309 lines
13 KiB
Markdown
309 lines
13 KiB
Markdown
# Wormhole Protocol
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The Wormhole protocol is a way of transferring assets between a **root chain** and multiple **foreign chains**.
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It makes use of decentralized oracles called **guardians** to relay transfer information about token transfers
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between the chains.
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## The role of guardians
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Guardians are responsible for monitoring the root and foreign chains for token transfers to bridge *smart contracts*.
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This can be done using full or light clients of the particular network.
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They need to make sure to monitor finality of transactions (e.g. track number of confirmations) before relaying messages.
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A guardian is identified by an **admin key** and **voter key**.
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The **admin key** is supposed to be held in cold-storage and is used to manage rewards and assign a signer key.
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The **signer key** is a hot-key that is used to confirm asset transfers between chains by reporting lockups of tokens
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on a foreign chain on the root chain or the other way around.
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## Protocol
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The following section describes the protocol and design decisions made.
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### Signature scheme
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In order to implement a decentralized bridge, there needs to be a consensus mechanism to measure whether there is a quorum
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on a cross chain transfer to prevent a single malicious actor from unlocking or minting an infinite amount of assets.
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There are multiple ways to measure whether enough validators have approved a decision:
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#### Multiple signatures - MultiSig
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The most simple solution is by using a *MultiSig* mechanism. This means that each guardian would sign a message
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and submit it via a P2P gossip network.
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Once the consensus threshold has been reached, a guardian will aggregate all signatures into a VAA and execute/submit it
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on the chain.
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The downside here is that gas costs increase with larger guardian sets bringing verification costs to
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`(5k+5k)*n` (`ECRECOVER+GTXDATANONZERO*72`).
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To prevent lagging and complex gas price handling by validators or relayers, we always submit VAAs to Solana where txs
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are negligibly cheap. In the case of a Solana -> ETH transfer. Guardians would publish a signed VAA on Solana and a user
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or independently paid relayer would publish said VAA on Ethereum, paying for gas costs. This mechanism is similar to a
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check issued by the guardians (a VAA) which can be used on another chain to claim assets.
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#### Threshold signatures
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Most of the disadvantages of the MultiSig solution come down to the high gas costs of verifying multiple transactions
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and tracking individual guardian key changes / set changes on other chains.
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In order to prove a quorum on a single signature, there exist different mechanisms for so-called Threshold signatures.
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A single signature is generated using a multi party computation process or aggregation of signatures from different
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parties of a group and only valid if a previously specified quorum has participated in the generation of such signature.
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This would essentially mean that such a signature could be published on the Solana chain and relayed by anyone to
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authorize an action on another chain, the same concept as described above but implemented with the cost of only
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sending and verifying one signature.
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Since we target Ethereum as primary foreign chain, there are 3 viable options of threshold signatures:
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**t-ECDSA**
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Threshold ECDSA signatures generated using [GG20](https://eprint.iacr.org/2020/540.pdf).
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This is a highly complex, cutting edge cryptographic protocol that requires significant amounts of compute to generate
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signatures with larger quorums.
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Still, it generates plain ECDSA signatures that can easily be verified on Ethereum (`5k gas`) or even be used for Bitcoin
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transactions.
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**BLS**
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Boneh–Lynn–Shacham threshold signatures are very lightweight because they don't require a multi-round process and can
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simply be aggregated from multiple individual signatures. This would eliminate the need for a p2p layer for MPC
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communication.
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However, verifying a BLS signature on Ethereum costs about 130k gas using the precompiled pairing functions over bn128.
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Also there's very little prior work on this scheme especially in the context of Solidity.
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**Schnorr-Threshold**
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Schnorr threshold signatures require a multi-round computation and distributed key generation.
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They can be verified on Ethereum extremely cheaply (https://blog.chain.link/threshold-signatures-in-chainlink/) and scale
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well with more signing parties.
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There's been significant prior work in the blockchain space, several implementations over different curves and a proposal
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to implement support on Bitcoin (BIP340).
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---
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A great overview can be found [here](https://github.com/Turing-Chain/TSSKit-Threshold-Signature-Scheme-Toolkit)
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#### Design choices
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For transfers we implement a simple MultiSig schema.
