Abstract:
An operation method and apparatus over Galois Field GF(2 m ) using a subfield GF(2 m/2 ). The operation apparatus includes a conversion circuit for converting the elements represented by a basis of GF(2 m ) into the elements represented by a basis of GF(2 m/2 ); an operation circuit for performing an operation over GF(2 m/2 ) with respect to the elements represented by the basis of GF(2 m/2 ); and a reversion circuit for reverting the operated elements represented by the basis of GF(2 m/2 ) to the elements represented by the basis of GF(2 m ), thereby performing high speed operation and simplifying circuit construction.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an operation method and apparatus over Galois Field GF(2 m ), and more particularly relates to an operational method and apparatus over GF(2 m ) using a subfield GF(2 m/2 ). 
     A finite field operation has recently been adapted to an error correcting code theory, a switching theory and a coding theory field etc., according to the improvement of digital signal processing. Specifically, in the case of implementing multipliers and dividers which are basic among the finite field, operation requires high speed operation and reduction in the complexity of a total system using the finite field operation by simplifying computation. 
     Generally, the representation method of respective elements over the finite field GF(2 m ) which have 2 m  elements can be classified as a vector representation or an exponential representation. 
     In exponential representation operations, it&#39;s comparatively easy to multiply and divide but difficult to add. Accordingly, in implementing of hardware by exponential representation, the multiplier and the divider are comparatively simple but the adder is complex. And at the same time, in vector representation operations, it&#39;s easy to add but difficult to multiply and divide. Accordingly, in implementing of hardware by vector representation, an adder is simple while a multiplier and a divider are very complex. 
     For example, when implementing the multiplier of the finite field GF(2 8 ), 64 AND gates and 73 XOR gates are needed, and an inversion circuit for the divider requires 304 AND gates and 494 OR gates. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide an operational method over GF(2 m ) using a subfield GF(2 m/2 ) which can perform a high speed operation. 
     It is an another object of the present invention to provide an operational apparatus over GF(2 m ) using a subfield GF(2 m/2 ) which can simplify a circuit construction. 
     To achieve the above objects, the operation method over GF(2 m ) using a subfield GF(2 m/2 ) according to the present invention comprises steps of: 
     converting elements represented by a basis of GF(2 m ) into elements represented by a basis of subfield GF(2 m/2 ); 
     performing an operation over subfield GF(2 m/2 ) with respect to the elements represented by the basis of subfield GF(2 m/2 ); and 
     reverting the operated elements represented by the basis of subfield GF(2 m/2 ) to the elements represented by the basis of GF(2 m ). 
     To achieve another object, the operation apparatus over GF(2 m ) using a subfield GF(2 m/2 ) according to the present invention comprises: 
     a conversion means for converting the elements represented by a basis of GF(2 m ) into the elements represented by a basis of subfield GF(2 m/2 ); 
     an operation means for performing an operation over subfield GF(2 m/2 ) with respect to the elements represented by the basis of subfield GF(2 m/2 ); and 
     a reversion means for reverting the operated elements represented by the basis of subfield GF(2 m/2 ) to the elements represented by the basis of GF(2 m ). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a logic circuit diagram of a conversion circuit for converting to the elements over GF(2 4 ) according to the present invention. 
     FIG. 2 is a logic circuit diagram of a reversion circuit for reverting to the elements over GF(2 8 ) according to the present invention. 
     FIG. 3 is a block diagram of a multiplier using a subfield GF(2 4 ) according to the present invention. 
     FIG. 4 is a block diagram of an inversion circuit using a subfield GF(2 4 ) according to the present invention. 
     FIG. 5 is a block diagram of a divider using a subfield GF(2 4 ) according to the present invention. 
     FIG. 6 is a logic circuit diagram of a multiplier over GF(2 4 ) according to the present invention. 
     FIG. 7 is a logic circuit diagram of an inversion circuit over GF(2 4 ) according to the present invention. 
     FIG. 8 is a logic circuit diagram of a square and γ multiplier over GF(2 4 ) according to the present invention. 
     FIG. 9 is a logic circuit diagram of a γ multiplier over GF(2 4 ) according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The operational method and circuit over GF(2 8 ) using a subfield GF(2 4 ) as an embodiment of the present invention are as follows. 
     Suppose that α 4  is an arbitrary element over GF(2 8 ). It is represented as α 4  =a+bβ, where a,bε GF(2 4 ) and βε GF(2 8 ). If so, suppose that an arbitrary element over GF(2 8 ) is ##EQU1## where λ i is a basis. 
     It can also be represented as ##EQU2## where γε GF(2 8 ) and γε GF(2 4 ) 
     Here, {Zi}={0,1} 
     {λi}={1, γ, γ 2 , γ 3 , β,βγ, βγ 2 , βγ 3  }{λi} is mutually linear independent. 
     In the present invention, a subfield GF(2 4 ) of GF(2 8 ) is used and a basis over a subfield GF(2 4 ) of GF(2 8 ) is defined as {1, β}, where βε GF(2 8 ). Suppose that a root of a primitive polynomial P(χ) of GF(2 8 ) is α, where P(χ)=χ 8  +χ 4  +χ 3  +χ 2  +1. The primitive polymonial is represented as α 8  +α 4  +α 3  +α 2  +1=0. Suppose also that a root of a primitive polynomial P(χ) of GF(2 4 ) is γ, where P(χ)=χ 4  +χ 3  +1. This primitive polynomial is represented as γ 4  +γ 3  +1=0. And the element γ of GF(2 8 ) to satisfy γ 4  +γ 3  +1=0 is α 11  9 and the element β of GF(2 8 ) to satisfy β 2  =α 11  9 +1. β is α 7 . 
     According to the above, the basis over GF(2 4 ) of GF(2 8 ) is 
     {1, γ, γ 2 , γ 3 , β, βγ, βγ 2 , βγ 3  }={1, α 11  9, α 2  3 8, α 1  0 2, α 7 , α 12  6, α 2  4 5, α 1  0 9 } 
     And, an arbitrary element Z is represented by the above basis as follows. ##EQU3## 
     i) From the equation (1), a conversion from elements represented by the basis of GF(2 4 ) into elements represented by the basis of GF(2 8 ) is as follows. 
     
