309 lines
13 KiB
Markdown
309 lines
13 KiB
Markdown
# 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](https://eprint.iacr.org/2020/540.pdf).
|
||
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](https://github.com/Turing-Chain/TSSKit-Threshold-Signature-Scheme-Toolkit)
|
||
|
||
#### 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 + (40+128) * 6962 + (1340+128) * 6962
|
||
11569832 lamports = 0.0116 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 + (40+128) * 6962
|
||
1349616 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 <TODO> 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.
|