How to get the size of the entire X509 certificate

c, certificate, openssl, x509

Solution

For sending certificate over the network, I recommend using DER format. The reason is that PEM is Base64-encoded DER plus some additional text (prefix/suffix).

To estimate the size, you actually need to encode the certificate (this is DER):

size_t get_length(X509 *cer)
{
    int len;

    len = i2d_X509(cer, NULL);
    return len > 0 ? len : 0;
}

For PEM it is trickier:

unsigned char *data;
BIO *bio = BIO_new(BIO_s_mem());
PEM_write_bio_X509(bio, cer);
len = BIO_get_mem_data(bio, &data);
// here - data is a pointer to encoded data, len - length of data.

BIO_free(bio); // free _after_ you no longer need data

Problem

I am reading a .crt certificate which I generate using openSSL. I have the certificate in my C program in a X509 file. I would like to know the size of the whole certificate that I have just read. How can this be there? Is there a specific function that returns the size of the certificate? Thanks Best Regards

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