ProtectedData.Protect intermittent failure

Question

I'm writing a password encryption routine. I've written the below app to illustrate my problem. About 20% of the time, this code works as expected. The rest of the time, the decryption throws a cryptographic exception - "The data is invalid".

I believe the problem is in the encryption portion, because the decryption portion works the same every time. That is, if the encryption routine yields a value that the decryption routine can decrypt, it can always decrypt it. But if the encryption routine yields a value that chokes the decryption routine, it always chokes. So the decrypt routine is consistent; the encrypt routine is not.

I suspect my use of Unicode encoding is incorrect, but I've tried others with the same result.

What am I doing wrong?

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Security.Cryptography;

namespace DataProtectionTest
{
    public partial class Form1 : Form
    {
        private static readonly byte[] entropy = { 1, 2, 3, 4, 1, 2, 3, 4 };
        private string password;
        public Form1()
        {
            InitializeComponent();
        }

        private void btnEncryptIt_Click(object sender, EventArgs e)
        {
            Byte[] pw = Encoding.Unicode.GetBytes(textBox1.Text);
            Byte[] encryptedPw = ProtectedData.Protect(pw, entropy, DataProtectionScope.LocalMachine);
            password = Encoding.Unicode.GetString(encryptedPw);     
        }

        private void btnDecryptIt_Click(object sender, EventArgs e)
        {
            Byte[] pwBytes = Encoding.Unicode.GetBytes(password);
            try
            {
                Byte[] decryptedPw = ProtectedData.Unprotect(pwBytes, entropy, DataProtectionScope.LocalMachine);
                string pw = Encoding.Unicode.GetString(decryptedPw);
                textBox2.Text = pw;
            }
            catch (CryptographicException ce)
            {
                textBox2.Text = ce.Message;
            }
        }
    }
}

Answer 1

On the advice of a colleague, I opted for Convert.ToBase64String. Works well. Corrected program below.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Security.Cryptography;

namespace DataProtectionTest
{
    public partial class Form1 : Form
    {
        private static readonly byte[] entropy = { 1, 2, 3, 4, 1, 2, 3, 4 };
        private string password;
        public Form1()
        {
            InitializeComponent();
        }

        private void btnEncryptIt_Click(object sender, EventArgs e)
        {
            Byte[] pw = Encoding.Unicode.GetBytes(textBox1.Text);
            Byte[] encryptedPw = ProtectedData.Protect(pw, entropy, DataProtectionScope.LocalMachine);
            //password = Encoding.Unicode.GetString(encryptedPw);       
            password = Convert.ToBase64String(encryptedPw);
        }

        private void btnDecryptIt_Click(object sender, EventArgs e)
        {
            //Byte[] pwBytes = Encoding.Unicode.GetBytes(password);
            Byte[] pwBytes = Convert.FromBase64String(password);
            try
            {
                Byte[] decryptedPw = ProtectedData.Unprotect(pwBytes, entropy, DataProtectionScope.LocalMachine);
                string pw = Encoding.Unicode.GetString(decryptedPw);
                textBox2.Text = pw;
            }
            catch (CryptographicException ce)
            {
                textBox2.Text = ce.Message;
            }
        }
    }
}