Senior / Staff Backend Engineer

About the job

We're building a core system that underpins digital identification, validation, and certification services - the kind of infrastructure that financial institutions, government services, and enterprises depend on to operate securely. This is PKI, digital identity management, and Trust Service Provider (TSP) infrastructure: highly regulated, demanding and critical.

This role is for experienced engineers who thrive on complex technical challenges, take ownership seriously, and want to work on systems where quality and precision aren't negotiable.

What you'll be working on

You'll be developing core infrastructure for cryptography and key management, and digital identity services that operate under strict regulatory frameworks (ETSI, eIDAS, PSD2, CA/Browser Forum). This means:

• Building critical infrastructure system from scratch: High availability, fault tolerance, and security are crucial
• Navigating technical and regulatory complexity: You'll work within strict standards while solving genuinely hard engineering problems.
• Owning critical components: You'll have direct impact on core infrastructure decisions. Our team is experienced and collaborative, but we expect you to drive your domain with accountability
• Adapting to evolving requirements: Requirements will evolve as we navigate compliance, integration, and operational realities. You need to be comfortable with ambiguity and pragmatic trade-offs.

What we're looking for

• 5+ years building production systems in regulated, high-stakes environments (financial services, critical infrastructure, security, healthcare, identity management)
• Proven experience with complex, multi-component backend systems
• Track record of shipping and maintaining systems under compliance or audit requirements

Technology scope:

We work across multiple stacks and technologies. You don't need to know everything, but you need to be able to adapt quickly and learn independently:

Backend & Architecture

• Strong grasp of OOP, SOLID principles, design patterns
• Experience designing RESTful APIs (OpenAPI/Swagger/Scalar)
• Knowledge of OAuth2, OpenID Connect, JWT, service-to-service authentication
• Experience with long-running background processes, schedulers, and queue-based processing
• Familiarity with distributed systems and multi-instance deployments

Primary Stack (need strong experience in at least one):

• .NET
• NET 6+ / .NET 10 (ASP.NET Core, Web API)
• C#, async/await, background workers
• Experience with REST API programming, asynchronous programming and background processing in .NET
• Entity Framework Core
• Experience with multithreading and concurrency

Java

• Java 11+ / 17+
• Spring Boot, Spring Security, Spring Data
• Experience with multithreading and concurrency

Infrastructure & DevOps

• Docker (writing and maintaining Dockerfiles and running services in containerized environments)
• CI/CD pipelines (GitLab CI, Azure DevOps, GitHub Actions)
• Linux environments and basic system administration
• Configuration management (environment variables, secrets handling)
• Container orchestration concepts (Kubernetes experience is a plus)

Data & Databases

• Strong SQL knowledge and schema design skills
• PostgreSQL or MS SQL Server
• Experience with transactions, locking, indexing, and performance tuning

Highly valued (not required, but significant advantage):

• Understanding of PKI concepts: certificates, private keys, CRL, OCSP, RSA/ECC, hashing algorithms
• Experience with digital signatures (PAdES, XAdES, CAdES)
• HSM integration, secure key storage, encryption handling
• Security-focused application design

What makes this team work:

Serious about the work. We're an experienced team that respects the gravity of what we're building while keeping things collaborative and human.

• Compliance is handled: Dedicated PMs manage regulatory navigation and support the requirements management. Everyone collaborates and shares context.
• We've been here before: The team has shipped regulated, critical systems. You won't be explaining why testing matters or why shortcuts aren't acceptable.
• High trust, high accountability: We expect you to own your work, ask for help when needed, and deliver what you commit to.

What we offer:

• Competitive salary with performance-based bonuses
• Private health insurance
• Flexible working hours and hybrid work model option
• Note: due to the project nature, we expect presence in our Belgrade office
• Exposure to advanced security, cryptography, and PKI systems
• Long-term, stable project with clear technical ownership