Question

I am looking for a way to extract the AuthorityKeyIdentifier extension from an X509Certificate2 instance. I did not see any built-in support for this but since windows can properly construct a certificate chain I know the functionality has to exist at some level. If the answer is to roll a DER parser, is there a good implementation that can be referenced?

Answer 1

Iterate through the extensions in the X509Certificate2.Extensions property and look for an extension with the OID 2.5.29.35 (as per http://www.alvestrand.no/objectid/2.5.29.35.html). That is the AuthorityKeyIdentifier extension.

[Edit: Added the following.]

Each member of the Extensions property is an ASN encoded. So you can do the following to get it in a human readable or machine parsable format:

using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;

...

X509Extension extension; // The OID 2.5.29.35 extension
AsnEncodedData asndata = new AsnEncodedData(extension.Oid, extension.RawData);
Console.WriteLine(asndata.Format(true));

For one of the Microsoft intermediate CA certificates, it the Format() method returns the following:

[1]Authority Info Access
     Access Method=Certification Authority Issuer (1.3.6.1.5.5.7.48.2)
     Alternative Name:
          URL=http://www.microsoft.com/pki/certs/MicrosoftRootCert.crt

It is certainly not easy to parse but you can look for a line starting with the regular expression \[\d+\]Authority Info Access then find a line beneath it with the regular expression URL=(.+) (the eight spaces are unclear in the formatting) and use the URL in the parenthesized group.