The other alternative is to brute-force the entire keyspace, which puts you back in 2^128 (or 2^512) territory but with the option of trying billions of keys a second. A few Universe lifetimes, and my .emacs will be all yours.
Oh, but I use a long password anyway. The key, IMO, is to avoid using a password you have ever used for anything else. You might not have to testify against yourself, but your employer or mail provider will be happy to cough up your password for the authorities. So don't give it to them.
Got it: http://github.com/jrockway/dotfiles/blob/master/emacs
First you can use a dictionary - then munch it (with Jack or something) to create a large dictionary within the keyspace. It's surprising how much that covers.
Then you can parallel the process; I know a few commercial companies (ourselves included) with a large computer capacity for these purposes. I'd be surprised if the govt. didn't have some form of similar access.
If you can reduce the search space by a factor of 1,000 and run your cracker on 1,000 machines it makes a dramatic difference.
But now it takes 150 years to brute-force the keyspace.
I doubt anyone is going to devote 150 years of computer time to someone going through customs.
The forensics guys will clone your drive, then feed huge fuzzed dictionary attacks using the same algorithm until they find something that works, and they'll do it with machines a lot faster than your laptop.
They don't have to brute-force the keyspace. They only have to brute-force your password-space.
Then you just start building a customized keyspace from that information. Include some common substitutions: letters for numbers, numbers for letters. If you have other passwords, look for general patterns. Some people use long passphrases, but they have a pattern or algorithm on how they build all their passphrases! There you just might need a human expert to take a look.
In the end, yes, it is very difficult but it is still a lot better than plain brute-force.
But yeah, I doubt Customs have the equipment or knowledge to do this. That is something that NSA would do. They would have access to the best tools. I can imagine there might be some kind of very constrained cooperation between them. "You send us a disk clone + personal info of the owner, and we'll try to send you back the cracked passphrase. You don't ask as questions how we got it, and you don't even mention that we got involved at all."
:)