Comment by nerdsniper

Upon entering the US, CBP can ask you to unlock your phone, then connect it to a little box that hacks into the phone and downloads everything. Search for "Cellebrite Universal Forensic Extraction Device (UFED)" or "Grayshift GrayKey". The border agent doesn't have to know anything about phones/computers, it's just "plug in, press button". With modern phones, they really only work if you unlock your phone before handing it to them, and they'll make you do that. If you don't unlock the phone and let them walk off with it for awhile, they'll refuse you entry into the USA and send you back.

US citizens are, of course, allowed in even if they refuse, but they will confiscate a citizen's phone in exchange for a custody receipt (Form 6051-D) and they are supposed to return it to the US citizen after they break into the phone / crack the encryption. If they can't crack it, they can choose to never return the phone to the US citizen. And it can be a very stressful situation in which citizens may not know what their rights are in the moment (or can't afford to replace their phone or lose access to it because how would you even get an Uber from the airport or coordinate a pickup if you don't have a phone).

You can choose to bring burner phones or make sure your phone is freshly factory reset, but if you're a non-citizen that can also be a reason to be refused entry, and if you are a citizen that can "get you on a list", leading to getting "SSSS" stamped on every boarding pass for every flight you take, in every country in the world, for the next many years. If your boarding pass gets "SSSS" written on it, you will get pulled aside by security and all your bags get individually hand-searched prior to every single flight (even transfers/connections/layovers). This will be a global thing, not limited to USA flights.

Non-citizens are also sometimes asked for a list of your social media accounts and the passwords to their social media accounts. Refusing to provide your passwords can be used as a reason to refuse entry to the USA. If the USA believes you have a social media account that you failed to tell them about, that can also be a reason to refuse entry.