# 2. Ethereum¶

Since all proofs always include the blockheader it is crucial to verify the correctness of these data as well. But verification depends on the consensus of the underlying blockchain. (For details, see Ethereum Verification and MerkleProof.)

## 2.2. Proof of Work¶

Currently, the public chain uses proof of work. This makes it very hard to verify the header since anybody can produce such a header. So the only way to verify that the block in question is an accepted block is to let registered nodes sign the blockhash. If they are wrong, they lose their previously stored deposit. For the client, this means that the required security depends on the deposit stored by the nodes.

This is why a client may be configured to require multiple signatures and even a minimal deposit:

client.sendRPC('eth_getBalance', [account, 'latest'], chain, {
minDeposit: web3.utils.toWei(10,'ether'),
signatureCount: 3
})


The minDeposit lets the client preselect only nodes with at least that much deposit. The signatureCount asks for multiple signatures and so increases the security.

Since most clients are small devices with limited bandwith, the client is not asking for the signatures directly from the nodes but, rather, chooses one node and lets this node run a subrequest to get the signatures. This means not only fewer requests for the clients but also that at least one node checks the signatures and “convicts” another if it lied.

## 2.3. Proof of Authority¶

The good thing about proof of authority is that there is already a signature included in the blockheader. So if we know who is allowed to sign a block, we do not need an additional blockhash signed. The only critical information we rely on is the list of validators.

Currently, there are two consensus algorithms:

### 2.3.1. Aura¶

Aura is only used by Parity, and there are two ways to configure it:

• static list of nodes (like the Kovan network): in this case, the validatorlist is included in the chain-spec and cannot change, which makes it very easy for a client to verify blockheaders.
• validator contract: a contract that offers the function getValidators(). Depending on the chain, this contract may contain rules that define how validators may change. But this flexibility comes with a price. It makes it harder for a client to find a secure way to detect validator changes. This is why the proof for such a contract depends on the rules laid out in the contract and is chain-specific.

### 2.3.2. Clique¶

Clique is a protocol developed by the Geth team and is now also supported by Parity (see Görli testnet).

Instead of relying on a contract, Clique defines a protocol of how validator nodes may change. All votes are done directly in the blockheader. This makes it easier to prove since it does not rely on any contract.

The Incubed nodes will check all the blocks for votes and create a validatorlist that defines the validatorset for any given blockNumber. This also includes the proof in form of all blockheaders that either voted the new node in or out. This way, the client can ask for the list and automatically update the internal list after it has verified each blockheader and vote. Even though malicious nodes cannot forge the signatures of a validator, they may skip votes in the validatorlist. This is why a validatorlist update should always be done by running multiple requests and merging them together.

## 2.4. Ethereum Verification¶

The Incubed is also often called Minimal Verifying Client because it may not sync, but still is able to verify all incoming data. This is possible since ethereum is based on a technology allowing to verify almost any value.

Our goal was to verify at least all standard eth_... rpc methods as described in the Specification.

In order to prove something, you always need a starting value. In our case this is the BlockHash. Why do we use the BlockHash? If you know the BlockHash of a block, you can easily verify the full BlockHeader. And since the BlockHeader contains the stateRoot, transationRoot and receiptRoot, these can be verified as well. And the rest will simply depend on them.

There is also another reason why the BlockHash is so important. This is the only value you are able to access from within a SmartContract, because the evm supports a OpCode (BLOCKHASH), which allows you to read the last 256 Blockhashes, which gives us the chance to even verify the blockhash onchain.

Depending on the method, different proofs are needed, which are described in this document.

### 2.4.1. BlockProof¶

BlockProofs are used whenever you want to read data of a Block and verify them. This would be:

The eth_getBlockBy... methods return the Block-Data. In this case all we need is somebody verifying the blockhash, which is don by requiring somebody who stored a deposit and would lose it, to sign this blockhash.

The Verification is then simply by creating the blockhash and comparing this to the signed one.

