Cloudflare helps verify the security of end-to-end encrypted messages by auditing key transparency for WhatsApp

End-to-end encrypted messaging apps such as WhatsApp, Signal, and iMessage rely on public-private key exchange for secure conversations. However, an implicit trust in the correct distribution of public keys by the app infrastructure is often overlooked. To protect the integrity of end-to-end encrypted conversations, several methods have been deployed, including in-person verification of QR code fingerprints. WhatsApp has paved the way with their Key Transparency announcement, and Cloudflare now verifies WhatsApp's Key Transparency audit proofs. The Auditable Key Directory (AKD) is a tree-shaped data structure that forms the architectural foundation of Key Transparency. Cloudflare's role in Key Transparency involves checking that the AKD tree has been constructed correctly and consistently over time.