bitcoin-filter

Bitcoin connection Bloom filtering (BIP37)

bitcoin-filter is used to set connection Bloom filters for Bitcoin light clients, so that we only receive relevant transactions. It should be used with the bitcoin-net package.

Usage

npm install bitcoin-filter

var PeerGroup = require('bitcoin-net')
var Filter = require('bitcoin-filter')

var peers = new PeerGroup(params)
var filter = new Filter(peers)
filter.add(new Buffer('818895f3dc2c178629d3d2d8fa3ec4a3f8179821', 'hex'))
filter.add(walletObject)

API

Filter

var filter = new Filter(peers, [opts])

Creates a new Bloom filter, which gets sent to peers in peers (a bitcoin-net PeerGroup instance). Elements can be added to the filter, and it will be updated on the remote peers. It will also be sent to new peers which get connected through the PeerGroup.

opts can contain the following properties:

• falsePositiveRate, Number (default: 0.01) - The filter parameters (size and number of hash functions) will be adjusted to maintain this false positive rate as elements are added. A higher value uses more bandwidth since more transactions are sent, but a lower value is less anonymous since remote peers have a better idea of which transactions are yours.
• resizeThreshold, Number (default: 0.6) - When the actual false positive rate exceeds falsePositiveRate * resizeThreshold, the filter will be resized to bring the false positive rate back to the target.

filter.add(element)

Adds an element or an object that implements the Filterable interface to the filter.

element should be a Buffer or Filterable.

If element is a Buffer, it will be kept in memory since it will be used if the filter needs to be recalculated.

filter.remove(element)

Removes an element or an object that implements the Filterable interface from the filter.

element should be a Buffer or Filterable.

Interface: Filterable

Objects can implement this interface so their elements can be easily added to the Bloom filter without being kept in memory. This can be useful for instance for a wallet that manages many keys which should be added to the filter.

Using this interface saves a lot of memory over just adding the elements as Buffers, since we only need the elements when recalculating the filter and we can usually just recalculate them (e.g. deriving HD keys).

filterable.filterElements()

This method will be called when the Filterable is first added to the filter, or when the filter needs to recalculate. It should return an array of Buffers, which are all of the filter elements this Filterable has created so far (e.g. the keys of a wallet).

Event: filteradd

• Buffer - An element that should be added to the filteradd

This event should be emitted whenever a new element should be added to the filter.