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convertsidtostringsid (advapi32)
 
.
Summary
The ConvertSidToStringSid function converts a security identifier (SID) to a string format suitable for display, storage, or transmission.

C# Signature:

[DllImport("advapi32", CharSet=CharSet.Auto, SetLastError=true)]
static extern bool ConvertSidToStringSid(
    [MarshalAs(UnmanagedType.LPArray)] byte [] pSID,
    out IntPtr ptrSid);

[DllImport("advapi32", CharSet = CharSet.Auto, SetLastError = true)]
static extern bool ConvertSidToStringSid(IntPtr pSid, out string strSid);

VB Signature:

Declare Auto Function ConvertSidToStringSid Lib "advapi32.dll" (ByVal pSID() As Byte, _
   ByRef ptrSid As IntPtr) As Boolean

Notes:

Warning

As explained here, memory for the returned unmanaged string must be freed by calling LocalFree. Therefore, you must define the string as an IntPtr in managed code and call LocalFree yourself. Otherwise, the marshaler would call CoTaskMemFree which is not correct in this case.

Tips & Tricks:

Dissent
Don't use an IntPtr like above, just use a System.Text.StringBuilder. You shouldn't need to worry about calling LocalFree in this case either, but I have not confirmed this 100%.

Sample Code:

// C# sample
public static string GetSidString(byte[] sid)
{
   IntPtr ptrSid;
   string sidString;
   if (!ConvertSidToStringSid(sid,out ptrSid))
     throw new System.ComponentModel.Win32Exception();
   try
   {
     sidString = Marshal.PtrToStringAuto(ptrSid);
   }
   finally
   {
     LocalFree(ptrSid);
   }
   return sidString;
}

// Another C# Sample that converts a sid from a DirectoryEntry object

private string GetTextualSID(DirectoryEntry objGroup){
  string sSID = string.Empty;
  byte[] SID = objGroup.Properties["objectSID"].Value as byte[];
  IntPtr sidPtr = Marshal.AllocHGlobal( SID.Length);
  sSID = "";
  System.Runtime.InteropServices.Marshal.Copy(SID, 0, sidPtr, SID.Length);
  ConvertSidToStringSid((IntPtr)sidPtr, ref sSID);
  System.Runtime.InteropServices.Marshal.FreeHGlobal( sidPtr );
  return sSID;}


'VB Sample
Public Shared Function ByteArrayToStringSid(ByRef bArray As Byte()) As String
   Dim ptrSID As IntPtr = Nothing
   Try
     Dim sSID As String = String.Empty
     If ConvertSidToStringSid(bArray, ptrSID) = True Then
       'The PtrToStringXXX call here needs to match the CharSet used on your
       'ConvertSidToStringSid DllImport.  The default is CharSet.Ansi.
       sSID = System.Runtime.InteropServices.Marshal.PtrToStringAnsi(ptrSID)
     End If
     Return sSID
   Finally
     LocalFree(ptrSID)
   End Try
End Function

'Alternative VB Sample
Public Shared Function ByteArrayToStringSid(ByRef bArray As Byte()) As String
   Dim ptrSID As IntPtr = Nothing
   Try
     Dim sSID As String = String.Empty
     If ConvertSidToStringSid(bArray, ptrSID) = True Then
       sSID = System.Runtime.InteropServices.Marshal.PtrToStringAuto(ptrSID)
     End If
     Return sSID
   Finally
     System.Runtime.InteropServices.Marshal.FreeHGlobal(ptrSID)
   End Try
End Function

Alternative Managed API:

System.Security.Principal.SecurityIdentifier.ToString()

(see "Another C# Example" below)

Alternative Managed Code:

// C# Example
private string ConvertByteToStringSid(Byte[] sidBytes)
    {
        short sSubAuthorityCount = 0;
        StringBuilder strSid = new StringBuilder();
        strSid.Append("S-");
        try
        {
        // Add SID revision.
        strSid.Append(sidBytes[0].ToString());

        sSubAuthorityCount = Convert.ToInt16(sidBytes[1]);

        // Next six bytes are SID authority value.
        if (sidBytes[2] != 0 || sidBytes[3] != 0)
        {
            string strAuth = String.Format("0x{0:2x}{1:2x}{2:2x}{3:2x}{4:2x}{5:2x}",
                           (Int16) sidBytes[2],
                           (Int16) sidBytes[3],
                           (Int16) sidBytes[4],
                           (Int16) sidBytes[5],
                           (Int16) sidBytes[6],
                           (Int16) sidBytes[7]);
            strSid.Append("-");
            strSid.Append(strAuth);
        }
        else
        {
            Int64 iVal = sidBytes[7] +
                 (sidBytes[6] << 8) +
                 (sidBytes[5] << 16) +
                 (sidBytes[4] << 24);
            strSid.Append("-");
            strSid.Append(iVal.ToString());
        }

        // Get sub authority count...
        int idxAuth = 0;
        for (int i = 0; i < sSubAuthorityCount; i++)
        {
            idxAuth = 8 + i*4;
            UInt32 iSubAuth = BitConverter.ToUInt32(sidBytes, idxAuth);
            strSid.Append("-");
            strSid.Append(iSubAuth.ToString());
        }
        }
        catch (Exception ex)
        {
        Trace.TraceWarning(ex.Message);
        throw;
        }
        return strSid.ToString();
    }

Another C# Example

using System.Security.Principal;

private string ConvertSidBytesToString(byte[] sidBytes)
{
   //SecurityIdentifier is defined in the System.Security.Principal namespace.
   SecurityIdentifier si = new SecurityIdentifier(sidBytes, 0);

   return si.ToString();
}

A PowerShell Example

     $strSID='S-1-5-21-XXXXXXXXXX-XXXXXXXXX-XXXXXXXXXX-1026'
     $binarySid = New-Object byte[] $sid.BinaryLength
     ([System.Security.Principal.SecurityIdentifier]$strSid).GetBinaryForm($binarySid,0)

Documentation

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