iTunes backup password unlock
Then 1234 may be the password that you just entered the very first time you're asked. Have you tried the passwords that you have used before or now you use? One of them may be the password you used for your iPhone encrypted backup.
The same has happened to me. As I remember, I solved using a tool called "Tenorshare iPhone Backup Password Unlocker". With its "Dictionary Attack" mode, I got back my forgotten encrypted iPhone backup password within minutes!
Or the Makeuseof Q&A of how to unlock iPhone backup password for iTunes will help you: