The most prominent cases involving password to decrypt…
In one case, the fellow’s computer was viewed by border guards and illegal images were seen on it. It was shut down, and when they went to get the images again, they found they needed to decrypt the files.
In the other case, the encrypted partition had files pertaining to the woman’s illegal activities as an accountant. The police had wiretaps of her telling her husband “the report of X that they need is safely encrypted on that disk.”
In both cases, the authorities knew what they were after. They had specific knowledge of specific items and were demanding that the evidence be turned over. That is something (outside of passwords) the court can do. “You have your tax return for 2007? Hand it over”. they don’t care if it is paper, microfilm, or bytes. They don’t care if it is in your files, or you wrapped it in plastic and buried it in the north 40. You hand over what you have.
This issue gets more complex when you get into mechanism. If they can ask you for a password, and then peruse the complete contents of the drive, at what point is it a fishing expedition? “We were looking for tax fraud on the disk, but by going through 10,000 photos we found 5 that were of his grow-op, so we want warrants to look at the following building…”
There’s the “plain sight” rule for evidence. If the police search you house with a warrant, and the drugs are right there on the coffee table, they can seize those and charge you too. However, if the warrant says “looking for guns and ammo” they cannot take or rifle through your papers looking for tax fraud; since a gun would obviously not be in a file folder full of papers.
So how deeply into your files can the police look once they have access to your computer? That’s a specific question AFAIK the SCOTUS has not yet addressed, except in a peripheral way in analogous cases. Someday they’ll answer these sorts of questions that are key to 20th century life.
As for destruction of evidence… As C Pham points out, the trick is to prove intent. Did you specifically destroy these record knowing the police would be wanting them? If you destroy all communication as a matter of course, that’s hard to prove. If the only glaring holes in your files are the ones related to this case, the DA’s job got a lot easier. If the disk wipe program was downloaded and installed the day after the investigation was announced, more evidence.
Recall that Martha Stewart was not convicted of insider trading. She was convicted of obstruction and destroying evidence. When she found there was an investigation, she deleted the entry about the phone call from her computer log. After she thought about it for a while, she changed her mind and retyped the entry. That deletion was good for a conviction. IIRC, she did this in front of her secretary, so basically it was kind of difficult to deny it happened. (The feds thoughtfully threatened if she didn’t roll over and take the plea, they would also charge and so destroy the lives of a bunch of mid-level low paid flunkies who could not afford good lawyers - her secretary, the stockbroker’s assistant, etc.).