The ID5 API script (id5-api.js) is an integral component of the ID5 identity solution, designed to address the challenges of user identification in a cookie-less or privacy-restricted digital advertising landscape. This script operates on a website to generate, retrieve, and manage a universal, encrypted user identifier (ID5 ID). It leverages various signals to create a persistent, pseudonymous identifier for individual users, which can then be shared with demand-side platforms (DSPs) and supply-side platforms (SSPs) within the programmatic advertising ecosystem. By deploying this script, websites can maintain or improve their addressability, allowing for more relevant ad targeting, frequency capping, and campaign measurement without reliance on traditional third-party cookies. It helps publishers monetize their inventory effectively and enables advertisers to reach their target audiences while adhering to evolving privacy regulations and browser restrictions. The script typically interacts with ID5 servers to perform identity synchronization and returns the ID5 ID to the client-side, making it available for subsequent ad requests.