Pascal (Lazarus/Delphi)

How to Tell if a File is .p7m or .p7s

See more Misc Examples

This example explains how to detect if a file is a .p7m or .p7s. It provides as solution to the following problem:

Sometimes people submit P7S/P7M with wrong extensions. Can I check if file is P7S or P7M with Chilkat?

Also see What is a P7M or P7S File?

Pascal (Lazarus/Delphi)

program ChilkatDemo;

// Demonstrates using the Chilkat Pascal wrapper via the C bridge DLL.
// Builds as a console application under Lazarus (FPC) or Delphi.

{$IFDEF FPC}
  {$MODE DELPHI}
{$ENDIF}
{$APPTYPE CONSOLE}

uses
  {$IFDEF UNIX}
  cthreads,
  {$ENDIF}
  SysUtils,
  CkDllLoader,
  Chilkat.BinData,
  Chilkat.StringBuilder;

// ---------------------------------------------------------------------------

procedure RunDemo;
var
  success: Boolean;
  bd: TBinData;
  sb: TStringBuilder;
  caseSensitive: Boolean;

begin
  success := False;

  //  A .p7m or .p7s file is binary and contains PKCS7.
  //  PKCS7 is ASN.1, and always begins with a SEQUENCE tag (a byte equal to 0x30)
  //  followed by an encoded length.
  //  If the file is larger than 128 bytes (and it SHOULD be), the encoded length will begin with either 0x80, 0x81, 0x82, or 0x83.
  //  It can also begin with 0x84, but only for very large files (where the length is too large for 3 bytes).
  //  See How ASN.1 Encodes Lengths

  //  Therefore, if we have a file that might be .p7m or .p7s, we can check by
  //  examining the 1st 2 bytes of the file.

  bd := TBinData.Create;

  success := bd.LoadFile('qa_data/p7m/a.p7m');
  if (success = False) then
    begin
      WriteLn('Failed to load the file.');
      Exit;
    end;

  sb := TStringBuilder.Create;
  sb.Append(bd.GetEncodedChunk(0,2,'hex'));

  caseSensitive := True;
  if (sb.ContentsEqual('3080',caseSensitive) or sb.ContentsEqual('3081',caseSensitive) or sb.ContentsEqual('3082',caseSensitive) or sb.ContentsEqual('3083',caseSensitive)) then
    begin
      WriteLn('This is a .p7m or .p7s file.');
    end
  else
    begin
      WriteLn('This is not a .p7m or .p7s file.');
    end;


  bd.Free;
  sb.Free;

end;

// ---------------------------------------------------------------------------

begin

  try
    RunDemo;
  except
    on E: Exception do
      WriteLn('Unhandled exception: ', E.ClassName, ': ', E.Message);
  end;

  WriteLn;
  {$IFDEF MSWINDOWS}
  WriteLn('Press Enter to exit...');
  ReadLn;
  {$ENDIF}
end.