I've always felt like SecureString was a little odd, but assumed most of my issues with it were due to security problems I don't understand. Today I decided to sit down and teach myself about it, but I've hit what seems like a fatal snag.
The scenario I envision is "user enters password into text box, that password is hashed and compared to a stored hash". At first I was worried that the text box contained the string, but then I realized you could roll a custom text box that uses SecureString as its store. Cool. It's the "that password is hashed and compared..." part that is giving me trouble.
My first hack at the problem in VB .NET was naive and wrong:
Dim passwordHandle As IntPtr
Dim insecurePassword As String = Nothing
Try
passwordHandle = Marshal.SecureStringToBSTR(_password)
insecurePassword = Marshal.PtrToStringBSTR(passwordHandle)
Catch ex As Exception
Finally
If passwordHandle <> IntPtr.Zero Then
Marshal.ZeroFreeBSTR(passwordHandle)
End If
End Try
If insecurePassword <> Nothing Then
' Do hash and comparison
End If
This just stuffs the password into a regular string and defeats the purpose of using SecureString in the first place. So I kept searching and found a blog post that solves the problem nicely in C#: the string is made into a BSTR, copied into a pinned string, then both the BSTR and pinned string are zeroed after use. This seems like a much better idea because it minimizes the amount of time that the insecure string is in memory. However, it doesn't look like there's a way to pull this off in VB .NET. C# is using its unsafe code feature to do pointer manipulations, but VB .NET cannot do this. I had a look at Marhsall.Copy(), but it looks like it's oriented towards arrays. I thought about trying to cast the IntPtr variables for the object and BSTR to Strings, but that still left me using a method like String.Replace(), which will create a new string.
Is it not possible to do this from VB .NET at all, or is there something I'm missing?
edit I'm accepting AMissico's answer with only slight reservations. The Marshal.ReadByte() method will copy a byte from unmanaged memory and create a byte in unmanaged memory. This produces the small chance that an attacker could find the individual characters of the password. I think that's far less than the odds of finding an entire string, but the C# in the (apparently defunct) article I referenced was able to use unsafe code to avoid this neatly. The thought process was it used GCHandle to pin a string in memory, then used unsafe code to get around the immutability of .NET strings. Clever trick that seems impossible in VB .NET. I'll try to come back with the C# code itself.
The link to "Marshaling SecureString Passwords to String - Mark Nicholson " at http://dotnet.org.za/markn/archive/2008/10/04/handling-passwords.aspx finally displayed.
So your statement, "C# is using its unsafe code feature to do pointer manipulations," seems to be addressed with "HOW TO: Wrap a UCOMIStream in a Stream Class in Visual Basic .NET" at http://support.microsoft.com/kb/321695. (I searched on "AddrOfPinnedObject".)
I did not read your whole question (link to post always timed out), but are these classes and testing code helpful? The password never lives as a System.String; therefore, you need a SecureStringTextBox implementation as noted in your question.
I dislike adding all this code. Let me know what code is helpful, and I will edit the answer to keep only the helpful stuff.
Imports System.Security
Imports System.Security.Principal
Imports System.Security.Permissions
Imports System.Runtime.InteropServices
''' <summary>
''' Helper class to programmatically impersonate a user, load and unload a user's profile, and perform other maintenance-related tasks for impersonating a user.
''' </summary>
Public Class ImpersonationHelper
Implements IDisposable
#Region " IDisposable Implementaton "
Private _disposed As Boolean
Protected Overrides Sub Finalize()
Dispose(False)
MyBase.Finalize()
End Sub
''' <summary>
''' Implementation of the <b>IDisposable</b> interface.
''' </summary>
''' <remarks>This method calls <see>Undo</see> if impersonation is still being performed. This method calls the common language runtime version of the Dispose method.</remarks>
Public Overloads Sub Dispose() Implements IDisposable.Dispose
Dispose(True)
System.GC.SuppressFinalize(Me)
End Sub
''' <summary>
''' Implementation of the <b>IDisposable</b> interface.
