In the US the current rules for personal hard drives are bound by the 5th amendment which has been interpreted as "a reasonable expectation for privacy." What happens is the police say "Give us your password and we'll drop whatever sentence by 75% for helping the investigation." You don't have to give your password but the NSA works pretty extensively with law enforcement and the FBI (most US cases that require password cracks are federal cases but thats a separate issue).
Anything that ever touches an ISP is a totally separate issue though. In that case, in the US, any information stored by an ISP can be retrieved without a warrant 6 months (I'd need to confirm its not 120 days) after the incident. Those cases fall under the interpretation of a message overheard. In 5th Amendment cases, if you say a message in a crowded room, you don't have a reasonable expectation of privacy and this is how any message on the internet is interpreted legally. There was also a court case this spring where the DoD sued an ISP to give over IP addresses sooner than the 6 month (120 days?) limit (anyone got a link?). The DoD dropped the case though.
Do you think the NSA is going to reveal to foreign governments that they've broken AES by going after some guy with child porn on his laptop? I personally doubt it.
Could the NSA cooperate with the FBI? Yes. Will they? Not if it means they can't spy on Russia anymore.
They could just provide the password without divulging how they did it. The best way would be for them to get the drive, if they crack the password, and the password looks like it could have been guessed or generated from some contextual info about the subject or the case, then they return it the police. If they do crack it but the password is actually a random string and disclosing it would betray NSA's abilities, they they simply refuse to disclose it.
On the other hand maybe it would be better to create disinformation that they have cracked all kinds of ciphers or at least their popular implementations? Maybe it will lead enemies to try to implement their own or use alternate implementations that are actually less secure. This will be similar to Airforce's disinformation related to captured UFO tech in the 50s and 60s...
It has nothing to do with holes in AES, NSA just has better brute force capabilities than the FBI or any other law enforcement. And while you sit in jail awaiting trial, they take the months it takes to brute force a key.
"Breaking AES" is not at all necessary. All it takes is one implementation hole, or some plaintext unknowingly cached by a program.
The NSA don't merely employ scores of cryptanalysts to sit around all day to try to break ciphers (though I expect they do this too). Exploiting mistakes is their bread and butter.
Anything that ever touches an ISP is a totally separate issue though. In that case, in the US, any information stored by an ISP can be retrieved without a warrant 6 months (I'd need to confirm its not 120 days) after the incident. Those cases fall under the interpretation of a message overheard. In 5th Amendment cases, if you say a message in a crowded room, you don't have a reasonable expectation of privacy and this is how any message on the internet is interpreted legally. There was also a court case this spring where the DoD sued an ISP to give over IP addresses sooner than the 6 month (120 days?) limit (anyone got a link?). The DoD dropped the case though.