using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.IO;
using System.Linq;
using System.Security.Cryptography;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Windows.Forms.VisualStyles;

namespace Decryption_App
{
    public partial class frmMain : Form
    {
        public frmMain()
        {
            InitializeComponent();
        }

        private void textBox1_TextChanged(object sender, EventArgs e)
        {

        }

        private void textBox2_TextChanged(object sender, EventArgs e)
        {

        }

        private void btnDecrypt_Click(object sender, EventArgs e)
        {
            var inputString = txtEncrypted.Text;
            var thePassword = txtPassword.Text;

            // Get the initial value from the last 16 characters of the case note string
            // and remove from the ciphertext
            var initialValue = inputString.Substring(inputString.Length - 16);
            var cipherText = inputString.Substring(0, inputString.Length - 20);

            string decrypted = DecryptString(cipherText, thePassword, initialValue);

            //txtDecrypted.Text = decrypted;

            txtDecrypted.Text = "Saved as file.";

            SaveAsFile(decrypted);
        }

        static string DecryptString(string encrypted, string password, string initialValue)
        {
            // The ciphertext is stored as URL-safe Base64-encoded text. Convert to regular Base64.
            string cipherTextUrlSafe = encrypted;
            string cipherTextBase64 = cipherTextUrlSafe.Replace('-', '+').Replace('_', '/');

            // The ciphertext needs padding to work with AES CBC. Trimming bytes would corrupt the data.
            int mod4 = cipherTextBase64.Length % 4;
            if (mod4 > 0) cipherTextBase64 = cipherTextBase64.PadRight(cipherTextBase64.Length + (4 - mod4), '=');
            byte[] encryptedBytes = Convert.FromBase64String(cipherTextBase64);

            // The vendor didn't use a key derivation function. The decryption key is literally the bytes of the password!
            byte[] passwordBytes = Encoding.UTF8.GetBytes(password);
            byte[] ivBytes = Encoding.UTF8.GetBytes(initialValue);

            using (Aes aes = Aes.Create())
            {
                aes.Key = passwordBytes;
                aes.IV = ivBytes;
                aes.Mode = CipherMode.CBC;

                // If this works with PKCS5, we can probably remove.
                aes.Padding = PaddingMode.PKCS7;

                using (MemoryStream ms = new MemoryStream())
                using (CryptoStream cs = new CryptoStream(ms, aes.CreateDecryptor(), CryptoStreamMode.Write))
                {
                    cs.Write(encryptedBytes, 0, encryptedBytes.Length);
                    cs.FlushFinalBlock(); 
                    return Encoding.UTF8.GetString(ms.ToArray());
                }
            }
        }


        private void SaveAsFile(string decrypted)
        { 
            string fileName = DateTime.Now.ToString("dd-MM-yyyy-HH-mm-ss");
            File.WriteAllText(@"C:\\Users\\Public\\Documents\"+fileName +".html", decrypted);

            MessageBox.Show("Saved as " + fileName + ".html");
            //frmSaved.Show(this);
        }

        private void ShowSavedDialogue()
        {
            var frmSaved = new Form();
            frmSaved.Show(this);
        }

        private void frmMain_Load(object sender, EventArgs e)
        {

        }

        private void btnInformation_Click(object sender, EventArgs e)
        {
            frmInformation form2 = new frmInformation();
            form2.Show();
        }
    }
}