Swift at Apple: Migrating the Password Monitoring Service from Java

Summary

Swift at Apple: migrating the Password Monitoring service from Java June 2, 2025 Swift is heavily used in production for building cloud services at Apple, with incredible results. Last year, the Password Monitoring service was rewritten in Swift, handling multiple billions of requests per day from devices all over the world. In comparison with the previous Java service, the updated backend delivers a 40% increase in performance, along with improved scalability, security, and availability. The Passwords app, introduced in the fall of 2024, helps users manage their passwords, passkeys, and verification codes. It allows them to store, autofill, and generate strong passwords that can be shared across all their devices, as well as share passwords with trusted contacts. One security feature included in the app is Password Monitoring, which warns users if one of their saved passwords shows up in a data leak. This feature has a server component that is maintained by Apple. On a regular interval, Password Monitoring checks a user’s passwords against a continuously updated and curated list of passwords that are known to have been exposed in a leak. Importantly, this task is handled in a thoughtful, privacy-preserving way that never reveals users’ passwords to Apple. A detailed discussion of how this is done using the cryptographic private set intersection protocol is in the Password Monitoring section of the Apple Platform Security guide. The migration from Java to Swift was motivated by a need to scale the Password Monitoring service in a performant way. The layered encryption module used by Password Monitoring requires a significant amount of computation for each request, yet the overall service needs to respond quickly even when under high load. For years, our team relied on Java to power large-scale, mission-critical services because of its proven stability and performance. However, Java’s memory management approach no longer aligns with our growing demands and efficiency...