diff --git a/ChangeLog.d/constant_time_module.txt b/ChangeLog.d/constant_time_module.txt
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+Changes
+    * The mbedcrypto library includes new a source code module constant_time.c,
+      containing various functions meant to resist timing side channel attacks.
+      This module does not have a separate configuration option, and functions
+      from this module will be included in the build as required. Currently
+      the interface of this module is private and may change at any time.