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We'll create a portable "action blob" with a threshold signature to allow anyone to relay action approvals
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between chains. We call this structure: **VAA** (Verifiable Action Approval).
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A validator action approval guarantees eventual consistency across chains - if the validators have submitted a VAA to a token lockup
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on Solana, this VAA can be used to unlock the tokens on the specified foreign chain.
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While for the above mentioned transfers from Solana => foreign chain we use Solana for data availability of the VAAs,
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in the other direction data availability i.e. the guardians posting the VAA on the foreign chain (where the transfer
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was initiated) is optional because in most cases it will be substantially cheaper for the guardians to directly submit
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the VAA on Solana itself to unlock/mint the transferred tokens there.
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### VAA - Verifiable Action Approval
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Verifiable action approvals are used to approve the execution of a specified action on a chain.
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They are structured as follows:
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```
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Header:
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uint8 version (0x01)
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uint32 guardian set index
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uint8 len signatures
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per signature:
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uint8 index of the signer (in guardian keys)
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[65]uint8 signature
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body:
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uint32 unix seconds
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uint8 action
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[payload_size]uint8 payload
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```
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The `guardian set index` does not need to be in the signed body since it is verifiable using the signature itself which
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is created using the guardian set's key.
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It is a monotonically number that's increased every time a validator set update happens and tracks the public key of the
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set.
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#### Actions
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##### Guardian set update
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ID: `0x01`
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Payload:
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```
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uint32 new_index
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uint8 len(keys)
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[][20]uint8 guardian addresses
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```
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The `new_index` must be monotonically increasing and is manually specified here to fix a potential guardian_set index
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desynchronization between the any of the chains in the system.
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##### Transfer
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ID: `0x10`
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Payload:
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```
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uint32 nonce
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uint8 source_chain
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uint8 target_chain
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[32]uint8 source_address
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[32]uint8 target_address
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uint8 token_chain
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[32]uint8 token_address
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uint8 decimals
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uint256 amount
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```
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### Cross-Chain Transfers
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#### Transfer of assets Foreign Chain -> Root Chain
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If this is the first time the asset is transferred to the root chain, the user inititates a `CreateWrapped` instruction
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on the root chain to initialize the wrapped asset.
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The user creates a token account for the wrapped asset on the root chain.
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The user sends a chain native asset to the bridge on the foreign chain using the `Lock` function.
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The lock function takes a Solana `address` as parameter which is the TokenAccount that should receive the wrapped token.
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Guardians will pick up the *Lock transaction* once it has enough confirmations on the foreign chain. The amount of
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confirmations required is a parameter that guardians can specify individually.
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They check for the validity, parse it and will then initiate a threshold signature ceremony on a deterministically
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produced VAA (`Transfer`) testifying that they have seen a foreign lockup. They will post this VAA on the root chain
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using the `SubmitVAA` instruction.
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This instruction will either mint a new wrapped asset or release tokens from custody.
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Custody is used for Solana-native tokens that have previously been transferred to a foreign chain, minting will be used
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to create new units of a wrapped foreign-chain asset.
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If this is the first time a foreign asset is minted, a new **Mint** (token) will be created on quorum.
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### Transfer of assets Root Chain -> Foreign Chain
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The user sends a **Lock** or **LockNative** instruction to the *Bridge program*.
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**Lock** has to be used for wrapped assets that should be transferred to a foreign chain. They will be burned on Solana.
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**LockNative** has to be used for Solana-native assets that should be transferred to a foreign chain. They will be held
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in a custody account until the tokens are transferred back from the foreign chain.
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The lock function takes a `chain_id` which identifies the foreign chain the tokens should be sent to and a `foreign_address`
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which is a left-zero-padded address on the foreign chain. This operation creates a **LockProposal** account
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that tracks the status of the transfer.
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Guardians will pick up the **LockProposal** once it has enough confirmations on the Solana network. It defaults to
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full confirmation (i.e. the max lockup, currently 32 slots), but can be changed to a different commitment levels
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on each guardian's discretion.
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They check for the validity of the tx, parse it and will initiate an off-chain signature aggregation ceremony which will
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output a **VAA** that can be used with a foreign chain smart contract to reclaim an unwrapped local asset or mint a
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wrapped `spl-token`.
