'Cannot find the requested object' exception while creating X509Certificate2 from string

Question

I am trying to create X509Certificate2 from string. Let me show an example:

string keyBase64String = Convert.ToBase64String(file.PKCS7);
var cert = new X509Certificate2(Convert.FromBase64String(keyBase64String));

and keyBase64String has a such content: "MIIF0QYJKoZI ........hvcNAQcCoIIFwjCCBb4CA0="

and file.PKCS7 is byte array which I downloaded from database.

I've got the following exception when creating X509Certificate2:

Cannot find the requested object

And the stack trace:

"Cannot find requested object" X509Certificate2 Exception "Cannot find requested object"} at System.Security.Cryptography.CryptographicException.ThrowCryptographicException(Int32 hr) at System.Security.Cryptography.X509Certificates.X509Utils._QueryCertBlobType(Byte[] rawData) at System.Security.Cryptography.X509Certificates.X509Certificate.LoadCertificateFromBlob(Byte[] rawData, Object password, X509KeyStorageFlags keyStorageFlags) at System.Security.Cryptography.X509Certificates.X509Certificate2..ctor(Byte[] rawData) at WebApp.SoupController.d__7.MoveNext() in D:\Projects\WebApp\Controllers\SoupController.cs:line 118

Please, say me what I am doing wrong. Any help would be greatly appreciated!

Answer 1

If file.PKCS7 represents a PKCS#7 SignedData blob (what gets produced from X509Certificate2.Export(X509ContentType.Pkcs7) or X509Certificate2Collection.Export(X509ContentType.Pkcs7)) then there are two different ways of opening it:

• new X509Certificate2(byte[])/new X509Certificate2(string)
  • The single certificate constructor will extract the signing certificate of the SignedData blob. If this was just being exported as a collection of certs, but not signing anything, there is no such certificate, and so it fails with Cannot find the original signer. (Win 2012r2, other versions could map it to a different string)
• X509Certificate2Collection::Import(byte[])/X509Certificate2Collection::Import(string)
  • The collection import will consume all of the "extra" certificates, ignoring the signing certificate.

So if it's really PKCS#7 you likely want the collection Import (instance) method. If it isn't, you have some odd variable/field/property names.

Answer 2

The constructor of of X509Certificate2 expects to get a the certificate file name, but you are giving it a key (X509Certificate2 Constructor (String))

I assume that keyBase64String is the certificate key, and that the certificate is installed on the machine that executes the code. Try this:

var store = new X509Store(StoreName.My, StoreLocation.LocalMachine);
store.Open(OpenFlags.ReadOnly);
var certCollection = store.Certificates.Find(X509FindType.FindByThumbprint, keyBase64String , false);
var cert = certCollection[0];

You can also try FindByKeyUsage, FindBySubjectKeyIdentifier, or other types of X509FindType Enumeration