Minimal example of loading a PEM certificate using pkijs (in nodejs)

pkijs-decode-pem-node.js

#!/usr/bin/env node
// Minimal example of loading a PEM certificate using pkijs (in node)

// babel-polyfill needs to be loaded for pkijs
// It uses webcrypto which needs browser shims
require('babel-polyfill')

const Pkijs = require('pkijs')
const Asn1js = require('asn1js')
const FS = require('fs')


function decodeCert(pem) {
    if(typeof pem !== 'string') {
        throw new Error('Expected PEM as string')
    }

    // Load certificate in PEM encoding (base64 encoded DER)
    const b64 = cert.replace(/(-----(BEGIN|END) CERTIFICATE-----|[\n\r])/g, '')

    // Now that we have decoded the cert it's now in DER-encoding
    const der = Buffer(b64, 'base64')

    // And massage the cert into a BER encoded one
    const ber = new Uint8Array(der).buffer

    // And now Asn1js can decode things \o/
    const asn1 = Asn1js.fromBER(ber)

    return new Pkijs.Certificate({ schema: asn1.result })
}

const certFile = process.argv[2]
if(certFile === undefined) {
    const file = require('path').basename(process.argv[1])
    console.error(`Usage: ${file} <pem_file>`)
    process.exit(1)
}

const cert = FS.readFileSync(certFile).toString()
// Prints out cert as a map data structure
console.log(JSON.stringify(decodeCert(cert), null, 2))

Excellent!

A little glitch: line 19 -> pem

DER is a subset of BER, so no conversion is needed. See here.

The code on line 25 (const ber = new Uint8Array(der).buffer) is taking the backing ArrayBuffer of the der Buffer. However, since a
Buffer is a view of an ArrayBuffer, and the backing ArrayBuffer can be bigger (see here), this could potentially return bytes that were not part of the original Buffer.

The safe way to do it would be to use ArrayBuffer.prototype.slice() to extract a new ArrayBuffer with just the relevant bytes:

const arrayBuffer = der.buffer.slice(der.byteOffset, der.byteOffset + der.length)

const asn1 = Asn1js.fromBER(arrayBuffer)

However, it seems that this is not needed. asn1js.fromBER() also takes an ArrayBufferView, which Buffer fits. So we can just pass the
original DER Buffer directly:

const asn1 = Asn1js.fromBER(der)