issue with X509_get_issuer_name returning different values under OpenSSL 3

[adv2011@xxxxxxxxxxxxxxx]

Hi all, I am starting to port some code to OpenSSL 3 (it's my first 
taste of it), and I'm stuck with a problem. I'm working under Ubuntu 22.

I saw that the function X509_issuer_and_serial_hash doesn't return the 
same value it did before (though not for an obvious reason), and since 
that value is used by my software to identify some certificates against 
a DB, I need to replicate the old behaviour.

To do so, I'm first trying to change the old function (from OpenSSL 1.1) 
so that it compiles under OpenSSL 3.

Here, a is of type X509, I always accessed most data from pointers. Now 
that they are gone, how do I read the following information to obtain 
exactly the same data?

- a->cert_info.issuer ...is it X509_get_issuer_name(a) exactly the same?

- a->cert_info.serialNumber.data ?

- a->cert_info.serialNumber.length ?

For completeness, my first, very raw code follows, where you can see how 
I'd use the values.

Thank you very much - Ubi


#if OPENSSL_VERSION_NUMBER >= 0x30000000L
#warning "I WILL HAVE MY LOCAL X509_issuer_and_serial_hash, UNDER OPENSSL 3"

unsigned long custom_X509_issuer_and_serial_hash(X509 *a)
{
    unsigned long ret = 0;
    EVP_MD_CTX *ctx = EVP_MD_CTX_new();
    unsigned char md[16];
    char *f = NULL;

    if (ctx == NULL)
        goto err;
        // cannot do this under OpenSSL 3 (code from v 1.1): f = 
X509_NAME_oneline(a->cert_info.issuer, NULL, 0);
        f = X509_NAME_oneline(X509_get_issuer_name(a), NULL, 0);
    if (f == NULL)
        goto err;
    if (!EVP_DigestInit_ex(ctx, EVP_md5(), NULL))
        goto err;
    if (!EVP_DigestUpdate(ctx, (unsigned char *)f, strlen(f)))
        goto err;
    if (!EVP_DigestUpdate
        // cannot do this under OpenSSL 3 (code from v 1.1): (ctx, 
(unsigned char *)a->cert_info.serialNumber.data,
        // ...but how do I get the data from here?
        (ctx, X509_get_serialNumber(a),
        // ...same problem here: how do I get the data length?
        (unsigned long)a->cert_info.serialNumber.length))
        goto err;
    if (!EVP_DigestFinal_ex(ctx, &(md[0]), NULL))
        goto err;
    ret = (((unsigned long)md[0]) | ((unsigned long)md[1] << 8L) |
           ((unsigned long)md[2] << 16L) | ((unsigned long)md[3] << 24L)
        ) & 0xffffffffL;
 err:
    OPENSSL_free(f);
    EVP_MD_CTX_free(ctx);
    return ret;
}

#endif