Encrypted iPhone Backup Over iTunes on Windows 10
Some good news and some bad news.

Good news is the password is one you entered. It’s not reused or picked up from anywhere else. You (or your agent at the keyboard) could choose the same password as was used elsewhere, but they are not automatic or linked in any way. 

To elaborate, iTunes does not encrypt backups by default, so someone typed in this password. That means you can think back to the first device you ever had that was an iPod shuffle, Classic iPod or iPhone and start making a list on paper or electronically.

List out the passwords you or someone with access to your computer may have ever used. Maybe start with easy things like the PIN you and your household use for gates, doors, devices, bank accounts. Me methodical and record which PIN you type. Also, go slow. If you want to try the pin 0000 and think you entered it but typed 00000 it may be a long time before you go back and try 0000.

Last good news is you are not really rate limited or at risk of destroying the backup by guessing wrong. You only lose time.

Bad news - the PIN or computer could be misbehaving - this is rare, but can happen. If the PIN was truly 1234 and that doesn’t work, your backup might be lost.

Neutral news - there is cracking software you may choose to pay for or trust. Like any internet software, even if the author didn’t plant malware, someone else could so consider care getting any software - but there are options to try and brute force passwords on backup files.

Apple isn’t offering to unlock it since they don’t set a default password and don’t want to get tricked into unlocking someone else’s private information let alone offer that sort of service. They do document how to move on if you exhaust your time or resources and can’t get into the backup you made.


https://support.apple.com/en-us/HT205220