Job offer : Senior Back-end Developer Rust / Next.js — security and cryptography

Our client is a French confidential file transfer service based in Lyon. The site is already online and operational. It offers automatic browser encryption, optional private end-to-end encryption for the sender, and OneTimeView, a sending mode that can only be viewed once. We are now entering a new stage: new features and services are in preparation, which we cannot yet talk about publicly. To build them, we are looking for a developer for whom security comes before everything else. The structure is small: decisions are fast, you work in direct contact with the founder, and your work is immediately visible in production. Job description: You will evolve the existing platform and develop the next modules. Specifically: evolve the back-end of the Next.js application in production: new features, review and hardening of existing code, dependency updates; design and develop new components in Rust; work on encryption flows (OpenPGP, browser-side encryption), key management, and file protection during transfer and storage; integrate security by design: threat modeling, access control, abuse protection, logging; write tests and rely on our automated test suite; communicate directly with the founder on technical choices and priorities. Freelance mission of at least 6 months, hybrid, starting mid-November 2026. Location Lyon. Qualifications: At least 7 years of experience in back-end development, with a significant portion on security topics. Very good command of Rust. Good command of Next.js and the JavaScript / TypeScript ecosystem. Solid foundations in applied cryptography (OpenPGP, AES-GCM, key management), and above all a clear idea of what should never be done. Good knowledge of web vulnerabilities (OWASP Top 10) and secure development practices. Comfortable with Linux and server deployment. Autonomy, rigor, and discretion. Confidentiality is at the heart of our business, and a non-disclosure agreement will be signed. A plus: open source contributions in Rust, experience in security auditing or bug bounty, zero-knowledge architectures.