         b.sub.0 =z.sub.0 +z.sub.1 +z.sub.2 +z.sub.6 +z.sub.7 
    
     
         b.sub.1 =z.sub.1 +z.sub.2 +z.sub.5 
    
     
         b.sub.2 =z.sub.3 +z.sub.5 +z.sub.7 
    
     
         b.sub.3 =z.sub.2 +z.sub.6 +z.sub.7 
    
     
         b.sub.4 =z.sub.1 +z.sub.7 
    
     
         b.sub.5 =z.sub.5 +z.sub.6 +z.sub.7 
    
     
         b.sub.6 =z.sub.3 +z.sub.5 +z.sub.6 
    
     
         b.sub.7 =z.sub.1 +z.sub.4 +z.sub.6 +z.sub.7                (2) 
    
     FIG. 1 shows a logic circuit diagram for implementing the equation (2) by using thirteen XOR gates 10. 
     ii) From the equations (2), a conversion from elements represented by the basis of GF(2 8 ) into elements represented by the basis of GF(2 4 ) is as follows. 
     
         z.sub.0 =b.sub.0 +b.sub.1 +b.sub.5 
    
     
         z.sub.1 =b.sub.1 +b.sub.3 +b.sub.5 
    
     
         z.sub.2 =b.sub.2 +b.sub.3 +b.sub.6 
    
     
         z.sub.3 =b.sub.1 +b.sub.3 +b.sub.4 +b.sub.6 
    
     
         z.sub.4 =b.sub.1 +b.sub.2 +b.sub.3 +b.sub.5 +b.sub.6 +b.sub.7 
    
     
         z.sub.5 =b.sub.2 +b.sub.5 +b.sub.6 
    
     
         z.sub.6 =b.sub.1 +b.sub.2 +b.sub.3 +b.sub.4 +b.sub.5 +b.sub.6 
    
     
         z.sub.7 =b.sub.1 +b.sub.3 +b.sub.4 +b.sub.5                (3) 
    
