x0x1)⊕x2⊕(x1x2)⊕(x3x4)⊕(x1x5)⊕(x2x5)⊕(x1x6)⊕(x4x6)⊕x7⊕(x2x8)⊕(x3x8);


and said second equation comprises


y0=(x1x3)⊕x4⊕(x0x1x1)⊕x5⊕(x2x5)⊕(x3x4x5)⊕x6⊕(x0x6)⊕(x1x6)⊕(x3x6)⊕(x2x4x6)⊕(x1x5x6)⊕(x4x5x6);



y1=(x0x1)⊕(x1x4)⊕(x2x4)⊕x5⊕(x1x2x5)⊕(x0x3x5)⊕x5⊕(x0x2x3)⊕(x3x6)⊕(x4x5x6)‘1’;



y2=x0⊕(x0x3)⊕(x2x3)⊕(x1x2x4)⊕(x0x3x4)⊕(x1x5)⊕(x0x2x5)⊕(x0x5)⊕(x0x1x6)⊕(x2x6)⊕(x1x6)⊕‘1’;



y3=x1⊕(x0x1x2)⊕(x1x4)⊕(x3x4)⊕(x0x5)⊕(x0x1x5)⊕(x2x3x5)⊕(x1x4x5)⊕(x2x6)⊕(x1x3x6);



y4=(x0x2)⊕x3⊕(x1x3)⊕(x1x4)⊕(x0x1x4)⊕(x2x3x4)⊕(x0x5)⊕(x1x3x5)⊕(x0x4x5)⊕(x1x6)⊕(x3x6)⊕(x0x3x6)⊕(x5x6)⊕‘1’;



y5=x2⊕(x0x2)⊕(x0x3)⊕(x1x2x3)⊕(x0x1x4)⊕(x0x5)⊕(x2x5)⊕(x4x5)⊕(x1x6)⊕(x1x2x6)⊕(x0x3x6)⊕(x3x4x6)⊕(x2x5x6)⊕‘1’;



y6=(x1x2)⊕(x1x1x3)⊕(x0x4)⊕(x1x5)⊕(x3x5)⊕x6⊕(x0x1x6)⊕(x2x3x6)⊕(x1x4x5)⊕(x0x5x6);

8. An encryption apparatus for dividing a first plaintext bit stream of length 2n into first and second sub-bit streams of length n, dividing a second plaintext bit stream of length 2n into third and fourth sub-bit streams of length n, and generating a ciphertext bit stream of length 2n from the first, second, third and fourth sub-bit streams using 2-rounds of encryption, the apparatus comprising:
a first ciphering unit for receiving the first and second sub-bit streams, and generating first and second ciphertext bit streams of length n by encrypting the first and second sub-bit streams with predetermined first encryption codes KO1,1, KO1,2, KO1,3, KI1,1, KI1,2, and KI1,3 an odd number of times, wherein the first ciphertext bit stream is encrypted a first number of times and the second ciphertext bit stream is encrypted a second number of times, the second number of times exceeding the first number of times;
an operating unit for generating a first operated ciphertext bit stream by performing a logical exclusive-OR-operation on the first ciphertext bit stream and the third sub-bit stream and generating a second operated ciphertext bit stream by performing a logical exclusive-OR-operation on the second ciphertext bit stream with the fourth sub-bit stream; and
a second ciphering unit for receiving the first operated ciphertext bit stream and the second operated ciphertext bit stream comprising the predetermined time delay, generating third and fourth ciphertext bit streams of length n by encrypting the first operated ciphertext bit stream and the second operated ciphertext bit stream with predetermined second encryption codes KO2,1, KO2,2, KO2,3, KI2,1, KI2,2, and KI2,3 an odd number of times, and concurrently outputting the third and fourth ciphertext bit streams, wherein the third ciphertext bit stream is encrypted a third number of times and the fourth ciphertext bit stream is encrypted the fourth number of times, the fourth number of times exceeding the third number of times,
wherein the last encryption for the second ciphertext bit stream in the first-round of encryption is performed at the same time of performing the first encryption for the first operated ciphertext bit stream in the second-round of encryption.

