How to get reliable download for old Android Samsung?
Use a transparent source

Open-source is transparent, so it is reliable when there are a big community with many eyes   inspecting.  http://lineageos.org  and Heimdall are the most (positivally) cited, 


Reliable download of an open firmware... E.g. from https://download.lineageos.org/i9300   (open sources  at Git/android_device_samsung_i9300) resulting e.g. lineage-14.1-20190117-nightly-i9300-signed.zip
Reliable download of an open TWRP... E.g. from https://dl.twrp.me/i9300/  (open sources  at Git/android_bootable_recovery)
Confirm sha256 of the firmware, E.g. by the Linux command sha256sum lineage-14.1-20190117-nightly-i9300-signed.zip and comparing with lineageFile.zip's sha256
Reliable install of Heimdall.E.g. at reliable UBUNTU installation use sudo apt install heimdall-flash check version with the command heimdall version. PS: it is the last from gitlab.com/BenjaminDobell/Heimdall
Follow the instructions to install firmware in your smartphone using Heimdall. E.g. https://wiki.lineageos.org/devices/i9300/install