marsze marsze - 1 year ago 615
C# Question

Read Private Key from PFX-file

I know, there are many posts about this, but still I cannot find a solution to get this to work. I have generated a PFX-file with openssl on my machine like this:

openssl x509 -req -days 365 -in "myReqest.csr" -signkey "myPrivateKey.pem" -out "myCertificate.crt"
openssl pkcs12 -export -out "myCertificate.pfx" -inkey "myPrivateKey.pem" -in "myCertificate.crt" -certfile "myCertificate.crt"

In my C# app, I access the private key like this:

var cert = new X509Certificate2("myCertificate.pfx", "myPassword");
cert.HasPrivateKey; // This is always true!
cert.PrivateKey; // Works on my machine (only)

This works perfectly fine (on my machine), but when I run the same code on another machine, it throws: "Key set not found", even though
! Shouldn't the private key be included in the *.pfx-file? Can you tell me:

  1. Was the certificate/private key somehow automatically installed on my machine by openssl when I created it?

  2. How can I read the private key from the *.PFX-file (or alternatively from the *.PEM-file)?

StackTrace of Exception:

at System.Security.Cryptography.Utils.CreateProvHandle(CspParameters parameters, Boolean randomKeyContaier)
at System.Security.Cryptography.Utils.GetKeyPairHelper(CspAlgorithmType keyType, CspParameters parameters, Boolean randomKeyContaier, Int32 dwKeySize, SafeProvHandle& safeProvHandle, SafeKeyHandle& safeKeyHandle)
at System.Security.Cryptography.RSACryptoServiceProvider.GetKeyPair()
at System.Security.Cryptography.RSACryptoServiceProvider..ctor(Int32 dwKeySize, CspParameters parameters, Boolean useDefaultKeySize)
at System.Security.Cryptography.RSACryptoServiceProvider..ctor(CspParameters parameter)
at System.Security.Cryptography.X509Certificates.X509Certificate2.get_PrivateKey()


I've found out, that the following does work:

// on my machine
// read certificate from file (exportable!)
X509Certificate2 cert = new X509Certificate2("filename.pfx", "password", X509KeyStorageFlags.Exportable)
// sign data etc.
// export private key to XML-file
File.WriteAllText("filename.xml", cert.PrivateKey.ToXmlString(true));

// on the other machine
// create new RSA object
RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
// import private key from xml
// verify data etc.

However, to me, this is only a workaround, I would like to do it an a more conventional/standard compliant way!

Answer Source

It seems, there is no straight-forward way to do this in .NET. Therefore I've decided now to load the certificate directly from the certificate store:

X509Store store = new X509Store(StoreName.My, StoreLocation.CurrentUser);
store.Open(OpenFlags.OpenExistingOnly | OpenFlags.ReadOnly);
X509Certificate2Collection certificates = store.Certificates.Find(X509FindType.FindByThumbprint, CERTIFICATE_THUMB_PRINT, false);
if (certificates.Count == 0)
    // "Certificate not installed."
    certificate = certificates[0];

For that, of course, if has to be installed on the machine.

I think this is a nice solution to this problem, because it adds an additional layer of security to it (the app must be run on the machine, on which the certificate is installed and as the user who installed it, also the file itself can be stored in a safe place somewhere else).

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