Store a private key and a certificate in C++/OpenSSL - c++

Question

I need to store certificates and their private key in memory.
Certificates can be in the 4 following formats : PEM, PKCS12, PKCS7, DER.
I'ill need to write them back as PEM later.
All the snippets i see are storing only the public certificate in a X509 struct.
What about the private part ??
I've found a way using X509_INFO, but i got a major problem with it :
I haven't find a way to get a X509_INFO from DER/PKCS7/PKCS12 files
For the moment i got the following code :
QList<X509_INFO*>* Certificat::stringPEMToX509_INFO(QString stringPem)
{
QList <X509_INFO*>* liste_certificats = new QList<X509_INFO*>;
STACK_OF(X509_INFO)* pile_certificats = NULL;
X509_INFO* certificat;
BIO* bio = BIO_new(BIO_s_mem());
const char* pem = stringPem.toAscii().constData();
BIO_puts(bio, pem);
//https://github.com/openssl/openssl/blob/master/crypto/pem/pem_info.c
pile_certificats = PEM_X509_INFO_read_bio(bio, NULL, NULL, NULL);
for (int i = 0; i < sk_X509_INFO_num(pile_certificats); i++)
{
certificat = sk_X509_INFO_value(pile_certificats, i);
liste_certificats->push_back(certificat);
}
sk_X509_INFO_pop_free(pile_certificats, X509_INFO_free);
BIO_free_all(bio);
return liste_certificats;
}
My goal would be to have the same function but for DER, PKCS12 and PKCS7.
I tried to get a X509_INFO from a DER like this :
p12 = d2i_PKCS12_bio(bio, NULL);
certificat = X509_INFO_new();
certificat->x509 = cert;
certificat->x_pkey = pkey;
But x_pkey is a X509_PKEY and pkey an EVP_PKEY...
If there is no way to store it as a single struct, would it be possible to store my certificates as X509 + a EVP_PKEY for the private key, and still output both private and public part in a PEM ?

Answer 1

PKCS7 is only meant for public keys. DER and PEM are simply ways of encoding a PKCS (and many other) objects. Since you want to store everything into a single structure, you would probably most benefit from PKCS12. OpenSSL provides functions to parse PKCS12 data and get both the cert and key out of it.