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How to validate digital signature using .NET 2.0

Posted on 2008-06-12
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Last Modified: 2013-12-17
I'm creating web application using .NET Framework 2.0. One of the requirements is that the data entered by user should be signed using certificate issued by CA, before is submitted for further processing. Certificate is chosen by the user from Certificate Store. For that purpose I use JavaScript and CAPICOM to sign the data. Everything is OK on that part. Data is signed and submitted to the server. On the server side I need to first validate the user certificate and then validate signature.
1. Using X509Chain I'm unable to validate certificate. ChainStatus return following statuses
PartialChain: A certificate chain could not be built to a trusted root authority.
RevocationStatusUnknown: The revocation function was unable to check revocation for the certificate.
OfflineRevocation: The revocation function was unable to check revocation because the revocation server was offline.
2. How to validate digiatal signature using System.Security.Cryptography classes. I tried using RSACryptoServiceProvider. Code sample is bellow.

Does enyone know where is my error, and what am I doing wrong?
Can someone give me URLs with this topic?


1)
X509Certificate2 aCertificate = new X509Certificate2(UnicodeEncoding.Unicode.GetBytes(Signature)); 
X509Chain chain = new X509Chain();
chain.ChainPolicy.RevocationFlag = X509RevocationFlag.EntireChain;
chain.ChainPolicy.RevocationMode = X509RevocationMode.Online;
chain.ChainPolicy.UrlRetrievalTimeout = new TimeSpan(0, 1, 0);
chain.ChainPolicy.VerificationFlags = X509VerificationFlags.NoFlag;
bool chainIsValid = chain.Build(aCertificate);
 
2)
byte[] data = Encoding.Unicode.GetBytes(Data); //original data
SHA1 sha = new SHA1CryptoServiceProvider();
byte[] hash = sha.ComputeHash(data);
RSACryptoServiceProvider RSAP = (RSACryptoServiceProvider)aCert.PublicKey.Key; 
//aCert is X509Certificate2 from code above
RSAPKCS1SignatureDeformatter RSADeformatter = new RSAPKCS1SignatureDeformatter(RSAP);
RSADeformatter.SetHashAlgorithm("SHA1");
if (RSADeformatter.VerifySignature(hash, sig))
{
    return true;
}
else
{
    return false;
}

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Question by:re-aktiv
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3 Comments
 
LVL 8

Accepted Solution

by:
the_crazed earned 500 total points
ID: 21874036
I know its a bit lame, but we use CAPICOM through interop.
we coudn't get the RSAPKCS1SignatureDeformatter to verify our capicom sigs either, its too hard to guess what CAPICOM does under the hood.
here is our verification code:
using CAPICOM;
 
'..........
 
		public static bool VerifyTheSignedString(string strSignedData, string strRawData, out SignedDataClass objSignedData, bool blnDetatchedSignature, out string strErrorReason)
		{
			bool blnRet = true;
			strErrorReason = "";
 
 
			objSignedData = new SignedDataClass();
			try
			{
				if (blnDetatchedSignature) objSignedData.Content = strRawData;
				objSignedData.Verify(strSignedData, blnDetatchedSignature,CAPICOM_SIGNED_DATA_VERIFY_FLAG.CAPICOM_VERIFY_SIGNATURE_AND_CERTIFICATE);
 
			}
			catch(Exception e)
			{
				blnRet = false;
				strErrorReason = e.Message;
 
			}
                            return blnRet;
}

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