How to convert SecureString to System.String?

Question

All reservations about unsecuring your SecureString by creating a System.String out of it aside, how can it be done?

How can I convert an ordinary System.Security.SecureString to System.String?

I'm sure many of you who are familiar with SecureString are going to respond that one should never transform a SecureString to an ordinary .NET string because it removes all security protections. I know. But right now my program does everything with ordinary strings anyway, and I'm trying to enhance its security and although I'm going to be using an API that returns a SecureString to me I am not trying to use that to increase my security.

I'm aware of Marshal.SecureStringToBSTR, but I don't know how to take that BSTR and make a System.String out of it.

For those who may demand to know why I would ever want to do this, well, I'm taking a password from a user and submitting it as an html form POST to log the user into a web site. So... this really has to be done with managed, unencrypted buffers. If I could even get access to the unmanaged, unencrypted buffer I imagine I could do byte-by-byte stream writing on the network stream and hope that that keeps the password secure the whole way. I'm hoping for an answer to at least one of these scenarios.

Answer 1

Use the System.Runtime.InteropServices.Marshal class:

String SecureStringToString(SecureString value) {
  IntPtr valuePtr = IntPtr.Zero;
  try {
    valuePtr = Marshal.SecureStringToGlobalAllocUnicode(value);
    return Marshal.PtrToStringUni(valuePtr);
  } finally {
    Marshal.ZeroFreeGlobalAllocUnicode(valuePtr);
  }
}

If you want to avoid creating a managed string object, you can access the raw data using Marshal.ReadInt16(IntPtr, Int32):

void HandleSecureString(SecureString value) {
  IntPtr valuePtr = IntPtr.Zero;
  try {
    valuePtr = Marshal.SecureStringToGlobalAllocUnicode(value);
    for (int i=0; i < value.Length; i++) {
      short unicodeChar = Marshal.ReadInt16(valuePtr, i*2);
      // handle unicodeChar
    }
  } finally {
    Marshal.ZeroFreeGlobalAllocUnicode(valuePtr);
  }
}

Answer 2

Obviously you know how this defeats the whole purpose of a SecureString, but I'll restate it anyway.

If you want a one-liner, try this: (.NET 4 and above only)

string password = new System.Net.NetworkCredential(string.Empty, securePassword).Password;

Where securePassword is a SecureString.

Answer 3

Dang. right after posting this I found the answer deep in this article. But if anyone knows how to access the IntPtr unmanaged, unencrypted buffer that this method exposes, one byte at a time so that I don't have to create a managed string object out of it to keep my security high, please add an answer. :)

static String SecureStringToString(SecureString value)
{
    IntPtr bstr = Marshal.SecureStringToBSTR(value);

    try
    {
        return Marshal.PtrToStringBSTR(bstr);
    }
    finally
    {
        Marshal.FreeBSTR(bstr);
    }
}

Answer 4

In my opinion, extension methods are the most comfortable way to solve this.

I took Steve in CO's excellent answer and put it into an extension class as follows, together with a second method I added to support the other direction (string -> secure string) as well, so you can create a secure string and convert it into a normal string afterwards:

public static class Extensions
{
    // convert a secure string into a normal plain text string
    public static String ToPlainString(this System.Security.SecureString secureStr)
    {
        String plainStr = new System.Net.NetworkCredential(string.Empty, 
                          secureStr).Password;
        return plainStr;
    }

    // convert a plain text string into a secure string
    public static System.Security.SecureString ToSecureString(this String plainStr)
    {
        var secStr = new System.Security.SecureString(); secStr.Clear();
        foreach (char c in plainStr.ToCharArray())
        {
            secStr.AppendChar(c);
        }
        return secStr;
    }
}

With this, you can now simply convert your strings back and forth like so:

// create a secure string
System.Security.SecureString securePassword = "MyCleverPwd123".ToSecureString(); 

// convert it back to plain text (normal string)
String plainPassword = securePassword.ToPlainString();  

But keep in mind the decoding method should only be used for testing.

