I also couldn't restore my phone (5s) from backup because I didn't have the correct password. When I called the store where I had purchased the phone, they knew it immediately. When they transferred my information from my iPhone 4 to the iPhone 5s (Verizon), the store put in a password. Evidently, the password follows the phone, not the computer where the backup is stored. No wonder I couldn't guess the password - I never knew it in the first place!
I only got into this mess because I was trying to free up some memory on the phone (the "other" category) - and followed a procedure outlined elsewhere in Apple support. It did ask for a 4-digit password, and, although I didn't remember encrypting the phone, I put in one of my standard ones - which seemed to work - as the restore proceeded. It seems to me that Apple should build in a check for for the correct password BEFORE one erases the whole phone.