diff --git a/.gitignore b/.gitignore
index a5fd5c2..88b65f8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -34,5 +34,4 @@ node_modules
 # Optional REPL history
 .node_repl_history
 
-lib
 dist
diff --git a/.travis.yml b/.travis.yml
index e1d6320..4fe6550 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,8 +1,15 @@
 sudo: false
 language: node_js
-node_js:
-  - 4
-  - 5
+matrix:
+  include:
+  - node_js: 4
+    env: CXX=g++-4.8
+  - node_js: 6
+    env:
+      - SAUCE=true
+      - CXX=g++-4.8
+  - node_js: stable
+    env: CXX=g++-4.8
 
 # Make sure we have new NPM.
 before_install:
@@ -13,12 +20,17 @@ script:
   - npm test
   - npm run coverage
 
-addons:
-  firefox: 'latest'
-
 before_script:
   - export DISPLAY=:99.0
   - sh -e /etc/init.d/xvfb start
 
 after_success:
   - npm run coverage-publish
+
+addons:
+  firefox: 'latest'
+  apt:
+    sources:
+      - ubuntu-toolchain-r-test
+    packages:
+      - g++-4.8
\ No newline at end of file
diff --git a/README.md b/README.md
index 9221ed9..052e61f 100644
--- a/README.md
+++ b/README.md
@@ -9,6 +9,10 @@
 [![Coverage Status](https://coveralls.io/repos/github/ipfs/js-ipfs-block/badge.svg?branch=master)](https://coveralls.io/github/ipfs/js-ipfs-block?branch=master)
 [![Dependency Status](https://david-dm.org/ipfs/js-ipfs-block.svg?style=flat-square)](https://david-dm.org/ipfs/js-ipfs-block)
 [![js-standard-style](https://img.shields.io/badge/code%20style-standard-brightgreen.svg?style=flat-square)](https://github.com/feross/standard)
+![](https://img.shields.io/badge/npm-%3E%3D3.0.0-orange.svg?style=flat-square)
+![](https://img.shields.io/badge/Node.js-%3E%3D4.0.0-orange.svg?style=flat-square)
+
+[![Sauce Test Status](https://saucelabs.com/browser-matrix/ipfs-js-block.svg)](https://saucelabs.com/u/ipfs-js-block)
 
 > [IPFS][ipfs] implementation of the Block data structure in JavaScript.
 
@@ -25,10 +29,9 @@
   - [Browser: `<script>` Tag](#browser-script-tag)
 - [API](#api)
   - [Block](#block)
-    - [`new Block(data, [type])`](#new-blockdata-type)
+    - [`new Block(data)`](#new-blockdata)
     - [`block.data`](#blockdata)
-    - [`block.key`](#blockkey)
-  - [`block.extension`](#blockextension)
+    - [`block.key([hashAlg,] callback)`](#blockkeyhashAlg-callback)
 - [Contribute](#contribute)
 - [License](#license)
 
@@ -56,7 +59,7 @@ const Block = require('ipfs-block')
 // create a block
 const block = new Block('hello world')
 console.log(block.data)
-console.log(block.key)
+block.key((err, key) => console.log(err, key))
 ```
 
 ### Browser: Browserify, Webpack, other bundlers
@@ -67,7 +70,7 @@ it and use with your favourite bundler without having to adjust asset management
 process.
 