---

If you're interested in the details: NetworkCredential uses internally

using System.Runtime.InteropServices;
using System.Security;

private string MarshalToString(SecureString sstr)
{
    if (sstr == null || sstr.Length == 0)
    {
        return string.Empty;
    }
    IntPtr intPtr = IntPtr.Zero;
    string empty = string.Empty;
    try
    {
        intPtr = Marshal.SecureStringToGlobalAllocUnicode(sstr);
        return Marshal.PtrToStringUni(intPtr);
    }
    finally
    {
        if (intPtr != IntPtr.Zero)
        {
            Marshal.ZeroFreeGlobalAllocUnicode(intPtr);
        }
    }
}

private unsafe SecureString MarshalToSecureString(string str)
{
    if (string.IsNullOrEmpty(str))
    {
        return new SecureString();
    }
    fixed (char* ptr = str)
    {
        char* value = ptr;
        return new SecureString(value, str.Length);
    }
}

to convert a SecureString into a string and vice versa.

And SecureString internally uses

using System.Runtime.InteropServices;

[DllImport("crypt32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
internal static extern bool CryptProtectMemory(SafeBuffer pData, uint cbData, uint dwFlags);

[DllImport("crypt32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
internal static extern bool CryptUnprotectMemory(SafeBuffer pData, uint cbData, uint dwFlags);

to encrypt / decrypt the string data (pData buffer) in situ (i.e. in place, without copying it). This means, you don't know how a SecureString is being internally encrypted. But since you can decrypt it with zero effort, it isn't really secure - it just prevents someone looking into a memory dump from finding plain text strings too easily. In other words, if you don't find a string in a memory dump easily, you obviously can't decrypt it (security through obscurity).

Answer 5

I think it would be best for SecureString dependent functions to encapsulate their dependent logic in an anonymous function for better control over the decrypted string in memory (once pinned).

The implementation for decrypting SecureStrings in this snippet will:

1. Pin the string in memory (which is what you want to do but appears to be missing from most answers here).
2. Pass its reference to the Func/Action delegate.
3. Scrub it from memory and release the GC in the finally block.

This obviously makes it a lot easier to "standardize" and maintain callers vs. relying on less desirable alternatives:

• Returning the decrypted string from a string DecryptSecureString(...) helper function.
• Duplicating this code wherever it is needed.

Notice here, you have two options:

1. static T DecryptSecureString<T> which allows you to access the result of the Func delegate from the caller (as shown in the DecryptSecureStringWithFunc test method).
2. static void DecryptSecureString is simply a "void" version which employ an Action delegate in cases where you actually don't want/need to return anything (as demonstrated in the DecryptSecureStringWithAction test method).

Example usage for both can be found in the StringsTest class included.

Strings.cs

using System;
using System.Runtime.InteropServices;
using System.Security;

namespace SecurityUtils
{
    public partial class Strings
    {
        /// <summary>
        /// Passes decrypted password String pinned in memory to Func delegate scrubbed on return.
        /// </summary>
        /// <typeparam name="T">Generic type returned by Func delegate</typeparam>
        /// <param name="action">Func delegate which will receive the decrypted password pinned in memory as a String object</param>
        /// <returns>Result of Func delegate</returns>
        public static T DecryptSecureString<T>(SecureString secureString, Func<string, T> action)
        {
            var insecureStringPointer = IntPtr.Zero;
            var insecureString = String.Empty;
            var gcHandler = GCHandle.Alloc(insecureString, GCHandleType.Pinned);

            try
            {
                insecureStringPointer = Marshal.SecureStringToGlobalAllocUnicode(secureString);
                insecureString = Marshal.PtrToStringUni(insecureStringPointer);

                return action(insecureString);
            }
            finally
            {
                //clear memory immediately - don't wait for garbage collector
                fixed(char* ptr = insecureString )
                {
                    for(int i = 0; i < insecureString.Length; i++)
                    {
                        ptr[i] = '\0';
                    }
                }

                insecureString = null;

                gcHandler.Free();
                Marshal.ZeroFreeGlobalAllocUnicode(insecureStringPointer);
            }
        }

        /// <summary>
        /// Runs DecryptSecureString with support for Action to leverage void return type
        /// </summary>
        /// <param name="secureString"></param>
        /// <param name="action"></param>
        public static void DecryptSecureString(SecureString secureString, Action<string> action)
        {
            DecryptSecureString<int>(secureString, (s) =>
            {
                action(s);
                return 0;
            });
        }
    }
}

StringsTest.cs

using Microsoft.VisualStudio.TestTools.UnitTesting;
using System.Security;

namespace SecurityUtils.Test
{
    [TestClass]
    public class StringsTest
    {
        [TestMethod]
        public void DecryptSecureStringWithFunc()
        {
            // Arrange
            var secureString = new SecureString();

            foreach (var c in "UserPassword123".ToCharArray())
                secureString.AppendChar(c);

            secureString.MakeReadOnly();

