diff --git a/library/constant_time.h b/library/constant_time.h
index 598b0eb..5988f52 100644
--- a/library/constant_time.h
+++ b/library/constant_time.h
@@ -46,6 +46,8 @@
  */
 unsigned mbedtls_cf_uint_mask( unsigned value );
 
+#if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC)
+
 /** Turn a value into a mask:
  * - if \p value == 0, return the all-bits 0 mask, aka 0
  * - otherwise, return the all-bits 1 mask, aka (size_t) -1
@@ -59,6 +61,8 @@
  */
 size_t mbedtls_cf_size_mask( size_t value );
 
+#endif /* MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC */
+
 #if defined(MBEDTLS_BIGNUM_C)
 
 /** Turn a value into a mask: