docs(token-bridge): add draft for MetaChain Token & Bridge (#102)

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#### What this PR does / why we need it:

As per the title, this PR is a working draft version of MetaChain Token
& Bridge.

Signed-off-by: Fuxing Loh <[email protected]>
Co-authored-by: Eli <[email protected]>

Author: fuxingloh

docs/:300:MetaChain Protocol/:300:token-bridge.md

@@ -6,3 +6,116 @@ title: MetaChain Tokens & Bridge
 Currently work in progress — this article/page is consider draft, the information reflected here might not be
 up-to-date.
 :::
+
+A blockchain bridge, also known as a network bridge, allows assets to be moved between different Blockchains. Bridges
+facilitate information transfer between separate networks, enabling assets to be moved between blockchain
+ecosystems. Generally, there are 2 types of bridges, Trusted Bridges and Trustless Bridges.
+
+## Types of bridge
+
+> To better understand MetaChain Tokens & Bridge design, you will first need to understand the different types of
+> bridges deployed today. See [ethereum.org/bridges/#types-of-bridge](https://ethereum.org/en/bridges/#types-of-bridge).
+
+| **Trusted Bridges**                                                                                                                                     | **Trustless Bridges**                                                                            |
+| ------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------ |
+| Trusted bridges depend upon a central entity or system for their operations.                                                                            | Trustless bridges operate using smart contracts and algorithms.                                  |
+| They have trust assumptions with respect to the custody of funds and the security of the bridge. Users mostly rely on the bridge operator's reputation. | They are trustless; the security of the bridge is the same as that of the underlying blockchain. |
+| Users need to give up control of their crypto assets.                                                                                                   | Through smart contracts, trustless bridges enable users to remain in control of their funds.     |
+
+In a nutshell, we can say that trusted bridges have trust assumptions, whereas trustless bridges are trust-minimized and
+don’t make new trust assumptions beyond those of the underlying domains. Here’s how these terms can be described:
+
+- **Trustless**: having equivalent security to the underlying domains.
+- **Trust assumptions**: moving away from the security of the underlying domains by adding external verifiers in the
+  system, thus making it less crypto-economically secure.
+
+## MetaChain Trustless Bridge
+
+DeFiChain Native<->Meta Bridge is a trustless bidirectional asset bridging between NativeChain (UTXO) and MetaChain
+(Account). MetaChain Bridge has the equivalent security of a trustless bridge without the security worries of the underlying
+domains. MetaChain and NativeChain operate under the same domain, and the same [consensus](/consensus) mechanism within
+the same ecosystem keeps the validators honest.
+
+DeFiChain introduces a set of Smart Contracts on each platform/chain to perform the bidirectional minting and burning.
+The supply of all DFI comes from the NativeChain; all DFI on MetaChain are bridged from the NativeChain.
+
+### Trustless Bridge Smart Contract
+
+```mermaid
+classDiagram
+  class DfTxMetaBridge {
+      +BigDecimal balance
+      +send(pubKey, amount)
+      +receive(MetaChain.txid, amount)
+  }
+
+  class MetaChainBridgeContract {
+      +BigDecimal balance
+      +send(pubKey, amount)
+      +receive(NativeChain.txid, amount)
+  }
+```
+
+**NativeChain Asset Bridge Native Contract**
+
+- `DfTx.MetaChain.send(pubKey, amount)`
+- `DfTx.MetaBridge.receive(MetaChain.txid, amount)`
+
+**MetaChain Asset Bridge EVM Contract**
+
+- `MetaChain.BridgeContract.send(pubKey, amount)`
+- `MetaChain.BridgeContract.receive(NativeChain.txid, amount)`
+
+### Validating & Finalizing
+
+MetaChain trustless bridge actions are atomic and finalized within a single block. Each `send` requires an
+equivalent `receive` on the other chain within the same block. When a `txid = DfTx.MetaBridge.send(pubKey, 1.0)`
+transaction is present on the NativeChain, an equivalent `MetaChain.BridgeContract.receive(txid, 1.0)` must be present
+on the MetaChain.
+
+All `*.receive` transactions will be automatically created by the DeFiChain consensus and included in the coinbase
+transaction for block import. Validators connecting blocks with the bridge transactions validate the legitimacy of each
+block by validating that each `*.receive` has an equivalent `*.send` transaction.
+
+```mermaid
+stateDiagram-v2
+
+state "User Initiated & Prepare Bridge Transaction" as 0
+state "NativeChain to MetaChain" as NM
+state "MetaChain to NativeChain" as MN
+
+[*] --> 0
+0 --> NM : Broadcast
+0 --> MN : Broadcast
+
+state NM {
+  state "MetaBridge.send(0x00, 10.0)" as NM1
+  state "BridgeContract.receive(txid, 10.0)" as NM2
+
+  [*] --> NM1
+  NM1 --> NM2 : Bridge Consensus
+  NM2 --> [*]
+}
+
+state MN {
+  state "BridgeContract.send(0x00, 15.0)" as MN1
+  state "MetaBridge.receive(txid, 15.0)" as MN2
+
+  [*] --> MN1
+  MN1 --> MN2 : Bridge Consensus
+  MN2 --> [*]
+}
+
+MN --> Validators : Added to Block
+NM --> Validators : Added to Block
+
+state Validators {
+  state "Connect Block" as cb1
+  state "Process *.receive Transactions" as cb2
+
+  [*] --> cb1
+  cb1 --> cb2
+  cb2 --> False: missing *.send
+  cb2 --> True : found equivalent *.send
+}
+```