Dart

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?

Dart

import 'package:chilkat/chilkat.dart';

void main() {
  // 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.

  final bd = CkBinData();

  try {
    bd.loadFile('qa_data/p7m/a.p7m');
  } on ChilkatException {
    print('Failed to load the file.');
    return;
  }

  final sb = CkStringBuilder();
  sb.append(bd.getEncodedChunk(0, 2, 'hex'));

  final caseSensitive = true;
  if (sb.contentsEqual('3080', caseSensitive) || (sb.contentsEqual('3081', caseSensitive) || (sb.contentsEqual('3082', caseSensitive) || sb.contentsEqual('3083', caseSensitive)))) {
    print('This is a .p7m or .p7s file.');
  } else {
    print('This is not a .p7m or .p7s file.');
  }
}