namespace Kyrathasoft.Cryptography.TripleDES.FileInFileOutUsingPassword {	
	
    using System;
    using System.IO;
    using System.Security.Cryptography;
    using System.Windows.Forms;
    
    class clsKyrCrypt {

        // Encrypt a file into another file using a password 

        public static void Encrypt(string fileIn,
                    string fileOut, string Password) {                 

                // First we are going to open the file streams 

                FileStream fsIn = new FileStream(fileIn,
                    FileMode.Open, FileAccess.Read);
                FileStream fsOut = new FileStream(fileOut,
                    FileMode.OpenOrCreate, FileAccess.Write);

                // Then we are going to derive a Key and an IV from the
                // Password and create an algorithm 

                PasswordDeriveBytes pdb = new PasswordDeriveBytes(Password,
                    new byte[] {0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 
                    0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76});

                Rijndael alg = Rijndael.Create();
                alg.Key = pdb.GetBytes(32);
                alg.IV = pdb.GetBytes(16);

                // Now create a crypto stream through which we are going
                // to be pumping data. 
                // Our fileOut is going to be receiving the encrypted bytes. 

                CryptoStream cs = new CryptoStream(fsOut,
                    alg.CreateEncryptor(), CryptoStreamMode.Write);

                // Now will will initialize a buffer and will be processing
                // the input file in chunks. 
                // This is done to avoid reading the whole file (which can
                // be huge) into memory. 

                int bufferLen = 4096;
                byte[] buffer = new byte[bufferLen];
                int bytesRead;

                do {
                    // read a chunk of data from the input file 
                    bytesRead = fsIn.Read(buffer, 0, bufferLen);
                    // encrypt it 
                    cs.Write(buffer, 0, bytesRead);
                } while (bytesRead != 0);

                // close everything 
                // this will also close the unrelying fsOut stream

                cs.Close();
                fsIn.Close();                                            
        }



        // Decrypt a file into another file using a password 

        public static void Decrypt(string fileIn,
                    string fileOut, string Password) {

            // First we are going to open the file streams 
            FileStream fsIn = new FileStream(fileIn,
                        FileMode.Open, FileAccess.Read);
            FileStream fsOut = new FileStream(fileOut,
                        FileMode.OpenOrCreate, FileAccess.Write);

            // Then we are going to derive a Key and an IV from
            // the Password and create an algorithm 

            PasswordDeriveBytes pdb = new PasswordDeriveBytes(Password,
                new byte[] {0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 
            0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76});
            Rijndael alg = Rijndael.Create();

            alg.Key = pdb.GetBytes(32);
            alg.IV = pdb.GetBytes(16);

            // Now create a crypto stream through which we are going
            // to be pumping data. 
            // Our fileOut is going to be receiving the Decrypted bytes. 

            CryptoStream cs = new CryptoStream(fsOut,
                alg.CreateDecryptor(), CryptoStreamMode.Write);

            // Now will will initialize a buffer and will be 
            // processing the input file in chunks. 
            // This is done to avoid reading the whole file (which can be
            // huge) into memory. 

            int bufferLen = 4096;
            byte[] buffer = new byte[bufferLen];
            int bytesRead;

            do {
                // read a chunk of data from the input file 
                bytesRead = fsIn.Read(buffer, 0, bufferLen);
                // Decrypt it 
                cs.Write(buffer, 0, bytesRead);
            } while (bytesRead != 0);

            // close everything 
            cs.Close(); // this will also close the unrelying fsOut stream 
            fsIn.Close();
        }

    }
}

File-in File-out TripleDES encryption using password

C#