Mbed TLS: When There's Simply No Room for OpenSSL

A familiar situation: you have a microcontroller that needs to securely send data to a server. Memory is scarce, the processor is modest, and you need to squeeze a full-fledged TLS implementation in there. OpenSSL? A great library, but its appetite may be too much for your little "baby." What do you do? Write your own TLS implementation from scratch? Relax, there's a more elegant solution.

Today we'll look under the hood of Mbed TLS — a library that has become a true lifesaver for the embedded systems and IoT world.

What is Mbed TLS and Who Needs It?

To put it briefly, Mbed TLS is a C library for implementing TLS/DTLS protocols and working with X.509 certificates. Its main feature, as the developers themselves emphasize, is the "small code footprint" — minimal code size. It's designed to take up as little memory space as possible and not require a powerful processor.

The project was originally called PolarSSL, but after being acquired by ARM, it received a new name and became part of their ecosystem for embedded solutions. Today, Mbed TLS is an open-source project under the umbrella of TrustedFirmware.org, which speaks to a serious focus on security and reliability.

The primary audience for the project is, of course, developers for embedded systems and the Internet of Things (IoT). But thanks to its clean code and flexibility, Mbed TLS has found applications in other areas where minimalism and control over dependencies are important.

Why Do Developers Love Mbed TLS?

Let's explore the key features that make this library so attractive.

1. A Cryptography Constructor

One of the most powerful capabilities of Mbed TLS is its flexible configuration. Imagine you don't need support for legacy ciphers or exotic elliptic curves. In most libraries, they'll still be included in the build, "just in case," eating up precious kilobytes.

In Mbed TLS, everything is different. You have a header file mbedtls_config.h where, like a constructor, you can enable and disable almost everything:

• Specific encryption algorithms (AES, ChaCha20)
• Hash functions (SHA-256, SHA-512)
• TLS modes of operation (for example, TLS 1.3 support)
• Certificate handling functions

This allows you to build exactly the version of the library needed for your project, without a single extra byte. And for automating this process, there's a convenient Python script scripts/config.py.

2. Portability and Minimal Dependencies

The library is written in standard C99 and designed with maximum portability in mind. It doesn't require a complex operating system or exotic dependencies. If you have a C99 compiler and the standard library, you'll most likely be able to run Mbed TLS.

The README clearly states the minimum platform requirements:

• Bytes must be 8-bit.
• int and size_t must be at least 32-bit.
• Signed integers use two's complement representation.

This makes porting to new architectures and operating systems predictable and relatively painless.

3. Clean and Readable Code

Anyone who has ever tried to understand OpenSSL source code knows it's not a task for the faint of heart. Mbed TLS, on the contrary, is famous for its clean and well-documented code. This isn't just an aesthetic advantage. For a cryptographic library, code readability is part of security. When the logic is easy to trace, the likelihood of finding or, conversely, avoiding bugs is much higher.

Because of this, Mbed TLS is often used for educational purposes to learn how security protocols actually work.

4. Modern Architecture and Integration

Under the hood, Mbed TLS consists of three main components that can even be used separately:

• libtfpsacrypto: a low-level cryptographic library implementing the PSA Cryptography API.
• libmbedx509: a library for working with X.509 certificates.
• libmbedtls: the actual implementation of TLS and DTLS protocols.

The modern approach is also evident in the build system. The project uses CMake, which greatly simplifies integration into other projects. Adding Mbed TLS as a dependency is just a matter of a few lines in CMakeLists.txt:

find_package(MbedTLS REQUIRED)

target_link_libraries(my_awesome_app
    PUBLIC MbedTLS::mbedtls
           MbedTLS::mbedx509
           MbedTLS::tfpsacrypto)

This automatically links the required libraries and adds paths to header files. Simple, clean, and efficient.

Where Will Mbed TLS Be Particularly Useful?

• IoT devices: from smart bulbs to industrial sensors. Everywhere that needs a secure connection with limited resources.
• Microservices and lightweight applications: when you need a TLS client or server but don't want to drag heavy dependencies like OpenSSL into your project.
• Projects with high code audit requirements: thanks to its readability, Mbed TLS is easier to audit for vulnerabilities.
• Education: an excellent "textbook" for those who want to understand the inner workings of cryptography and network security.

Conclusion: Is It Worth Trying?

Absolutely yes. Mbed TLS is a great example of what a modern specialized library should look like. It doesn't try to be everything for everyone, like OpenSSL. Instead, it perfectly solves its main task: providing reliable and lightweight cryptography for resource-constrained environments.

If you write in C/C++ for embedded systems or simply value minimalism and full control over dependencies, definitely add Mbed TLS to your toolkit. And for a quick start, check out the programs/ folder in the repository — you'll find plenty of useful examples there.