 ```js
-var Block = require('ipfs-block')
+const Block = require('ipfs-block')
 ```
 
 ### Browser: `<script>` Tag
@@ -89,26 +92,24 @@ const Block = require('ipfs-block')
 
 ### Block
 
-#### `new Block(data, [type])`
+#### `new Block(data)`
+
+- `data: Buffer|String`
 
-Creates a new block with raw data `data`. `type` can be either `'protobuf'` or `'ipld'`
+Creates a new block with raw data `data`.
 
 #### `block.data`
 
 The raw data of the block. Its format matches whatever was provided in its
 constructor.
 
-#### `block.key`
-
-The [multihash][multihash] of the block's data, as a buffer.
-
+#### `block.key([hashAlg,] callback)`
 
-### `block.extension`
+- `hashAlg: String`, optional. Default `sha2-256`.
+- `callback: Function`
 
-The extension on how to store the blog, depends on the type:
+The callback will be called with the [multihash][multihash] of the block's data, as a buffer.
 
-- `'protobuf'`: `'data'`
-- `'ipld'`: `'ipld'`
 
 [ipfs]: https://ipfs.io
 [multihash]: https://github.com/jbenet/js-multihash
diff --git a/package.json b/package.json
index 11aea09..c3a7275 100644
--- a/package.json
+++ b/package.json
@@ -2,8 +2,7 @@
   "name": "ipfs-block",
   "version": "0.4.0",
   "description": "JavaScript Implementation of IPFS Block",
-  "main": "lib/index.js",
-  "jsnext:main": "src/index.js",
+  "main": "src/index.js",
   "scripts": {
     "lint": "aegir-lint",
     "build": "aegir-build",
@@ -37,12 +36,17 @@
   },
   "homepage": "https://github.com/ipfs/js-ipfs-block#readme",
   "devDependencies": {
-    "aegir": "^8.0.0",
+    "aegir": "^9.0.1",
+    "async": "^2.1.2",
     "chai": "^3.5.0",
     "multihashes": "^0.2.2"
   },
   "dependencies": {
-    "multihashing": "^0.2.0"
+    "async": "^2.1.2",
+    "multihashing-async": "^0.2.0"
+  },
+  "engines": {
+    "node": ">=4.0.0"
   },
   "contributors": [
     "David Dias <daviddias.p@gmail.com>",
@@ -51,4 +55,4 @@
     "greenkeeperio-bot <support@greenkeeper.io>",
     "npmcdn-to-unpkg-bot <npmcdn-to-unpkg-bot@users.noreply.github.com>"
   ]
-}
\ No newline at end of file
+}
diff --git a/src/index.js b/src/index.js
index cbb6035..f81b601 100644
--- a/src/index.js
+++ b/src/index.js
@@ -1,29 +1,63 @@
 'use strict'
 
-const multihashing = require('multihashing')
+const multihashing = require('multihashing-async')
+const setImmediate = require('async/setImmediate')
 
-function Block (data) {
-  if (!data) {
-    throw new Error('Block must be constructed with data')
-  }
+module.exports = Block
 
+// Immutable block of data
+function Block (data) {
   if (!(this instanceof Block)) {
     return new Block(data)
   }
 
-  if (data instanceof Buffer) {
-    this.data = data
-  } else {
-    this.data = new Buffer(data)
+  if (!data) {
+    throw new Error('Block must be constructed with data')
   }
 
-  this.key = (hashFunc) => {
+  this._cache = {}
+
+  data = ensureBuffer(data)
+
+  Object.defineProperty(this, 'data', {
+    get () {
+      return data
+    },
+    set () {
+      throw new Error('Tried to change an immutable block')
+    }
+  })
+
+  this.key = (hashFunc, callback) => {
+    if (typeof hashFunc === 'function') {
+      callback = hashFunc
+      hashFunc = null
+    }
+
     if (!hashFunc) {
       hashFunc = 'sha2-256'
     }
 
-    return multihashing(this.data, hashFunc)
+    if (this._cache[hashFunc]) {
+      return setImmediate(() => {
+        callback(null, this._cache[hashFunc])
+      })
+    }
+
+    multihashing(this.data, hashFunc, (err, multihash) => {
