diff --git a/library/constant_time.c b/library/constant_time.c
index 0c5c04c..604859f 100644
--- a/library/constant_time.c
+++ b/library/constant_time.c
@@ -111,3 +111,28 @@
 #pragma warning( pop )
 #endif
 }
+
+/*
+ * Turn a bit into a mask:
+ * - if bit == 1, return the all-bits 1 mask, aka (size_t) -1
+ * - if bit == 0, return the all-bits 0 mask, aka 0
+ *
+ * This function can be used to write constant-time code by replacing branches
+ * with bit operations using masks.
+ *
+ * This function is implemented without using comparison operators, as those
+ * might be translated to branches by some compilers on some platforms.
+ */
+size_t mbedtls_cf_size_mask( size_t bit )
+{
+    /* MSVC has a warning about unary minus on unsigned integer types,
+     * but this is well-defined and precisely what we want to do here. */
+#if defined(_MSC_VER)
+#pragma warning( push )
+#pragma warning( disable : 4146 )
+#endif
+    return -bit;
+#if defined(_MSC_VER)
+#pragma warning( pop )
+#endif
+}