Going to the U.S.
U.S. customs and border protection officials have the legal authority to conduct a search of you and your property when you are seeking entry into the U.S. They do not need to have any suspicions or reasonable grounds to do so. Their broad powers allow them to ask for the password to your device or ask you to open it with voice recognition or fingerprint access.
Once your device is open, border officials may review what’s on it, copy files, and analyze the data.
The Government's interest in preventing the entry of unwanted persons and effects is at its zenith at the international border. Time and again, [the Supreme Court has] stated that 'searches made at the border, pursuant to the longstanding right of the sovereign to protect itself by stopping and examining persons and property crossing into this country, are reasonable simply by virtue of the fact that they occur at the border.’
United States v. Flores-Montano, 541 U.S. 149, (2004) at 152-53
Officials, however, may only look at information that is on your device. According to a U.S. Customs and Border Patrol Directive, dated January 4, 2018, they may not use the device to examine remotely-stored information.
The best way to protect confidentiality and solicitor-client privilege is to travel with a clean device, store sensitive information on the cloud, and access this information remotely.