Why do you consider anything encrypted as an extension of oneself? The 5th amendment protects you from being forced to testify against yourself. It does not protect you from being forced to turn over evidence. That is where obstruction of justice charges come into play. In your example of refusing to turn over a key to a diary is no different to the government than you burning that diary. You are actively preventing the government from accessing preexisting evidence.
One could argue that governments can take possession of a computer with a search warrant, but even if they did, it violates the right against self-incrimination to have to give up your passwords and encryption keys. That's where the fifth amendment comes into play. It's my understanding that this is not settled law, and I'm not a lawyer, but that's the general idea.
Memorizing an unbreakable password to an encrypted file is a lot more convenient that memorizing the entire file and then destroying the bits.
However legally, I believe it should have the same net effect, because using technology to augment our increasingly limited biological memory shouldn't eliminate the protection against the government accessing that memory to use against us.
Maybe I'm 10-100 years early on this, but I think time will tell.
The 5th Amendment grants you the right not to incriminate yourself in legal proceedings. Diaries found with a proper warrant can totally be used against you.
