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using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;
using System.Security.Cryptography;


public partial class CS : System.Web.UI.Page
{
protected void EncryptFile(object sender, EventArgs e)
{
    //Get the Input File Name and Extension.
    string fileName = Path.GetFileNameWithoutExtension(FileUpload1.PostedFile.FileName);
    string fileExtension = Path.GetExtension(FileUpload1.PostedFile.FileName);

    //Build the File Path for the original (input) and the encrypted (output) file.
    string input = Server.MapPath("~/Files/") + fileName + fileExtension;
    string output = Server.MapPath("~/Files/") + fileName + "_enc" + fileExtension;

    //Save the Input File, Encrypt it and save the encrypted file in output path.
    FileUpload1.SaveAs(input);
    this.Encrypt(input, output);

    //Download the Encrypted File.
    Response.ContentType = FileUpload1.PostedFile.ContentType;
    Response.Clear();
    Response.AppendHeader("Content-Disposition", "attachment; filename=" + Path.GetFileName(output));
    Response.WriteFile(output);
    Response.Flush();

    //Delete the original (input) and the encrypted (output) file.
    File.Delete(input);
    File.Delete(output);

    Response.End();
}

private void Encrypt(string inputFilePath, string outputfilePath)
{
    string EncryptionKey = "MAKV2SPBNI99212";
    using (Aes encryptor = Aes.Create())
    {
        Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(EncryptionKey, new byte[] { 0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76 });
        encryptor.Key = pdb.GetBytes(32);
        encryptor.IV = pdb.GetBytes(16);
        using (FileStream fsOutput = new FileStream(outputfilePath, FileMode.Create))
        {
            using (CryptoStream cs = new CryptoStream(fsOutput, encryptor.CreateEncryptor(), CryptoStreamMode.Write))
            {
                using (FileStream fsInput = new FileStream(inputFilePath, FileMode.Open))
                {
                    int data;
                    while ((data = fsInput.ReadByte()) != -1)
                    {
                        cs.WriteByte((byte)data);
                    }
                }
            }
        }
    }
}

protected void DecryptFile(object sender, EventArgs e)
{
    //Get the Input File Name and Extension
    string fileName = Path.GetFileNameWithoutExtension(FileUpload1.PostedFile.FileName);
    string fileExtension = Path.GetExtension(FileUpload1.PostedFile.FileName);

    //Build the File Path for the original (input) and the decrypted (output) file
    string input = Server.MapPath("~/Files/") + fileName + fileExtension;
    string output = Server.MapPath("~/Files/") + fileName + "_dec" + fileExtension;

    //Save the Input File, Decrypt it and save the decrypted file in output path.
    FileUpload1.SaveAs(input);
    this.Decrypt(input, output);

    //Download the Decrypted File.
    Response.Clear();
    Response.ContentType = FileUpload1.PostedFile.ContentType;
    Response.AppendHeader("Content-Disposition", "attachment; filename=" + Path.GetFileName(output));
    Response.WriteFile(output);
    Response.Flush();

    //Delete the original (input) and the decrypted (output) file.
    File.Delete(input);
    File.Delete(output);

    Response.End();
}

private void Decrypt(string inputFilePath, string outputfilePath)
{
    string EncryptionKey = "MAKV2SPBNI99212";
    using (Aes encryptor = Aes.Create())
    {
        Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(EncryptionKey, new byte[] { 0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76 });
        encryptor.Key = pdb.GetBytes(32);
        encryptor.IV = pdb.GetBytes(16);
        using (FileStream fsInput = new FileStream(inputFilePath, FileMode.Open))
        {
            using (CryptoStream cs = new CryptoStream(fsInput, encryptor.CreateDecryptor(), CryptoStreamMode.Read))
            {
                using (FileStream fsOutput = new FileStream(outputfilePath, FileMode.Create))
                {
                    int data;
                    while ((data = cs.ReadByte()) != -1)
                    {
                        fsOutput.WriteByte((byte)data);
                    }
                }
            }
        }
    }
}
}

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="CS.aspx.cs" Inherits="CS" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
    <style type="text/css">
        body
        {
            font-family: Arial;
            font-size: 10pt;
        }
    </style>
</head>
<body>
    <form id="form1" runat="server">
    <asp:FileUpload ID="FileUpload1" runat="server" />
    <hr />
    <asp:Button ID="btnEncrypt" Text="Encrypt File" runat="server" OnClick="EncryptFile" />
    <asp:Button ID="btnDecrypt" Text="Decrypt File" runat="server" OnClick="DecryptFile" />
    </form>
</body>
</html>