http://www.codeproject.com/KB/files/encryption.aspx
/**/
///<summary>
/// Steve Lydford - 12/05/2008.
///
/// Encrypts a file using Rijndael algorithm.
///</summary>
///<param name="inputFile"></param>
///<param name="outputFile"></param>
private
void
EncryptFile(
string
inputFile,
string
outputFile)

...
{
try

...{
string password = @"myKey123"; // Your Key Here
UnicodeEncoding UE = new UnicodeEncoding();
byte[] key = UE.GetBytes(password);
string cryptFile = outputFile;
FileStream fsCrypt = new FileStream(cryptFile, FileMode.Create);
RijndaelManaged RMCrypto = new RijndaelManaged();
CryptoStream cs = new CryptoStream(fsCrypt,
RMCrypto.CreateEncryptor(key, key),
CryptoStreamMode.Write);
FileStream fsIn = new FileStream(inputFile, FileMode.Open);
int data;
while ((data = fsIn.ReadByte()) != -1)
cs.WriteByte((byte)data);
fsIn.Close();
cs.Close();
fsCrypt.Close();
}
catch

...{
MessageBox.Show("Encryption failed!", "Error");
}
}

/**/
///<summary>
/// Steve Lydford - 12/05/2008.
///
/// Decrypts a file using Rijndael algorithm.
///</summary>
///<param name="inputFile"></param>
///<param name="outputFile"></param>
private
void
DecryptFile(
string
inputFile,
string
outputFile)

...
{

...{
string password = @"myKey123"; // Your Key Here
UnicodeEncoding UE = new UnicodeEncoding();
byte[] key = UE.GetBytes(password);
FileStream fsCrypt = new FileStream(inputFile, FileMode.Open);
RijndaelManaged RMCrypto = new RijndaelManaged();
CryptoStream cs = new CryptoStream(fsCrypt,
RMCrypto.CreateDecryptor(key, key),
CryptoStreamMode.Read);
FileStream fsOut = new FileStream(outputFile, FileMode.Create);
int data;
while ((data = cs.ReadByte()) != -1)
fsOut.WriteByte((byte)data);
fsOut.Close();
cs.Close();
fsCrypt.Close();
}
}