Add NFT design doc

Change-Id: I83162d5632b0fab6c49333ca06fc59077fbb18e5

design/0006_nft_bridge.md

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+# NFT bridge App
+
+[TOC]
+
+## Objective
+
+To use the Wormhole message passing protocol to transfer NFTs between different connected chains.
+
+## Background
+
+NFTs are a new asset class that has grown in popularity recently. It especially attracts new users and companies to
+crypto. NFTs, just like traditional tokens, are minted on a single blockchain and cannot be transferred to other chains.
+Howevre as more chains introduce NFT standards and marketplaces there is demand for ways to transfer NFTS across chains
+to access these markets and collect them in a single wallet.
+
+## Goals
+
+We want to implement a generalized NFT bridge using the Wormhole message passing protocol that is able to bridge any
+standards-compliant NFT between chains, creating unique wrapped representations on each connected chain on demand.
+
+* Allow transfer of standards-compliant NFTs between chains.
+* Use a universal NFT representation that is compatible with most VM data types.
+
+## Non-Goals
+
+* Support EIP1155
+* Manage / Transfer chain-specific NFT metadata that isn't broadly applicable to all chains.
+* Automatically relay NFT transfer messages to the target chain.
+
+## Overview
+
+On each chain of the NFT bridge network there will be a NFT bridge endpoint program.
+
+These programs will manage authorization of payloads (emitter filtering), wrapped representations of foreign chain
+NFTs ("Wrapped NFTs") and custody locked NFTs.
+
+We aim to support:
+
+- EIP721: Ethereum, BSC
+- Metaplex SPL Meta: Solana
+
+## Detailed Design
+
+For outbound transfers, the contracts will have a lock method that either locks up a native NFT and produces a
+respective Transfer message that is posted to Wormhole, or burns a wrapped NFT and produces/posts said message.
+
+For inbound transfers they can consume, verify and process Wormhole messages containing a NFT bridge payload.
+
+There will be three different payloads:
+
+* Transfer - Will trigger the release of locked NFTs or minting of wrapped NFTs.
+
+Identical to the NFT bridge:
+
+* RegisterChain - Register the NFT bridge contract (emitter address) for a foreign chain.
+* UpgradeContract - Upgrade the contract.
+
+In order to transfer an NFT to another chain, a user needs to call the transfer method of the bridge contract with the
+recipient details. The contract will either hold the NFTs in a custody account (in case it is a native NFT) or burn
+wrapped NFTs. Wrapped NFTs can be burned because they can be freely minted once they are transferred back. After the
+lockup the contract will post a Transfer payload message to Wormhole. Once the message has been signed by the guardians,
+it can be posted to the target chain of the transfer. The target chain will then either release the native NFT from
+custody or mint a wrapped NFT depending on whether it's a native NFT there. The program will keep track of consumed
+message digests for replay prevention.
+
+Since the method for posting a VAA to the NFT bridge is authorized by the message signature itself, anyone can post any
+message.
+
+Since every NFT has unique metadata the Transfer messages contain all metadata, a transfer (even the first on per NFT)
+only requires a single Wormhole message to be passed compared to the Token Bridge. On the first transfer action of an
+NFT (address / symbol / name) a wrapped asset (i.e. master edition or new contract) is created. When the wrapped asset (
+contract) is already initialized or was just initialized, the (new) token_id and metadata URL are registered.
+
+### API / database schema
+
+Proposed bridge interface:
+
+transfer(address token, uint256 token_id, uint16 recipient_chain, bytes32 recipient) - Initiate a Transfer
+
+completeTransfer(Message transfer) - Execute a Transfer message
+
+registerChain(Message registerChain) - Execute a RegisterChain governance message
+
+upgrade(Message upgrade) - Execute a UpgradeContract governance message
+
+---
+Payloads:
+
+Transfer:
+
+```
+PayloadID uint8 = 1
+// Address of the NFT. Left-zero-padded if shorter than 32 bytes
+NFTAddress [32]uint8
+// Chain ID of the NFT
+NFTChain uint16
+// Name of the NFT
+Name [32]uint8
+// Symbol of the NFT
+Symbol [10]uint8
+// ID of the token (big-endian uint256)
+TokenID [32]uint8
+// URL of the NFT
+URLLength u8
+URL [n]uint8
+// Address of the recipient. Left-zero-padded if shorter than 32 bytes
+To [32]uint8
+// Chain ID of the recipient
+ToChain uint16
+```
+
+RegisterChain:
+
+```
+PayloadID uint8 = 2
+// Chain ID
+ChainID uint16
+// Emitter address. Left-zero-padded if shorter than 32 bytes
+EmitterAddress [32]uint8
+```
+
+UpgradeContract:
+
+```
+PayloadID uint8 = 3
+// Address of the new contract
+NewContract [32]uint8
+```
+
+## Caveats
+
+There is no guarantee for completion of transfers. If a user initiates a transfer and doesn't call completeTransfer on
+the target chain, a transfer might not be completed. In case a guardian set change happens in-between and the original
+signer guardian set expires, the transfer will be stuck indefinitely.
+
+Since there is no way for a NFT bridge endpoint to know which other chain already has wrapped assets set up for the
+native asset on its chain, there may be transfers initiated for assets that don't have wrapped assets set up yet on the
+target chain. However, the transfer will become executable once the wrapped asset is set up (which can be done any time)
+.
+
+Currently Solana only supports u64 token ids which is incompatible with Ethereum which specifically mentions the use of
+UUIDs as token ids (utilizing all bytes of the uint256). There will either need to be a mechanism to translate ids i.e.
+a map of `[32]u8 -> incrementing_u64` (in the expectation there will never be more than MaxU64 editions) or Solana needs
+to change their NFT contract.