Public Report – WhatsApp Auditable Key Directory (AKD) Implementation Review

In August 2023, Meta engaged NCC Group’s Cryptography Services practice to perform an implementation review of their Auditable Key Directory (AKD) library, which provides an append-only directory of public keys mapped to user accounts and a framework for efficient cryptographic validation of this directory by an auditor. The library is being leveraged to provide an AKD for WhatsApp and is meant to serve as a reference implementation for auditors of the WhatsApp AKD, as well as to allow other similar services to implement key transparency. The review was performed remotely by 3 consultants over a two-week period with a total of 20 person-days spent. The project concluded with a retest phase a few weeks after the original engagement that confirmed all findings were fixed.

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