C# SecureString Question

Question

Is there any way to get the value of a SecureString without comprising security? For example, in the code below as soon as you do PtrToStringBSTR the string is no longer secure because strings are immutable and garbage collection is non-deterministic for strings.

IntPtr ptr = Marshal.SecureStringToBSTR(SecureString object);
string value = Marshal.PtrToStringBSTR(ptr);

What if there were a way to get a char[] or byte[] of the unmanaged BSTR string? Would that mean garbage collection is more predictable (since you would be using a char[] or byte[] rather than a string? Is this assumption correct, and if so, how would you get back the char[] or byte[]?

Answer 1

Here's a class I've written especially for this purpose. Is it completely, 100% hackproof? No - there's very little you can do to make an application 100% safe, but this class goes about as far as you can to protect yourself if you need to convert a SecureString into a String.

Here's how you use the class:

using(SecureStringToStringMarshaler sm = new SecureStringToStringMarshaler(secureString))
{
    // Use sm.String here.  While in the 'using' block, the string is accessible
    // but pinned in memory.  When the 'using' block terminates, the string is zeroed
    // out for security, and garbage collected as usual.
}

Here's the class

/// Copyright (C) 2010 Douglas Day
/// All rights reserved.
/// MIT-licensed: http://www.opensource.org/licenses/mit-license.php

using System;
using System.Collections.Generic;
using System.Text;
using System.Security;
using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;

namespace DDay.Base
{
    public class SecureStringToStringMarshaler : IDisposable
    {
        #region Private Fields

        private string _String;
        private SecureString _SecureString;
        private GCHandle _GCH;

        #endregion

        #region Public Properties

        public SecureString SecureString
        {
            get { return _SecureString; }
            set
            {
                _SecureString = value;
                UpdateStringValue();
            }
        }

        public string String
        {
            get { return _String; }
            protected set { _String = value; }
        } 

        #endregion

        #region Constructors

        public SecureStringToStringMarshaler()
        {
        }

        public SecureStringToStringMarshaler(SecureString ss)        
        {
            SecureString = ss;
        }

        #endregion

        #region Private Methods

        void UpdateStringValue()
        {
            Deallocate();

            unsafe
            {
                if (SecureString != null)
                {
                    int length = SecureString.Length;
                    String = new string('\0', length);

                    _GCH = new GCHandle();

                    // Create a CER (Contrained Execution Region)
                    RuntimeHelpers.PrepareConstrainedRegions();
                    try { }
                    finally
                    {
                        // Pin our string, disallowing the garbage collector from
                        // moving it around.
                        _GCH = GCHandle.Alloc(String, GCHandleType.Pinned);
                    }

                    IntPtr stringPtr = IntPtr.Zero;
                    RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(
                        delegate
                        {
                            // Create a CER (Contrained Execution Region)
                            RuntimeHelpers.PrepareConstrainedRegions();
                            try { }
                            finally
                            {
                                stringPtr = Marshal.SecureStringToBSTR(SecureString);
                            }

                            // Copy the SecureString content to our pinned string
                            char* pString = (char*)stringPtr;
                            char* pInsecureString = (char*)_GCH.AddrOfPinnedObject();
                            for (int index = 0; index < length; index++)
                            {
                                pInsecureString[index] = pString[index];
                            }
                        },
                        delegate
                        {
                            if (stringPtr != IntPtr.Zero)
                            {
                                // Free the SecureString BSTR that was generated
                                Marshal.ZeroFreeBSTR(stringPtr);
                            }
                        },
                        null);
                }
            }
        }

        void Deallocate()
        {            
            if (_GCH.IsAllocated)
            {
                unsafe
                {
                    // Determine the length of the string
                    int length = String.Length;

                    // Zero each character of the string.
                    char* pInsecureString = (char*)_GCH.AddrOfPinnedObject();
                    for (int index = 0; index < length; index++)
                    {
                        pInsecureString[index] = '\0';
                    }

                    // Free the handle so the garbage collector
                    // can dispose of it properly.
                    _GCH.Free();
                }
            }
        } 

        #endregion

        #region IDisposable Members

        public void Dispose()
        {
            Deallocate();
        }

        #endregion
    }
}

This code requires that you can compile unsafe code, but it works like a charm.

Regards,

-Doug