tlv.js

This package en- and decodes TLV objects as they are used for BER (Basic Encoding Rules) data structures. (But don't expect BER to be included! This package could be used to implement a BER en- and decoder.)

This is basically the foundation if you want to read X.509 certificats by hand.

For verifacation purposes there exists a handy dandy online viewer for in- and outputs of this package: The ASN.1 JavaScript decoder by Lapo Luchini.

The ruleset is described in ISO/IEC 7816. This document is your deep dive, if you don't understand the terminology used in the API description down below.

API

const TLV = require('tlv');

Constructor

const obj = new TLV(opts);

Creates a new TLV object. opts states the properties the object shall be initialised with. Cf. the property description below for further details.

Furthermore opts can have the property inherit which states another TLV instance of which the properties class, tag and type are inherited.

Property: class

obj.class = 'context';
const class = obj.class;

Sets or gets the class of the TLV object. Valid classes are: 'universal', 'application', 'context' and 'private'.

Property: type

obj.type = 'primitive';
const type = obj.type;

Sets or gets the type of the TLV object. 'primitive' objects hold binary data. 'constructed' objects hold further TLV objects.

Property: tag

obj.tag = 0x10;
const tag = obj.tag;

Sets or gets the tag of obj.

Property: length and fullLength

const length = obj.length;
const fullLength = obj.fullLength;

Gets the length in bytes. length is the size of the object's value. fullLength also include the object's header size.

Property: value

const value = obj.value

Gets the hold value. For primitive objects value returns a Buffer containing its value. If the TLV object is constructed, value is an Array of TLV instances.

const int = new TLV({class: 'univeral', type: 'primitive', tag: 0x02, value: 0x42});
obj.value = int;

Stores int insided of obj. obj.type will change to constructed.

const data = Buffer.from('hello');
obj.value = data;

Stores data insided of obj. obj.type will change to primitive.

Property: next

const int = new TLV({class: 'univeral', type: 'primitive', tag: 0x02, value: 0x42});
obj.next = int;

Stores int after (and not inside of) obj;

const next = obj.next;

Returns the next TLV object after obj or null if obj is the last item.

Method: toBuffer()

const buf = obj.toBuffer();

Converts the TLV object into a Buffer.

Method: is()

const SEQ = new TLV({class: 'universal', type: 'constructed', tag: 0x10});
const isSeq = obj.is(SEQ);

isSeq is true, if obj has the same class, type and tag as SEQ. Value and length are not compared.

Method: assert()

const SEQ = new TLV({class: 'universal', type: 'constructed', tag: 0x10});
obj.assert(SEQ);

Throws an Error, if obj is not a BER SEQ.