Protocol Stack Engineer (80–100%)

Threema, the world's best‑selling secure messenger for private users and businesses, is looking for a Protocol Stack Engineer to join its growing team. Since 2012 the company has built a global user base of millions, delivering privacy‑focused communication across Android, iOS, desktop and web platforms. The role is offered at 80‑100 % capacity and will be based in Pfäffikon SZ, in the Greater Zurich Area. The engineer will be responsible for designing, implementing and maintaining the cryptographic protocols that power every Threema message, including the forward‑secrecy Ibex protocol. Daily tasks involve creating new protocol features such as multi‑device sync, group messaging and calls, and drafting precise specifications for them. The code will be written primarily in Rust and reviewed across all client platforms. The position also requires threat modelling, handling edge cases like message reordering or compromised keys, and collaborating with external security researchers and auditors to address findings and ship fixes. Maintaining backward compatibility for a large installed base while advancing the protocol, as well as mentoring fellow engineers and establishing standards for security‑critical code, are also key parts of the job. Candidates must bring at least six years of software‑engineering experience, with several years focused on network or cryptographic protocols. A solid grasp of applied cryptography, understanding authenticated encryption, key exchange, forward secrecy and post‑compromise security, is essential, as is the ability to write and read protocol specifications that are clear enough for others to implement. Strong proficiency in at least one systems language is required; Rust knowledge is considered a plus. The role demands meticulous attention to detail, since even a missing length check can become a vulnerability, and it expects good written and spoken German and English at a minimum B2 level. Soft‑skill expectations include a strong sense of responsibility, precise work habits, the ability to explain trade‑offs to non‑specialists, pragmatic judgment on design readiness, and openness to having one's own designs examined critically. Nice‑to‑have experience includes using formal analysis tools such as Tamarin, ProVerif or TLA+, familiarity with the Signal protocol, MLS or the Noise framework, and a track record of published security research, CVEs, audit reports or open‑source contributions. Background in mobile platform constraints, push delivery, background limits and battery usage, is also valued. Familiarity with frontier AI models for productivity in design and coding is mentioned, though the final accountability for the protocol remains with the engineer. The position offers a young, motivated team, flexible working hours, up to two days per week remote work, a two‑week "work from anywhere" option each year, and a range of wellbeing perks such as fitness facilities, monthly massages, language courses and a great coffee machine. The role starts immediately or by agreement, and Threema welcomes applicants from diverse backgrounds. What the role asks for: - Minimum six years software engineering experience - Several years experience with network or cryptographic protocols - Deep understanding of authenticated encryption, key exchange, forward secrecy - Proven ability to write and read precise protocol specifications - Strong proficiency in at least one systems language (Rust a plus) - Meticulous attention to detail for security‑critical code - Good written and spoken German and English (minimum B2) - Nice to have: Experience with formal analysis tools (Tamarin, ProVerif, TLA+) - Nice to have: Familiarity with Signal protocol, MLS, or Noise framework - Nice to have: Published security research, CVEs, audit reports, or open‑source contributions - Nice to have: Background in mobile platform constraints (push delivery, battery usage) - Nice to have: Proficiency with frontier AI models for design and coding assistance