LegoLog: A configurable transparency log

Transparency logs are critical for a wide range of applications, from web certificates to end-to-end encrypted messaging. Today, many transparency log designs exist for various applications and workloads, and developers must fully understand the design space to find the best design for their needs. Worse, if a developer needs a transparency log for an application and workload without an existing transparency log, the developer (who might not be an expert) must design a new log. To address these challenges, we introduce the paradigm of a configurable transparency log, which takes as input a description of the application work-load and constraints of different entities and automatically outputs a transparency log uniquely suited to the application. We present the first configurable transparency log design, LegoLog, which we implement and empirically evaluate end-to-end for three specialized transparency logs. We also show that LegoLog can express six different applications, and we compare the asymptotic complexity of LegoLog and existing transparency logs tailored to individual applications. We find that configurability does not come at the cost of performance: LegoLog can capture a variety of applications while performing comparably to existing, special-purpose transparency logs.