Actually, re my second paragraph, the question isn’t prohibited – the cops can ask you pretty much whatever you want – but for the reasons cited they can’t compel you to answer.
Also, in the interests of completeness: there is a second component of the fifth amendment right against self-incrimination – the right to not testify, e.g., a defendant does not have to take the witness stand. If he does take the stand, though, he has to answer the prosecution’s questions on cross and can only plead the fifth for those specific questions that he has adequate grounds to do so.
I think this will illustrate the difference in what I’m talking about. Take two criminal suspects, Arthur and Bob. Arthur is suspected of trafficking in child pornography. Bob is suspected of embezzling funds.
The cops ask Arthur: “give us the password to the encrypted folder labeled ‘kiddie porn.’” In this case, Arthur probably has a valid fifth amendment defense to telling them the password – it would be tantamount to an admission that he possessed child pornography. Since possession of child pornography is a crime, the statement would be self-incriminating. Thus, Arthur can plead the fifth.
The cops ask Bob: “give us the password to the encrypted folder labeled ‘offshore bank account numbers.’” In this case, Bob probably does NOT have a valid fifth amendment defense to revealing the password. Having a list of bank account numbers on your computer is not a crime. Revealing the password would only be an admission that Bob possessed a list of bank account numbers. Since that is not a criminal act, the statement is not self-incriminating and Bob can’t plead the fifth.
This is an interesting question overall because the cops aren’t asking WHERE something is – normally, asking where some piece of evidence is located would indicate complicity in a crime, and thus the fifth amendment comes into play. If the cops ask a suspected murderer “where did you put the gun,” knowledge of that location indicates participating in the murder. Here, the cops know exactly where they want to look – the PC – but need access to the PC. Thus I suspect in most instances the fifth amendment wouldn’t apply – providing the password is like providing the cops with a key to your home when they come with a valid search warrant.
Hmm, I’m not sure what you mean by “doesn’t exist.” All you need is a sufficiently random set of of bits and a XOR.
There are methods for getting very random bits from Nature; one example is timing the intervals between radioactive decay events. I suppose maybe Chaos Theory could predict patterns in such cases, but I don’t know anything about it.
There are rumors that the hotline between the White House and the Kremlin used a one-time pad, but that’s the only instance I’ve ever heard of. I am by no means an expert on the history of cryptography, though.
The issue is developing a “sufficiently random” set of bits, and transmiting it securely. The longer a sequence you use, the more deviations from true randomness matter.
Also, it’s pretty rare that the need for the security provided by a one-time pad offsets the cost of producing one. RSA, DEC, and other popular algorithms are “good enough for government work”, so to speak. That’s more what I had in mind.