Increase bcrypt cost factor, add future cost factor auto-optimization (…

…#30)

* Increase bcrypt default cost function; add auto-increase

- Increases default bcrypt cost factor to 12
- Increases bcrypt cost factor +1 automatically every 1.5 years from 1/1/17
  to maintain cost increases without needing updates

* Force 0 multiplier in case of negative floor due to pre-1/1/17 clock skew

* Move bcrypt constants to header per @daffl

packages/authentication-local/src/utils/hash.js

@@ -1,8 +1,16 @@
 import bcrypt from 'bcryptjs';
 
+const BCRYPT_WORK_FACTOR_BASE = 12;
+const BCRYPT_DATE_BASE = 1483228800000;
+const BCRYPT_WORK_INCREASE_INTERVAL = 47300000000;
+
 export default function hasher (password) {
   return new Promise((resolve, reject) => {
-    bcrypt.genSalt(10, function (error, salt) {
+    let BCRYPT_CURRENT_DATE = new Date().getTime();
+    let BCRYPT_WORK_INCREASE = Math.max(0, Math.floor((BCRYPT_CURRENT_DATE - BCRYPT_DATE_BASE) / BCRYPT_WORK_INCREASE_INTERVAL));
+    let BCRYPT_WORK_FACTOR = Math.min(19, BCRYPT_WORK_FACTOR_BASE + BCRYPT_WORK_INCREASE);
+
+    bcrypt.genSalt(BCRYPT_WORK_FACTOR, function (error, salt) {
       if (error) {
         return reject(error);
       }