MD5 Checksum Validation
In order for netplay to work, both you and your opponent must have the same ROM on both computers. Players using different ROM variants can lead to desyncing. Oftentimes, verifying the validity of ROMs can be a pain, leading folks to reacquire them unnecessarily in a game of guess and check. To solve this issue, MD5 checksum calculation and validation has been made available.
To access this, just right click on any ROM name, and click "Calculate MD5 Checksum". A dialog will pop up showing the MD5 checksum of the selected ROM, and a button to easily copy it to the clipboard to share with your opponent.
To verify that you have the same ROM as your opponent, just copy the MD5 checksum given to you, and click the "Paste & Verify" button. If your ROMs match, you'll see a green "Verified!" by the Close button. If the hashes do not match, it will say "Mismatch." in red.
flycast_roms.jsonfile is found, it should contain a listing of MD5 checksums for approved ROMs between users. This format is intended for communities to share common ROM metadata between users to ensure that everyone has the same files.
If your ROM is found in this file, the MD5 Checksum window will say "Validation File Match". So long as you see this, you should be good to go!