The backup encryption passphrase is set the first time you do a backup after checking "Encrypt backup" either intentionally or because a corporate security profile required it. At that time you are prompted for a passcode, and asked to confirm it. After that, unless you intentionally change it, the passcode never changes. It is most likely a passcode you have used in the past, because most people reuse passcodes. The good news is that you get unlimited guesses. To speed up the guessing process uncheck "Encrypt backup" and you will be prompted immediately for the current passcode. There is a good backup passcode cracker made by Elcomsoft. It is intended for law enforcement use, and is priced accordingly. However, the trial version will return the first 2 characters of the passcode if it can crack it. Elcomsoft's FAQ says that it is not guaranteed to work, and if you are in the habit of using strong passcodes it won't be able to crack it.