RE: Verifying X509Certificate signature

Hi Peter,

As you said that you want some information about verifying X509 certificate
signature, I'd like to confirm what's the exact verification you want.

My understanding is that you may have a X509 certificate which assocate
with a publickey/private key pair( of RSA or DSA encypt provider) and
you've also used the private key to digital signed some data, and want to
verify the data, correct?

If this is the case, generally you will need to do two things here:

1. Retrieve the public/private key from the certificate (in certificate
store of windows)

2. use the key info associated with the certificate to do encryption or
digital sign.

Here are some web articles introducing some code on this:

and here is a simple test function I've written which include accessing
cert store to retrieve key info in cert and do some RSA signing and

private void btnTest_Click(object sender, EventArgs e)
RSACryptoServiceProvider rsa;
RSAParameters key;
SHA1CryptoServiceProvider sha = new SHA1CryptoServiceProvider();

bool result;

signSrc = Encoding.UTF8.GetBytes("ABCDEFG");

string tp = "2b6f8ac51a85cbaf429474a55304313968667611";
X509Store store = new X509Store(StoreName.My,

X509Certificate2 cert2 =
store.Certificates.Find(X509FindType.FindByThumbprint, tp, true)[0];


rsa = cert2.PrivateKey as RSACryptoServiceProvider;

signDes = rsa.SignData(signSrc, sha);

result = rsa.VerifyData(signSrc, sha, signDes);

MessageBox.Show("valid: " + result);

/** here try exporting the CERT to a exportable file */

byte[] pfx_bytes =

//txtContent.Text = Convert.ToBase64String(pfx_bytes);

X509Certificate2 filecert = new X509Certificate2();
filecert.Import(pfx_bytes, "Password01!",

RSACryptoServiceProvider rsa1 = filecert.PrivateKey as

result = rsa1.VerifyData(signSrc, sha, signDes);

MessageBox.Show("valid: " + result);

string expkey = Convert.ToBase64String(rsa.ExportCspBlob(true));

txtExportedKey.Text = expkey;


Hope this helps some.


Steven Cheng

Microsoft MSDN Online Support Lead

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From: =?Utf-8?B?UGV0ZXIgUml0Y2hpZSBbQyMgTVZQXQ==?=
Subject: Verifying X509Certificate signature
Date: Wed, 30 Jul 2008 18:23:00 -0700

Sorry for the cross-post; the last post didn't use the correct MSDN

Can anyone point me in the right direction for verifying an
signature? i.e. that it was truly signed by a known/trusted certificate

Thanks -- Peter

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Microsoft MVP, Visual Developer - Visual C#