Our solution was to make sure our users did not have to give us their bank and credit card credentials. Instead, we provide an optional, downloadable application, the Wesabe Uploader, which keeps their credentials on their own computer. The Uploader contacts the bank and credit card sites directly, and uses the user's credentials to log in and download their data. It then strips sensitive information out of the data file (such as the user's account number), and uploads just the transaction data to Wesabe. The Uploader acts as a privacy agent for the user. (We also provide a way for the user to manually upload a data file they've downloaded from their bank or credit card web site, though this requires more effort on their part.)
The advantages of the client model are that the user need not invest as much trust in the web application as they would otherwise, and that we do not have a central database of thousands of users' bank credentials (a very tempting target for an attacker). As a small startup, not having to ask our users for as much trust is great -- we can grow without needing people to be willing to give us their bank credentials from the start. Likewise, as a user of the site, you can try it out without having to surrender these credentials just to experiment. The Uploader approach has been extremely successful for us -- our users have (as of early April 2007) uploaded nearly half a billion dollars in transaction data, with over 80% of that information coming through the Uploader.
The most significant disadvantage of the Uploader model is that it places a significant security burden on the user. If the user's machine is vulnerable, storing bank passwords on their machine does not protect them. (Note, however, that if the user's machine is vulnerable, an attacker can go after those same credentials via the web browser, so in some sense the burden on the user is the same.) Asking a user to download a client application is also a usability burden, since it requires greater commitment and trust that the client application does what it says it does and does not contain spyware or trojans. Finally, if we were to become very successful, the Uploader application could itself become a specific target for trojans, degrading its benefit.
Overall, we believe that a local client is a good privacy tool for new companies, and for applications where some data should absolutely never be placed on a server. Wesabe will continue to provide a local client for all users, but we will also move to providing other data syncing tools that do not require a client download, since we believe that over time people will be more comfortable with those approaches and will want the convenience of not running the Uploader. For now, though, a local client has been a great approach for us, and should be considered whenever an application involves data the user legitimately would hesitate to ever upload.
Use a Privacy Wall to Separate Public and Private Data
The first people we asked to upload data to Wesabe were some of our closest friends. Many of them replied, "Um, will you be able to see all my bank data, then?" Even people who trusted us were, understandably, very reluctant to participate. We devised a method, the "privacy wall," for protecting their information even from us as developers of the site. We believe this model is a good approach to ensuring that employees of a company have the least possible access to users' data, and to minimizing the harm that would come from a security breach on the site.