''' </summary>
''' <param name="disposing">If <b>true</b>, the object to be disposed is finalized and collected by the garbage collector; otherwise, <b>false</b>.</param>
''' <remarks>This method calls Undo if impersonation is still being performed. This method calls the common language runtime version of the Dispose method.</remarks>
Protected Overloads Sub Dispose(ByVal disposing As Boolean)
If Not _disposed Then
If disposing Then
If Not IsNothing(_impersonationContext) Then
_impersonationContext.Undo()
_impersonationContext.Dispose()
End If
End If
_impersonationContext = Nothing
End If
_disposed = True
End Sub
#End Region
'2009.02.12 AMJ
' Modified From:
' How to implement impersonation in an ASP.NET application (KB306158)
' http://support.microsoft.com/kb/306158
' Implemented IDisposable based on ImpersonationHelper class of
' Namespace: Microsoft.Office.Excel.Server.Addins.ComputeCluster.Security
' Assembly: Microsoft.Office.Excel.Server.Addins.ComputeCluster (in microsoft.office.excel.server.addins.computecluster.dll)
Const LOGON32_LOGON_INTERACTIVE As Integer = 2
Const LOGON32_LOGON_BATCH As Integer = 4
Const LOGON32_LOGON_SERVICE As Integer = 5
Const LOGON32_PROVIDER_DEFAULT As Integer = 0
Const LOGON32_PROVIDER_WINNT35 As Integer = 1
Private Enum SECURITY_IMPERSONATION_LEVEL
SecurityAnonymous = 0
SecurityIdentification = 1
SecurityImpersonation = 2
SecurityDelegation = 3
End Enum
Private Declare Auto Function LogonUser Lib "advapi32.dll" ( _
ByVal username As String, _
ByVal domain As String, _
ByVal password As IntPtr, _
ByVal logonType As Integer, _
ByVal logonProvider As Integer, _
ByRef token As IntPtr) As Boolean
Private Declare Auto Function DuplicateToken Lib "advapi32.dll" ( _
ByVal ExistingTokenHandle As IntPtr, _
ByVal ImpersonationLevel As SECURITY_IMPERSONATION_LEVEL, _
ByRef DuplicateTokenHandle As IntPtr) As Integer
Private Declare Auto Function RevertToSelf Lib "advapi32.dll" () As Long
Private Declare Auto Function CloseHandle Lib "kernel32.dll" (ByVal handle As IntPtr) As Long
Dim _impersonationContext As WindowsImpersonationContext
Dim _domain As String
Dim _login As String
Dim _password As SecureString
#Region " Standard Constructor & Properties "
''' <summary>
''' Initializes a new instance of the ImpersonationHelper class.
''' </summary>
''' <param name="domain">The domain or computer name of the user to impersonate.</param>
''' <param name="userName">The user name of the user to impersonate.</param>
''' <param name="password">The secure string password of UserName. For more information about secure strings, see the <see cref="System.Security.SecureString">SecureString</see> class.</param>
<DebuggerNonUserCode()> _
Public Sub New(ByVal domain As String, ByVal userName As String, ByVal password As SecureString)
Me.Domain = domain
Me.Login = userName
Me.Password = password
End Sub
''' <summary>
''' Do not allow a new instance of the ImpersonationHelper class without credentials.
''' </summary>
Private Sub New()
End Sub
''' <summary>
''' Gets or sets the domain of the user to impersonate.
''' </summary>
''' <value>The domain of the user.</value>
<DebuggerNonUserCode()> _
Public Property Domain() As String
Get
Return _domain
End Get
Set(ByVal value As String)
_domain = value
End Set
End Property
''' <summary>
''' Gets or sets the user name of the user to impersonate.
''' </summary>
''' <value>The user name.</value>
<DebuggerNonUserCode()> _
Public Property Login() As String
Get
Return _login
End Get
Set(ByVal value As String)
_login = value
End Set
End Property
''' <summary>
''' Sets the encrypted password of the user to impersonate.
''' </summary>
''' <value>The encrypted password.</value>
<DebuggerNonUserCode()> _
Public WriteOnly Property Password() As SecureString
Set(ByVal value As SecureString)
_password = value
End Set
End Property
#End Region
''' <summary>
''' Performs the impersonation of the user based on the parameters provided in the constructor.
