Senator Chuck Grassley from R-IA just proposed a new bill targetting cryptocurrency users to declare what and how much worth of cryptocurrency people entering USA have.
The bill goes by name and number U.S. Bill S.1241
The Exact Cryptocurrency Targeted Line
If this goes out when entering USA you will be obligated to say what kind of currencies you have on your devices and claim that with U.S. Customs and Border Protection agency.
When getting to USA you currently have to declare if you have more than 10 000 USD of cash on yourself and Customs may seek your bagage if they believe you have more and didnt confirm you do.
With cryptocurrencies this gets more complex - you can have 0 on your phone but can have all stored ie on exchange to which you have password. With such interpretation it would mean you have to tell of ALL CRYPTOCURRENCIES you hold even outside USA when entering.
Due to complications this may cause itself some lawyers predict Foreign Account Tax Compliance Act will get extended so that USA knows of everyones holdings - even if you dont live there or never been.
Keep in mind - this is just in talks and the bill didnt pass yet.
We can see where its going to go in future even if not this time.
Full text to read is on https://www.congress.gov/bill/115th-congress/senate-bill/1241/text