(Node.js) How to Tell if a File is .p7m or .p7s

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?

Install Chilkat for Node.js and Electron using npm at Chilkat npm packages for Node.js Chilkat npm packages for Electron on Windows, Linux, MacOSX, and ARM

var os = require('os');
if (os.platform() == 'win32') {  
    if (os.arch() == 'ia32') {
        var chilkat = require('@chilkat/ck-node21-win-ia32');
    } else {
        var chilkat = require('@chilkat/ck-node21-win64'); 
    }
} else if (os.platform() == 'linux') {
    if (os.arch() == 'arm') {
        var chilkat = require('@chilkat/ck-node21-arm');
    } else if (os.arch() == 'x86') {
        var chilkat = require('@chilkat/ck-node21-linux32');
    } else {
        var chilkat = require('@chilkat/ck-node21-linux64');
    }
} else if (os.platform() == 'darwin') {
    if (os.arch() == 'arm64') {
        var chilkat = require('@chilkat/ck-node21-mac-m1');
    } else {
        var chilkat = require('@chilkat/ck-node21-macosx');
    }
}

function chilkatExample() {

    // 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.

    var bd = new chilkat.BinData();

    var success = bd.LoadFile("qa_data/p7m/a.p7m");
    if (success == false) {
        console.log("Failed to load the file.");
        return;
    }

    var sb = new chilkat.StringBuilder();
    sb.Append(bd.GetEncodedChunk(0,2,"hex"));

    var caseSensitive = true;
    if (sb.ContentsEqual("3080",caseSensitive) || sb.ContentsEqual("3081",caseSensitive) || sb.ContentsEqual("3082",caseSensitive) || sb.ContentsEqual("3083",caseSensitive)) {
        console.log("This is a .p7m or .p7s file.");
    }
    else {
        console.log("This is not a .p7m or .p7s file.");
    }


}

chilkatExample();