C# – How to convert SecureString to System.String

.netc++encryptionsecurity

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.

Best Solution

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);
  }
}