To establish reasonable doubt all the defense has to do is come up with some alternative way all of your facts can be true without Bob having the key. I can think of two from the top of my head.
1. Bob has mischievous friends, or worse, enemies at school. He leaves his laptop unattended/exposed where someone installs the encryption then wipes their fingerprints, or perhaps has worn gloves. Bob takes his laptop home and tries to regain access to the computer.
2. Bob unwittingly acquires the laptop from a criminal (he may have bought it, or maybe he fixes computers) who encrypted the drive and wiped away all fingerprints. Bob tries to gain access to the computer.
To be free from self-incrimination Bob can simply refuse to answer any questions about the laptop in question at all.
Doesn't work; it's an age old argument ("wasn't me guv, was my mate wearing my clothes") and it will be struck from the record if you claim it with no evidence.
Remember; reasonable doubt is not just the production of an alternate theory, it requires legitimate evidence to verify.
Both the example theories you cite would usually be easy to disprove as well. The first because you could look at various aspects of activity on the computer either side of the creation of the encrypted file and show that it resembles their usual activity (for example, there are numerous other ways to do it).
Now, this is where it gets clunky. I'm speculating here, but from direct experience so... take it with caution.
If you're under investigation for something and refuse to hand over a password then you're unlikely to automatically go to jail over it. The case that probably exists is that there is evidence to support the accusation, but no actual images/material. The latter is needed for a prosecution to succeed. I've never seen a case that looks like a blank go as far as demanding encryption keys - unless you are insanely careful there will always be traces left outside the encrpted file.
(BTW, Pro Tip - if you want to be secure from investigation, scrap windows (it logs way too much) and switch to Linux. Much of the forensics stuff is Windows focused so you instantly throw the [get the right file system and the main forensic tools won't even recognise it...]. Couple that with encrypted containers and you're on to a winner)
I think you're confusing "reasonable doubt" with "any doubt at all." Postulating malicious data-encrypting malfeasance is not reasonable doubt, it's conspiracy theory.
>One of the earliest attempts to quantify reasonable doubt was a 1971 article by Rita Simon and Linda Mahan, "Quantifying Burdens of Proof: A View from the Bench, the Jury, and the Classroom." In a later analysis of the question ("Distributions of Interest for Quantifying Reasonable Doubt and Their Applications," 2006[10]) , three students at Valparaiso University presented a trial to groups of students. Half of the students decided the guilt or innocence of the defendant. The other half recorded their perceived likelihood, given as a percentage, that the defendant committed the crime. They then matched the highest likelihoods of guilt with the guilty verdicts and the lowest likelihoods of guilt with the innocent verdicts. From this, the researchers gauged that the cutoff for reasonable doubt fell somewhere between the highest likelihood of guilt matched to an innocent verdict and the lowest likelihood of guilt matched to a guilty verdict. From these samples, they concluded that the standard was between 0.70 and 0.74.
1. Bob has mischievous friends, or worse, enemies at school. He leaves his laptop unattended/exposed where someone installs the encryption then wipes their fingerprints, or perhaps has worn gloves. Bob takes his laptop home and tries to regain access to the computer.
2. Bob unwittingly acquires the laptop from a criminal (he may have bought it, or maybe he fixes computers) who encrypted the drive and wiped away all fingerprints. Bob tries to gain access to the computer.
To be free from self-incrimination Bob can simply refuse to answer any questions about the laptop in question at all.