I have what may perhaps seem like a strange question. Is there any commonly used software for encrypting and decrypting web pages?
Yes, SSL .. and it's been around for quite a while.
Let me explain that a little better: imagine a web site which has content destined for specific individuals. For each individual there is separate content on separate pages, and no one but the individual for whom the content is destined should be able to read the content, not even the creator of the content!
Why not just SSL the site, and then restrict access to it using certificates (still X.509, but separate from the one used for transport security)
In other words, is there a private/public key mechanism similar to PGP (or even a PGP web page plugin) that will work transparently while browsing the web? The transparently part would mean that a user can provide a private key to a browser and any pages encrypted with the user's public key would automatically be decrypted for him when he views them.
Again, this can be easily provided by issuing X.509 certificates to the end-users and then requiring those certificates to authenticate to the webserver. Transport security (as it pertains to TOR, etc.) is provided by a separate X.509 certificate who's purpose is to sign the encrypted channel over which the data is transfered. You would manage the X.509 certificates assigned to your users by yourself, so you could handle revolkations (although Verisign, et.al. will happily sell you a commercial X.509 solution for client auth).
If you had a scenario where you needed to deploy a webserver in "hostile territory" and needed to ensure the security of the data thereon, you could conceivably gzip and GPG each .html page and associated items with multiple public keys based on some other criteria (like what cert the browser provided) and then let the end-user decrypt it with their private .. but this definitely won't be "automatic" .. but you could wrap it in Java to make it somewhat portable if you wanted. You could also write an ActiveX or XPI plug-in to incorporate it into the browser .. but then you're putting a lot of "trust" in a 3rd party with your GPG keys.
~Mike.