            // Act
            var result = Strings.DecryptSecureString<bool>(secureString, (password) =>
            {
                return password.Equals("UserPassword123");
            });

            // Assert
            Assert.IsTrue(result);
        }

        [TestMethod]
        public void DecryptSecureStringWithAction()
        {
            // Arrange
            var secureString = new SecureString();

            foreach (var c in "UserPassword123".ToCharArray())
                secureString.AppendChar(c);

            secureString.MakeReadOnly();

            // Act
            var result = false;

            Strings.DecryptSecureString(secureString, (password) =>
            {
                result = password.Equals("UserPassword123");
            });

            // Assert
            Assert.IsTrue(result);
        }
    }
}

Obviously, this doesn't prevent abuse of this function in the following manner, so just be careful not to do this:

[TestMethod]
public void DecryptSecureStringWithAction()
{
    // Arrange
    var secureString = new SecureString();

    foreach (var c in "UserPassword123".ToCharArray())
        secureString.AppendChar(c);

    secureString.MakeReadOnly();

    // Act
    string copyPassword = null;

    Strings.DecryptSecureString(secureString, (password) =>
    {
        copyPassword = password; // Please don't do this!
    });

    // Assert
    Assert.IsNull(copyPassword); // Fails
}

Happy coding!

Answer 6

I created the following extension methods based on the answer from rdev5. Pinning the managed string is important as it prevents the garbage collector from moving it around and leaving behind copies that you're unable to erase.

I think the advantage of my solution has is that no unsafe code is needed.

/// <summary>
/// Allows a decrypted secure string to be used whilst minimising the exposure of the
/// unencrypted string.
/// </summary>
/// <typeparam name="T">Generic type returned by Func delegate.</typeparam>
/// <param name="secureString">The string to decrypt.</param>
/// <param name="action">
/// Func delegate which will receive the decrypted password as a string object
/// </param>
/// <returns>Result of Func delegate</returns>
/// <remarks>
/// This method creates an empty managed string and pins it so that the garbage collector
/// cannot move it around and create copies. An unmanaged copy of the the secure string is
/// then created and copied into the managed string. The action is then called using the
/// managed string. Both the managed and unmanaged strings are then zeroed to erase their
/// contents. The managed string is unpinned so that the garbage collector can resume normal
/// behaviour and the unmanaged string is freed.
/// </remarks>
public static T UseDecryptedSecureString<T>(this SecureString secureString, Func<string, T> action)
{
    int length = secureString.Length;
    IntPtr sourceStringPointer = IntPtr.Zero;

    // Create an empty string of the correct size and pin it so that the GC can't move it around.
    string insecureString = new string('\0', length);
    var insecureStringHandler = GCHandle.Alloc(insecureString, GCHandleType.Pinned);

    IntPtr insecureStringPointer = insecureStringHandler.AddrOfPinnedObject();

    try
    {
        // Create an unmanaged copy of the secure string.
        sourceStringPointer = Marshal.SecureStringToBSTR(secureString);

        // Use the pointers to copy from the unmanaged to managed string.
        for (int i = 0; i < secureString.Length; i++)
        {
            short unicodeChar = Marshal.ReadInt16(sourceStringPointer, i * 2);
            Marshal.WriteInt16(insecureStringPointer, i * 2, unicodeChar);
        }

        return action(insecureString);
    }
    finally
    {
        // Zero the managed string so that the string is erased. Then unpin it to allow the
        // GC to take over.
        Marshal.Copy(new byte[length], 0, insecureStringPointer, length);
        insecureStringHandler.Free();

        // Zero and free the unmanaged string.
        Marshal.ZeroFreeBSTR(sourceStringPointer);
    }
}

/// <summary>
/// Allows a decrypted secure string to be used whilst minimising the exposure of the
/// unencrypted string.
/// </summary>
/// <param name="secureString">The string to decrypt.</param>
/// <param name="action">
/// Func delegate which will receive the decrypted password as a string object
/// </param>
/// <returns>Result of Func delegate</returns>
/// <remarks>
/// This method creates an empty managed string and pins it so that the garbage collector
/// cannot move it around and create copies. An unmanaged copy of the the secure string is
/// then created and copied into the managed string. The action is then called using the
/// managed string. Both the managed and unmanaged strings are then zeroed to erase their
/// contents. The managed string is unpinned so that the garbage collector can resume normal
/// behaviour and the unmanaged string is freed.
/// </remarks>
public static void UseDecryptedSecureString(this SecureString secureString, Action<string> action)
{
    UseDecryptedSecureString(secureString, (s) =>
    {
        action(s);
        return 0;
    });
}

