Worked on the signalapp/Signal-Server repository to deliver a security-focused enhancement for user onboarding. Developed and enforced the SPARSE_POST_QUANTUM_RATCHET capability during user registration, ensuring that only devices supporting this post-quantum security feature could complete the process. The implementation involved validating device capabilities at registration and updating the underlying data models to capture this new requirement. Leveraged Java for backend development and API design, with a strong emphasis on security best practices. The work addressed a specific need for robust cryptographic onboarding, resulting in a single feature addition and no bug fixes during the month, reflecting focused, in-depth engineering effort.