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

Senior Applied Cryptography Engineer (80 - 100%)

Your Role

As an applied cryptography engineer at Threema, you will help us design, build and scrutinize the cryptographic protocols behind our messenger, and contribute to the effective privacy protection of millions of people.

Your Tasks

Protocol design and quantum-safe migration:

• Develop and specify cryptographic communication protocols

• Design cryptographic systems, such as key management and multi-device synchronization

• Lead Threema's migration to quantum-safe cryptography, together with the cryptography researchers at IBM Research

• Own threat modeling for new product features and protocol designs, in close collaboration with the product and engineering teams

Security analysis:

• Analyze protocols, algorithms and implementations for security weaknesses, and assess what new attack vectors mean for our products

• Assess the vulnerability reports that reach us through our bug bounty partner and coordinate the fix with the engineering teams

Research and community:

• Follow current developments in applied cryptography and turn relevant findings into concrete product improvements

• Maintain contacts with the security research community and, if you enjoy it, represent Threema at conferences

Your Skills

Cryptography and engineering:

• Degree in computer science, mathematics or information security, ideally with a focus on cryptography, or equivalent practical experience

• Solid understanding of modern cryptographic primitives (AEAD, key exchange, KDFs, digital signatures) and of how they fail when combined incorrectly

• Working knowledge of key encapsulation mechanisms and of post-quantum cryptography, including hybrid constructions such as X25519MLKEM768

• Experience designing or reviewing security protocols, ideally in end-to-end encryption, key agreement or multi-device settings

• You write code yourself, in at least one of Rust, Kotlin, Swift, TypeScript or something comparable

• Experience reviewing implementations for side channels, nonce reuse, misuse-prone APIs and similar problems

• Nice to have: practical experience with hybrid post-quantum handshakes, formal protocol verification, published research or CVEs, experience with coordinated vulnerability disclosure

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 a cryptographic trade-off to people who are not cryptographers

• 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 English (at least B2), German is a nice-to-have

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 (macOS 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)

Start date: Immediately or by agreement

Have 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.

Applications sent via email cannot be considered. Please use the official "Apply" button below so we don't accidentally miss your resume in our crowded inboxes

For agencies: we are filling this role directly and do not accept unsolicited candidate profiles.