9. The encryption apparatus of claim 8, wherein the first ciphering unit comprises:
a first block comprising a first exclusive-OR operator for performing a logical exclusive-OR operation on the first sub-bit stream and the first encryption code KO1,1, a first sub-cipher for encrypting the exclusive-OR-operated bit stream with the first encryption code KI1,1, and a second exclusive-OR operator for generating a first signal by performing a logical exclusive-OR operation on the encrypted signal with the second sub-bit stream being delayed to provide time for the encryption;
a second block comprising a third exclusive-OR operator for performing a logical exclusive-OR operation on the second sub-bit stream and the first encryption code KO1,2, a second sub-cipher for encrypting the exclusive-OR-operated bit stream with the first encryption code KI1,2, and a fourth exclusive-OR operator for generating the first operated ciphertext bit stream by performing a logical exclusive-OR operation on the encrypted signal and the first signal; and
a third block comprising a fifth exclusive-OR operator for performing a logical exclusive-OR operation on the first signal and the first encryption code KO1,3, a third sub-cipher for encrypting the exclusive-OR-operated bit stream with the first encryption code KI1,3, and a sixth exclusive-OR operator for generating the second operated ciphertext bit stream by performing a logical exclusive-OR-operation on the encrypted signal and the first sub-bit stream delayed by time required for the encryption.

10. The encryption apparatus of claim 8, wherein the second ciphering unit comprises:
a fourth block comprising a seventh exclusive-OR operator for exclusive-OR-operating the first operated ciphertext bit stream with the second encryption code KO2,1, a fourth sub-cipher for encrypting the exclusive-OR-operated bit stream with the second encryption code KI2,1, and an eighth exclusive-OR operator for generating a second signal by performing a logical exclusive-OR-operation on the encrypted signal and the second operated ciphertext bit stream;
a fifth block comprising a ninth exclusive-OR operator for exclusive-OR-operating the second operated ciphertext bit stream with the second encryption code KO2,2, a fifth sub-cipher for encrypting the exclusive-OR-operated bit stream with the second encryption code KI2,2, and a tenth exclusive-OR operator for generating the third ciphertext bit stream by performing a logical exclusive-OR-operation on the encrypted signal and the second signal delayed by time required for the encryption; and
a sixth block comprising an eleventh exclusive-OR operator for performing a logical exclusive-OR operation on the second signal with the second encryption code KO2,3, a sixth sub-cipher for encrypting the exclusive-OR-operated bit stream with the second encryption code KI2,3, and a twelfth exclusive-OR operator for generating the fourth ciphertext bit stream by performing a logical exclusive-OR operation on the encrypted signal and the third ciphertext bit stream.

11. The encryption apparatus of claim 10, wherein each of the first to sixth sub-ciphers includes first and second sub-ciphering units, and a register for storing the outputs of the first and second sub-ciphering units and simultaneously retrieving the outputs according to an external clock signal.

12. The encryption apparatus of claim 11, wherein each of the first and second sub-ciphering units divides a 16-bit input bit stream into a 9-bit stream and a 7-bit stream, and generates a 9-bit ciphertext bit stream from the 9-bit stream using a third equation, and a 7-bit ciphertext bit stream from the 7-bit stream using a fourth equation, said third equation comprising


y0=(x0x2)⊕x3⊕(x2x5)⊕(x5x6)⊕(x0x7)⊕(x1x7)⊕(x2x7)⊕(x4x8)⊕(x5x3)⊕(x7x8)⊕‘1’;



y1=x1⊕(x0x1)⊕(x2x3)⊕(x0x4)⊕(x1x4)⊕(x0x5)⊕(x3x5)⊕x6⊕(x1x7)⊕(x2x7)⊕(x5x8)⊕‘1’;



