int VSS_verify_shares()

in src/shamir.c [317:366]

• 38 lines of code
• 5 McCabe index (conditional complexity)

int VSS_verify_shares(int k, const octet *X_j, const octet * Y_j, const octet *C)
{
    int rc;

    ECP_SECP256K1 G;
    ECP_SECP256K1 V;

    BIG_256_56  x;
    BIG_256_56 xn;
    BIG_256_56 q;
    DBIG_256_56 w;

    BIG_256_56_rcopy(q, CURVE_Order_SECP256K1);
    BIG_256_56_fromBytesLen(x, X_j->val, X_j->len);

    // Initialize accumulator and exponent
    rc = ECP_SECP256K1_fromOctet(&V, C);
    if (rc != 1)
    {
        return VSS_INVALID_CHECKS;
    }
    BIG_256_56_one(xn);

    for (int i = 1; i < k; i++)
    {
        rc = ECP_SECP256K1_fromOctet(&G, C+i);
        if (rc != 1)
        {
            return VSS_INVALID_CHECKS;
        }

        BIG_256_56_mul(w, xn, x);
        BIG_256_56_dmod(xn, w, q);

        ECP_SECP256K1_mul(&G, xn);
        ECP_SECP256K1_add(&V, &G);
    }

    // Compute ground truth
    ECP_SECP256K1_generator(&G);
    BIG_256_56_fromBytesLen(x, Y_j->val, Y_j->len);
    ECP_SECP256K1_mul(&G, x);

    if (!ECP_SECP256K1_equals(&G, &V))
    {
        return VSS_INVALID_SHARES;
    }

    return VSS_OK;
}