diff --git a/README.md b/README.md
index 8c84f65d..00f94865 100644
--- a/README.md
+++ b/README.md
@@ -51,11 +51,32 @@ This repo contains the JavaScript implementation of the crypto primitives needed
 npm install --save libp2p-crypto
 ```
 
+## Usage
+
+```js
+const crypto = require('libp2p-crypto')
+
+// Now available to you:
+//
+// crypto.aes
+// crypto.hmac
+// crypto.keys
+// etc.
+//
+// See full API details below...
+```
+
+### Web Crypto API
+
+The `libp2p-crypto` library depends on the [Web Crypto API](https://developer.mozilla.org/en-US/docs/Web/API/Web_Crypto_API) in the browser. Web Crypto is available in all modern browsers, however browsers restrict its usage to [Secure Contexts](https://developer.mozilla.org/en-US/docs/Web/Security/Secure_Contexts).
+
+**This means you will not be able to use some `libp2p-crypto` functions in the browser when the page is served over HTTP.**
+
 ## API
 
 ### `crypto.aes`
 
-Expoes an interface to AES encryption (formerly Rijndael), as defined in U.S. Federal Information Processing Standards Publication 197.
+Exposes an interface to AES encryption (formerly Rijndael), as defined in U.S. Federal Information Processing Standards Publication 197.
 
 This uses `CTR` mode.
 
diff --git a/src/hmac/index-browser.js b/src/hmac/index-browser.js
index 3592291d..1eeb7dfc 100644
--- a/src/hmac/index-browser.js
+++ b/src/hmac/index-browser.js
@@ -1,6 +1,6 @@
 'use strict'
 
-const webcrypto = require('../webcrypto.js')
+const webcrypto = require('../webcrypto')
 const lengths = require('./lengths')
 
 const hashTypes = {
@@ -10,13 +10,13 @@ const hashTypes = {
 }
 
 const sign = async (key, data) => {
-  return Buffer.from(await webcrypto.subtle.sign({ name: 'HMAC' }, key, data))
+  return Buffer.from(await webcrypto.get().subtle.sign({ name: 'HMAC' }, key, data))
 }
 
 exports.create = async function (hashType, secret) {
   const hash = hashTypes[hashType]
 
-  const key = await webcrypto.subtle.importKey(
+  const key = await webcrypto.get().subtle.importKey(
     'raw',
     secret,
     {
diff --git a/src/keys/ecdh-browser.js b/src/keys/ecdh-browser.js
index ee7c2edd..d12d831a 100644
--- a/src/keys/ecdh-browser.js
+++ b/src/keys/ecdh-browser.js
@@ -1,6 +1,6 @@
 'use strict'
 
-const webcrypto = require('../webcrypto.js')
+const webcrypto = require('../webcrypto')
 const BN = require('asn1.js').bignum
 const { toBase64, toBn } = require('../util')
 
@@ -11,7 +11,7 @@ const bits = {
 }
 
 exports.generateEphmeralKeyPair = async function (curve) {
-  const pair = await webcrypto.subtle.generateKey(
+  const pair = await webcrypto.get().subtle.generateKey(
     {
       name: 'ECDH',
       namedCurve: curve
@@ -25,7 +25,7 @@ exports.generateEphmeralKeyPair = async function (curve) {
     let privateKey
 
     if (forcePrivate) {
-      privateKey = await webcrypto.subtle.importKey(
+      privateKey = await webcrypto.get().subtle.importKey(
         'jwk',
         unmarshalPrivateKey(curve, forcePrivate),
         {
@@ -40,7 +40,7 @@ exports.generateEphmeralKeyPair = async function (curve) {
     }
 
     const keys = [
-      await webcrypto.subtle.importKey(
+      await webcrypto.get().subtle.importKey(
         'jwk',
         unmarshalPublicKey(curve, theirPub),
         {
@@ -53,7 +53,7 @@ exports.generateEphmeralKeyPair = async function (curve) {
       privateKey
     ]
 
-    return Buffer.from(await webcrypto.subtle.deriveBits(
+    return Buffer.from(await webcrypto.get().subtle.deriveBits(
       {
         name: 'ECDH',
         namedCurve: curve,
@@ -64,7 +64,7 @@ exports.generateEphmeralKeyPair = async function (curve) {
     ))
   }
 
-  const publicKey = await webcrypto.subtle.exportKey('jwk', pair.publicKey)
+  const publicKey = await webcrypto.get().subtle.exportKey('jwk', pair.publicKey)
 
   return {
     key: marshalPublicKey(publicKey),
diff --git a/src/keys/rsa-browser.js b/src/keys/rsa-browser.js
index 5cefbdb3..be268fb1 100644
--- a/src/keys/rsa-browser.js
+++ b/src/keys/rsa-browser.js
@@ -1,12 +1,12 @@
 'use strict'
 
-const webcrypto = require('../webcrypto.js')
+const webcrypto = require('../webcrypto')
 const randomBytes = require('../random-bytes')
 
 exports.utils = require('./rsa-utils')
 
 exports.generateKey = async function (bits) {
-  const pair = await webcrypto.subtle.generateKey(
+  const pair = await webcrypto.get().subtle.generateKey(
     {
       name: 'RSASSA-PKCS1-v1_5',
       modulusLength: bits,
@@ -27,7 +27,7 @@ exports.generateKey = async function (bits) {
 
 // Takes a jwk key
 exports.unmarshalPrivateKey = async function (key) {
-  const privateKey = await webcrypto.subtle.importKey(
+  const privateKey = await webcrypto.get().subtle.importKey(
