From bc079594073cd93ba0c59b3c481dd5f031be108e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?B=C3=A9n=C3=A9dikt=20Tran?=
 <10796600+picnixz@users.noreply.github.com>
Date: Tue, 15 Jul 2025 14:03:21 +0200
Subject: [PATCH] gh-72570: mention the incompatibility of XOFs with HMAC
 (GH-136676) (cherry picked from commit
 a02cf19deed353d1e0e7564468f10aced61c12e8)
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Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
---
 Doc/library/hmac.rst | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Doc/library/hmac.rst b/Doc/library/hmac.rst
index d6692033b2d4c3..57076c38086c79 100644
--- 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)