Bridge
A bridge moves assets between blockchains. Tokens do not travel: the original is locked on one network while a wrapped copy is issued on the other. Bridges are the most attacked part of crypto infrastructure.
How it works
You send a token into the bridge's contract on the source network. It is locked there, and an equivalent amount of a wrapped token is minted on the destination. Going back burns the copy and unlocks the original.
A wrapped token is worth exactly as much as the bridge's promise to return the original. If the locked funds are stolen, the copy is worthless even though it still sits in your balance.
So a bridge concentrates funds in one contract, guarded by one set of keys or validators. An attacker need not break a blockchain, only the bridge.
Why this is dangerous
The largest bridge exploits, all within a single year.
| Ronin, March 2022 | ≈ $625m |
| Wormhole, February 2022 | ≈ $325m |
| Nomad, August 2022 | ≈ $190m |
In Ronin's case the attacker gained control of five of nine validator keys, which was enough to sign a withdrawal. The weakness was not in the cryptography but in how many signatures were required and who held them.
How to reduce the risk
- Do not park money in a bridgeA bridge is transport, not storage. Move the asset and put it to use straight away.
- The network's own bridgeA Layer 2 usually has its own bridge: slower than third-party ones, but without an extra set of trusted signers.
- Look at who signsHow many keys a withdrawal needs and who holds them is the bridge's security model.
- Move large sums in partsSeveral transfers with a check between them cost less than one mistake with everything.