''' </summary>
''' <remarks>
''' <para>If logon fails using the supplied credentials, an exception is thrown. The exception is thrown because this method is unable to duplicate the logged-on user's token for purposes of impersonation or is unable to create a Windows identity from the user's impersonated token.</para>
''' <para>For details about the direct cause of the impersonation failure, you can inspect the inner exception.</para>
''' </remarks>
<PermissionSetAttribute(SecurityAction.Demand, Name:="FullTrust")> _
Public Sub ImpersonateUser()
Dim fResult As Boolean = False 'assume impersonation failed
Dim hPassword As IntPtr = IntPtr.Zero
Dim hToken As IntPtr = IntPtr.Zero
Dim hTokenDuplicate As IntPtr = IntPtr.Zero
Dim oException As ImpersonationException = Nothing
If RevertToSelf <> 0 Then
hPassword = Marshal.SecureStringToGlobalAllocUnicode(_password)
If LogonUser(Me.Login, Me.Domain, hPassword, LOGON32_LOGON_INTERACTIVE, LOGON32_PROVIDER_DEFAULT, hToken) Then
If DuplicateToken(hToken, SECURITY_IMPERSONATION_LEVEL.SecurityImpersonation, hTokenDuplicate) <> 0 Then
_impersonationContext = New WindowsIdentity(hTokenDuplicate).Impersonate()
If Not _impersonationContext Is Nothing Then
fResult = True
End If
End If
Else
oException = New ImpersonationException(Me.Login, Me.Domain)
End If
If hPassword.Equals(IntPtr.Zero) = False Then
Marshal.ZeroFreeGlobalAllocUnicode(hPassword)
End If
End If
If Not hTokenDuplicate.Equals(IntPtr.Zero) Then
CloseHandle(hTokenDuplicate)
End If
If Not hToken.Equals(IntPtr.Zero) Then
CloseHandle(hToken)
End If
If Not (oException Is Nothing) Then
Throw oException
End If
End Sub
''' <summary>
''' Undoes the impersonation of the user, if it is impersonated.
''' </summary>
''' <remarks>Use this method to free the objects associated with impersonation.</remarks>
<PermissionSetAttribute(SecurityAction.Demand, Name:="FullTrust")> _
<DebuggerNonUserCode()> _
Public Sub Undo()
_impersonationContext.Undo()
_impersonationContext = Nothing
End Sub
Public Shared Function InvokeAsUser(ByVal userName As String, ByVal domain As String, ByVal password As SecureString, ByVal methodToCall As [Delegate], ByVal ParamArray parameters() As Object) As Object
Dim oResult As Object = Nothing
Using oImpersonation As New ImpersonationHelper(domain, userName, password)
oImpersonation.ImpersonateUser()
oResult = methodToCall.DynamicInvoke(parameters)
End Using
Return oResult
End Function
End Class
Public Class ImpersonationException
Inherits System.Exception
Public ReadOnly Login As String
Public ReadOnly Domain As String
Public Sub New(ByVal userName As String, ByVal domain As String)
MyBase.New(String.Format("Impersonation failure: {1}\{0}", userName, domain), New System.ComponentModel.Win32Exception)
End Sub
End Class
Imports Missico.Personal
Imports System.Security
Imports Microsoft.VisualStudio.TestTools.UnitTesting
<TestClass()> _
Public Class ImpersonationHelperTest
Private testContextInstance As TestContext
Public Property TestContext() As TestContext
Get
Return testContextInstance
End Get
Set(ByVal value As TestContext)
testContextInstance = value
End Set
End Property
<TestMethod()> _
Public Sub ImpersonationHelperTest()
'testing only, never initialize the characters of the password in this fashion
'replace with valid password
Dim oPassword As New System.Security.SecureString
oPassword.AppendChar("o"c)
oPassword.AppendChar("o"c)
oPassword.AppendChar("p"c)
oPassword.AppendChar("s"c)
oPassword.AppendChar("!"c)
oPassword.AppendChar(" "c)
oPassword.AppendChar("n"c)
oPassword.AppendChar("o"c)
oPassword.AppendChar(" "c)
oPassword.AppendChar("p"c)
oPassword.AppendChar("a"c)
oPassword.AppendChar("s"c)
oPassword.AppendChar("s"c)
oPassword.AppendChar("w"c)
oPassword.AppendChar("o"c)
oPassword.AppendChar("r"c)
oPassword.AppendChar("d"c)
Using oImpersonation As New ImpersonationHelper("ANTHONY", "amissico", oPassword)
oImpersonation.ImpersonateUser()
'...
End Using
Try
Using oImpersonation As New ImpersonationHelper("INVALID", "amissico", oPassword)
oImpersonation.ImpersonateUser()
'...
End Using
Catch ex As ImpersonationException
'expected
' due to invalid domain
End Try
Try
Using oImpersonation As New ImpersonationHelper("ANTHONY", "INVALID", oPassword)
oImpersonation.ImpersonateUser()
'...
End Using
Catch ex As ImpersonationException
'expected
' due to invalid user
End Try
Try
oPassword.AppendChar(" "c) 'invalidate password
Using oImpersonation As New ImpersonationHelper("ANTHONY", "amissico", oPassword)
oImpersonation.ImpersonateUser()
'...
End Using
Catch ex As ImpersonationException
'expected
' due to invalid password
End Try
End Sub
End Class
Imports System.Security
Imports System.Runtime.InteropServices
Imports System.Runtime.CompilerServices
Public Module SecureStringExtensions
''' <summary>
''' Determines whether the specified <see cref="System.Security.SecureString">System.Security.SecureString</see> instances are considered equal.
''' </summary>
''' <param name="valueA">The first <see cref="System.Security.SecureString">System.Security.SecureString</see> to compare.</param>
''' <param name="valueB">The second <see cref="System.Security.SecureString">System.Security.SecureString</see> to compare.</param>
''' <returns>True if valueA is equal to valueB; otherwise, False.</returns>
<Extension()> _
Public Function Equals(ByVal valueA As SecureString, ByVal valueB As SecureString) As Boolean
Return IsEqual(valueA, valueB)
End Function
''' <summary>
''' Determines whether the specified <see cref="System.Security.SecureString">System.Security.SecureString</see> instances are considered equal.
''' </summary>
''' <param name="valueA">The first <see cref="System.Security.SecureString">System.Security.SecureString</see> to compare.</param>
''' <param name="valueB">The second <see cref="System.Security.SecureString">System.Security.SecureString</see> to compare.</param>
''' <returns>True if valueA is equal to valueB; otherwise, False.</returns>
''' <remarks>Comparison loop based on Microsoft souce code for String.EqualsHelper method.</remarks>
<Extension()> _
Public Function IsEqual(ByVal valueA As SecureString, ByVal valueB As SecureString) As Boolean
Dim fResult As Boolean = False 'assume failure
'short-circuit if lengths are not the same
If valueA.Length <> valueB.Length Then
'cannot be the same value
Return False
End If
Using oCopyA As SecureString = valueA.Copy, oCopyB As SecureString = valueB.Copy
Dim iLength As Integer = oCopyA.Length
Dim oPtrA As IntPtr = Marshal.SecureStringToBSTR(oCopyA)
Dim oPtrB As IntPtr = Marshal.SecureStringToBSTR(oCopyB)
Try
Do While (iLength > 0)
If Marshal.ReadByte(oPtrA, iLength) <> Marshal.ReadByte(oPtrB, iLength) Then
Exit Do
End If
iLength -= 1
Loop
fResult = (iLength <= 0)
Finally
Marshal.ZeroFreeBSTR(oPtrA)
Marshal.ZeroFreeBSTR(oPtrA)
End Try
End Using
Return fResult
End Function
End Module
Imports System.Security
Imports System.Diagnostics
Imports Microsoft.VisualStudio.TestTools.UnitTesting
Imports Missico.Security.SecureStringExtensions
<TestClass()> _
Public Class SecureStringExtensionsFixture
#Region " TestContext "
Private testContextInstance As TestContext
Public Property TestContext() As TestContext
Get
Return testContextInstance
End Get
Set(ByVal value As TestContext)
testContextInstance = value
End Set
End Property
#End Region
<TestMethod()> _
Public Sub EqualsTest()
Dim oValueA As New SecureString
Dim oValueB As New SecureString
oValueA.AppendChar("p"c)
oValueA.AppendChar("a"c)
oValueA.AppendChar("s"c)
oValueA.AppendChar("s"c)
oValueA.AppendChar("w"c)
oValueA.AppendChar("o"c)
oValueA.AppendChar("r"c)
oValueA.AppendChar("d"c)
oValueB.AppendChar("p"c)
oValueB.AppendChar("a"c)
oValueB.AppendChar("s"c)
oValueB.AppendChar("s"c)
oValueB.AppendChar("w"c)
oValueB.AppendChar("o"c)
oValueB.AppendChar("r"c)
oValueB.AppendChar("d"c)
'The Object.Equal method does not work because you cannot compare to secure strings.
If oValueA.Equals(oValueB) Then
'expected, but does not work
'you cannot compare two secure strings
Else
'always fails
End If
'Using the fully-qualified path to the Equal extension method.
If Missico.Security.SecureStringExtensions.Equals(oValueA, oValueB) Then
'expected
Else
Assert.Fail("SecureString values are not equal, which is not expected.")
End If
'Using the IsEqual extension method that does not conflict with the Object.Equal method.
If oValueA.IsEqual(oValueB) Then
'expected
Else
Assert.Fail("SecureString values are not equal, which is not expected.")
End If
'change the second value
oValueB.AppendChar(" "c)
If oValueA.IsEqual(oValueB) Then
Assert.Fail("SecureString values are equal, which is not expected.")
Else
'expected
End If
End Sub
End Class