Can fake SHA256 checksum be created trivially for any file by attacker after padding some data after original file i.e. using length extension attack?
Alice creates fake linux installation ISO file using length extension attack by padding extra data after ISO and calculates fake SHA256 checksum of the ISO
Bob downloads Alice's fake ISO file and calculates the SHA256 of the ISO
Bob compares the SHA256 checksum that he generated from fake ISO file to the checksum found on official linux distribution's home page. Because Bob's fake ISO checksum matches the official ISO checksum, Bob doesn't notice that he has downloaded fake ISO
If Bob had compared the fake ISO file size to the original ISO file size found on official linux distribution's home page in addition to calculating the checksum, he would have noticed that he has fake ISO not the original
Is this trivial to do?
In other words, is SHA256 completely insecure when a file's file size is not manually checked by computer user?
Does the file size always need to be checked when using SHA256?