Visual C＃独立解密错误：要解密的数据长度无效

Question

In my visual C# encryption/decryption program I am trying to encrypt and decrypt independantly while I try to decrypt the previously encrypted data I get a length of data to decrypt is invalid error

String filename;
        String filename2;

        private void button1_Click(object sender, EventArgs e)
        {
            OpenFileDialog ofd = new OpenFileDialog();
            if (ofd.ShowDialog() == DialogResult.OK) {
                filename = ofd.FileName;
                textBox1.Text = filename;
                button2.Enabled = true;
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            StreamReader sr = new StreamReader(filename);
            String toencrypt = sr.ReadToEnd();
            sr.Dispose();

            MD5CryptoServiceProvider md5 = new MD5CryptoServiceProvider();
            UTF8Encoding utf8 = new UTF8Encoding();
            TripleDESCryptoServiceProvider tripleDES = new TripleDESCryptoServiceProvider();
            tripleDES.Key = md5.ComputeHash(utf8.GetBytes(toencrypt));
            tripleDES.Mode = CipherMode.ECB;
            tripleDES.Padding = PaddingMode.PKCS7;
            ICryptoTransform trans = tripleDES.CreateEncryptor();
            String encrypted = BitConverter.ToString(trans.TransformFinalBlock(utf8.GetBytes(textBox2.Text), 0, utf8.GetBytes(textBox2.Text).Length));

            StreamWriter sw = new StreamWriter(filename);
            sw.Write(encrypted);
            sw.Dispose();
        }

        private void button3_Click(object sender, EventArgs e)
        {
            OpenFileDialog ofd = new OpenFileDialog();
            if (ofd.ShowDialog() == DialogResult.OK)
            {
                filename2 = ofd.FileName;
                textBox3.Text = filename2;
                button4.Enabled = true;
            }
        }

        private void button4_Click(object sender, EventArgs e)
        {
            StreamReader sr = new StreamReader(filename2);
            String todecrypt = sr.ReadToEnd();
            sr.Dispose();

            MD5CryptoServiceProvider md5 = new MD5CryptoServiceProvider();
            UTF8Encoding utf8 = new UTF8Encoding();
            TripleDESCryptoServiceProvider tripleDES = new TripleDESCryptoServiceProvider();
            tripleDES.Key = md5.ComputeHash(utf8.GetBytes(textBox4.Text));
            tripleDES.Mode = CipherMode.ECB;
            tripleDES.Padding = PaddingMode.PKCS7;
            ICryptoTransform trans = tripleDES.CreateDecryptor();
            String decrypted = BitConverter.ToString(trans.TransformFinalBlock(utf8.GetBytes(todecrypt), 0, utf8.GetBytes(todecrypt).Length));

            StreamWriter sw = new StreamWriter(filename2);
            sw.Write(decrypted);
            sw.Dispose();
        }

Basically what i'm trying to do here is open up a txt file get the data inside to encrypt (which works fine) then decrypt (were I get the error) when it gives me the error I don't know what to change to fix this because the length of data is using the same key and padding and everything else that applies...

Answer 1

First of all:

String encrypted = BitConverter.ToString(trans.TransformFinalBlock(utf8.GetBytes(textBox2.Text), 0, utf8.GetBytes(textBox2.Text).Length));

The string 'encrypted' is formatted as

0C-4C-9B-01-00-0D-81-EC-D6-C5-2C-9A-EC-CE-08-95

But you try to decrypt the Utf8 bytes of the formatted string. That fails; and most of the time due to an incorrect length. You want to convert the string to a byte-array using BitConverter - NOT using Utf8.

Apart from that: 1) Dont use ECB 2) do use an IV 3) using the hash of a file as key is questionable at best

Edit: formatting