1 Reply by eyrmin

(6 replies, posted in wolfCrypt)

Hi,

I'm working on reading the ApplePay VAS loyalty passes. I have got the code working fine with OpenSSL but we can't build it for our  payment terminal which is why we tried to achieve the same with WolfSSL as it provides the OpenSSL compatibility layer.

The first step of the algorithm says:
"Set the randomly generated ECDH public key as the X9.62 compressed coordinate of a point over GFp for group P-256 , assuming 0 as the y-bit."

This randomly generated ECDH public key comes from the phone.


Using OpenSSL code, we achieve this with the following code (the only missing piece when using WolfSSL is EC_POINT_set_compressed_coordinates_GFp ):

int message_public_key(size_t message_len, const uint8_t* message, EVP_PKEY *public_key)
{
    int rc;
    if (message_len < ECIES_PUBLIC_KEY_SIZE) {
        rc = 0;
    }
    else {
        BIGNUM *bn = NULL;
        EC_GROUP *group = NULL;
        EC_POINT *ecp = NULL;
        EC_KEY *ephemeral_key = NULL;

        do {
            bn = BN_new();
            BN_bin2bn(message, ECIES_PUBLIC_KEY_SIZE, bn);
            group = EC_GROUP_new_by_curve_name(NID_X9_62_prime256v1);
            ecp = EC_POINT_new(group);
            RC_CHECK(rc = EC_POINT_set_compressed_coordinates_GFp(group, ecp, bn, 0, NULL));
            ephemeral_key = EC_KEY_new_by_curve_name(NID_X9_62_prime256v1);
            RC_CHECK(rc = EC_KEY_set_public_key(ephemeral_key, ecp));
            RC_CHECK(rc = EVP_PKEY_set1_EC_KEY(public_key, ephemeral_key));
        } while (0);

        if (bn) BN_free(bn);
        if (group) EC_GROUP_free(group);
        if (ecp) EC_POINT_free(ecp);
        if (ephemeral_key) EC_KEY_free(ephemeral_key);
    }
    return rc;
}

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2 Topic by eyrmin

(6 replies, posted in wolfCrypt)

Hi,

Is there a support for compressed coordinates for EC points? I found in WolfSSL a function EC_POINT_set_affine_coordinates_GFp(), but there is no EC_POINT_set_compressed_coordinates_GFp() which can be found in OpenSSL.

Also, are there any APIs available to convert the point between compressed and affine form?

For decryption process I'm trying to implement, I get the randomly generated ECDH public key from a device and need to set is as the X9.62 compressed coordinate of a point over GFp for group P-256, assuming 0 as the y-bit.

Thanks

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