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This VAA will be posted on Solana by one of the guardians using the `SubmitVAA` instruction and will be stored in the
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`LockProposal`.
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The user can then get the VAA from the `LockProposal` and submit it on the foreign chain.
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### Fees
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Fees exist for 2 reasons: spam prevention and guardian cost coverage.
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Costs for guardians:
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Assuming no hosting costs for a guardian operation (blockchain and guardian nodes), the only costs
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that need to be covered by a guardian operator are Solana transaction fees as well as rent costs for newly
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created account (used to store application information).
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**For a transfer from Solana to a foreign chain (20 guardians; 14 quorum):**
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Transactions required: `3 (signatures + verify) + 1 (post VAA)`
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Accounts created: `1 ClaimedVAA + 1 SignatureState`
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Costs:
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```
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4 TX (14 secp signatures + 4 ed25519) + ClaimedVAA (exemption rent) + SignatureState (exemption rent)
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18 * 10_000 + (36+128) * 6962 + (1337+128) * 6962
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11521098 lamports = 0.0115 SOL
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```
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**For a transfer from a foreign chain to Solana (20 guardians; 14 quorum):**
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Transactions required: `3 (signatures + verify) + 1 (post VAA)`
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Accounts created: `1 ClaimedVAA + 1 SignatureState (temporary; evicted in PostVAA)`
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Costs:
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```
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4 TX (14 secp signatures + 4 ed25519) + ClaimedVAA (exemption rent)
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18 * 10_000 + (36+128) * 6962
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1321768 lamports = 0.0013 SOL
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```
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---
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In order to cover rent costs there exists a subsidy pool controlled by the bridge to cover rent payments.
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While the guardian needs to hold enough SOL to pay for the rent, it is automatically refunded by the pool,
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in case the pool has sufficient balance.
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This subsidy pool is funded by transaction fees.
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Additionally, the subsidy pool subsidizes the transactions fees paid by the guardian submitting the VAA.
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As long as the pool has a sufficient balance, it will try to refund transaction fees to the guardian.
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Since Wormhole does not require foreign chain users to own SOL, Wormhole can't charge subsidy fees on inbound
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transfers. Assuming a balance between inbound and outbound transfers, outbound transfers need to subsidize
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inbound Solana transfers.
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Additionally, foreign chain contracts might start charging additional fees in the future.
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---
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The bridge can handle at most <TODO> transactions per second. Therefore, the fees should prevent spam
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by dynamically adjusting to load. This is particularly useful on Solana where fees are low and spamming
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would be cheap.
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Dynamic fees should be cheap while the system is under low and medium load and high while the system is
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close or above its capacity.
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To prevent sudden fee changes, the fee system has inertia.
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Fees scale as follows `fee = (tps/tps_max)^6`.
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The result is the fee per transfer in SOL. So at max capacity, the price per transfer is 1SOL.
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TPS is measured over a 30 second window.
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The minimum fee is the equivalent of 2x the rent of SignatureState and ClaimedVAA to cover the cost
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of this transfer and about 10 inbound transfers.
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---
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The above design can currently not be implemented due to limitations in the Solana BPF VM.
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In the current design, tx fees are refunded, rents are subsidized by the bridge and transfers out of Solana
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cost a fixed fee of 2x (ClaimedVAA rent + SignatureState rent + VAA submission fee), which will roughly
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pay for 1 outbound + ~10 inbound transfers.
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### Config changes
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#### Guardian set changes
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The guardians need to make sure that the sets are synchronized between all chains.
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If the guardian set is changed, the guardian must also be replaced on all foreign chains. Therefore we
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conduct these changes via VAAs that are universally valid on all chains.
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That way, if a change is made on the root chain, the same signatures can be used to trigger the same
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update on the foreign chain. This allows all parties in the system to propagate bridge state changes across all
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chains.
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If all VAAs issued by the previous guardian set would immediately become invalid once a new guardian set takes over, that would
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lead to some payments being "stuck". Therefore we track a list of previous guardian sets. VAAs issued by old
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guardian sets stay valid for one day from the time that the change happens in the default configuration.
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