Encryption/Decryption not working

Question

This code isnt giving me the correct text decrypted...

 private void Encrypt(object sender, EventArgs e)
        {
            string keystring = Key.Text.Trim();
            byte[] key = Encoding.Unicode.GetBytes(keystring);
            byte[] input = Encoding.Unicode.GetBytes(EnterText.Text);
            string returnstring = "";
            int keyindex = 0;
            int counter = 0;
            foreach (byte CHAR in input)
            {
                returnstring += (char)(CHAR + 10);
                keyindex++;
                counter++;
            }
            Return.Text = returnstring;
        }

        private void Decrypt(object sender, EventArgs e)
        {
            string keystring = Key.Text.Trim();
            byte[] key = Encoding.Unicode.GetBytes(keystring);
            byte[] input = Encoding.Unicode.GetBytes(EnterText.Text);
            string returnstring = "";
            int keyindex = 0;
            int counter = 0;
            foreach (byte CHAR in input)
            {
                returnstring += (char)(CHAR - 10);
                keyindex++;
                counter++;
            }
            Return.Text = returnstring;
        }