How to convert SecureString to System.String?

Question

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);
  }
}

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.

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);
    }
}

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

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

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:

Pin the string in memory (which is what you want to do but appears to be missing from most answers here).
Pass its reference to the Func/Action delegate.
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:

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).
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!

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;
    });
}

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;
            });
        }
    }

How to convert SecureString to System.String?

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c#

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security

encryption

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How to convert SecureString to System.String?

Tags:

c#

.net

security

encryption

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