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 Role

The protocols underneath all of this are what we're hiring for.

You'll 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'll do

• Design new protocol features and extensions, such as multi-device sync, group messaging and calls, and write the specifications for them.
• Implement protocol logic in Rust and review implementations across our Android, i OS, desktop and web clients.
• Model threats and reason through edge cases like message reordering, device loss, malicious servers and compromised keys.
• Work with external security researchers and auditors, respond to findings and ship fixes.
• Keep backward compatibility for a large installed base while moving the protocol forward.
• Mentor other engineers and help set standards for how we write and review security-critical code.

What you bring

• At least 6 years of software engineering, with several years working on network or cryptographic protocols.
• A solid grasp of applied cryptography.

You know how authenticated encryption, key exchange, forward secrecy and post-compromise security work, and where implementations usually go wrong.

• Experience reading and writing protocol specifications precise enough for someone else to implement from.
• Strong skills in at least one systems language. Rust is a plus.
• Care about the details. In this job, a missing length check is a vulnerability.
• Good written English. German is helpful but not required.
• Nice to have
• Experience with formal analysis tools such as Tamarin, Pro Verif or TLA+.
• Familiarity with the Signal protocol, MLS or the Noise framework.
• Published research, CVEs, audit reports or open-source contributions in security.
• Background in mobile platforms and their constraints, such as push delivery, background limits and battery use.

Experience 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 design
• Whatever tools you use, the outcome is yours. AI can draft and suggest, but the person accountable for a protocol that holds is you

Soft skills

• Strong sense of responsibility, precise way of working
• You can write a protocol specification that someone else can implement, and explain trade-offs to people who are not protocol specialists
• Pragmatic judgment about when a design is good enough to ship and when it is not
• Willingness to have your own designs picked apart, and to pick apart other people's with care
• Good written and spoken German and English (at least B2)

We are also happy to accept applications from people who meet many but not all of these criteria.

What We Offer

• A young, motivated team with straightforward communication
• Flexible working hours, option to work from home up to two days per week
• Option to work from anywhere for two weeks per year
• Option to take unpaid leave
• You can choose your own hardware (mac OS or Linux)
• Public transportation discount or parking space (electric car charging available)
• Free use of fitness room, including a fitness trainer once a month
• Professional massage once a month
• Internal German or English courses
• A great coffee machine ;-)
• Regular company events and aperitifs
• The good feeling of contributing to the effective protection of the privacy of millions of people
• Place of work: Pfäffikon SZ (Greater Zurich Area)

Have we sparked your interest?

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