     FIG. 2 shows a logic circuit diagram for implementing the equations (3) which uses thirteen XOR gates 10. 
     By using the conversion method and circuit and reversion method and circuit for respectively converting and reverting the elements represented by the basis of GF(2 8 ) into the elements represented by the basis of GF(2 4 ) from the equations (2) and (3), implementation of a multiplier, an inversion circuit, and a divider over GF(2 4 ) are as follows. 
     Suppose that two elements A and B are the elements converted by the equations (2). 
     iii) Suppose that element C is the product of the elements A and B. This can be represented as: ##EQU4## where a 0 , a 1 , b 0 , b 1 , c 0 , c 1  ε GF(2 4 ). 
     Here, c 0  and c 1  are represented as: 
     
         c.sub.0 =a.sub.0 b.sub.0 +a.sub.1 b.sub.1 γ 
    
     
         c.sub.1 =a.sub.0 b.sub.1 +a.sub.1 b.sub.0 +a.sub.1 b.sub.1 (4) 
    
     FIG. 3 shows a block diagram for implementing the equations (4) by using three multiplies over GF(2 4 ) 50, four adders over GF(2 4 ) 40, and a γ multiplier over GF(2 4 ) 60. 
     iv) Assuming that the inverse of Z is Z -1 , and 
     
         Z=x.sub.0 +x.sub.1 β 
    
     where x 0 ,x 1  ε GF(2 4 ), and 
     
         Z.sup.-1 =y.sub.0 +y.sub.1 β 
    
     where y 0 ,y 1  ε GF(2 4 ), then, ##EQU5## where β 2  is represented by f 0  +f 1  β, and f 0 ,f 1  ε GF(2 4 ). In equation (5), if β 2  =γ+β is taken to make f1 equal to 1, then following equation is derived from equation (5). 
     
         Z.Z.sup.-1 =(x.sub.0 y.sub.0 +x.sub.1 γy.sub.1)+(x.sub.1 y.sub.0 +(x.sub.0 +x.sub.1)y.sub.1)β=1 
    
     and, ##EQU6## 
     From the equation (6), the inverting results y 0  and y 1  are represented as: ##EQU7## 
     FIG. 4 shows a block diagram for implementing the equation (7) by using three multipliers over GF(2 4 ) 50, an adder over GF(2 4 ) 40, a square and γ multiplier over GF(2 4 ) 70, and an inversion circuit over GF(2 4 ) 80. 
     v) Suppose that the quotient of elements A and B is D and let ##EQU8## where a 0 ,a 1 ,b 0 ,b 1 ,d 0 ,d 1  ε GF(2 4 ). 
     Then, the quotient of d 0  and d 1  is represented as: ##EQU9## 
     FIG. 5 shows a block diagram for implementing the equations (8) by using six multipliers over GF(2 4 ) 50, five adders over GF(2 4 ) 40, an inversion circuit over GF(2 4 ) 80, a γ multiplier 60, and a square and γ multiplier over GF(2 4 ) 70. 
     Steps for the implementation of a multiplier, an inversion circuit, a square and γ multiplier, and γ multiplier over GF(2 4 ) which constitute the apparatuses shown in FIGS. 3, 4, 5 are as follows. 
     i) Suppose that the product of elements X and Y is Z and let ##EQU10## 
     FIG. 6 shows a logic circuit diagram for implementing the equations (9) by using sixteen AND gates 20 and fifteen XOR gates 10. 
     ii) Suppose that the inverse of element A is I, the inversion table is as follows. 
     
         ______________________________________ii) Suppose that the inverse of element A is I,the inversion table is as follows.a.sub.0   a.sub.1 a.sub.2                   a.sub.3                         I.sub.0                             I.sub.1                                   I.sub.2                                       I.sub.3______________________________________1       1     0       0   0     1   0     0   0γ 0     1       0   0     0   0     1   1γ.sup.2   0     0       1   0     0   1     1   0γ.sup.3   0     0       0   1     1   1     0   0γ.sup.4   1     0       0   1     1   0     1   1γ.sup.5   1     1       0   1     0   1     0   1γ.sup.6   1     1       1   1     1   0     1   0γ.sup.7   1     1       1   0     0   1     1   1γ.sup.8   0     1       1   1     1   1     1   0γ.sup.9   1     0       1   0     1   1     1   1γ.sup.10   0     1       0   1     1   1     0   1γ.sup.11   1     0       1   1     1   0     0   1γ.sup.12   1     1       0   0     0   0     0   1γ.sup.13   0     1       1   0     0   0     1   0γ.sup.14   0     0       1   1     0   1     0   0______________________________________ 
    
