int main() {
int i, j;
int size = sizeof(a_l)/sizeof(int);
for (j = 0; j < size; j++)
{
a_l[j] = rand() % MAXRANDINT;
}
precomputation();
computation(3, size);
postcomputation();
return 0;
}
void mexp(int k, mpz_t b[k], mpz_t e[k], mpz_t *m, mpz_t *r) {
// Main multiexp alg from paper
int i, j, jnew, J, l, precompindex, offset,km;
unsigned det;
int msb_set;
unsigned bitlen, maxbitlen = 0;
// Setup and precomputation
bases = b;
modulus = m;
numberOfFactors = k;
// Compute window size
for (i = 0; i < k; i++) {
bitlen = mpz_sizeinbase(e[i], 2);
if (bitlen > maxbitlen) maxbitlen = bitlen;
}
// TODO: Make this more elegant
double lmb = log(maxbitlen)/log(2);
winSize = (int) ceil((lmb - 2*(log(lmb)/log(2))) / k);
if(winSize <= 0) winSize =1;
// Build up precomputation table
precomputation();
km = k -1;
mpz_set_ui(*r, 1);
// Find maximal bit length of all exponents
// Type conversion from unsigned to int. Only a problem for exponents
// that have more binary digits than max(int). This is unlikely to occur in practice.
// Maybe add check on arg to m_exp() to test for this possible problem?
j = maxbitlen - 1;
// A-while
while (j >= 0) {
// check if bit #maxbitlen is zero for all exponents
// TODO: in the first iteraration this is the case by def of maxbitlen
// -> can we skip first iteration to improve efficiency?
msb_set = 0;
for(i=0; i< k && msb_set == 0; i++) msb_set = mpz_tstbit(e[i], j);
// B-if
if (msb_set == 0) {
mpz_powm_ui(*r, *r, 2, *m);
j--;
}
// C-else
else {
if ( (j-winSize) > -1) jnew = j-winSize; else jnew = -1;
J = jnew + 1;
// D-while (is implemented as do-while)
for(;;) {
i = km;
msb_set = 0;
for(i=0; i< k && msb_set == 0; i++) msb_set = mpz_tstbit(e[i], j);
if (msb_set == 0)
J++;
else
break;
}
// E-for
precompindex = 0;
offset = winSize - (j - J) -1;
for (i = km; i >= 0; i--) {
// Compute access index into pre-computation table
precompindex <<= offset;
for (l = j; l >= J; l--) {
precompindex <<= 1;
precompindex = precompindex | mpz_tstbit(e[i], l);
}
}
// F-while
det = 1;
det <<= ((j-J)+1);
mpz_powm_ui(*r, *r,det, *m);
j = J-1;
mpz_mul(*r, *r, preCompTable[precompindex]);
mpz_mod(*r, *r, *m);
// G-while
if (j > jnew) {
det =1;
det <<= (j-jnew);
mpz_powm_ui(*r, *r,det, *m);
j = jnew + 1;
}
}
}
freeMemory();
}
