refactor: the whole thing (#102)

src/hmac/index.js

@@ -0,0 +1,21 @@
+'use strict'
+
+const crypto = require('crypto')
+const lengths = require('./lengths')
+
+exports.create = function (hash, secret, callback) {
+  const res = {
+    digest (data, cb) {
+      const hmac = crypto.createHmac(hash.toLowerCase(), secret)
+
+      hmac.update(data)
+
+      setImmediate(() => {
+        cb(null, hmac.digest())
+      })
+    },
+    length: lengths[hash]
+  }
+
+  callback(null, res)
+}