static int ecdsa_do_verify(const unsigned char *dgst, int dgst_len,
                           const ECDSA_SIG *sig, EC_KEY *eckey)
{
    int ret = -1, i;
    BN_CTX *ctx;
    BIGNUM *order, *u1, *u2, *m, *X;
    EC_POINT *point = NULL;
    const EC_GROUP *group;
    const EC_POINT *pub_key;

    /* check input values */
    if (eckey == NULL || (group = EC_KEY_get0_group(eckey)) == NULL ||
        (pub_key = EC_KEY_get0_public_key(eckey)) == NULL || sig == NULL) {
        ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ECDSA_R_MISSING_PARAMETERS);
        return -1;
    }

    ctx = BN_CTX_new();
    if (!ctx) {
        ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_MALLOC_FAILURE);
        return -1;
    }
    BN_CTX_start(ctx);
    order = BN_CTX_get(ctx);
    u1 = BN_CTX_get(ctx);
    u2 = BN_CTX_get(ctx);
    m = BN_CTX_get(ctx);
    X = BN_CTX_get(ctx);
    if (!X) {
        ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_BN_LIB);
        goto err;
    }

    if (!EC_GROUP_get_order(group, order, ctx)) {
        ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_EC_LIB);
        goto err;
    }

    if (BN_is_zero(sig->r) || BN_is_negative(sig->r) ||
        BN_ucmp(sig->r, order) >= 0 || BN_is_zero(sig->s) ||
        BN_is_negative(sig->s) || BN_ucmp(sig->s, order) >= 0) {
        ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ECDSA_R_BAD_SIGNATURE);
        ret = 0;                /* signature is invalid */
        goto err;
    }
    /* calculate tmp1 = inv(S) mod order */
    if (!BN_mod_inverse(u2, sig->s, order, ctx)) {
        ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_BN_LIB);
        goto err;
    }
    /* digest -> m */
    i = BN_num_bits(order);
    /*
     * Need to truncate digest if it is too long: first truncate whole bytes.
     */
    if (8 * dgst_len > i)
        dgst_len = (i + 7) / 8;
    if (!BN_bin2bn(dgst, dgst_len, m)) {
        ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_BN_LIB);
        goto err;
    }
    /* If still too long truncate remaining bits with a shift */
    if ((8 * dgst_len > i) && !BN_rshift(m, m, 8 - (i & 0x7))) {
        ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_BN_LIB);
        goto err;
    }
    /* u1 = m * tmp mod order */
    if (!BN_mod_mul(u1, m, u2, order, ctx)) {
        ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_BN_LIB);
        goto err;
    }
    /* u2 = r * w mod q */
    if (!BN_mod_mul(u2, sig->r, u2, order, ctx)) {
        ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_BN_LIB);
        goto err;
    }

    if ((point = EC_POINT_new(group)) == NULL) {
        ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_MALLOC_FAILURE);
        goto err;
    }
    if (!EC_POINT_mul(group, point, u1, pub_key, u2, ctx)) {
        ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_EC_LIB);
        goto err;
    }
    if (EC_METHOD_get_field_type(EC_GROUP_method_of(group)) ==
        NID_X9_62_prime_field) {
        if (!EC_POINT_get_affine_coordinates_GFp(group, point, X, NULL, ctx)) {
            ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_EC_LIB);
            goto err;
        }
    }
#ifndef OPENSSL_NO_EC2M
    else {                      /* NID_X9_62_characteristic_two_field */

        if (!EC_POINT_get_affine_coordinates_GF2m(group, point, X, NULL, ctx)) {
            ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_EC_LIB);
            goto err;
        }
    }
#endif
    if (!BN_nnmod(u1, X, order, ctx)) {
        ECDSAerr(ECDSA_F_ECDSA_DO_VERIFY, ERR_R_BN_LIB);
        goto err;
    }
    /*  if the signature is correct u1 is equal to sig->r */
    ret = (BN_ucmp(u1, sig->r) == 0);
 err:
    BN_CTX_end(ctx);
    BN_CTX_free(ctx);
    if (point)
        EC_POINT_free(point);
    return ret;
}