This article details how Meta scaled a critical security feature, Key Transparency, to Messenger's massive user base. Engineers can learn about distributed system challenges, cryptographic key management, and infrastructure resilience for high-volume, security-sensitive applications.
End-to-end encryption on Messenger already ensures that the content of your direct messages and calls are protected from the moment they leave your device to the moment they reach the receiver’s device. As part of our end-to-end encrypted chat platform, we believe it’s also important that anyone can verify that the public keys (used by the sender’s device for encrypting each message) belong to the intended recipients and haven’t been tampered with.
This launch builds upon the valuable work and experiences shared by others in the industry. WhatsApp’s implementation of key transparency in 2023 demonstrated the feasibility of this technology for large-scale encrypted messaging. We’ve extended these pioneering efforts in our Messenger implementation to deliver a robust and reliable solution with similar security properties.
Key transparency provides messaging users with a verifiable and auditable record of public keys. It allows them to confirm that their conversations are indeed encrypted with the correct keys for their contacts, and that these keys haven’t been maliciously swapped by a compromised server. This means you can be more confident that your messages are only accessible to the people you intend to communicate with.
You can already check your keys for end-to-end encrypted chats on Messenger, but this can be cumbersome for people who have logged in to Messenger on multiple devices, each of which has its own key. Moreover, these keys change when new devices are added or are re-registered, which necessitates another check of the key every time this happens.
To address this, we’ve added a new security feature, based on key transparency, that allows users to verify these keys without having to compare them manually with their contacts. Of course, anyone who wishes to continue manually verifying their keys is free to do so.
Our key transparency implementation leverages the Auditable Key Directory (AKD) library, mirroring the system already in place for WhatsApp. This system allows Meta to securely distribute and verify users’ public keys. To further enhance the security of this process, we use Cloudflare’s key transparency auditor to provide an additional layer of verification, ensuring that the distribution of keys is transparent and verifiable by anyone. Cloudflare’s auditor maintains a live log of the latest entries on the Key Transparency Dashboard, for both the WhatsApp and Messenger directories.
Implementing key transparency on the scale of Messenger presented unique engineering challenges. One significant factor was the sheer volume and frequency of key updates. Messenger indexes keys for each and every device someone has logged in on, which means that a single user often has multiple, frequently-changing keys associated with their account.
This increased complexity leads to a much higher frequency of key updates being sequenced into our key transparency directory. Currently, we’re observing an epoch frequency of approximately 2 minutes per publish, with hundreds of thousands of new keys added in each epoch. Since we began indexing, our database has already grown to billions of key entries. We’ve implemented a number of advancements in our infrastructure and libraries to help manage this massive and constantly growing dataset, while ensuring high availability and real-time verification:
We improved the algorithmic efficiency of the existing key lookup and verification operations in the AKD library by optimizing for smaller proof sizes, even as the number of updates (versions) for a single key grows. Previously, these proofs grew linearly with the height of the transparency tree, which was still difficult to manage given the number of nodes in the tree.
We also updated our existing infrastructure to be more resilient to temporary outages and improved the process for recovering from long delays in key sequencing. These improvements were adapted from lessons learned from running WhatsApp’s key transparency log for the past two years.
With key transparency now live on Messenger, users will have the ability to automatically verify the authenticity of their contacts’ encryption keys for one-on-one chats. This represents another step forward in our ongoing investment in providing a secure and private service.
Stay tuned for more updates as we continue to enhance the security and privacy of end-to-end encryption in Messenger.
The post Key Transparency Comes to Messenger appeared first on Engineering at Meta.
Continue reading on the original blog to support the author
Read full articleIt demonstrates how to implement privacy-preserving security features in end-to-end encrypted environments. Engineers can learn how to balance cryptographic privacy primitives like PIR and OPRF with the practical performance requirements of large-scale real-time messaging.
WhatsApp's migration demonstrates that Rust is production-ready for massive-scale, cross-platform applications. It proves memory-safe languages can replace legacy C++ to eliminate vulnerabilities while improving performance and maintainability.
This article details how Meta scaled invisible video watermarking, a critical technology for content provenance. It's vital for engineers tackling challenges like detecting AI-generated media and ensuring content authenticity at massive scale with operational efficiency.
Scaling security updates across massive codebases is traditionally slow and error-prone. By combining secure-by-default frameworks with AI-powered codemods, Meta demonstrates how to automate large-scale security migrations, reducing developer friction and improving app safety at scale.