The Blockhash is calculated by serializing the blockdata with rlp and hashing it:

blockHeader = rlp.encode([
bytes32( parentHash ),
bytes32( sha3Uncles ),
bytes32( stateRoot ),
bytes32( transactionsRoot ),
bytes32( receiptsRoot || receiptRoot ),
bytes256( logsBloom ),
uint( difficulty ),
uint( number ),
uint( gasLimit ),
uint( gasUsed ),
uint( timestamp ),

... sealFields
? sealFields.map( rlp.decode )
: [
bytes32( b.mixHash ),
bytes8( b.nonce )
]
])


For POA-Chains the blockheader will use the sealFields (instead of mixHash and nonce) which are already rlp-encoded and should be added as raw data when using rlp.encode.

if (keccak256(blockHeader) !== singedBlockHash)
throw new Error('Invalid Block')


In case of the eth_getBlockTransactionCountBy... the proof contains the full blockHeader already serilalized + all transactionHashes. This is needed in order to verify them in a merkleTree and compare them with the transactionRoot

### 2.4.2. Transaction Proof¶

TransactionProofs are used for the following transaction-methods:

In order to verify we need :

1. serialize the blockheader and compare the blockhash with the signed hash as well as with the blockHash and number of the transaction. (See BlockProof)
2. serialize the transaction-data
transaction = rlp.encode([
uint( tx.nonce ),
uint( tx.gasPrice ),
uint( tx.gas || tx.gasLimit ),
uint( tx.value ),
bytes( tx.input || tx.data ),
uint( tx.v ),
uint( tx.r ),
uint( tx.s )
])

1. verify the merkleProof of the transaction with
verifyMerkleProof(
keccak256(proof.txIndex), /* key or path */
proof.merkleProof, /* serialized nodes starting with the root-node */
transaction /* expected value */
)


The Proof-Data will look like these:

{
"jsonrpc": "2.0",
"id": 6,
"result": {
"blockHash": "0xf1a2fd6a36f27950c78ce559b1dc4e991d46590683cb8cb84804fa672bca395b",
"blockNumber": "0xca",
"from": "0x7e5f4552091a69125d5dfcb7b8c2659029395bdf",
"gas": "0x55f0",
"gasPrice": "0x0",
"hash": "0xe9c15c3b26342e3287bb069e433de48ac3fa4ddd32a31b48e426d19d761d7e9b",
"input": "0x00",
"value": "0x3e8"
...
},
"in3": {
"proof": {
"type": "transactionProof",
"merkleProof": [  /* serialized nodes starting with the root-node */
],
"txIndex": 0,
"signatures": [...]
}
}
}


### 2.4.3. Receipt Proof¶

Proofs for the transactionReceipt are used for the following transaction-method:

In order to verify we need :

1. serialize the blockheader and compare the blockhash with the signed hash as well as with the blockHash and number of the transaction. (See BlockProof)
2. serialize the transaction receipt
transactionReceipt = rlp.encode([
uint( r.status || r.root ),
uint( r.cumulativeGasUsed ),
bytes256( r.logsBloom ),
r.logs.map(l => [
l.topics.map( bytes32 ),
bytes( l.data )
])
].slice(r.status === null && r.root === null ? 1 : 0))

1. verify the merkleProof of the transaction receipt with
verifyMerkleProof(
keccak256(proof.txIndex), /* key or path */
proof.merkleProof, /* serialized nodes starting with the root-node */
transactionReceipt /* expected value */
)

1. Since the merkle-Proof is only proving the value for the given transactionIndex, we also need to prove that the transactionIndex matches the transactionHash requested. This is done by adding another MerkleProof for the Transaction itself as described in the Transaction Proof

### 2.4.4. Log Proof¶

Proofs for logs are only for the one rpc-method:

Since logs or events are based on the TransactionReceipts, the only way to prove them is by proving the TransactionReceipt each event belongs to.

That’s why this proof needs to provide

• all blockheaders where these events occured
• all TransactionReceipts + their MerkleProof of the logs
• all MerkleProofs for the transactions in order to prove the transactionIndex

The Proof data structure will look like this:

  Proof {
type: 'logProof',
logProof: {
[blockNr: string]: {  // the blockNumber in hex as key
block : string  // serialized blockheader
receipts: {
[txHash: string]: {  // the transactionHash as key
txIndex: number // transactionIndex within the block
txProof: string[] // the merkle Proof-Array for the transaction
proof: string[] // the merkle Proof-Array for the receipts
}
}
}
}
}


In order to verify we need :

1. deserialize each blockheader and compare the blockhash with the signed hashes. (See BlockProof)
2. for each blockheader we verify all receipts by using
verifyMerkleProof(
keccak256(proof.txIndex), /* key or path */
proof.merkleProof, /* serialized nodes starting with the root-node */
transactionReceipt /* expected value */
)

1. The resulting values are the receipts. For each log-entry, we are comparing the verified values of the receipt with the returned logs to ensure that they are correct.

### 2.4.5. Account Proof¶

Prooving an account-value applies to these functions:

#### 2.4.5.1. eth_getProof¶

For the Transaction or Block Proofs all needed data can be found in the block itself and retrieved through standard rpc calls, but if we want to approve the values of an account, we need the MerkleTree of the state, which is not accessable through the standard rpc. That’s why we have created a EIP to add this function and also implemented this in geth and parity:

This function accepts 3 parameter :

1. account - the address of the account to proof
2. storage - a array of storage-keys to include in the proof.
3. block - integer block number, or the string “latest”, “earliest” or “pending”
{
"jsonrpc": "2.0",
"id": 1,
"method": "eth_getProof",
"params": [
"0x7F0d15C7FAae65896648C8273B6d7E43f58Fa842",
"latest"
]
}


The result will look like this:

{
"jsonrpc": "2.0",
"result": {
"accountProof": [
"0xf90211a...0701bc80",
"0xf90211a...0d832380",
"0xf90211a...5fb20c80",
"0xf90211a...0675b80",
"0xf90151a0...ca08080"
],
"balance": "0x0",
"nonce": "0x0",
"storageProof": [
{
"proof": [
"0xf90211a...0701bc80",
"0xf90211a...0d832380"
],
"value": "0x1"
}
]
},
"id": 1
}


In order to run the verification the blockheader is needed as well.

The Verification of such a proof is done in the following steps:

1. serialize the blockheader and compare the blockhash with the signed hash as well as with the blockHash and number of the transaction. (See BlockProof)
2. Serialize the account, which holds the 4 values:
account = rlp.encode([
uint( nonce),
uint( balance),
bytes32( storageHash || ethUtil.KECCAK256_RLP),
bytes32( codeHash || ethUtil.KECCAK256_NULL)
])

1. verify the merkle Proof for the account using the stateRoot of the blockHeader:
verifyMerkleProof(
block.stateRoot, // expected merkle root
accountProof.accountProof.map(bytes), // array of Buffer with the merkle-proof-data
isNotExistend(accountProof) ? null : serializeAccount(accountProof), // the expected serialized account
)


In case the account does exist yet, (which is the case if none == startNonce and codeHash == '0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470'), the proof may end with one of these nodes:

• the last node is a branch, where the child of the next step does not exist.
• the last node is a leaf with different relative key

Both would prove, that this key does not exist.

1. Verify each merkle Proof for the storage using the storageHash of the account:
verifyMerkleProof(
bytes32( accountProof.storageHash ),   // the storageRoot of the account
util.keccak(bytes32(s.key)),  // the path, which is the hash of the key
s.proof.map(bytes), // array of Buffer with the merkle-proof-data
s.value === '0x0' ? null : util.rlp.encode(s.value) // the expected value or none to proof non-existence
))


### 2.4.6. Call Proof¶

Call Proofs are used whenever you are calling a read-only function of smart contract:

Verifying the result of a eth_call is a little bit more complex. Because the response is a result of executing opcodes in the vm. The only way to do so, is to reproduce it and execute the same code. That’s why a Call Proof needs to provide all data used within the call. This means :

• all referred accounts including the code (if it is a contract), storageHash, nonce and balance.
• all storage keys, which are used ( This can be found by tracing the transaction and collecting data based on th SLOAD-opcode )
• all blockdata, which are referred at (besides the current one, also the BLOCKHASH-opcodes are referring to former blocks)

For Verifying you need to follow these steps:

1. serialize all used blockheaders and compare the blockhash with the signed hashes. (See BlockProof)
2. Verify all used accounts and their storage as showed in Account Proof
3. create a new VM with a MerkleTree as state and fill in all used value in the state:
  // create new state for a vm
const state = new Trie()
const vm = new VM({ state })

// fill in values
for (const adr of Object.keys(accounts)) {

// create an account-object
const account = new Account([ac.nonce, ac.balance, ac.stateRoot, ac.codeHash])

// if we have a code, we will set the code
if (ac.code) account.setCode( state, bytes( ac.code ))

// set all storage-values
for (const s of ac.storageProof)
account.setStorage( state, bytes32( s.key ), rlp.encode( bytes32( s.value )))

// set the account data
}

// add listener on each step to make sure it uses only values found in the proof
vm.on('step', ev => {
const contract = toHex( ev.address ) // address of the current code
const storageKey = bytes32( ev.stack[ev.stack.length - 1] ) // last element on the stack is the key
if (!getStorageValue(contract, storageKey))
throw new Error(incomplete data: missing key \${storageKey})
}
/// ... check other opcodes as well
})

// create a transaction
const tx = new Transaction(txData)

// run it
const result = await vm.runTx({ tx, block: new Block([block, [], []]) })

// use the return value
return result.vm.return


In the future we will be using the same approach to verify calls with ewasm.