If you’re a U.S. citizen: CBP cannot deny you entry to the United States merely because you refuse to unlock the phone. If you’re a non-citizen seeking admission: refusal is much riskier.
The important wrinkle is that CBP’s published policy expressly guarantees that a person being admitted as a U.S. citizen won’t be denied entry solely because CBP couldn’t inspect the device. It doesn’t give lawful permanent resident (green card holders) that same explicit statement. Instead, it says refusal by a “foreign national” can be considered in an admissibility determination.
this only applies if they don't refuse to acknowledge your papers as valid and/or they haven't previously put you on some hidden list of people of interest, in which case the instance where you get to prove you're who you say you are will be mediated, like the rest of the (as per the current system) nonpeople, by as many layers of humilliation and risk to your life and health as they can place.
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Isn't the guarantee for citizens a bit stronger than CBP's policy? I don't think they can lawfully deny entry of a citizen, period.