I agree that one cannot switch course midway and as per the Oliver North case, it is best to adopt the 5th from the get go.

My reply to the original post was aimed at: "If something is private, keep it off websites and other such things. If something is really private, keep in encrypted and/or stored in a secure location (like a good safe)."

Keeping things encrypted or in a safe location (like a safe) is inadequate.

I agree with only the final sentence: "If something is really, really private, don't have a record of it at all, keep it just in your head."

The United States Supreme Court has squarely held that the Fifth Amendment privilege deflects any official compulsion to answer incriminating questions until after immunity has been granted.

In the case of a private key, the immunity is extended only to the content of the key itself. So if the key is comprised only of digits then the immunity is not worth much. On the other hand if the key is "iKilledMyMother01.12.1972", then that is what the immunity covers, it does not also include the content of any incriminating files that one has possession, custody or control over.

You are ignoring p2p networks like FreeNet, where files are encrypted and highly fragmented with the intention of guaranteeing anonymity - to facilitate free speech in oppressive regimes like China and to a lesser extent the USA.

In the USA for example a court can compel you to provide a decryption key, failure to do so resulting in contempt of court charges. There is little plausible deniability for something the authorities pull from your safe.