+      if (err) {
+        return callback(err)
+      }
+      this._cache[hashFunc] = multihash
+      callback(null, multihash)
+    })
   }
 }
 
-module.exports = Block
+function ensureBuffer (data) {
+  if (Buffer.isBuffer(data)) {
+    return data
+  }
+
+  return new Buffer(data)
+}
diff --git a/test/block.spec.js b/test/block.spec.js
index 50c548d..81cb700 100644
--- a/test/block.spec.js
+++ b/test/block.spec.js
@@ -2,41 +2,87 @@
 'use strict'
 
 const expect = require('chai').expect
+const parallel = require('async/parallel')
+const series = require('async/series')
+const mh = require('multihashes')
+
 const Block = require('../src')
-const multihash = require('multihashes')
+
+function expectKey (block, hashFn, expectedKey, callback) {
+  const cb = (err, key) => {
+    if (err) {
+      return callback(err)
+    }
+    expect(mh.toB58String(key)).to.be.eql(expectedKey)
+    callback()
+  }
+
+  if (typeof expectedKey === 'function') {
+    callback = expectedKey
+    expectedKey = hashFn
+    block.key(cb)
+  } else {
+    block.key(hashFn, cb)
+  }
+}
 
 describe('block', () => {
-  it('create', () => {
+  it('create', (done) => {
     const b = new Block('random-data')
-    expect(multihash.toB58String(b.key())).to.equal('QmeoBGh5g5kHgK3xppJ1YPwB9xgH2GoqhMSuQVpzDdvtJG')
-    expect(multihash.toB58String(b.key('sha1'))).to.equal('5dsvLgRV9RVj9eSgtxMrXQjbpfFeHY')
     expect(b.data).to.exist
+    parallel([
+      (cb) => expectKey(b, 'QmeoBGh5g5kHgK3xppJ1YPwB9xgH2GoqhMSuQVpzDdvtJG', cb),
+      (cb) => expectKey(b, 'sha1', '5dsvLgRV9RVj9eSgtxMrXQjbpfFeHY', cb)
+    ], done)
   })
 
-  it('create /wo new', () => {
+  it('create /wo new', (done) => {
     const b = Block('random-data')
-    expect(multihash.toB58String(b.key())).to.equal('QmeoBGh5g5kHgK3xppJ1YPwB9xgH2GoqhMSuQVpzDdvtJG')
-    expect(multihash.toB58String(b.key('sha1'))).to.equal('5dsvLgRV9RVj9eSgtxMrXQjbpfFeHY')
     expect(b.data).to.exist
+    parallel([
+      (cb) => expectKey(b, 'QmeoBGh5g5kHgK3xppJ1YPwB9xgH2GoqhMSuQVpzDdvtJG', cb),
+      (cb) => expectKey(b, 'sha1', '5dsvLgRV9RVj9eSgtxMrXQjbpfFeHY', cb)
+    ], done)
   })
 
   it('fail to create an empty block', () => {
     expect(() => new Block()).to.throw()
   })
 
-  it('2 different blocks have different hashes', () => {
+  it('2 different blocks have different hashes', (done) => {
     const b1 = new Block('random-data')
     const b2 = new Block('more-random-data')
-    expect(b1).to.not.deep.equal(b2)
+
+    parallel([
+      (cb) => b1.key(cb),
+      (cb) => b2.key(cb)
+    ], (err, keys) => {
+      expect(err).to.not.exist
+      expect(keys[0]).to.not.deep.equal(keys[1])
+      done()
+    })
+  })
+
+  it('caches key calls', (done) => {
+    const b = new Block('random')
+    series([
+      (cb) => b.key(cb),
+      (cb) => b.key(cb)
+    ], (err, res) => {
+      expect(err).to.not.exist
+      expect(b._cache['sha2-256']).to.be.eql(res[0])
+      expect(res[0]).to.be.eql(res[1])
+      done()
+    })
   })
 
-  it.skip('block stays immutable', () => {
-    // it from the original implementation
-    // It doesn't stricly verify the immutability of the Block object
+  it('block stays immutable', () => {
     const block = new Block("Can't change this!")
-    let key = block.key()
-    key = new Buffer('new key')
 
-    expect(key.equals(block.key())).to.equal(false)
+    expect(
+      () => { block.data = 'fail' }
+    ).to.throw(
+      /immutable/
+    )
   })
 })