     'jwk',
     key,
     {
@@ -57,7 +57,7 @@ exports.unmarshalPrivateKey = async function (key) {
 exports.getRandomValues = randomBytes
 
 exports.hashAndSign = async function (key, msg) {
-  const privateKey = await webcrypto.subtle.importKey(
+  const privateKey = await webcrypto.get().subtle.importKey(
     'jwk',
     key,
     {
@@ -68,7 +68,7 @@ exports.hashAndSign = async function (key, msg) {
     ['sign']
   )
 
-  const sig = await webcrypto.subtle.sign(
+  const sig = await webcrypto.get().subtle.sign(
     { name: 'RSASSA-PKCS1-v1_5' },
     privateKey,
     Uint8Array.from(msg)
@@ -78,7 +78,7 @@ exports.hashAndSign = async function (key, msg) {
 }
 
 exports.hashAndVerify = async function (key, sig, msg) {
-  const publicKey = await webcrypto.subtle.importKey(
+  const publicKey = await webcrypto.get().subtle.importKey(
     'jwk',
     key,
     {
@@ -89,7 +89,7 @@ exports.hashAndVerify = async function (key, sig, msg) {
     ['verify']
   )
 
-  return webcrypto.subtle.verify(
+  return webcrypto.get().subtle.verify(
     { name: 'RSASSA-PKCS1-v1_5' },
     publicKey,
     sig,
@@ -99,13 +99,13 @@ exports.hashAndVerify = async function (key, sig, msg) {
 
 function exportKey (pair) {
   return Promise.all([
-    webcrypto.subtle.exportKey('jwk', pair.privateKey),
-    webcrypto.subtle.exportKey('jwk', pair.publicKey)
+    webcrypto.get().subtle.exportKey('jwk', pair.privateKey),
+    webcrypto.get().subtle.exportKey('jwk', pair.publicKey)
   ])
 }
 
 function derivePublicFromPrivate (jwKey) {
-  return webcrypto.subtle.importKey(
+  return webcrypto.get().subtle.importKey(
     'jwk',
     {
       kty: jwKey.kty,
diff --git a/src/webcrypto.js b/src/webcrypto.js
index 0f9a557b..1663369a 100644
--- a/src/webcrypto.js
+++ b/src/webcrypto.js
@@ -1,5 +1,24 @@
-/* global self */
+/* eslint-env browser */
 
 'use strict'
 
-module.exports = self.crypto || self.msCrypto
+// Check native crypto exists and is enabled (In insecure context `self.crypto`
+// exists but `self.crypto.subtle` does not).
+exports.get = (win = self) => {
+  const nativeCrypto = win.crypto || win.msCrypto
+
+  if (!nativeCrypto || !nativeCrypto.subtle) {
+    throw Object.assign(
+      new Error(
+        'Missing Web Crypto API. ' +
+        'The most likely cause of this error is that this page is being accessed ' +
+        'from an insecure context (i.e. not HTTPS). For more information and ' +
+        'possible resolutions see ' +
+        'https://github.com/libp2p/js-libp2p-crypto/blob/master/README.md#web-crypto-api'
+      ),
+      { code: 'ERR_MISSING_WEB_CRYPTO' }
+    )
+  }
+
+  return nativeCrypto
+}
diff --git a/test/browser.js b/test/browser.js
new file mode 100644
index 00000000..264c41a6
--- /dev/null
+++ b/test/browser.js
@@ -0,0 +1,61 @@
+/* eslint-env mocha */
+'use strict'
+
+const chai = require('chai')
+const dirtyChai = require('dirty-chai')
+const expect = chai.expect
+chai.use(dirtyChai)
+const crypto = require('../')
+const webcrypto = require('../src/webcrypto')
+
+async function expectMissingWebCrypto (fn) {
+  try {
+    await fn()
+  } catch (err) {
+    expect(err.code).to.equal('ERR_MISSING_WEB_CRYPTO')
+    return
+  }
+  throw new Error('Expected missing web crypto error')
+}
+
+describe('Missing web crypto', () => {
+  let webcryptoGet
+  let rsaPrivateKey
+
+  before(async () => {
+    rsaPrivateKey = await crypto.keys.generateKeyPair('RSA', 512)
+  })
+
+  before(() => {
+    webcryptoGet = webcrypto.get
+    webcrypto.get = () => webcryptoGet({})
+  })
+
+  after(() => {
+    webcrypto.get = webcryptoGet
+  })
+
+  it('should error for hmac create when web crypto is missing', () => {
+    return expectMissingWebCrypto(() => crypto.hmac.create('SHA256', Buffer.from('secret')))
+  })
+
+  it('should error for generate ephemeral key pair when web crypto is missing', () => {
+    return expectMissingWebCrypto(() => crypto.keys.generateEphemeralKeyPair('P-256'))
+  })
+
+  it('should error for generate rsa key pair when web crypto is missing', () => {
+    return expectMissingWebCrypto(() => crypto.keys.generateKeyPair('rsa', 256))
+  })
+
+  it('should error for unmarshal RSA private key when web crypto is missing', () => {
+    return expectMissingWebCrypto(() => crypto.keys.unmarshalPrivateKey(crypto.keys.marshalPrivateKey(rsaPrivateKey)))
+  })
+
+  it('should error for sign RSA private key when web crypto is missing', () => {
+    return expectMissingWebCrypto(() => rsaPrivateKey.sign(Buffer.from('test')))
+  })
+
+  it('should error for verify RSA public key when web crypto is missing', () => {
+    return expectMissingWebCrypto(() => rsaPrivateKey.public.verify(Buffer.from('test'), Buffer.from('test')))
+  })
+})