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POC# How to Verify Digital Signature from email ( Encoding SeveBit )
 

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  1. POC# How to Verify Digital Signature from email ( Encoding SeveBit )
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    <p>I get the message body and the smime.p7s file that contains the digital signature. And I want to verify if the mail is signed by that signature. I'm using the following code.</p> <pre><code> private bool VerifyCommand(string text, byte[] signature, string certPath) { // Load the certificate file to use to verify the signature from a file // If using web service or ASP.NET, use: X509Certificate2 cert = new X509Certificate2(Request.ClientCertificate.Certificate); X509Certificate2 cert = new X509Certificate2(certPath); // Get public key RSACryptoServiceProvider csp = (RSACryptoServiceProvider)cert.PublicKey.Key; // Hash the text, the text is the expected command by the client application. // Remember hased data cannot be unhash. It is irreversable SHA1Managed sha1 = new SHA1Managed(); UnicodeEncoding encoding = new UnicodeEncoding(); byte[] data = encoding.GetBytes(text); byte[] hash = sha1.ComputeHash(data); // Verify the signature with the hash return csp.VerifyHash(hash, CryptoConfig.MapNameToOID("SHA1"), signature); } </code></pre> <p><code>byte[] signature</code> is the signature from mail after Convert.FromBase64String(mailsignature). <code>string certPath</code> is the path tot the smime.p7s file. ( the smime.p7s is attached to the mail)</p> <p>This is the part where the body mail is:</p> <pre><code>------=_NextPart_001_0039_01CC77C1.AFC97230 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit FINAL TEST SIGNED ------=_NextPart_001_0039_01CC77C1.AFC97230 </code></pre> <p>This is a part of the Signature attachment:</p> <pre><code>------=_NextPart_000_0038_01CC77C1.AFC4B740 Content-Type: application/x-pkcs7-signature; name="smime.p7s" Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename="smime.p7s" MIAGCSqGSIb3DQEHAqCAMIACAQExCzAJBgUrDgMCGgUAMIAGCSqGSIb3DQEHAQAAoIIWADCCA7Ew ggKZoAMCAQICEBErBTlXKN63QvT+VRPTt1EwDQYJKoZIhvcNAQEFBQAwQzEXMBUGA1UEChMOQWxj YXRlbCBMdWNlbnQxKDAmBgNVBAMTH0FsY2F0ZWwgTHVjZW50IEludGVybmFsIFJvb3QgQ0EwHhcN MDgxMTAzMTU0MTE2WhcNMjgxMTAzMTU0MTE2WjBDMRcwFQYDVQQKEw5BbGNhdGVsIEx1Y2VudDEo MCYGA1UEAxMfQWxjYXRlbCBMdWNlbnQgSW50ZXJuYWwgUm9vdCBDQTCCASIwDQYJKoZIhvcNAQEB BQADggEPADCCAQoCggEBAL5IGBVth8afQdnpuLDI0Z37GgIcPWznOOzFJUV1gVbztqQ5CIxkVL4K ................... </code></pre> <p>Is the method that I'm using correct? is the Encoding write? or I have to use a 7-bit?</p> <p>enter code here</p> <hr> <p>Thnx Henning Krause. I searched, and I'm stuck again :( .</p> <pre><code>public static bool Verify(byte[] signature, X509Certificate2 certificate) { X509Certificate2 cert=new X509Certificate2(@"D:\Work\Digital Signature\smime.p7s"); certificate = cert; if(signature == null) throw new ArgumentNullException("signature"); if(certificate == null) throw new ArgumentNullException("certificate"); //the text from the body of the mail string text = "FINAL TEST SIGNED"; //hash the text // Methode 3 for Hashing System.Security.Cryptography.SHA1 hash3 = System.Security.Cryptography.SHA1.Create(); System.Text.UnicodeEncoding encoder = new System.Text.UnicodeEncoding(); byte[] combined = encoder.GetBytes(text); byte[] hash3byte = hash3.ComputeHash(combined); //Adding the text from the email, to a contentInfo ContentInfo content = new ContentInfo(hash3byte); // decode the signature SignedCms verifyCms = new SignedCms(content,true); verifyCms.Decode(signature); // verify it try { verifyCms.CheckSignature(new X509Certificate2Collection(certificate), false); return true; } catch(CryptographicException) { return false; } } </code></pre> <p>I get the CryptographicException "The hash value is not correct." I tried only <code>verifyCms.CheckSignature(true)</code>; (same error) I tried to add in ContentInfo the whole mail (Sender , Subject , Body, HTML Sectione ...) (same error)</p> <p>Can you please be more specific how can I use the SignedCms for my problem?</p>
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