[breser]

You probably want to read EFF's excellent "Guide to Protecting Electronic Devices and Data at the U.S. Border": https://www.eff.org/deeplinks/2010/11/effs-guide-protecting-...

I'd expect that if CBP is asking for your encrytion key and you are not a U.S. citizen and you refuse to provide it you're probably going to be refused entry. Being refused entry isn't to be taken lightly. It may very well mean you will never be allowed entry again.

As others have pointed out it's probably better to simply not know the encryption key or to download your data after arriving. Which are covered in the guide I linked.

Your post mentioned the TSA but I'm not aware of the TSA asking for encryption keys. I'd be surprised if they ever do or have since their mandate is aircraft safety. Searching data would not likely be allowable under the search exceptions they operate under. Your concern should be towards CBP doing the search while clearing passport control/customs.