Answer 7

Final working solution according to sclarke81 solution and John Flaherty fixes is:

    public static class Utils
    {
        /// <remarks>
        /// This method creates an empty managed string and pins it so that the garbage collector
        /// cannot move it around and create copies. An unmanaged copy of the the secure string is
        /// then created and copied into the managed string. The action is then called using the
        /// managed string. Both the managed and unmanaged strings are then zeroed to erase their
        /// contents. The managed string is unpinned so that the garbage collector can resume normal
        /// behaviour and the unmanaged string is freed.
        /// </remarks>
        public static T UseDecryptedSecureString<T>(this SecureString secureString, Func<string, T> action)
        {
            int length = secureString.Length;
            IntPtr sourceStringPointer = IntPtr.Zero;

            // Create an empty string of the correct size and pin it so that the GC can't move it around.
            string insecureString = new string('\0', length);
            var insecureStringHandler = GCHandle.Alloc(insecureString, GCHandleType.Pinned);

            IntPtr insecureStringPointer = insecureStringHandler.AddrOfPinnedObject();

            try
            {
                // Create an unmanaged copy of the secure string.
                sourceStringPointer = Marshal.SecureStringToBSTR(secureString);

                // Use the pointers to copy from the unmanaged to managed string.
                for (int i = 0; i < secureString.Length; i++)
                {
                    short unicodeChar = Marshal.ReadInt16(sourceStringPointer, i * 2);
                    Marshal.WriteInt16(insecureStringPointer, i * 2, unicodeChar);
                }

                return action(insecureString);
            }
            finally
            {
                // Zero the managed string so that the string is erased. Then unpin it to allow the
                // GC to take over.
                Marshal.Copy(new byte[length * 2], 0, insecureStringPointer, length * 2);
                insecureStringHandler.Free();

                // Zero and free the unmanaged string.
                Marshal.ZeroFreeBSTR(sourceStringPointer);
            }
        }

        /// <summary>
        /// Allows a decrypted secure string to be used whilst minimising the exposure of the
        /// unencrypted string.
        /// </summary>
        /// <param name="secureString">The string to decrypt.</param>
        /// <param name="action">
        /// Func delegate which will receive the decrypted password as a string object
        /// </param>
        /// <returns>Result of Func delegate</returns>
        /// <remarks>
        /// This method creates an empty managed string and pins it so that the garbage collector
        /// cannot move it around and create copies. An unmanaged copy of the the secure string is
        /// then created and copied into the managed string. The action is then called using the
        /// managed string. Both the managed and unmanaged strings are then zeroed to erase their
        /// contents. The managed string is unpinned so that the garbage collector can resume normal
        /// behaviour and the unmanaged string is freed.
        /// </remarks>
        public static void UseDecryptedSecureString(this SecureString secureString, Action<string> action)
        {
            UseDecryptedSecureString(secureString, (s) =>
            {
                action(s);
                return 0;
            });
        }
    }

Answer 8

I derived from This answer by sclarke81. I like his answer and I'm using the derivative but sclarke81's has a bug. I don't have reputation so I can't comment. The problem seems small enough that it didn't warrant another answer and I could edit it. So I did. It got rejected. So now we have another answer.

sclarke81 I hope you see this (in finally):

Marshal.Copy(new byte[length], 0, insecureStringPointer, length);

should be:

Marshal.Copy(new byte[length * 2], 0, insecureStringPointer, length * 2);

And the full answer with the bug fix:

    /// 
    /// Allows a decrypted secure string to be used whilst minimising the exposure of the
    /// unencrypted string.
    /// 
    /// Generic type returned by Func delegate.
    /// The string to decrypt.
    /// 
    /// Func delegate which will receive the decrypted password as a string object
    /// 
    /// Result of Func delegate
    /// 
    /// This method creates an empty managed string and pins it so that the garbage collector
    /// cannot move it around and create copies. An unmanaged copy of the the secure string is
    /// then created and copied into the managed string. The action is then called using the
    /// managed string. Both the managed and unmanaged strings are then zeroed to erase their
    /// contents. The managed string is unpinned so that the garbage collector can resume normal
    /// behaviour and the unmanaged string is freed.
    /// 
    public static T UseDecryptedSecureString(this SecureString secureString, Func action)
    {
        int length = secureString.Length;
        IntPtr sourceStringPointer = IntPtr.Zero;

