diff --git a/src/pbkdf2.js b/src/pbkdf2.js
--- a/src/pbkdf2.js
+++ b/src/pbkdf2.js
@@ -1,3 +1,4 @@
+// https://tools.ietf.org/html/rfc2898
 import {int322bytesBE} from "./util.js";
 import {blake2s} from "./blake.js";