diff --git a/boot/zephyr/include/config-rsa.h b/boot/zephyr/include/config-rsa.h
index 0552420..f07c457 100644
--- a/boot/zephyr/include/config-rsa.h
+++ b/boot/zephyr/include/config-rsa.h
@@ -64,6 +64,7 @@
 #define MBEDTLS_MD_C
 #define MBEDTLS_OID_C
 #define MBEDTLS_SHA256_C
+#define MBEDTLS_SHA256_SMALLER
 #define MBEDTLS_SHA224_C
 #define MBEDTLS_AES_C