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+---
+title: 'How to verify child chain state on parent chain'
+description: Learn how to verify child chain state on its parent chain
+user_story: As a developer, I want to understand how to verify child chain state on parent chain.
+content_type: how-to
+---
+
+Arbitrum implements a **fraud proof** system that ensures that the state of any given child chain is safely maintained by its parent chain. In this system, a validator is responsible for periodically transmitting information about the child chain's state to its parent chain.
+This information is included in "rollup blocks", which we refer to as **RBlocks** throughout our docs. See [Inside Arbitrum Nitro](../inside-arbitrum-nitro/inside-arbitrum-nitro.mdx#arbitrum-rollup-protocol) to learn more about RBlocks.
+
+These RBlocks are effectively _assertions about the impact that a series of child chain blocks (containing transactions) ought to have on its parent chain's state_. When received by the parent chain, these RBlocks are decoded by a contract hosted by the parent chain (an "on-chain contract"); this information can then be optionally relayed to off-chain tools for arbitrary purposes (for example, off-chain validation).
+
+Before we begin, we will introduce the key component: rblock, assertion and send roots.
+
+# Rblock and Assertion
+
+The rollup contract contains a series of components used to maintain the operation of the layer2 network, including rblocks.
+
+Here is what an rblock contains:
+
+```
+struct Node {
+    // Hash of the state of the chain as of this node
+    bytes32 stateHash;
+    // Hash of the data that can be challenged
+    bytes32 challengeHash;
+    // Hash of the data that will be committed if this node is confirmed
+    bytes32 confirmData;
+    // Index of the node previous to this one
+    uint64 prevNum;
+    // Deadline at which this node can be confirmed
+    uint64 deadlineBlock;
+    // Deadline at which a child of this node can be confirmed
+    uint64 noChildConfirmedBeforeBlock;
+    // Number of stakers staked on this node. This includes real stakers and zombies
+    uint64 stakerCount;
+    // Number of stakers staked on a child node. This includes real stakers and zombies
+    uint64 childStakerCount;
+    // This value starts at zero and is set to a value when the first child is created. After that it is constant until the node is destroyed or the owner destroys pending nodes
+    uint64 firstChildBlock;
+    // The number of the latest child of this node to be created
+    uint64 latestChildNumber;
+    // The block number when this node was created
+    uint64 createdAtBlock;
+    // A hash of all the data needed to determine this node's validity, to protect against reorgs
+    bytes32 nodeHash;
+}
+```
+
+When creating a new rblock, a new assertion will be made too:
+
+```
+struct Assertion {
+    ExecutionState beforeState;
+    ExecutionState afterState;
+    uint64 numBlocks;
+}
+```
+
+As we can see above, an rblock has a series of field, they are useful when validators try to challenge or confirm this rblock.
+What we can use here is the `confirmData`, the `confirmData` is the keccak256 hash of child chain block Hash and sendRoot.
+As for Assertion, it has 2 `ExecutionState` which is the start state and the end state of this assertion, and `ExecutionState` contains the information about child chain blockhash and related sendroot, so we can extract `blockhash` from there.
+
+# Send roots
+
+The send root mapping is stored in the outbox contract. This mapping is used to store the Merkle root of each batch of child chain -> parent chain transactions called send root and its corresponding child chain block hash.
+
+When an rblock is confirmed, the corresponding send root will be recorded to outbox contract from rollup contract so when an user wants to triger the child chain -> parent chain transaction on parent chain the transaction requests can be verified.
+
+```
+mapping(bytes32 => bytes32) public roots; // maps root hashes => child chain block hash
+```
+
+This mapping will save the `blockhash`, so we can get the child chain blockhash from the outbox contract too.
+
+# Verify child chain state on parent chain
+
+Assume that there is a contract called `foo` on child chain, and its contract address is `fooAddress`, now we want to prove its state value at storage `slot`.
+
+To verify the state, we need a Merkle Trie Verifier contract, one example is [Lib_MerkleTrie.sol](https://github.com/ethereum-optimism/optimism-legacy/blob/8205f678b7b4ac4625c2afe351b9c82ffaa2e795/packages/contracts/contracts/libraries/trie/Lib_MerkleTrie.sol).
