asn1js is a JavaScript generic ASN.1 parser/decoder that can decode any valid ASN.1 DER or BER structures.

An example page that can decode Base64-encoded (raw base64, PEM armoring and begin-base64 are recognized) or Hex-encoded (or local files with some browsers) is included and can be used both online on the official website or offline (ZIP file).

This package can be installed with either npm or yarn via the following commands:

npm install @lapo/asn1js
# or other tools
pnpm install @lapo/asn1js
yarn add @lapo/asn1js

You can import the classes like this:

import { ASN1 } from '@lapo/asn1js';

A submodule of this package can also be imported:

import { Hex } from '@lapo/asn1js/hex.js';

If your code is still not using ES6 Modules (and is using CommonJS) you can require it normally since NodeJS 22 (with parameter --experimental-require-module):

const
  { ASN1 } = require('@lapo/asn1js'),
  { Hex } = require('@lapo/asn1js/hex.js');
console.log(ASN1.decode(Hex.decode('06032B6570')).content());

On older NodeJS you instead need to use async import:

async function main() {
  const
    { ASN1 } = await import('@lapo/asn1js'),
    { Hex } = await import('@lapo/asn1js/hex.js');
  console.log(ASN1.decode(Hex.decode('06032B6570')).content());
}
main();

Can be tested on JSFiddle.

<script>
import { ASN1 } from 'https://unpkg.com/@lapo/asn1js@2.0.0/asn1.js';
import { Hex } from 'https://unpkg.com/@lapo/asn1js@2.0.0/hex.js';

document.body.innerText = ASN1.decode(Hex.decode('06032B6570')).content();
</script>

Since unfortunately ESM modules are not working on file: protocol due to CORS issues, there is a bundled single-file version working locally. It doesn't work online (due to CSP restrictions about inline content) but can be saved locally and opened in a browser. (known bug: dark mode is currently broken in this mode)

You can dump an ASN.1 structure from the command line using the following command (no need to even install it):

npx @lapo/asn1js ed25519.cer

ASN.1 JavaScript decoder Copyright (c) 2008-2024 Lapo Luchini lapo@lapo.it

Permission to use, copy, modify, and/or distribute this software for any purpose with or without fee is hereby granted, provided that the above copyright notice and this permission notice appear in all copies.

THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

• OBJECT IDENTIFIER values are recognized using data taken from Peter Gutmann's dumpasn1 program.
• BMPString support added by Felipe Gasper
• extended tag support added by Péter Budai
• patches by Gergely Nagy
• Relative OID support added by Mistial Developer
• dark mode and other UI improvements by Oliver Burgmaier
• patches by Nicolai Søborg

• official website
• dedicated domain
• InDefero tracker
• GitHub mirror
• Ohloh code stats