Protocol Stack Engineer (80–100%)

Threema is the world’s best-selling secure messenger for both private users and businesses. Since 2012, we have been working tirelessly to ensure that our users can communicate freely without worrying about their privacy. Our growing user base includes millions of private customers and thousands of businesses and organizations from all over the world.We are growing and looking for a new team member to join as a Protocol Stack Engineer (80–100%).Your RoleThe protocols underneath all of this are what we’re hiring for. You will design, implement, and maintain the cryptographic protocols that carry every Threema message, including Ibex, our forward-secrecy protocol. Your work will be read by our own engineers and also by outside researchers, auditors, and curious users. We expect it to hold up to that.What You Will DoDesign new protocol features and extensions, such as multi-device sync, group messaging, and calls, and write the specifications for themImplement protocol logic in Rust and review implementations across our Android, iOS, desktop, and web clientsModel threats and reason through edge cases like message reordering, device loss, malicious servers, and compromised keysWork with external security researchers and auditors, respond to findings, and ship fixesKeep backward compatibility for a large installed base while moving the protocol forwardMentor other engineers and help set standards for how we write and review security-critical codeWhat you bringAt least six years of software engineering, with several years working on network or cryptographic protocolsA solid grasp of applied cryptography: you know how authenticated encryption, key exchange, forward secrecy, and post-compromise security work, and where implementations usually go wrongExperience in reading and writing protocol specifications precise enough for someone else to implement fromStrong skills in at least one systems language; Rust is a plusCare about the details: in this job, a missing length check is a vulnerabilityNice to HaveExperience with formal analysis tools such as Tamarin, ProVerif, or TLA+Familiarity with the Signal protocol, MLS, or the Noise frameworkPublished research, CVEs, audit reports, or open-source contributions in securityBackground in mobile platforms and their constraints, such as push delivery, background limits, and battery useExperience with AI Tools:You’re familiar with current frontier AI models and use them as a productivity multiplier in design work as well as in coding, for example, to review specification drafts, survey recent literature, or poke holes in a protocol designWhatever tools you use, the outcome is yours; AI can draft and suggest, but the person accountable for a protocol that holds is youSoft Skills:Strong sense of responsibility, precise way of workingYou can write a protocol specification that someone else can implement, and explain trade-offs to people who are not protocol specialistsPragmatic judgment about when a design is good enough to ship and when it is notWillingness to have your own designs picked apart, and to pick apart other people’s with careGood written and spoken German and English (at least B2)What We OfferA young, motivated team with straightforward communicationFlexible working hours, option to work from home up to two days per weekOption to work from anywhere for two weeks per yearOption to take unpaid leaveYou can choose your own hardware (macOS or Linux)Public transportation discount or parking space (electric car charging available)Free use of fitness room, including a fitness trainer once a monthProfessional massage once a monthInternal German or English coursesA great coffee machine ;-)Regular company events and aperitifsThe good feeling of contributing to the effective protection of the privacy of millions of peoplePlace of work: Pfäffikon SZ (Greater Zurich Area)Start date: Immediately or by agreementHave we sparked your interest?We believe that diverse teams build better products. We welcome and encourage applications from people of all backgrounds, and perspectives. Your unique experience is an asset, and we’d love to hear from you. #J-18808-Ljbffr