I have a new phone and would like to restore the backup, but the backup is encrypted and I have now idea on where to find the password.

Then you won't be able to restore the data from that backup.

Without the password you used when you enabled the encryption, it's useless.... That's pretty much the entire point of encrypting anything.

You put a password on your backup, whether you remember doing so or not. That’s the only way encrypted backups get created. Either you checked Encrypt Backup, or, if you have a company email account on your phone, your Exchange administrator required it. But either way you were prompted to enter a password - twice. However, that could have been years ago; you are only prompted the first time you made an encrypted backup, you will never be prompted again until you go to restore a backup.

When you entered the backup password if you have a Mac and have Keychain enabled you were given the option of saving the password in Keychain (this is the default). So check Keychain for it.

If you haven’t erased the phone yet you can create a new backup with a new password—>About encrypted backups on your iPhone, iPad, or iPod touch - Apple Support

If neither of those work you get unlimited guesses, so try to guess what password you might have entered back when. If you are like most people it will be a password that you reused. And to speed up the process click to change the password and you will be prompted immediately.

Note that if you do NOT encrypt your computer backup then the backup will be incomplete; it will not include health data or passwords. iCloud backups are protected by default because they require your Apple ID and password to access, so this is not an issue for iCloud backups.

I have experienced the same thing. Only, I did not encrypt it deliberately. My wife has the same issue, but she was able to restore to her phone but did not get everything. When we tried to Check the backup, it is now encrypted. Neither of us have deliberately put a password on the backups. I lost 10 years of grandchildren’s and national parks pictures. And I have had to manually reinstall and configure almost 100 apps.