Software Security Engineer

Why Castelion, Why Now Castelion is moving incredibly fast to develop and deliver advanced defense systems at a time when execution matters more than ever. We believe focus, ownership, and excellence are decisive advantages - and we're building a world-class team to turn bold ideas into real capability. This is a rare opportunity to join at an early stage, where you'll have significant ownership, collaborate with exceptional teammates, and make a direct, measurable impact on our mission and the future of the company - regardless of your function. Software Security Engineer As a Software Security Engineer at Castelion, you will build and implement the cryptographic foundations that secure our flight systems. You will own encryption, code signing, secure boot, and key management across embedded platforms, ensuring the integrity, authenticity, and confidentiality of mission-critical systems. You will work closely with embedded, avionics, and infrastructure engineers to integrate security directly into the full software and hardware stack from manufacturing provisioning through flight operations. We seek engineers who think in systems, understand threat models deeply, and can translate cryptographic principles into practical, resilient implementations in constrained environments. Responsibilities • Secure Boot & Root of Trust: Design and implement secure boot chains, hardware-backed root of trust mechanisms, and firmware verification processes for embedded flight systems. • Cryptography & Encryption: Develop and integrate cryptographic protocols and libraries (e.g., symmetric/asymmetric encryption, key exchange, digital signatures) for embedded and distributed systems. • Code Signing & Update Security: Own software signing infrastructure and verification workflows to ensure authenticity and integrity of firmware, flight software, and field updates. • Key Management & Provisioning: Design secure key generation, storage, rotation, and provisioning systems across development, manufacturing, and deployed environments. • Secure Communications: Implement and review authenticated and encrypted communication channels between vehicle, ground systems, and internal subsystems. • Threat Modeling & Hardening: Conduct threat modeling and security reviews across the software stack. Identify vulnerabilities and implement practical mitigations aligned with mission constraints. • Cross-Functional Integration: Partner with embedded, avionics, hardware, and infrastructure teams to embed security principles into system architecture from initial design through deployment. Basic Qualifications • Bachelors degree in Computer Science, Computer Engineering, Electrical Engineering, or related STEM field • 3+ years of professional experience in software engineering with exposure to applied cryptography or security engineering • Experience implementing encryption, authentication, or digital signature systems in C, C++, Rust, or similar systems languages • Strong understanding of cryptographic fundamentals (PKI, TLS, key exchange, hashing, signatures) • Experience working in Linux-based or embedded environments Preferred Skills and Experience • Experience implementing secure boot or hardware root-of-trust mechanisms • Experience with code signing pipelines and artifact verification • Familiarity with TPMs, HSMs, or secure elements • Experience in embedded, aerospace, defense, or other safety-critical systems • Experience designing secure provisioning workflows in manufacturing environments • Understanding of real-time systems and resource-constrained devices • Experience with mTLS, certificate lifecycle management, and replay protection mechanisms Castelion offers a generous benefits package. Please refer to the bottom of our Careers page for more details. Other Duties Please note this job description is not designed to cover or contain a comprehensive listing of activities, duties or responsibilities that are required of the employee for this job. Duties, responsibilities and activities may change at any time with or without notice. Additional Eligibility Requirements This position may require access to classified information or restricted U.S. Government sites, systems, or information, as determined by the Company and/or applicable U.S. Government requirements. If the position is so designated, your employment in the role may be contingent upon your ability to obtain and maintain the required U.S. Government security clearance or other government authorization, and to satisfy any citizenship or other eligibility requirements imposed by applicable law, regulation, executive order, or government contract requirements. You will be notified if and when such requirements apply. Affirmative Action/EEO Statement Castelion is an Equal Opportunity Employer. We are committed to providing equal employment opportunities to all applicants and employees without regard to race, color, religion, sex, sexual orientation, gender identity, national origin, age, disability, genetic information, protected veteran status, or any other characteristic protected by applicable federal, state, or local law. Castelion is committed to providing reasonable accommodations to qualified individuals with disabilities throughout the application and hiring process. If you require a reasonable accommodation to complete an application, participate in the interview process, or otherwise participate in the hiring process, please contact hr@castelion.com. Requests for accommodation will be considered on an individual basis and handled in accordance with applicable law. Castelion is committed to fostering a workplace where employment decisions are based on qualifications, business needs, and the ability to perform the essential functions of the role, with or without reasonable accommodation. EAR/ITAR Requirements This position requires access to export-controlled information, and as such, employment (or hiring of a contractor) is contingent upon the candidate’s ability to access all applicable export-controlled information without additional export licensing being required by the Bureau of Industry and Security and/or the Directorate of Defense Trade Controls.