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.gitignore add Apr 26, 2017



This project provides loggers which implement a basic blockchain structure. Data is put into the following block format and stored according to the chosen strategy:

  "data": "your data here",
  "meta": {
    "timestamp": "when it was created",
    "previousHash": "a sha256 hash of the previous block"
  "hash": "a sha256 hash of this block’s contents"


yarn add blockchain-logger # or npm install blockchain-logger

Local file and Twitter loggers

Local file logger

This will store logs in date-stamped files in the directory of your choice.

import { LocalFileLogger } from 'blockchain-logger'

const logger = new LocalFileLogger({
  genesisHash: 'abc123',
  logPath: './logs/',
  logFilePrefix: 'actions',

Twitter logger

This will store logs in the alt-text of an image you repeatedly upload to your Twitter account. See @tweetblockchain for an example. (Click on an image for gallery view, and then inspect element to see the alt-text.)

import { TwitterLogger } from 'blockchain-logger'

const logger = new TwitterLogger({
    accessTokenKey: 'get-this-from-twitter',
    accessTokenSecret: 'get-this-from-twitter',
    baseImageLocation: './path/to/some/media/file.jpg',
    consumerKey: 'get-this-from-twitter',
    consumerSecret: 'get-this-from-twitter',
    genesisHash: 'abc123',
    screenName: '@handle',

Log something

const data = { whatever: 'you want' }

Retrieve logs


Blockchain logger

There’s also a logger for the Bitcoin blockchain or the Bitcoin Testnet. This stores data in transactions with an OP_RETURN output. Since the length of data that can be stored in such a transaction is limited to 80 bytes, this logger does not provide the blockchain properties of the other loggers, but can be used to store small amounts of data, or in conjunction with the other loggers to ensure integrity by storing hashes in a decentralised database.

Create a Blockchain logger

import { BlockchainLogger } from 'blockchain-logger'

const logger = new BlockchainLogger({
  maxFee: 5000, // satoshis
  prefix: 'BL', // prepended to everything you log (uses up characters!)
  privateKey: 'generate-this-using-secure-tools',
  testnet: true,

Only the private key is required. The private key is needed to sign the transactions created to store logs in OP_RETURN outputs. The transaction sends 0 satoshis to the OP_RETURN script, and the rest back to the input address, minus a mining fee.

The logger uses’s recommended fees to calculate what fee to pay for the logging transaction, but you can set a limit with maxFee.

Log something'some text')'some arbitrary data'))

The limit is 80 bytes minus the length of your prefix (if you provided one).

Retrieve logs


This returns an array of strings of text that has been stored using OP_RETURN transactions for the specified address, which include the prefix (if you provided one). The prefix is stripped off.

Interplay with other loggers

The local file and Twitter loggers can use the Blockchain logger to validate hash integrity. Just provide either one with an additional config option blockchainOptions, which should be a nested object containing the Blockchain logger options. E.g.:

import { LocalFileLogger } from 'blockchain-logger'

const logger = new LocalFileLogger({
  genesisHash: 'abc123',
  logPath: './logs/',
  logFilePrefix: 'actions',
  blockchainOptions: {
    maxFee: 5000,
    prefix: 'BL',
    privateKey: 'generate-this-using-secure-tools',
    testnet: true,

Now if you call logger.getLoggedData(ensureHashConsistency=true), it will loop through the hashes stored on the Bitcoin/Testnet blockchain, and make sure that the hashes for your logs appear in that order on the relevant blockchain.