I'm building a system for Ethereum authentication through the browser, which also checks whether the user owns a specific token.
For this, I'm sending from the back-end a nonce for the user to sign using their private key with Metamask. Then I validate that signature on the back-end to check if it's correct for that address, and whether the user owns said tokens on that address.
Now suppose the token I want to validate is on chain A and chain B. Since the chain is selected simply through Metamask, how do I confirm that the user is on a specific chain, if the signed message is valid on both chains?
From what I was checking it seems like transactions have a different signature across different chains, but not messages.