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Wormhole Protocol
The Wormhole protocol is a way of transferring assets between a root chain and multiple foreign chains. It makes use of decentralized oracles called guardians to relay transfer information about token transfers between the chains.
The role of guardians
Guardians are responsible for monitoring the root and foreign chains for token transfers to bridge smart contracts. This can be done using full or light clients of the particular network. They need to make sure to monitor finality of transactions (e.g. track number of confirmations) before relaying messages.
A guardian is identified by an admin key and voter key.
The admin key is supposed to be held in cold-storage and is used to manage rewards and assign a signer key.
The signer key is a hot-key that is used to confirm asset transfers between chains by reporting lockups of tokens on a foreign chain on the root chain or the other way around.
Protocol
The following section describes the protocol and design decisions made.
Signature scheme
In order to implement a decentralized bridge, there needs to be a consensus mechanism to measure whether there is a quorum on a cross chain transfer to prevent a single malicious actor from unlocking or minting an infinite amount of assets.
There are multiple ways to measure whether enough validators have approved a decision:
Multiple signatures - MultiSig
The most simple solution is by using a MultiSig mechanism. This means that each guardian would sign a message and submit it via a P2P gossip network.
Once the consensus threshold has been reached, a guardian will aggregate all signatures into a VAA and execute/submit it on the chain.
The downside here is that gas costs increase with larger guardian sets bringing verification costs to
(5k+5k)*n (ECRECOVER+GTXDATANONZERO*72).
To prevent lagging and complex gas price handling by validators or relayers, we always submit VAAs to Solana where txs are negligibly cheap. In the case of a Solana -> ETH transfer. Guardians would publish a signed VAA on Solana and a user or independently paid relayer would publish said VAA on Ethereum, paying for gas costs. This mechanism is similar to a check issued by the guardians (a VAA) which can be used on another chain to claim assets.
Threshold signatures
Most of the disadvantages of the MultiSig solution come down to the high gas costs of verifying multiple transactions and tracking individual guardian key changes / set changes on other chains.
In order to prove a quorum on a single signature, there exist different mechanisms for so-called Threshold signatures. A single signature is generated using a multi party computation process or aggregation of signatures from different parties of a group and only valid if a previously specified quorum has participated in the generation of such signature.
This would essentially mean that such a signature could be published on the Solana chain and relayed by anyone to authorize an action on another chain, the same concept as described above but implemented with the cost of only sending and verifying one signature.
Since we target Ethereum as primary foreign chain, there are 3 viable options of threshold signatures:
t-ECDSA
Threshold ECDSA signatures generated using GG20. This is a highly complex, cutting edge cryptographic protocol that requires significant amounts of compute to generate signatures with larger quorums.
Still, it generates plain ECDSA signatures that can easily be verified on Ethereum (5k gas) or even be used for Bitcoin
transactions.
BLS
Boneh–Lynn–Shacham threshold signatures are very lightweight because they don't require a multi-round process and can simply be aggregated from multiple individual signatures. This would eliminate the need for a p2p layer for MPC communication. However, verifying a BLS signature on Ethereum costs about 130k gas using the precompiled pairing functions over bn128. Also there's very little prior work on this scheme especially in the context of Solidity.
Schnorr-Threshold
Schnorr threshold signatures require a multi-round computation and distributed key generation. They can be verified on Ethereum extremely cheaply (https://blog.chain.link/threshold-signatures-in-chainlink/) and scale well with more signing parties. There's been significant prior work in the blockchain space, several implementations over different curves and a proposal to implement support on Bitcoin (BIP340).
A great overview can be found here
Design choices
For transfers we implement a simple MultiSig schema. We'll create a portable "action blob" with a threshold signature to allow anyone to relay action approvals between chains. We call this structure: VAA (Verifiable Action Approval).
A validator action approval guarantees eventual consistency across chains - if the validators have submitted a VAA to a token lockup on Solana, this VAA can be used to unlock the tokens on the specified foreign chain.
While for the above mentioned transfers from Solana => foreign chain we use Solana for data availability of the VAAs, in the other direction data availability i.e. the guardians posting the VAA on the foreign chain (where the transfer was initiated) is optional because in most cases it will be substantially cheaper for the guardians to directly submit the VAA on Solana itself to unlock/mint the transferred tokens there.
VAA - Verifiable Action Approval
Verifiable action approvals are used to approve the execution of a specified action on a chain.
They are structured as follows:
Header:
uint8 version (0x01)
uint32 guardian set index
uint8 len signatures
per signature:
uint8 index of the signer (in guardian keys)
[65]uint8 signature
body:
uint32 unix seconds
uint8 action
[payload_size]uint8 payload
The guardian set index does not need to be in the signed body since it is verifiable using the signature itself which
is created using the guardian set's key.
It is a monotonically number that's increased every time a validator set update happens and tracks the public key of the
set.
Actions
Guardian set update
ID: 0x01
Payload:
uint32 new_index
uint8 len(keys)
[][20]uint8 guardian addresses
The new_index must be monotonically increasing and is manually specified here to fix a potential guardian_set index
desynchronization between the any of the chains in the system.
Transfer
ID: 0x10
Payload:
uint32 nonce
uint8 source_chain
uint8 target_chain
[32]uint8 source_address
[32]uint8 target_address
uint8 token_chain
[32]uint8 token_address
uint8 decimals
uint256 amount
Cross-Chain Transfers
Transfer of assets Foreign Chain -> Root Chain
If this is the first time the asset is transferred to the root chain, the user inititates a CreateWrapped instruction
on the root chain to initialize the wrapped asset.