y2=x1⊕(x0x3)⊕(x3x4)⊕(x0x5)⊕(x2x6)⊕(x3x6)⊕(x5x6)⊕(x4x7)⊕(x5x7)⊕(x6x7)⊕x8⊕(x0x8)⊕‘1’;



y3=x0⊕(x1x2)⊕(x0x3)⊕(x2x4)⊕x5⊕(x0x6)⊕(x1x6)⊕(x4x7)⊕(x0x8)⊕(x1x8)⊕(x7x8);



y4=(x0x1)⊕(x1x3)⊕x4⊕(x0x5)⊕(x3x6)⊕(x0x7)⊕(x6x7)⊕(x1x8)⊕(x2x3)⊕(x3x8);



y5=x2⊕(x1x4)⊕(x4x5)⊕(x0x6)⊕(x1x6)⊕(x3x7)⊕(x4x7)⊕(x6x7)⊕(x5x3)⊕(x6x8)⊕(x7x8)⊕‘1’;



y6=x0⊕(x2x3)⊕(x1x5)⊕(x2x5)⊕(x4x5)⊕(x3x6)⊕(x4x6)⊕(x5x6)⊕x7⊕(x1x8)⊕(x3x8)⊕(x5x8)⊕(x7x8);



y7=(x0x1)⊕(x0x2)⊕(x1x2)⊕x3⊕(x0x3)⊕(x2x3)⊕(x4x5)⊕(x2x6)⊕(x3x5)⊕(x2x7)⊕(x5x7)⊕x8⊕‘1’;



y8=(x0x1)⊕x2⊕(x1x2)⊕(x3x4)⊕(x1x5)⊕(x2x5)⊕(x1x6)⊕(x4x6)⊕x7⊕(x2x8)⊕(x3x8);


and said fourth equation comprising


y0=(x1x3)⊕x4⊕(x0x1x4)⊕x5⊕(x2x5)⊕(x3x4x5)⊕x6⊕(x0x6)⊕(x1x6)⊕(x3x6)⊕(x2x4x6)⊕(x1x5x6)⊕(x4x5x6);



y1=(x0x1)⊕(x1x4)⊕(x2x4)⊕x5⊕(x1x2x5)⊕(x0x3x5)⊕x6⊕(x0x2x3)⊕(x3x6)⊕(x4x5x6)⊕‘1’;



y2=x0⊕(x0x3)⊕(x2x3)⊕(x1x2x4)⊕(x0x3x4)⊕(x1x5)⊕(x0x2x5)⊕(x0x5)⊕(x0x4x6)⊕(x2x6)⊕(x4x6)⊕‘1’;



y3=x1⊕(x0x1x2)⊕(x1x4)⊕(x3x4)⊕(x0x5)⊕(x0x1x5)⊕(x2x3x5)⊕(x1x4x5)⊕(x2x6)⊕(x1x3x6);



y4=(x0x2)⊕x3⊕(x1x3)⊕(x1x4)⊕(x0x1x4)⊕(x2x3x4)⊕(x0x5)⊕(x1x3x5)⊕(x0x4x5)⊕(x1x6)⊕(x3x6)⊕(x0x3x6)⊕(x5x6)⊕‘1’;



y5=x2⊕(x0x2)⊕(x0x3)⊕(x1x2x3)⊕(x0x1x4)⊕(x0x5)⊕(x2x5)⊕(x4x5)⊕(x1x6)⊕(x1x2x6)⊕(x0x3x6)⊕(x3x4x6)⊕(x2x5x6)⊕‘1’;



y6=(x1x2)⊕(x1x1x3)⊕(x0x4)⊕(x1x5)⊕(x3x5)⊕x6⊕(x0x1x6)⊕(x2x3x6)⊕(x1x4x5)⊕(x0x5x6);