diff --git a/base64url b/base64url
new file mode 100755
index 0000000..1769126
--- /dev/null
+++ b/base64url
@@ -0,0 +1,5 @@
+#!/bin/bash
+# Like base64 but for the URL-safe alphabet A-Za-z0-9-_ (instead of A-Za-z0-9+/).
+# This works by replacing occurrences of - with + and vice-versa, and likewise with _ and /.
+# Does not support passing the filename by argument.
+tr -- '\-_+/' '+/\-_' | base64 "$@"