Is it possible to include the private key in a .CER certificate file?

Question

I have a use case for a .NET application that stores certificates in a database. One of the requirements is for the application to reject certificates that contain private keys. The user will upload a certificate file (specifically .CER or .CRT) and the application will import it as an X509Certificate2 object so that I can check the HasPrivakeKey property.

I know that .PFX files can contain private keys, but is it possible for .CER or .CRT files to also contain private keys? If so, how can I generate a test certificate in order to test the application logic?

Crypt32 · Accepted Answer · 2015-09-04T16:57:29.303

First, .NET do not support PEM format with private key. But if such format is presented the following outcome is defined:

1) if certificate header/footer is first in the file, .NET will ignore the rest content of the file (e.g. private key information) and creates valid X509Certificate2 object without private key (because PKCS#1 and PKCS#8 keys are not supported by CryptoAPI functions which are called by a X509Certificate2 constructor. Though, there are functions to work with PKCS#1).

2) if private key header/footer is first in the file, .NET will raise exception about invalid certificate.

p.s. this combination is possible only when Base64 encoding is used and each section uses header and footer (e.g. -----BEGIN CERTIFICATE----- and -----END CERTIFICATE-----). It is impossible to combine them in binary form without using PKCS#12 container.

update: if you want to test it yourself, here is an example of such PEM file:

when you instantiate an X509Certificate2 object from this file, the call will succeed. Swap sections and you will see exception about invalid format.

Not sure if it is documented. I'm just a man who is experienced in Windows cryptography. I know about this behavior because there is `CryptStringToBinary` function call to convert Base64 to binary form and this function behaves as I described: only first section with header/footer is returned, the rest is ignored. — Crypt32, Sep 04 '15 at 16:50

score 1 · Answer 2 · answered Sep 04 '15 at 13:18

An X509 certificate in PEM format is just a text file. It is not uncommon for people to append both the certificate and key to the same file, so you end up with something that looks like:

-----BEGIN CERTIFICATE-----
...certificate data...
-----END CERTIFICATE-----
-----BEGIN RSA PRIVATE KEY-----
...key data...
-----END RSA PRIVATE KEY-----

Software expecting to read a private key will ignore everything outside the BEGIN RSA PRIVATE KEY/END RSA PRIVATE KEY lines, and software expecting to read a public certificate will ignore everything outside the BEGIN CERTIFICATE/END CERTIFICATE lines.

The easiest way to test for a private key in this case is just to look for the BEGIN RSA PRIVATE KEY marker.

I don't believe it is possible to concatenate DER encoded certificates in this fashion.

If I have a .pfx file or a .cer + .pvk, can I use makecert or openssl to generate the PEM encode .cer file you describe above? I know I can get the RSA data using X509Certificate2.PrivateKey.ToXmlString() method and manually create the file, but I would prefer to have an automatic way. — Timothy Schoonover, Sep 04 '15 at 13:27
I don't know that there's an automated way to create a file like this. In my experience, people just manually concatenate things. — larsks, Sep 04 '15 at 13:39

Is it possible to include the private key in a .CER certificate file?

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