When downgrading firmware iTunes is giving an error code of 1 & 1015.

The error code of one is because you have not put the phone in proper DFU mode to downgrade. This has nothing to do with iTunes version and has only come up especially when downgrading from 1.1.3

Here is how to properly put your phone in DFU:

This is how to achieve DFU mode. You can do it on every phone:
Attach the phone to the pc
Turn the phone off
Hold power and home together for *exactly* 10 seconds
Release power but keep holding home until the pc beeps as a USB device is recognized.
At no point will the display come on. Now your restore should work.

When you finish you will get an error code of 1015 this is normal but you will not be able to boot the phone. Try any tool iPhuc, iNdependence, iBricker etc etc. The phone will still be in recovery mode boot loop.

Do a second restore using the option key on mac or shift key on windows.

After your second restore is finished you can close iTunes & boot you phone with iNdependence or iBricker.

This affects all iPhones regardless of bootloader and especially FW 1.1.3 downgrade. It has nothing to do with iTunes version or mac or pc. I have tested this on mac & pc in several different operating systems.

Unless you dont put iPhone in proper DFU mode you will get error code 1 on a downgrade.