The user creates a token account for the wrapped asset on the root chain.
The user sends a chain native asset to the bridge on the foreign chain using the Lock function.
The lock function takes a Solana address as parameter which is the TokenAccount that should receive the wrapped token.
Guardians will pick up the Lock transaction once it has enough confirmations on the foreign chain. The amount of confirmations required is a parameter that guardians can specify individually.
They check for the validity, parse it and will then initiate a threshold signature ceremony on a deterministically
produced VAA (Transfer) testifying that they have seen a foreign lockup. They will post this VAA on the root chain
using the SubmitVAA instruction.
This instruction will either mint a new wrapped asset or release tokens from custody. Custody is used for Solana-native tokens that have previously been transferred to a foreign chain, minting will be used to create new units of a wrapped foreign-chain asset.
If this is the first time a foreign asset is minted, a new Mint (token) will be created on quorum.
Transfer of assets Root Chain -> Foreign Chain
The user sends a Lock or LockNative instruction to the Bridge program.
Lock has to be used for wrapped assets that should be transferred to a foreign chain. They will be burned on Solana.
LockNative has to be used for Solana-native assets that should be transferred to a foreign chain. They will be held in a custody account until the tokens are transferred back from the foreign chain.
The lock function takes a chain_id which identifies the foreign chain the tokens should be sent to and a foreign_address
which is a left-zero-padded address on the foreign chain. This operation creates a LockProposal account
that tracks the status of the transfer.
Guardians will pick up the LockProposal once it has enough confirmations on the Solana network. It defaults to full confirmation (i.e. the max lockup, currently 32 slots), but can be changed to a different commitment levels on each guardian's discretion.
They check for the validity of the tx, parse it and will initiate an off-chain signature aggregation ceremony which will
output a VAA that can be used with a foreign chain smart contract to reclaim an unwrapped local asset or mint a
wrapped spl-token.
This VAA will be posted on Solana by one of the guardians using the SubmitVAA instruction and will be stored in the
LockProposal.
The user can then get the VAA from the LockProposal and submit it on the foreign chain.
Fees
Fees exist for 2 reasons: spam prevention and guardian cost coverage.
Costs for guardians:
Assuming no hosting costs for a guardian operation (blockchain and guardian nodes), the only costs that need to be covered by a guardian operator are Solana transaction fees as well as rent costs for newly created account (used to store application information).
For a transfer from Solana to a foreign chain (20 guardians; 14 quorum):
Transactions required: 3 (signatures + verify) + 1 (post VAA)
Accounts created: 1 ClaimedVAA + 1 SignatureState
Costs:
4 TX (14 secp signatures + 4 ed25519) + ClaimedVAA (exemption rent) + SignatureState (exemption rent)
18 * 10_000 + (36+128) * 6962 + (1337+128) * 6962
11521098 lamports = 0.0115 SOL
For a transfer from a foreign chain to Solana (20 guardians; 14 quorum):
Transactions required: 3 (signatures + verify) + 1 (post VAA)
Accounts created: 1 ClaimedVAA + 1 SignatureState (temporary; evicted in PostVAA)
Costs:
4 TX (14 secp signatures + 4 ed25519) + ClaimedVAA (exemption rent)
18 * 10_000 + (36+128) * 6962
1321768 lamports = 0.0013 SOL
In order to cover rent costs there exists a subsidy pool controlled by the bridge to cover rent payments. While the guardian needs to hold enough SOL to pay for the rent, it is automatically refunded by the pool, in case the pool has sufficient balance.
This subsidy pool is funded by transaction fees. Additionally, the subsidy pool subsidizes the transactions fees paid by the guardian submitting the VAA. As long as the pool has a sufficient balance, it will try to refund transaction fees to the guardian.
Since Wormhole does not require foreign chain users to own SOL, Wormhole can't charge subsidy fees on inbound transfers. Assuming a balance between inbound and outbound transfers, outbound transfers need to subsidize inbound Solana transfers.
Additionally, foreign chain contracts might start charging additional fees in the future.
The bridge can handle at most transactions per second. Therefore, the fees should prevent spam by dynamically adjusting to load. This is particularly useful on Solana where fees are low and spamming would be cheap.
Dynamic fees should be cheap while the system is under low and medium load and high while the system is close or above its capacity. To prevent sudden fee changes, the fee system has inertia.
Fees scale as follows fee = (tps/tps_max)^6.
The result is the fee per transfer in SOL. So at max capacity, the price per transfer is 1SOL.
TPS is measured over a 30 second window.
The minimum fee is the equivalent of 2x the rent of SignatureState and ClaimedVAA to cover the cost of this transfer and about 10 inbound transfers.
The above design can currently not be implemented due to limitations in the Solana BPF VM.
In the current design, tx fees are refunded, rents are subsidized by the bridge and transfers out of Solana cost a fixed fee of 2x (ClaimedVAA rent + SignatureState rent + VAA submission fee), which will roughly pay for 1 outbound + ~10 inbound transfers.
Config changes
Guardian set changes
The guardians need to make sure that the sets are synchronized between all chains. If the guardian set is changed, the guardian must also be replaced on all foreign chains. Therefore we conduct these changes via VAAs that are universally valid on all chains.
That way, if a change is made on the root chain, the same signatures can be used to trigger the same update on the foreign chain. This allows all parties in the system to propagate bridge state changes across all chains.
If all VAAs issued by the previous guardian set would immediately become invalid once a new guardian set takes over, that would lead to some payments being "stuck". Therefore we track a list of previous guardian sets. VAAs issued by old guardian sets stay valid for one day from the time that the change happens in the default configuration.