diff --git a/docs/architecture/tls13-experimental.md b/docs/architecture/tls13-experimental.md
index bcf3e34..3db16e0 100644
--- a/docs/architecture/tls13-experimental.md
+++ b/docs/architecture/tls13-experimental.md
@@ -38,3 +38,12 @@
 - The HKDF key derivation function on which the TLS 1.3 key schedule is based,
   is already present as an independent module controlled by `MBEDTLS_HKDF_C`
   independently of the development of the TLS 1.3 prototype.
+
+- The TLS 1.3-specific HKDF-based key derivation functions (see RFC 8446):
+  * HKDF-Expand-Label
+  * Derive-Secret
+  - Secret evolution
+  * The traffic {Key,IV} generation from secret
+  Those functions are implemented in `library/ssl_tls13_keys.c` and
+  tested in `test_suite_ssl` using test vectors from RFC 8448 and
+  https://tls13.ulfheim.net/.