C# - compare two SecureStrings for equality

Question

I have a WPF application with two PasswordBoxes, one for the password and another for the password to be entered a second time for confirmation purposes. I was wanting to use PasswordBox.SecurePassword to get the SecureString of the password, but I need to be able to compare the contents of the two PasswordBoxes to ensure equality before I accept the password. However, two identical SecureStrings are not considered equal:

var secString1 = new SecureString();
var secString2 = new SecureString();
foreach (char c in "testing")
{
    secString1.AppendChar(c);
    secString2.AppendChar(c);
}
Assert.AreEqual(secString1, secString2); // This fails

I was thinking comparing the Password property of the PasswordBoxes would defeat the point of accessing only SecurePassword because I'd be reading the plain-text password. What should I do to compare the two passwords without sacrificing security?

Edit: based on this question, I'm checking out this blog post about "using the Marshal class to convert the SecureString to ANSI or Unicode or a BSTR", then maybe I can compare those.

Answer 1

It looks like you could use this to compare the two SecureStrings.

It uses unsafe code to iterate through the strings:

bool SecureStringEqual(SecureString s1, SecureString s2)  
{  
    if (s1 == null)  
    {  
        throw new ArgumentNullException("s1");  
    }  
    if (s2 == null)  
    {  
        throw new ArgumentNullException("s2");  
    }  

    if (s1.Length != s2.Length)  
    {  
        return false;  
    }  

    IntPtr bstr1 = IntPtr.Zero;  
    IntPtr bstr2 = IntPtr.Zero;  

    RuntimeHelpers.PrepareConstrainedRegions();  

    try 
    {  
        bstr1 = Marshal.SecureStringToBSTR(s1);  
        bstr2 = Marshal.SecureStringToBSTR(s2);  

        unsafe 
        {  
            for (Char* ptr1 = (Char*)bstr1.ToPointer(), ptr2 = (Char*)bstr2.ToPointer();  
                *ptr1 != 0 && *ptr2 != 0;  
                 ++ptr1, ++ptr2)  
            {  
                if (*ptr1 != *ptr2)  
                {  
                    return false;  
                }  
            }  
        }  

        return true;  
    }  
    finally 
    {  
        if (bstr1 != IntPtr.Zero)  
        {  
            Marshal.ZeroFreeBSTR(bstr1);  
        }  

        if (bstr2 != IntPtr.Zero)  
        {  
            Marshal.ZeroFreeBSTR(bstr2);  
        }  
    }  
} 

I have modified it below to work without unsafe code (note however you are able to see the string in plain text when debugging):

  Boolean SecureStringEqual(SecureString secureString1, SecureString secureString2)
  {
     if (secureString1 == null)
     {
        throw new ArgumentNullException("s1");
     }
     if (secureString2 == null)
     {
        throw new ArgumentNullException("s2");
     }

     if (secureString1.Length != secureString2.Length)
     {
        return false;
     }

     IntPtr ss_bstr1_ptr = IntPtr.Zero;
     IntPtr ss_bstr2_ptr = IntPtr.Zero;

     try
     {
        ss_bstr1_ptr = Marshal.SecureStringToBSTR(secureString1);
        ss_bstr2_ptr = Marshal.SecureStringToBSTR(secureString2);

        String str1 = Marshal.PtrToStringBSTR(ss_bstr1_ptr);
        String str2 = Marshal.PtrToStringBSTR(ss_bstr2_ptr);

        return str1.Equals(str2);
     }
     finally
     {
        if (ss_bstr1_ptr != IntPtr.Zero)
        {
           Marshal.ZeroFreeBSTR(ss_bstr1_ptr);
        }

        if (ss_bstr2_ptr != IntPtr.Zero)
        {
           Marshal.ZeroFreeBSTR(ss_bstr2_ptr);
        }
     }
  }