diff --git a/boot/zephyr/include/config-boot.h b/boot/zephyr/include/config-boot.h
index fbfe01c..50bee10 100644
--- a/boot/zephyr/include/config-boot.h
+++ b/boot/zephyr/include/config-boot.h
@@ -68,7 +68,7 @@
 
 #ifdef MCUBOOT_SIGN_RSA
 #define MBEDTLS_RSA_C
-#define MBEDTLS_PKCS1_V15
+#define MBEDTLS_PKCS1_V21
 #endif
 
 /* mbed TLS modules */