+
+## 1. How to verify a confirmed child chain block hash
+
+For the security of verification, we will use the latest confirmation instead of the latest proposed rblock for verification:
+
+- Obtain the latest confirmed rblock from rollup contract: `nodeIndex = rollup.latestConfirmed()`, this step will return the corresponding rblock number: `nodeIndex`
+- Filter the event with the obtained rblock number: `nodeEvent = NodeCreated(nodeIndex)`, and get the corresponding assertion information: `assertion = nodeEvents[0].args.assertion`
+- Fetch blockhash via `blockhash = GlobalStateLib.getBlockHash(assertion.afterState.globalState)` (As mentioned above, you can also get the block hash from the outbox contract)
+- Fetch sendRoot via `sendRoot = GlobalStateLib.getSendRoot(assertion.afterState.globalState)`
+- After getting the blockhash, we need to compare it with the confirmdata in rblock, to get the confirm data: `confirmdata = keccak256(solidityPack(['bytes32','bytes32'], [blockHash, sendRoot]))`
+- Get the corresponding rblock: `rblock = rollup.getNode(nodeIndex)`
+- Compare if they have the same value: `rblock.confirmData == confirmdata`
+
+## 2. Proof the state root belong to the child chain block hash by supplying the blockheader
+
+After we obtain the block hash, we can obtain the corresponding block information from child chain provider: `l2blockRaw = eth_getBlockByHash (blockhash)`
+
+Next, we need to manually derive blockhash by hashing block header fields.
+
+```
+blockarray = [
+    l2blockRaw.parentHash,
+    l2blockRaw.sha3Uncles,
+    l2blockRaw.miner,
+    l2blockRaw.stateroot,
+    l2blockRaw.transactionsRoot,
+    l2blockRaw.receiptsRoot,
+    l2blockRaw.logsBloom,
+    BigNumber.from(l2blockRaw.difficulty).toHexString(),
+    BigNumber.from(l2blockRaw.number).toHexString(),
+    BigNumber.from(l2blockRaw.gasLimit).toHexString(),
+    BigNumber.from(l2blockRaw.gasUsed).toHexString(),
+    BigNumber.from(l2blockRaw.timestamp).toHexString(),
+    l2blockRaw.extraData,
+    l2blockRaw.mixHash,
+    l2blockRaw.nonce,
+    BigNumber.from(l2blockRaw.baseFeePerGas).toHexString(),
+    ]
+```
+
+- Calculate the block hash to verify whether the information in the obtained block is correct: `calculated_blockhash = keccak256(RLP.encode(blockarray))`
+- Verify whether the block hash is same with what we got from assertion or outbox contract: `calculated_blockhash === blockHash`
+
+If it is same, it can be used to prove that the information in the block header, especially the stateroot, is correct.
+
+## 3. Proof the account storage inside the state root
+
+After we obtain the correct state root, we can continue to verify the storage slot.
+
+- First, we need to obtain the proof of the corresponding state root from child chain:
+
+```
+proof = l2provider.send('eth_getProof', [
+    fooAddress,
+    [slot],
+    {blockHash}
+  ]);
+```
+
+- Get account proof: `accountProof = RLP.encode(proof.accountProof)`
+- Get proofKey: `proofKey = ethers.utils.keccak256(fooAddress)`
+- Call the verifier contract to verify:
+
+```
+[acctExists, acctEncoded] = verifier.get(
+    proofKey, accountProof, stateroot
+  )
+```
+
+- Check for equality: `acctExists == true`
+
+## 4. Proof the storage slot is in the account root
+
+- Get storage root: `storageRoot = RLP.decode(acctEncoded)[2]`
+- Get storage slot key: `slotKey = ethers.utils.keccak256(slot)`
+- Get storageProof: `storageProof = ethers.utils.RLP.encode((proof.storageProof as any[]).filter((x)=>x.key===slot)[0].proof)`
+- Call the merkle verifier contract to verify:
+
+```
+const [storageExists, storageEncoded] = await verifier.get(
+    slotKey, storageProof, storageRoot
+  )
+```
+
+- Check for equality: `storageExists == true`
+- Obtain the value of the storage as `slot`: `storageValue = ethers.utils.RLP.decode(storageEncoded)`
+
+Then we can successfuly prove and get a certain state value at a specific block height on child chain through parent chain.
+
+### Let's check this value on child chain directly
+
+- Call child chain rpc provider to get the value of the corresponding block number: `actualValue = l2provider.getStorageAt(fooAddress, slot, l2blockRaw.number)`
+- Check for equality: `storageValue === BigNumber.from(actualValue).toHexString()`