gh-72570: mention the incompatibility of XOFs with HMAC (#136676)

diff --git a/Doc/library/hmac.rst b/Doc/library/hmac.rst
index d6692033b2d4c3ad8adf939aade0d8304d876a5f..57076c38086c79b1877a7534e02e0335f9b8f51a 100644 (file)
--- a/Doc/library/hmac.rst
+++ b/Doc/library/hmac.rst
@@ -12,6 +12,9 @@
 --------------
 
 This module implements the HMAC algorithm as described by :rfc:`2104`.
+The interface allows to use any hash function with a *fixed* digest size.
+In particular, extendable output functions such as SHAKE-128 or SHAKE-256
+cannot be used with HMAC.
 
 
 .. function:: new(key, msg=None, digestmod)