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BOOL GetTextualSid(
    PSID pSid,            // binary SID
    LPTSTR TextualSid,    // buffer for Textual representation of SID
    LPDWORD lpdwBufferLen // required/provided TextualSid buffersize
    )
{
    PSID_IDENTIFIER_AUTHORITY psia;
    DWORD dwSubAuthorities;
    DWORD dwSidRev=SID_REVISION;
    DWORD dwCounter;
    DWORD dwSidSize;

    // Validate the binary SID.

    if(!IsValidSid(pSid)) return FALSE;

    // Get the identifier authority value from the SID.

    psia = GetSidIdentifierAuthority(pSid);

    // Get the number of subauthorities in the SID.

    dwSubAuthorities = *GetSidSubAuthorityCount(pSid);

    // Compute the buffer length.
    // S-SID_REVISION- + IdentifierAuthority- + subauthorities- + NULL

    dwSidSize=(15 + 12 + (12 * dwSubAuthorities) + 1) * sizeof(TCHAR);

    // Check input buffer length.
    // If too small, indicate the proper size and set last error.

    if (*lpdwBufferLen < dwSidSize)
    {
        *lpdwBufferLen = dwSidSize;
        SetLastError(ERROR_INSUFFICIENT_BUFFER);
        return FALSE;
    }

    // Add 'S' prefix and revision number to the string.

    dwSidSize=wsprintf(TextualSid, TEXT("S-%lu-"), dwSidRev );

    // Add SID identifier authority to the string.

    if ( (psia->Value[0] != 0) || (psia->Value[1] != 0) )
    {
        dwSidSize+=wsprintf(TextualSid + lstrlen(TextualSid),
                    TEXT("0x%02hx%02hx%02hx%02hx%02hx%02hx"),
                    (USHORT)psia->Value[0],
                    (USHORT)psia->Value[1],
                    (USHORT)psia->Value[2],
                    (USHORT)psia->Value[3],
                    (USHORT)psia->Value[4],
                    (USHORT)psia->Value[5]);
    }
    else
    {
        dwSidSize+=wsprintf(TextualSid + lstrlen(TextualSid),
                    TEXT("%lu"),
                    (ULONG)(psia->Value[5]      )   +
                    (ULONG)(psia->Value[4] <<  8)   +
                    (ULONG)(psia->Value[3] << 16)   +
                    (ULONG)(psia->Value[2] << 24)   );
    }

    // Add SID subauthorities to the string.
    //
    for (dwCounter=0 ; dwCounter < dwSubAuthorities ; dwCounter++)
    {
        dwSidSize+=wsprintf(TextualSid + dwSidSize, TEXT("-%lu"),
                    *GetSidSubAuthority(pSid, dwCounter) );
    }

    return TRUE;
}                    

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function ConvertSidToStringSid(SID: PSID; var StringSid: LPSTR): Boolean; stdcall;
    external 'advapi32.dll' name 'ConvertSidToStringSidA';



function GetAccountSid(const Server, User: WideString; var Sid: PSID): DWORD;
var
  dwDomainSize, dwSidSize: DWord;
  R                 : LongBool;
  wDomain           : WideString;
  Use               : DWord;
begin
  Result := 0;
  SetLastError(0);
  dwSidSize := 0;
  dwDomainSize := 0;
  R := LookupAccountNameW(PWideChar(Server), PWideChar(User), nil, dwSidSize,
    nil, dwDomainSize, Use);
  if (not R) and (GetLastError = ERROR_INSUFFICIENT_BUFFER) then
  begin
    SetLength(wDomain, dwDomainSize);
    Sid := GetMemory(dwSidSize);
    R := LookupAccountNameW(PWideChar(Server), PWideChar(User), Sid,
      dwSidSize, PWideChar(wDomain), dwDomainSize, Use);
    if not R then
    begin
      FreeMemory(Sid);
      Sid := nil;
    end;
  end
  else
    Result := GetLastError;
end;


procedure TForm1.Button1Click(Sender: TObject);
var
  SID: PSID;
  strSID: PChar;
  s: String;
  err: DWORD;
begin
  err := GetAccountSid('hal9000', 'Michael', SID);
  if err = 0 then
  begin
    if ConvertSidToStringSid(SID, strSID) then
      s := strSID
    else
      s := SysErrorMessage(err);
  end
  else
    s := SysErrorMessage(err);

  ShowMessage(s);
end;