Charon Bridge

The structure used by the charon bridge is similar to a normal bridge, except deposits to the origin chain (e.g. Ethereum) are accompanied by a zk proof. Included in the proof are: destination chain, token address (of bridged token), amount being bridged, and a nullifier (or secret). The proof is passed to destination chains (e.g. Optimism, Polygon, and Arbitrum) and added to a merkle tree. Then, on a given destination chain, users can anonymously transact (trade in a shielded state) or withdraw (e.g. mint on the destination chain) to any address so long they provide a valid proof that they deposited a specificized amount of a token with this chain as the destination.

I used circomlib for the circuits (a tornado cash nova fork w/ poseidon and multi-token/cross-chain functionality added) and then used tellor as the oracle (state roots or native bridges can also be used). It was pretty simple in the structure, the tricky part was getting the proofs across chain and decoding them.