@panva/asn1.js/lib/asn1/decoders/pem.js

ASN.1 encoder and decoder for Node.js with no dependencies (fork of asn1.js using native BigInt)

const { inherits } = require('util')

const DERDecoder = require('./der')

function PEMDecoder (entity) {
  DERDecoder.call(this, entity)
  this.enc = 'pem'
}
inherits(PEMDecoder, DERDecoder)

PEMDecoder.prototype.decode = function decode (data, options) {
  const lines = data.toString().split(/[\r\n]+/g)

  const label = options.label.toUpperCase()

  const re = /^-----(BEGIN|END) ([^-]+)-----$/
  let start = -1
  let end = -1
  for (let i = 0; i < lines.length; i++) {
    const match = lines[i].match(re)
    if (match === null) { continue }

    if (match[2] !== label) { continue }

    if (start === -1) {
      if (match[1] !== 'BEGIN') { break }
      start = i
    } else {
      if (match[1] !== 'END') { break }
      end = i
      break
    }
  }
  if (start === -1 || end === -1) { throw new Error(`PEM section not found for: ${label}`) }

  const base64 = lines.slice(start + 1, end).join('')
  // Remove excessive symbols
  base64.replace(/[^a-z0-9+/=]+/gi, '')

  const input = Buffer.from(base64, 'base64')
  return DERDecoder.prototype.decode.call(this, input, options)
}

module.exports = PEMDecoder