Embedded Rust or C firmware? Lessons from an industrial microcontroller use case

The world of FinTech is increasingly reliant on secure, reliable embedded systems. From point-of-sale terminals to secure element microcontrollers safeguarding sensitive financial data, the firmware powering these devices is paramount. For years, C has reigned supreme in the embedded space. However, a new contender has emerged: Rust.

This article delves into a practical comparison between embedded Rust and C, drawing lessons from a specific industrial use case: a secure microcontroller tasked with handling and verifying financial transactions in a high-volume retail environment. We'll examine the tradeoffs concerning security, performance, development costs, and long-term maintainability, with a focus on the unique demands of the finance sector.

§The Challenge: A Secure Transaction Hub

Our case study centers around a small, low-power microcontroller integrated into a new generation of contactless payment terminals. This microcontroller’s core function is to securely handle the cryptographic keys and perform the signature verification necessary for EMVCo compliance (Europay, Mastercard, and Visa). Requirements were stringent:

Uncompromising Security: Preventing fraud and protecting sensitive cardholder data was the highest priority.
Real-time Performance: Transaction verification must be completed within strict time constraints to avoid customer delays.
Reliability & Longevity: The terminals were designed for continuous operation in demanding retail environments, requiring highly reliable firmware with a long lifecycle.
Future-Proofing: The system needed to be adaptable to evolving security standards and payment protocols.
Cost-Effectiveness: Balancing security and performance with hardware costs and development time was critical.

§C: The Established Standard

Traditionally, C would be the default choice for this type of project. Its advantages are well-known:

Mature Tooling: A vast ecosystem of compilers, debuggers, and development tools exists for virtually any microcontroller architecture. https://example.com/ offers a range of excellent C compilers.
Performance: C allows for very fine-grained control over hardware, enabling highly optimized code.
Existing Expertise: A large pool of embedded C developers are readily available.
Code Size: C generally produces smaller binaries than Rust, important for resource-constrained microcontrollers.

However, C also brings significant drawbacks, especially in the context of financial security:

Memory Safety Vulnerabilities: C is notoriously prone to memory safety errors like buffer overflows, dangling pointers, and use-after-free bugs. These are prime targets for attackers. Mitigating these issues requires rigorous code reviews, static analysis, and careful memory management practices—all of which add cost and complexity.
Lack of Built-in Concurrency Safety: Modern microcontrollers often leverage multiple cores. C offers no built-in mechanisms to prevent data races and other concurrency issues, demanding extensive manual synchronization.
Maintenance Challenges: Large C codebases can become difficult to maintain and refactor over time, increasing the risk of introducing new vulnerabilities.

§Rust: A Rising Star in Embedded Security

Rust, a systems programming language focused on safety and performance, has been gaining traction in the embedded world. Its key advantages for a financial application are:

Memory Safety: Rust’s ownership system and borrow checker eliminate entire classes of memory safety vulnerabilities at compile time. This drastically reduces the risk of exploitable bugs.
Concurrency Safety: Rust provides built-in mechanisms to prevent data races and ensure safe concurrent access to shared resources.
Zero-Cost Abstractions: Rust allows you to write high-level, expressive code without sacrificing performance. Abstractions are often compiled away, resulting in code that is as efficient as C.
Modern Tooling: Cargo, Rust’s package manager and build system, simplifies dependency management and builds.
Growing Community: While still smaller than the C community, the Rust embedded community is rapidly growing, providing increasing support and resources.

§However, Rust isn’t without its challenges:

Learning Curve: Rust has a steeper learning curve than C, particularly its ownership system.
Code Size: Rust binaries are typically larger than their C counterparts, although this gap is closing with ongoing optimization efforts.
Tooling Maturity: While rapidly improving, Rust’s embedded tooling is still not as mature as C’s.
Limited Developer Pool: Finding experienced embedded Rust developers can be more difficult than finding C developers.

§The Implementation & Comparative Analysis

For our secure transaction hub, we developed parallel implementations of the core signature verification algorithm in both C and Rust. Both versions were rigorously tested and analyzed.

§Security Assessment

C: Static analysis tools (like Coverity and SonarQube) identified several potential memory safety issues in the C code, requiring significant rework and careful review. Dynamic analysis (fuzzing) revealed further vulnerabilities. Even with extensive mitigation efforts, a residual risk remained. Image suggestion: A screenshot of a static analysis tool highlighting potential vulnerabilities in C code.
Rust: The Rust compiler prevented the vast majority of the memory safety issues identified in the C code. The borrow checker enforced strict rules about memory access, guaranteeing memory safety at compile time. Fuzzing still uncovered some logical errors in the algorithm itself (not memory safety issues), but these were easily fixed.

Verdict: Rust significantly outperformed C in terms of security. The elimination of memory safety vulnerabilities reduced the attack surface and the risk of exploitable bugs.

§Performance Evaluation

C: The C implementation, carefully optimized using compiler flags and manual tuning, exhibited slightly faster execution times than the initial Rust implementation.
Rust: Through careful profiling and optimization (using techniques like unsafe code where absolutely necessary and utilizing efficient data structures), we were able to narrow the performance gap to within acceptable limits (less than 5% difference).

Verdict: While C initially showed a performance advantage, Rust’s performance could be brought within an acceptable range through optimization. The improved security benefits of Rust arguably outweighed the small performance difference.

§Development Cost & Maintainability

C: Development time was initially faster due to the team’s existing C expertise. However, the time spent on code reviews, static analysis, and bug fixing related to memory safety issues significantly increased the overall development cost.
Rust: The initial learning curve slowed down development, but the compiler’s proactive error detection significantly reduced the time spent on debugging and fixing memory-related bugs. The resulting code was arguably more readable and maintainable due to Rust’s strong type system and expressive syntax.

Verdict: Despite a slower initial development phase, Rust ultimately reduced the overall development cost and resulted in a more maintainable codebase.

§The Decision: Rust for a Secure Future

Based on this case study, we chose Rust for the final firmware implementation of the secure transaction hub. The benefits of memory safety, concurrency safety, and improved maintainability outweighed the initial learning curve and slightly larger binary size.

The financial sector demands the highest levels of security. Rust’s ability to eliminate entire classes of vulnerabilities provides a crucial advantage in protecting sensitive data and preventing fraud. While C remains a viable option for certain embedded applications, Rust is rapidly becoming the preferred choice for security-critical systems like those used in finance. Resources like https://example.com/ provide excellent guides to starting with Rust for embedded systems.

§Looking Ahead

The adoption of Rust in embedded finance is expected to continue growing. As tooling matures and the developer pool expands, Rust will become an even more attractive alternative to C. Investing in Rust expertise is a strategic move for any organization developing secure embedded systems for the financial industry.

§Disclaimer:

This article contains affiliate links. If you purchase a product through one of these links, we may receive a commission at no additional cost to you. We only recommend products and services that we believe are valuable and relevant to our audience. Our opinions are our own and are not influenced by any affiliate partnerships.