     The above table is simplified by a Karnaugh&#39;s map as follows: 
     
         I.sub.0 =a.sub.0 a.sub.1 +a.sub.0 a.sub.2 a.sub.3 +a.sub.1 a.sub.2 a.sub.3 
    
     
         I.sub.1 =a.sub.0 a.sub.3 +a.sub.1 a.sub.2 a.sub.3 +a.sub.0 a.sub.1 a.sub.2 +a.sub.0 a.sub.2 a.sub.3 
    
     
         I.sub.2 =a.sub.1 a.sub.2 +a.sub.2 a.sub.3 +a.sub.0 a.sub.1 a.sub.3 +a.sub.0 a.sub.1 a.sub.2 a.sub.3 
    
     
         I.sub.3 =a.sub.1 a.sub.2 +a.sub.0 a.sub.1 a.sub.3 +a.sub.0 a.sub.2 a.sub.3( 10) 
    
     FIG. 7 shows a logic circuit diagram for implementing the equation (10) by using sixteen AND gates 20, ten OR gates 30, and four inverters which are unshown and corresponding to a 0  through a 3 . 
     iii) To implement the square and γ multiplier, suppose that element A is a 0  +a 1  γ+a 2  γ 2  +a 3  γ 3 , where γ 4  =γ 3  +1. Then, γA 2  is represented by (a 2  +a 3 )+(a 0  +a 2  +a 3 )γ+a 3  γ 2  +(a 1  +a 2 ). FIG. 8 shows a logic circuit diagram for implementing the equation by using three XOR gates 10. 
     iv) To implement the γ multiplier, suppose that element A is a 0  +a 1  γ+a 2  γ 2  +a 3  γ 3 , where γ 4  =γ 3  +1. Then, γ A is represented by a 3  +a 0  γ+a 1  γ 2  +(a 2  +a 3 )γ 3 . 
     FIG. 9 shows a logic circuit diagram for implementing the equation by using one XOR gate 10. 
     Here, the total numbers of gates used in the multiplier, the divider, and the inversion circuit will be calculated. The multiplier, the divider, and the inversion circuit over GF(2 8 ) using the subfield GF(2 4 ) must have the circuits of FIGS. 1 and 2 basically. 
     In FIG. 3, the total number of gates breaks down as follows. 
     
         ______________________________________       AND gate XOR gate______________________________________GF(2.sup.8)              13 →GF(2.sup.4)three multi-  16 × 3 = 48                    16 × 3 = 45pliersγ multiplier        1four adders               4 × 4 = 16GF(2.sup.4)              13 →GF(2.sup.8)         48         88______________________________________ 
    
     In FIG. 4, the total number of gates breaks down as follows. 
     
         ______________________________________                      OR  AND gate XOR gate   gate    NOT gate______________________________________GF(2.sup.8)         13 →GF(2.sup.4)three multi-    16 × 3 = 48               15 × 3 = 45pliersinversion                      10    4circuita square and         3γ multipliertwo adders          4 × 2 = 8GF(2.sup.4)         13 →GF(2.sup.8)    64         82         10    4______________________________________ 
    
     In FIG. 5, the total number of gates breaks down as follows. 
     
         ______________________________________                      OR  AND gate XOR gate   gate    NOT gate______________________________________GF(2.sup.8)         13 →GF(2.sup.4)six multi-    16 × 6 = 96               15 × 6 = 90pliersfive adders          4 × 5 = 20γ multiplier   1inversion    16                    10    4circuita square and         3γ multiplierGF(2.sup.4)         13 →GF(2.sup.8)    112        140        10    4______________________________________ 
    
     Accordingly, with the implementation of the multiplier, the divider, and the inversion circuit over GF(2 8 ) by using the subfield GF(2 4 ), the present invention simplifies the circuit and performs high speed operation by decreasing the number of logic gates. The circuit of the present invention can be applied to the multiplier, the divider, and the inversion circuit, over GF(2 m ) where m is more than 8 or less. Also, the conversion and reversion process of the elements over GF(2 m ) can be more than once. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is for illustration and example only and is not to be taken as a limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.