        // Create an empty string of the correct size and pin it so that the GC can't move it around.
        string insecureString = new string('\0', length);
        var insecureStringHandler = GCHandle.Alloc(insecureString, GCHandleType.Pinned);

        IntPtr insecureStringPointer = insecureStringHandler.AddrOfPinnedObject();

        try
        {
            // Create an unmanaged copy of the secure string.
            sourceStringPointer = Marshal.SecureStringToBSTR(secureString);

            // Use the pointers to copy from the unmanaged to managed string.
            for (int i = 0; i < secureString.Length; i++)
            {
                short unicodeChar = Marshal.ReadInt16(sourceStringPointer, i * 2);
                Marshal.WriteInt16(insecureStringPointer, i * 2, unicodeChar);
            }

            return action(insecureString);
        }
        finally
        {
            // Zero the managed string so that the string is erased. Then unpin it to allow the
            // GC to take over.
            Marshal.Copy(new byte[length * 2], 0, insecureStringPointer, length * 2);
            insecureStringHandler.Free();

            // Zero and free the unmanaged string.
            Marshal.ZeroFreeBSTR(sourceStringPointer);
        }
    }

    /// 
    /// Allows a decrypted secure string to be used whilst minimising the exposure of the
    /// unencrypted string.
    /// 
    /// The string to decrypt.
    /// 
    /// Func delegate which will receive the decrypted password as a string object
    /// 
    /// Result of Func delegate
    /// 
    /// This method creates an empty managed string and pins it so that the garbage collector
    /// cannot move it around and create copies. An unmanaged copy of the the secure string is
    /// then created and copied into the managed string. The action is then called using the
    /// managed string. Both the managed and unmanaged strings are then zeroed to erase their
    /// contents. The managed string is unpinned so that the garbage collector can resume normal
    /// behaviour and the unmanaged string is freed.
    /// 
    public static void UseDecryptedSecureString(this SecureString secureString, Action action)
    {
        UseDecryptedSecureString(secureString, (s) =>
        {
            action(s);
            return 0;
        });
    }
}

Answer 9

The code accepted as the answer is correct, and will work in most circumstances, but as mentioned in the comments using BSTR would be better and will cover all circumstances:

private string SecureStringToString(SecureString value) {
    IntPtr valuePtr = IntPtr.Zero;
    try {
        valuePtr = Marshal.SecureStringToBSTR(value);
        return Marshal.PtrToStringBSTR(valuePtr);
    } finally {
        Marshal.ZeroFreeBSTR(valuePtr);
    }
}

Answer 10

Use the following:

var plaintextPwd = new System.Net.NetworkCredential("", <securestring with your encrypted password>).Password

Answer 11

This C# code is what you want.

%ProjectPath%/SecureStringsEasy.cs

using System;
using System.Security;
using System.Runtime.InteropServices;
namespace SecureStringsEasy
{
    public static class MyExtensions
    {
        public static SecureString ToSecureString(string input)
        {
            SecureString secureString = new SecureString();
            foreach (var item in input)
            {
                secureString.AppendChar(item);
            }
            return secureString;
        }
        public static string ToNormalString(SecureString input)
        {
            IntPtr strptr = Marshal.SecureStringToBSTR(input);
            string normal = Marshal.PtrToStringBSTR(strptr);
            Marshal.ZeroFreeBSTR(strptr);
            return normal;
        }
    }
}

Answer 12

// using so that Marshal doesn't have to be qualified
using System.Runtime.InteropServices;    
//using for SecureString
using System.Security;
public string DecodeSecureString (SecureString Convert) 
{
    //convert to IntPtr using Marshal
    IntPtr cvttmpst = Marshal.SecureStringToBSTR(Convert);
    //convert to string using Marshal
    string cvtPlainPassword = Marshal.PtrToStringAuto(cvttmpst);
    //return the now plain string
    return cvtPlainPassword;
}

Answer 13

If you use a StringBuilder instead of a string, you can overwrite the actual value in memory when you are done. That way the password won't hang around in memory until garbage collection picks it up.

StringBuilder.Append(plainTextPassword);
StringBuilder.Clear();
// overwrite with reasonably random characters
StringBuilder.Append(New Guid().ToString());