I am trying to write a program to perform point operations on a elliptic curve in a prime field I am using the standard formulaes for point additions and doubling in my code and these operations are performed by functions that are called but I am getting output for certain points but not all so please help me to fix the problem that are present in this code.
structure point_multiply(int x, int y, int k )
{
int xk;
int yk,m;
xk=x;
yk=y;
m=1;
int xL,yL,s,e;
e=findInverse((2*yk),211);
if((((3*(xk*xk))*e)% 211)>0)
{s = (((3*(xk*xk))*e)% 211);
}
else
s=(((3*(xk*xk))*e)% 211)+ 211;
if((((s*s)- (2*xk)) % 211)>0)
{xL=(((s*s)- (2*xk)) % 211);
}
else
xL=(((s*s)- (2*xk)) % 211) + 211;
if(((-yk+ s*(xk-xL)) % 211) > 0)
yL=(-yk+ s*(xk-xL)) % 211;
else
yL=(-yk+ s*(xk-xL)) % 211 + 211;
xk=xL;
yk=yL;
m=m+1;
while(k>m)
{
sn=point_addition(xk,yk,x,y);
xk=sn.a;
yk=sn.b;
m++;
}
s1.a=xk;
s1.b=yk;
return s1;
}
structure point_addition(int x1, int y1, int x2, int y2)
{
int s,xL,yL;
if((x1-x2)!=0)
{
if ( x1 == 0 && y1 == 0 )
{
xL = x2;
yL = y2;
s7.a=xL;
s7.b=yL;
return s7;
}
if ( x2 == 0 && y2 == 0 )
{
xL = x1;
yL = y1;
s7.a=xL;
s7.b=yL;
return s7;
}
if ( y1 == -y2 )
{
xL = yL = 0;
s7.a=xL;
s7.b=yL;
return s7;
}
l=findInverse((x1-x2),211);
if ((((y1-y2)*l) % 211)>=0)
s=((((y1-y2)*l) % 211));
else
s=(((y1-y2)*l) % 211) + 211;
if ((((s*s)-(x1+x2)) % 211)>0)
xL= (((s*s)-(x1+x2)) % 211) ;
else
xL= (((s*s)-(x1+x2)) % 211) + 211;
if(((-y1+s*(x1-xL)))>=0)
yL= ((-y1+s*(x1-xL)) % 211);
else
yL= ((-y1+s*(x1-xL)) % 211) + 211;
}
else
{
xL= 0 ;
yL= 0;
}
s7.a= xL;
s7.b= yL;
return s7 ;
}
int findInverse(int a, int b)
{
int x[3];
int y[3];
int quotient = a / b;
int remainder = a % b;
x[0] = 0;
y[0] = 1;
x[1] = 1;
y[1] = quotient * -1;
int i = 2;
for (; (b % (a%b)) != 0; i++)
{
a = b;
b = remainder;
quotient = a / b;
remainder = a % b;
x[i % 3] = (quotient * -1 * x[(i - 1) % 3]) + x[(i - 2) % 3];
y[i % 3] = (quotient * -1 * y[(i - 1) % 3]) + y[(i - 2) % 3];
}
//x[i — 1 % 3] is inverse of a
//y[i — 1 % 3] is inverse of b
if(x[(i - 1) % 3]<0)
return x[(i - 1) % 3]+211;
else
//x[i — 1 % 3] is inverse of a
//y[i — 1 % 3] is inverse of b
return x[(i - 1) % 3];
}
Edited and added main c code which uses these function to perform elliptic curve cryptography
int main()
{
int y,z=0,x=2,i[200],j[200],h=0,g,k;
while(x<200)
{
y=sqrt((x*x*x)-4);
z=modulo(y,211);
if(z!=0)
{
i[h]=x;
j[h]=z;
s[h].a=i[h];
s[h].b=j[h];
s[h+1].a=i[h];
s[h+1].b=(211 - j[h]);
printf("\nh=%d X= %d Y= %d \nh=%d X= %d Y= %d",h,s[h].a,s[h].b,h+1,s[h+1].a,s[h+1].b);
h=h+2;
}
x++;
}
printf("The total no of points we have on our elliptic curve for cryptography is %d",h-1);
x=5;
y=11;
printf("\n %d %d\n",x,y );
printf("\nEnter A number between 0 and the private key");
scanf("%d",&k);
s2=point_multiply(x,y,k);
printf("\n The public key is \n %d %d \n ",s2.a,s2.b );
printf("Enter a RANDOM number to generate the cipher texts");
scanf("\n%d",&g);
s3= point_multiply(x,y,g);
s4=point_multiply(s2.a,s2.b,g );
label:
printf("\n Enter a number to send");
scanf("%d",&h);
s6=point_addition(s4.a,s4.b,s[h].a,s[h].b);
printf("The points to be sent are X= %d Y=%d",s[h].a,s[h].b);
printf(" \n X= %d Y=%d\n X = %d Y= %d ",s3.a,s3.b,s6.a,s6.b);
//RECIEVER
s8=point_multiply(s3.a,s3.b,k);
s9=point_addition((s8.a) ,-((s8.b)%211),s6.a,s6.b);
printf(" The decrypted points are \n %d %d",s9.a,s9.b);
printf("\n If you have more no to send press 1 else press 0");
scanf("\n %d", &x1);
if(x1==1)
goto label;
else
return 0;
}
s1, s2, s3 etc are structures which hold a 2 integers which act as x and y co-ordinates I am getting output by entering k=3,g=4, h=5 and many other cases mostly with small numbers but not for larger numbers. What could be wrong with the code?
Further edit: I guess that normal square root method is not applicable to find square roots of a modular no?.. Please tell me how to find the modular square root of a no?