Delphi DLL

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?

Delphi DLL

uses
    Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
    Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, BinData, StringBuilder;

...

procedure TForm1.Button1Click(Sender: TObject);
var
success: Boolean;
bd: HCkBinData;
sb: HCkStringBuilder;
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 := CkBinData_Create();

success := CkBinData_LoadFile(bd,'qa_data/p7m/a.p7m');
if (success = False) then
  begin
    Memo1.Lines.Add('Failed to load the file.');
    Exit;
  end;

sb := CkStringBuilder_Create();
CkStringBuilder_Append(sb,CkBinData__getEncodedChunk(bd,0,2,'hex'));

caseSensitive := True;
if (CkStringBuilder_ContentsEqual(sb,'3080',caseSensitive) or CkStringBuilder_ContentsEqual(sb,'3081',caseSensitive) or CkStringBuilder_ContentsEqual(sb,'3082',caseSensitive) or CkStringBuilder_ContentsEqual(sb,'3083',caseSensitive)) then
  begin
    Memo1.Lines.Add('This is a .p7m or .p7s file.');
  end
else
  begin
    Memo1.Lines.Add('This is not a .p7m or .p7s file.');
  end;

CkBinData_Dispose(bd);
CkStringBuilder_Dispose(sb);

end;