It's complicated.
> Courts have generally found that compelling individuals to provide their numeric or alphanumeric passcode is potentially testimonial under the Fifth Amendment, as it forces the defendant to reveal “the contents of his own mind.” In Re Grand Jury Subpoena Duces Tecum 670 F.3d at 1345; see also U.S. v. Apple MacPro Computer, 851 F.3d 238 (3d Cir. 2017). It is analogous to compelling production of the combination to a wall safe, which is testimonial, as opposed to surrendering the key to a strongbox, which is not. See Doe v. U.S., 487 U.S. 201, 220 (1988). However, even if a court finds that providing the passcode is “testimonial,” it may still fall under the “foregone conclusion” exception
https://www.nacdl.org/Content/Compelled-Decryption-Primer
In short, you can't be compelled to give up the code in a dragnet attempt to find evidence against you (e.g. a boarder guard can't riffle through your text messages to see if you might have done something illegal), but if it's already certain that particular evidence exists on the device as a result of other evidence, they may be able to compel you to give up your passcode.
Note though that the cases where this has come up are very few and far between, and there isn't a super clear overriding precedent to follow.
In general though, the best choice here is to say nothing at all and work with a lawyer to figure out how to proceed.
