diff --git a/library/constant_time.c b/library/constant_time.c
index 9ddd21d..aa291d7 100644
--- a/library/constant_time.c
+++ b/library/constant_time.c
@@ -295,6 +295,12 @@
     return( ( mask & if1 ) | (~mask & if0 ) );
 }
 
+size_t mbedtls_cf_size_if( unsigned cond, size_t if1, size_t if0 )
+{
+    size_t mask = mbedtls_cf_size_mask( cond );
+    return( ( mask & if1 ) | (~mask & if0 ) );
+}
+
 /**
  * Select between two sign values in constant-time.
  *