Hm, should be easy enough to have some browser plugin that lets you select a text/data field and recipient list field and encrypt it with the appropriate key; and to do something similar for recognition and decryption of fields.
I think there are complications though - you need to be very sure that rogue javascript can't dig around in your plugin and extract my private key. I'm not sure how securely sandboxed plugins can be.
What's the normal procedure for making a call whose output depends on a file that must be kept secret? Is there a typical OS API pattern that's seen in the various programs like ssh, scp, and so on?
I think the one of the problems is that software like GPG and OpenSSL go to a lot of trouble to make sure private keys don't hang around in memory for any longer than absolutely required - minimising the risk of having the OS preempt the executing code and write the key out to swap (or having malicious code slurp it up out of ram). The bare-metal hoop-jumping required to get that right might not be possible in the context of a browser plugin.
If I implement this, will I become famous?