Turnstile cookies: A new root of trust for personhood credentials

Personhood credentials (PHC) have been introduced to verify that an individual is real online and distinguish them from AI bots. In this preliminary work, we propose using turnstile ticket checks — for example, to access metro lines — as a new form of evidence of human involvement. We define turnstile cookies based on a recent cryptographic proposal involving non-transferable anonymous tokens. We want to show that all the elements required for our proposal are available today.