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7. Finally, close the files.
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' VB ' Close the file handles encryptStream.Close() inFile.Close() outFile.Close() // C# // Close the file handles encryptStream.Close(); inFile.Close(); outFile.Close();
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Now, open a command prompt and use your application to encrypt a text file and an image file. Verify that the size of the encrypted files is approximately the same as the size of the unencrypted files. (It might be slightly larger due to padding added during encryption.) Attempt to open the encrypted files, and verify that they are unreadable.
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Exercise 2: Write a Console Application to Decrypt Files
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In this exercise, you create a console application that decrypts files using a password. 1. Create a new console application in either Visual Basic or C#. 2. Add the System.Security.Cryptography and System.IO namespaces to your project. 3. Similar to the application you wrote for Exercise 1, create a console application that creates an encryption algorithm object based on a password, reads an encrypted file, and writes a decrypted file. For example, the following code would work:
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' VB Sub Main(ByVal args As String()) ' Read the command-line parameters
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Lesson 3: Encrypting and Decrypting Data
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Dim inFileName As String = args(0) Dim outFileName As String = args(1) Dim password As String = args(2) ' Create the password key Dim saltValueBytes As Byte() = System.Text.Encoding.ASCII.GetBytes("This is my sa1t") Dim passwordKey As Rfc2898DeriveBytes = New Rfc2898DeriveBytes(password, saltValueBytes) ' Create the algorithm and specify the key and IV Dim alg As RijndaelManaged = New RijndaelManaged alg.Key = passwordKey.GetBytes(alg.KeySize / 8) alg.IV = passwordKey.GetBytes(alg.BlockSize / 8) ' Read the encrypted file into fileData Dim decryptor As ICryptoTransform = alg.CreateDecryptor Dim inFile As FileStream = _ New FileStream(inFileName, FileMode.Open, FileAccess.Read) Dim decryptStream As CryptoStream = _ New CryptoStream(inFile, decryptor, CryptoStreamMode.Read) Dim fileData(inFile.Length) As Byte decryptStream.Read(fileData, 0, CType(inFile.Length, Integer)) ' Write the contents of the unencrypted file Dim outFile As FileStream = _ New FileStream(outFileName, FileMode.OpenOrCreate, _ FileAccess.Write) outFile.Write(fileData, 0, fileData.Length) ' Close the file handles decryptStream.Close() inFile.Close() outFile.Close() End Sub // C# // Read the command-line parameters string inFileName = args[0]; string outFileName = args[1]; string password = args[2]; // Create the password key byte[] saltValueBytes = Encoding.ASCII.GetBytes("This is my sa1t"); Rfc2898DeriveBytes passwordKey = new Rfc2898DeriveBytes(password, saltValueBytes); // Create the algorithm and specify the key and IV RijndaelManaged alg = new RijndaelManaged(); alg.Key = passwordKey.GetBytes(alg.KeySize / 8); alg.IV = passwordKey.GetBytes(alg.BlockSize / 8); // Read the encrypted file into fileData ICryptoTransform decryptor = alg.CreateDecryptor();
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User and Data Security
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FileStream inFile = new FileStream(inFileName, FileMode.Open, FileAccess.Read); CryptoStream decryptStream = new CryptoStream(inFile, decryptor, CryptoStreamMode.Read); byte[] fileData = new byte[inFile.Length]; decryptStream.Read(fileData, 0, (int)inFile.Length); // Write the contents of the unencrypted file FileStream outFile = new FileStream(outFileName, FileMode.OpenOrCreate, FileAccess.Write); outFile.Write(fileData, 0, fileData.Length); // Close the file handles decryptStream.Close(); inFile.Close(); outFile.Close();
4. Add code to read the command-line parameters into strings. If you are using Visual Basic, you have to change the Main parameter declaration to accept an array of strings. The following code would work, though you should add errorhandling that displays usage information if the user does not provide the correct parameters:
' VB Dim inFileName As String = args(0) Dim outFileName As String = args(1) Dim password As String = args(2) // C# string inFileName = args[0]; string outFileName = args[1]; string password = args[2];
Now, open a command prompt and use your application to decrypt the text file and image files that you encrypted earlier. Attempt to open the decrypted files, and verify that they are readable. Attempt to decrypt the files with the incorrect password and note that the application throws a System.Security.Cryptography.CryptographicException. You should catch this exception and display a friendly error message to the user indicating that they probably mistyped the password.
Lesson Summary
Symmetric key encryption is a cryptographic technique for protecting the privacy of data in situations where both the encryptor and decryptor have access to the same secret key. There are four symmetric algorithm classes in the .NET Framework: RijndaelManaged, DES, TripleDES, and RC2. The primary disadvantage of symmetric key encryption is that you have to transfer the key between the sender and receiver, and the key can be very complex. Alternatively, you can generate a key based on a user password by using the Rfc2898DeriveBytes class.
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