You want to use a different password for each web site you visit--if one gets hacked into, the attackers won't have your login info for all the other sites you have an account on. But remembering a different secure password for each site is practically impossible.
SitePass requires one master password, which you set on first run, and use to login to the application. For each site you enter, it takes your master password, combines it with the site's name, then encodes the result into letters and numbers.
If they don't have the master password, they can't get into the app. The password is stored in your phone's secure Keychain.
You'll lose all the sites you entered, but you can simply delete the app, reinstall it, and choose a new master password.
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