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Lesson 3: Encrypting and Decrypting Data
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// C# // In practice, the user would provide the password string password = "P@S5w0r]>"; // Create an algorithm object RijndaelManaged myAlg = new RijndaelManaged(); // Derive the key and use it to define the algorithm byte[] salt = Encoding.ASCII.GetBytes("This is my sa1t"); Rfc2898DeriveBytes key = new Rfc2898DeriveBytes(password, salt); myAlg.Key = key.GetBytes(myAlg.KeySize / 8); myAlg.IV = key.GetBytes(myAlg.BlockSize / 8);
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How to Encrypt and Decrypt Messages Using Symmetric Keys
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After both the encryptor and decryptor have the same key, they can begin exchanging encrypted messages. The .NET Framework makes this process easy. In fact, using encryption is similar to reading and writing to standard files and streams, and it requires only a few additional lines of code. To encrypt or decrypt messages in your application, perform the following tasks: 1. Create a Stream object to interface with the memory or file that you will be reading from or writing to. 2. Create a SymmetricAlgorithm object. 3. Specify the algorithm s key, the IV, or both. 4. Call SymmetricAlgorithm.CreateEncryptor() or SymmetricAlgorithm.CreateDecryptor() to create a ICryptoTransform object. 5. Create a CryptoStream object using the Stream object and the ICryptoTransform object. 6. Read from or write to the CryptoStream object just like any other Stream object. The following console application demonstrates these steps by reading an unencrypted file (the C:\Boot.ini file), encrypting it with the Rijndael algorithm, and saving the encrypted results as a new file. The application requires the System.IO and System.Security.Cryptography namespaces.
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' VB Dim inFileName As String = "C:\Boot.ini" Dim outFileName As String = "C:\Boot.ini.enc" ' Step 1: Create the Stream objects Dim inFile As FileStream = New FileStream(inFileName, FileMode.Open, FileAccess.Read) Dim outFile As FileStream = New FileStream(outFileName, FileMode.OpenOrCreate, FileAccess.Write)
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' Step 2: Create the SymmetricAlgorithm object Dim myAlg As SymmetricAlgorithm = New RijndaelManaged ' Step 3: Specify a key (optional) myAlg.GenerateKey() ' Read the unencrypted file into fileData Dim fileData(inFile.Length - 1) As Byte inFile.Read(fileData, 0, CType(inFile.Length, Integer)) ' Step 4: Create the ICryptoTransform object Dim encryptor As ICryptoTransform = myAlg.CreateEncryptor ' Step 5: Create the CryptoStream object Dim encryptStream As CryptoStream = New CryptoStream(outFile, encryptor, CryptoStreamMode.Write) ' Step 6: Write the contents to the CryptoStream encryptStream.Write(fileData, 0, fileData.Length) ' Close the file handles encryptStream.Close() inFile.Close() outFile.Close() // C# string inFileName = @"C:\Boot.ini"; string outFileName = @"C:\Boot.ini.enc"; // Step 1: Create the Stream objects FileStream inFile = new FileStream(inFileName, FileMode.Open, FileAccess.Read); FileStream outFile = new FileStream(outFileName, FileMode.OpenOrCreate, FileAccess.Write); // Step 2: Create the SymmetricAlgorithm object SymmetricAlgorithm myAlg = new RijndaelManaged(); // Step 3: Specify a key (optional) myAlg.GenerateKey(); // Read the unencrypted file into fileData byte[] fileData = new byte[inFile.Length]; inFile.Read(fileData, 0, (int)inFile.Length); // Step 4: Create the ICryptoTransform object ICryptoTransform encryptor = myAlg.CreateEncryptor(); // Step 5: Create the CryptoStream object CryptoStream encryptStream = new CryptoStream(outFile, encryptor, CryptoStreamMode.Write); // Step 6: Write the contents to the CryptoStream encryptStream.Write(fileData, 0, fileData.Length);
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Lesson 3: Encrypting and Decrypting Data
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// Close the file handles encryptStream.Close(); inFile.Close(); outFile.Close();
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Because the key is randomly generated, running the application repeatedly generates different results each time. Because the key is not stored, the file can never be decrypted. The key is simply an array of bytes and can be stored by using the BinaryWriter object or by transferring the key across a network. The code for decrypting a file is almost identical to the code for encrypting a file, except that it must read the encryption key that was used to encrypt the data rather than randomly generate it, and it must call decryption methods instead of encryption methods. To reverse the process to decrypt a file, simply make the following changes to an application:
Change the code for step 3 to read the key and IV that was used to encrypt the data. Change the code for step 4 to use the CreateDecryptor method instead of CreateEncryptor. Change the code for step 5 to use the CryptoStreamMode.Read enumeration instead of CryptoStreamMode.Write. Change the code for step 6 to read from the CryptoStream object.
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