Digital Signature Verification in .NET

[hdsingh71]

We have two files, one is plain text another is signature file. We have to verify the integrity of the message.

The details of SIG file is as under:
Digital signature in PKCS7 format
Signatures are attached in detach mode
Algorithm : SHA1
Signature rule: End certificate of the User. .

What we are doing is
1. Calculating the hash of plain text file say it is hasPlain
2. Extracting the public key from SIG file
3. Extracting the Encrypted Hash from SIG file
4. Decrypting the above hash by using public key say it will give us hashSign
5. Comparing hashPlain and hasSign, if both are equal then signature is verified
6. We are using RSACryptoserviceprovider class of .net

We are able to extract the public key from SIG file but we are not able to extract Encrypted Hash from SIG File.

Please help

[Curtis Rutland]

So what have you tried so far?

[hdsingh71]

X509Certificate2 x509_2 = new X509Certificate2("c:\\EP00000100008052201.sig")
byte[] cer_data = x509_2.PublicKey.Key
 
FileStream fsw = new FileStream("C:\\cc.cer", FileMode.OpenOrCreate, FileAccess.Write);
            fsw.Write(cer_data, 0, cer_data.Length);
            fsw.Close();
// certificate has been generated 
 
            FileStream fs1 = new FileStream("c:\\EP00000100008052201.sig", FileMode.Open, FileAccess.Read);
            byte[] tot_bytes = new byte[fs1.Length];
            fs1.Read(tot_bytes, 0, tot_bytes.Length);
            fs1.Close();
            fsw = new FileStream("C:\\left.sig", FileMode.OpenOrCreate,FileAccess.Write);
 
            string shouldIWrite;
            int tot = 0;
            for (int i = 0; i < tot_bytes.Length; i++)
            {
                shouldIWrite = "Y";
                for (int j = 0; j < cer_data.Length; j++)
                {
                    if (tot_bytes[i] == cer_data[j])
                        shouldIWrite = "N";
                }
                if (shouldIWrite == "Y")
                {
                    tot++;
                    fsw.WriteByte(tot_bytes[i]);
                }
            }
            fsw.Close();
 
 
            FileStream fs = new FileStream("c:\\EP00000100008052201.txt", FileMode.Open, FileAccess.Read);
            byte[] plainbyte = new byte[fs.Length];
            fs.Read(plainbyte, 0, plainbyte.Length);
            SHA1 sha = new SHA1CryptoServiceProvider();
            byte[] plain_hash = sha.ComputeHash(plainbyte);
            fs.Close();
 
            fs = new FileStream("C:\\left.sig", FileMode.Open, FileAccess.Read);
            byte[] leftout_bytes = new byte[fs.Length];
            fs.Read(leftout_bytes, 0, leftout_bytes.Length);
            fs.Close();
 
            RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
            rsa.FromXmlString(x509_2.PublicKey.Key.ToXmlString(false));
 
            byte[] encrypted_data = rsa.Decrypt(leftout_bytes,true);
 
// here we are facing the problem
// once we will decrypt it we can get the hash and then compare with the hash of plain file calculated earlier
 
 
            if (plainbyte == encrypted_data)
                MessageBox.Show("Y");
            else
                MessageBox.Show("N");
 

[Frinavale]

Have you manually checked that the decryption is working properly?
Are you sure that you are reading leftout_bytes contains all of the bytes required for the decryption to work correctly?

-Frinny