void SSS_shamir_to_additive()

in src/shamir.c [230:282]

• 34 lines of code
• 2 McCabe index (conditional complexity)

void SSS_shamir_to_additive(int k, const octet *X_j, const octet *Y_j, const octet *X, octet *S)
{
    BIG_256_56 x_j;
    BIG_256_56 q;

    BIG_256_56 w;
    DBIG_256_56 dw;

    BIG_256_56 n;
    BIG_256_56 d;

    BIG_256_56_rcopy(q, CURVE_Order_SECP256K1);

    BIG_256_56_fromBytesLen(x_j, X_j->val, X_j->len);

    // Initialize accumulators for numerator and denominator
    BIG_256_56_one(n);
    BIG_256_56_one(d);

    // x_j = -x_j mod q
    BIG_256_56_sub(x_j, q, x_j);

    for (int i = 0; i < k-1; i++)
    {
        // n = prod(x_i)
        BIG_256_56_fromBytesLen(w, X[i].val, X[i].len);
        BIG_256_56_mul(dw, n, w);
        BIG_256_56_dmod(n, dw, q);

        // d = prod(x_i - x_j)
        BIG_256_56_add(w, w, x_j);
        BIG_256_56_norm(w);
        BIG_256_56_mul(dw, d, w);
        BIG_256_56_dmod(d, dw, q);
    }

    // s = n/d * y
    BIG_256_56_invmodp(d, d, q);
    BIG_256_56_mul(dw, n, d);
    BIG_256_56_dmod(w, dw, q);

    BIG_256_56_fromBytesLen(x_j, Y_j->val, Y_j->len);
    BIG_256_56_mul(dw, w, x_j);
    BIG_256_56_dmod(w, dw, q);

    // Output additive share
    BIG_256_56_toBytes(S->val, w);
    S->len = SGS_SECP256K1;

    // Clean memory
    BIG_256_56_zero(w);
    BIG_256_56_dzero(dw);
}