How to encrypt and decrypt string in .NET application

using System;
using System.IO;
using System.Security.Cryptography;
using System.Text;

namespace MyCompany.Library
{
public class Encryption
{
public static string EncryptString(string valueToEncrypt)
{
return EncryptString(valueToEncrypt, "My password key");
}

public static string DecryptString(string valueToDecrypt)
{
return DecryptString(valueToDecrypt, "My password key");
}

public static string EncryptString(string valueToEncrypt, string secretPhrase)
{
CryptoStream encryptStream = null; // Stream used to encrypt
RijndaelManaged rijndael = null; // Rijndael provider
ICryptoTransform rijndaelEncrypt = null; // Encrypting object
MemoryStream memStream = new MemoryStream(); // Stream to contain data
byte[] key;
byte[] IV;
GenerateKey(secretPhrase, out key, out IV);
try
{
if( valueToEncrypt.Length > 0 )
{
// Create the crypto objects
rijndael = new RijndaelManaged();
rijndael.Key = key;
rijndael.IV = IV;
rijndaelEncrypt = rijndael.CreateEncryptor();
encryptStream = new CryptoStream(
memStream, rijndaelEncrypt, CryptoStreamMode.Write);

// Write the encrypted value into memory
byte[] input = Encoding.UTF8.GetBytes(valueToEncrypt);
encryptStream.Write(input, 0, input.Length);
encryptStream.FlushFinalBlock();

// Retrieve the encrypted value and return it
return Convert.ToBase64String(memStream.ToArray());
}
else
{
return "";
}
}
finally
{
if (rijndael != null) rijndael.Clear();
if (rijndaelEncrypt != null) rijndaelEncrypt.Dispose();
if (memStream != null) memStream.Close();
}
}

public static string DecryptString(string valueToDecrypt, string secretPhrase)
{
CryptoStream decryptStream = null; // Stream used to decrypt
RijndaelManaged rijndael = null; // Rijndael provider
ICryptoTransform rijndaelDecrypt = null; // decrypting object
MemoryStream memStream = null; // Stream to contain data
byte[] key;
byte[] IV;
GenerateKey(secretPhrase, out key, out IV);
try
{
if( valueToDecrypt.Length > 0 )
{
// Create the crypto objects
rijndael = new RijndaelManaged();
rijndael.Key = key;
rijndael.IV = IV;

//Now decrypt the previously encrypted message using the decryptor
// obtained in the above step.

// Write the encrypted value into memory
byte[] encrypted = Convert.FromBase64String(valueToDecrypt);
memStream = new MemoryStream(encrypted);


rijndaelDecrypt = rijndael.CreateDecryptor();
decryptStream = new CryptoStream(memStream, rijndaelDecrypt, CryptoStreamMode.Read);

byte[] fromEncrypt = new byte[encrypted.Length];

//Read the data out of the crypto stream.
decryptStream.Read(fromEncrypt, 0, fromEncrypt.Length);

// Retrieve the encrypted value and return it
string decryptedString = new string(Encoding.UTF8.GetChars(fromEncrypt));
return decryptedString.TrimEnd(new char[] {'\0'});
}
else
{
return "";
}
}
finally
{
if (rijndael != null) rijndael.Clear();
if (rijndaelDecrypt != null) rijndaelDecrypt.Dispose();
if (memStream != null) memStream.Close();
}
}

/// Generates an encryption key based on the given phrase. The
/// phrase is hashed to create a unique 32 character (256-bit)
/// value, of which 24 characters (192 bit) are used for the
/// key and the remaining 8 are used for the initialization vector (IV).
private static void GenerateKey(string secretPhrase, out byte[] key, out byte[] IV)
{
// Initialize internal values
key = new byte[24];
IV = new byte[16];

// Perform a hash operation using the phrase. This will
// generate a unique 32 character value to be used as the key.
byte[] bytePhrase = Encoding.ASCII.GetBytes(secretPhrase);
SHA384Managed sha384 = new SHA384Managed();
sha384.ComputeHash(bytePhrase);
byte[] result = sha384.Hash;

// Transfer the first 24 characters of the hashed value to the key
// and the remaining 8 characters to the intialization vector.
for (int index=0; index<24; index++) key[index] = result[index];
for (int index=24; index<40; index++) IV[index-24] = result[index];
}
}
}