Windows Phone 7 Development : Implementing Data Security (part 2) - Using Rfc2898DeriveBytes and AES to Encrypt Data

Open Visual Studio Express for Windows Phone and create a new project called AESEncryption.

Make the MainPage.xaml page look like the one shown in Figure 3. For convenience, the XAML of this page is pasted here:

<!--LayoutRoot contains the root grid where all other page content is placed-->

Code the Encrypt method. The Encrypt method takes data to encrypt, password and salt as parameters, and returns a string. Notice how the Encrypt method creates the Rfc2898DerivedBytes class and uses that class to generate a strong key from the password and salt combination; that key is later used by the AesManaged class to encrypt data.

public string Encrypt(string dataToEncrypt, string password, string salt)
        {
            AesManaged aes = null;
            MemoryStream memStream = null;
            CryptoStream crStream = null;

            try
            {
//Generate a Key based on a Password and Salt
                Rfc2898DeriveBytes rfc2898 = new Rfc2898DeriveBytes(password,
Encoding.UTF8.GetBytes(salt));

                //Create AES algorithm with 256 bit key and 128-bit block size
                aes = new AesManaged();
                aes.Key = rfc2898.GetBytes(aes.KeySize / 8);
                aes.IV = rfc2898.GetBytes(aes.BlockSize / 8);

                memStream = new MemoryStream();
                crStream = new CryptoStream(memStream, aes.CreateEncryptor(),

					  

CryptoStreamMode.Write);

                byte[] data = Encoding.UTF8.GetBytes(dataToEncrypt);
                crStream.Write(data, 0, data.Length);
                crStream.FlushFinalBlock();

                //Return Base 64 String
                return Convert.ToBase64String(memStream.ToArray());
            }
            finally
            {
                //cleanup
                if (crStream != null)
                    crStream.Close();

                if (memStream != null)
                    memStream.Close();

                if (aes != null)
                    aes.Clear();
            }
        }

Code the Decrypt method. The Decrypt method is the inverse of Encrypt: it takes data to decrypt, password and salt as parameters, and returns an input string. Since AES is a symmetric algorithm, the same password and salt values must be used to decrypt data as were used to encrypt it. The Decrypt method initializes the Rfc2898Bytes key and uses it to create Decryptor for data.

public string Decrypt(string dataToDecrypt, string password, string salt)
        {
            AesManaged aes = null;
            MemoryStream memStream = null;
            CryptoStream crStream = null;

            try
            {
                Rfc2898DeriveBytes rfc2898 = new Rfc2898DeriveBytes(password,
Encoding.UTF8.GetBytes(salt));
                aes = new AesManaged();
                aes.Key = rfc2898.GetBytes(aes.KeySize / 8);
                aes.IV = rfc2898.GetBytes(aes.BlockSize / 8);

                memStream = new MemoryStream();
                crStream = new CryptoStream(memStream, aes.CreateDecryptor(),
CryptoStreamMode.Write);
                byte[] data = Convert.FromBase64String(dataToDecrypt);
                crStream.Write(data, 0, data.Length);
                crStream.FlushFinalBlock();

                byte[] decryptBytes = memStream.ToArray();
                return Encoding.UTF8.GetString(decryptBytes, 0, decryptBytes.Length);
            }
            finally

					  

{
                if (crStream != null)
                    crStream.Close();

                if (memStream != null)
                    memStream.Close();

                if (aes != null)
                    aes.Clear();
            }
        }

Add code to call the Encrypt method when the user clicks the Encrypt button. Double-click the Encrypt button in MainPage.xaml and add the following code to the click event handler:

txtEncryptedData.Text = Encrypt(txtDataToEncrypt.Text, txtPassword.Text,
txtSalt.Text);

Finally, add code to call the Decrypt method when the user clicks the Decrypt button. Double-click the Decrypt button in MainPage.xaml and add the following code to the Click event handler:

txtDecryptedData.Text = Decrypt(txtEncryptedData.Text, txtPassword.Text,
         txtSalt.Text);