Arm Mbed and Pelion Device Management support forum

Mbedtls code size optmization

Hi,

We have did a custom secure protocol which is similar to ssl for low end devices. Basically it uses hmac, &x.509 (only rsa2048)certificate for authentication and AES GCM for cipher. We try to keep the in-house designed protocol simple. TLS/DTLS might not be feasible as the channel is serial with 9600 baud rate, with ROM cap 150 KB.

Currently, the code size takes around 170 KB, for the program. We do handshake similar to ssl to authenticate device, symmetric key, & network time using certificate and hmac combined. THen we use AES GCM for encryption after handshake. Tried to disable unwanted modules, like chacha20, sha512, md5, ecdh, dsa, etc by removing them from build directory. However checkconfig is throwing many define errors, although they were never called in any of my program.

What is the best way to optimize the output code size

Thanks,
Gopi Krishnan

Hi @gopi219
Note that check_config is done in compile time, not in link time, so even if there are symbols that you don’t need, but being defined, they are being checked.

What is the best way to optimize the output code size

Unfortunately, there isn’t a fast way to do that. I would suggest you take a minimal set of configuration file, from the configs folder, such as config-mini-tls1_1.h ( with removing definition of MBEDTLS_SSL_PROTO_TLS1_1 and other TLS specific defines), and then define only what you need. If check_config.h throws a compilation error, you should either add the prerequisite of your algorithm, or remove the unwanted define.
Regards,
Mbed TLS Support
Ron