Google security

I've used Google Docs on a number of occasions for collaborative working. It might not be a perfect tool for collaborative work but it is free and does the job. In the past I did not pay much attention to the security aspect of those documents since they were never of contained highly sensitive information.

But what if they did?!

Sharing documents online has its advantages namely only one master copy exists and can be viewed and edited by multiple users simultaneously. Such documents are accessible (if not set as public) only to specific users (specified by the document owner) who must have a Google account too.

If those documents are protected by simple username/password combination, how secure are they? In fact such documents are as secure as the weakest authentication credentials (username/password) of users who share those documents.

Therefore I would like to be able to specify a certain criteria which users must fulfil before they can access my shared documents. To add an additional layer of security I should be able to specify:

1. "To view my documents the invited user must use a strong password", meaning Google “password strength” tool must indicate "Strong".

2. "To view my documents the invited user must use captcher when login-in".

3. “To view my documents the invited user will need a secret key to decrypt them.” Google docs (end emails in Gmail) should have offer encryption which would add additional layer of protection for sensitive documents.

All of the above should be optional so that those who see it as "just another obstacle" do not have to use it.

How much additional security would such options provide???

What about backup?? How secure should the backup be?