# Unique, anonymous online authentication

Is there a way to authenticate users online without knowing their real identity yet preventing them from registering twice? If not, what is the least amount of information you would need? If it helps, you may assume the user has a smart phone and you have access to all its sensors. Probabilistic guarantees are quite acceptable (e.g., 1-$\epsilon$ certainty of uniqueness for small $\epsilon$).

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