diff --git a/README.md b/README.md
index a63b53f..60e8cf9 100644
--- a/README.md
+++ b/README.md
@@ -326,6 +326,8 @@ This feature was contributed by [underscorediscovery](https://github.com/yhirose
 
 ### Authentication
 
+NOTE: OpenSSL is required for Digest Authentication, since cpp-httplib uses message digest functions in OpenSSL.
+
 ```cpp
 httplib::Client cli("httplib.org");
 cli.set_auth("user", "pass");