How to protect private keys?

[Robert J. Hansen]

> to export private keys, without need to enter passphrase. This is very
> dangerous to a multi-user computer.

Clearly, you don't trust the computer you share with other users.  So  
why, exactly, are you running GnuPG on it?

Running GnuPG on a computer you don't trust is folly.  If you don't  
have physical security over the machine, there is no possibility of  
electronic security in your communications.

Beware of all other answers you receive to this question.  Before you  
try to fix the "GnuPG problem", fix the much bigger and more pressing  
problem about how you're trying to run security-critical software on  
a computer you don't physically control.