Encryption Troubles

[Cmonkey123]

[edit]

Actually I'm having problems with decryption...

		public static void DecryptRewrite( string FilePath )
		{
			string filedata;

			//create streams
			System.IO.FileStream decryptfile;
			System.IO.StreamReader readfile;
			System.Security.Cryptography.CryptoStream cstream;

			//create crypto
			DESCryptoServiceProvider cryptic = new DESCryptoServiceProvider();

			//create key
			cryptic.Key = ASCIIEncoding.ASCII.GetBytes( "xxxxxxxx" );
			cryptic.IV = cryptic.IV = ASCIIEncoding.ASCII.GetBytes( "xxxxxxxx" );

			//open file to be decrypted
			decryptfile = new FileStream( FilePath, FileMode.Open, FileAccess.Read );

			//decrypt decryptfile
			cstream = new CryptoStream( decryptfile, cryptic.CreateDecryptor(), CryptoStreamMode.Read );

			//read file data and store in data string
			readfile = new StreamReader( FilePath );
			filedata = readfile.ReadToEnd();
			byte[] data = ASCIIEncoding.ASCII.GetBytes( filedata );

			//close streams
			readfile.Close();
			cstream.Close();

			//rewrite file with decrypted data
			System.IO.FileStream rewritefile = new FileStream( FilePath, FileMode.OpenOrCreate, FileAccess.Write );
			rewritefile.Write( data, 0, data.Length );
			rewritefile.Close();
		}

In the "rewrite file with decrypted data" section at the end of the method there's some kind of problem with the writing of the data into the file. When I run the program, it gives me the exception:

An unhandled exception of type 'System.NotSupportedException' occurred in mscorlib.dll

Additional information: Stream does not support writing.

How am I supposed rewrite the decrypted data into the file?

[Ravatar]

MSDN has a great example of DES in action:

// This sample demonstrates using a key based on the cryptographic service provider (CSP) version
// of the Data Encryption Standard (DES)algorithm to encrypt a string to a byte array, and then 
// to decrypt the byte array back to a string.

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

class CryptoMemoryStream
{
    // Main method.
    public static void Main()
    {
        // Create a new DES key.
        DESCryptoServiceProvider key = new DESCryptoServiceProvider();

        // Encrypt a string to a byte array.
        byte[] buffer = Encrypt("This is some plaintext!", key);

        // Decrypt the byte array back to a string.
        string plaintext =  Decrypt(buffer, key);

        // Display the plaintext value to the console.
        Console.WriteLine(plaintext);
    }
    
    // Encrypt the string.
    public static byte[] Encrypt(string PlainText, SymmetricAlgorithm key)
    {
        // Create a memory stream.
        MemoryStream ms = new MemoryStream();

        // Create a CryptoStream using the memory stream and the 
        // CSP DES key.  
        CryptoStream encStream = new CryptoStream(ms, key.CreateEncryptor(), CryptoStreamMode.Write);

        // Create a StreamWriter to write a string
        // to the stream.
        StreamWriter sw = new StreamWriter(encStream);

        // Write the plaintext to the stream.
        sw.WriteLine(PlainText);

        // Close the StreamWriter and CryptoStream.
        sw.Close();
        encStream.Close();

        // Get an array of bytes that represents
        // the memory stream.
        byte[] buffer = ms.ToArray();

        // Close the memory stream.
        ms.Close();

        // Return the encrypted byte array.
        return buffer;
    }

   // Decrypt the byte array.
    public static string Decrypt(byte[] CypherText, SymmetricAlgorithm key)
    {
        // Create a memory stream to the passed buffer.
        MemoryStream ms = new MemoryStream(CypherText);

        // Create a CryptoStream using the memory stream and the 
        // CSP DES key. 
        CryptoStream encStream = new CryptoStream(ms, key.CreateDecryptor(), CryptoStreamMode.Read);

        // Create a StreamReader for reading the stream.
        StreamReader sr = new StreamReader(encStream);

        // Read the stream as a string.
        string val = sr.ReadLine();

        // Close the streams.
        sr.Close();
        encStream.Close();
        ms.Close();
            
        return val;
    }
}