Apple - iPhone asks for Mac password (or Mac asks for iOS password)
When you reset all settings or restore a backup from iCloud (or set up a device as new), the Keychain / iCloud and local USB trust pairings are removed and you should be prompted to trust the Mac when it connects to your iPhone.
- https://support.apple.com/en-us/HT202778
After the trust setting dialog pops up (if you are connected over USB), there is now a screen that asks you to enter your iCloud password to finish setting up the iOS device. I have not seen the follow on passcode/password page ever until today. In my case, it wanted the passcode on another iPad and not a Mac ( I have two iPads, an iPhone, Apple Watch, two Mac connected to my iCloud account as primary accounts / keychain sync enabled on all).
Enter Passcode for Other iPad
Access to your account is protected by the passcode for "iPad". Your passcode is encrypted and cannot be read by Apple.
In my case, the iPad in question is named iPad Air 2 and there's a blue link with the text "Forgot iPad Air 2 passcode?" Tapping that lets me select another device to enter a passcode. I then selected my Mac Pro and the text changes to:
Enter Mac Pro Password
Access to your account is protected by the password for "Mike's Mac Pro". Your passcode is encrypted and cannot be read by Apple.
I do have two step authentication enabled for my iCloud account so this seems a legitimate change that Apple has implemented recently to secure backups and/or joining a "new" device or OS to the keychain syncing / trust chain.
After entering the password, there was a long delay (the iPad screen went dark) and the next step was the iCloud Keychain dialog which also was slow to continue when I tapped that.
I just went through this for the first time. To avoid entering the other device's password on the new iOS device, I followed this process:
Tap the "Forgot [other device] passcode?" link.
After several confirmation dialogs, this should bring up up a list of other devices to choose from.
Tap the "Forgot passcode for all devices" link at the bottom of the list.
After additional confirmation dialogs, you should receive an alert on your other device(s) prompting you to approve the new device.
Follow the prompts on one of your other devices to authorize the new device.
I had a similar password request and I have a screen shot.
This seems very strange to me. They could send me identification codes to all the devices but why do they ask for a password of a different device? Unless I see an official document by apple, I will not enter any passwords. I do not use icloud anyway.
This request comes without connecting to the mac.
So, here is the solution: Just press: "I forgot the password". The phone will warn you that all the top secret info in the cloud will be deleted. But if you do not have anything in the cloud you can safely ignore this. After this everything is fine. I think apple is a little bit confusing with asking for things it does not need (in cases of users that do not use icloud).