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diff --git a/docs/solidity-by-example.rst b/docs/solidity-by-example.rst index 752e8641..88ee7913 100644 --- a/docs/solidity-by-example.rst +++ b/docs/solidity-by-example.rst @@ -647,10 +647,10 @@ Safe Remote Purchase Micropayment Channel ******************** -In this section we will learn how to build a simple implementation -of a payment channel. It use cryptographics signatures to make +In this section we will learn how to build an example implementation +of a payment channel. It uses cryptographic signatures to make repeated transfers of Ether between the same parties secure, instantaneous, and -without transaction fees. To do it we need to understand how to +without transaction fees. For the example, we need to understand how to sign and verify signatures, and setup the payment channel. Creating and verifying signatures @@ -658,88 +658,69 @@ Creating and verifying signatures Imagine Alice wants to send a quantity of Ether to Bob, i.e. Alice is the sender and the Bob is the recipient. + Alice only needs to send cryptographically signed messages off-chain -(e.g. via email) to Bob and it will be very similar to writing checks. - -Signatures are used to authorize transactions, -and they are a general tool that is available to -smart contracts. Alice will build a simple -smart contract that lets her transmit Ether, but -in a unusual way, instead of calling a function herself -to initiate a payment, she will let Bob -do that, and therefore pay the transaction fee. +(e.g. via email) to Bob and it is similar to writing checks. + +Alice and Bob use signatures to authorise transactions, which is possible with smart contracts on Ethereum. +Alice will build a simple smart contract that lets her transmit Ether, but instead of calling a function herself +to initiate a payment, she will let Bob do that, and therefore pay the transaction fee. + The contract will work as follows: 1. Alice deploys the ``ReceiverPays`` contract, attaching enough Ether to cover the payments that will be made. - 2. Alice authorizes a payment by signing a message with their private key. + 2. Alice authorises a payment by signing a message with their private key. 3. Alice sends the cryptographically signed message to Bob. The message does not need to be kept secret - (you will understand it later), and the mechanism for sending it does not matter. + (explained later), and the mechanism for sending it does not matter. 4. Bob claims their payment by presenting the signed message to the smart contract, it verifies the authenticity of the message and then releases the funds. Creating the signature ---------------------- -Alice does not need to interact with Ethereum network to -sign the transaction, the process is completely offline. -In this tutorial, we will sign messages in the browser -using ``web3.js`` and ``MetaMask``. -In particular, we will use the standard way described in `EIP-762 <https://github.com/ethereum/EIPs/pull/712>`_, +Alice does not need to interact with the Ethereum network to sign the transaction, the process is completely offline. +In this tutorial, we will sign messages in the browser using `web3.js <https://github.com/ethereum/web3.js>`_ and `MetaMask <https://metamask.io>`_, using the method described in `EIP-762 <https://github.com/ethereum/EIPs/pull/712>`_, as it provides a number of other security benefits. :: + /// Hashing first makes things easier + var hash = web3.utils.sha3("message to sign"); + web3.eth.personal.sign(hash, web3.eth.defaultAccount, function () { + console.log("Signed") + }); - /// Hashing first makes a few things easier - var hash = web3.sha3("message to sign"); - web3.personal.sign(hash, web3.eth.defaultAccount, function () {...}); - - -Note that the ``web3.personal.sign`` prepends the length of the message to the signed data. -Since we hash first, the message will always be exactly 32 bytes long, -and thus this length prefix is always the same, making everything easier. +.. note:: + The ``web3.eth.personal.sign`` prepends the length of the message to the signed data. Since we hash first, the message will always be exactly 32 bytes long, and thus this length prefix is always the same. What to Sign ------------ -For a contract that fulfills payments, the signed message must include: +For a contract that fulfils payments, the signed message must include: - 1. The recipient's address - 2. The amount to be transferred - 3. Protection against replay attacks + 1. The recipient's address. + 2. The amount to be transferred. + 3. Protection against replay attacks. A replay attack is when a signed message is reused to claim authorization for a second action. -To avoid replay attacks we will use the same as in Ethereum transactions +To avoid replay attacks we use the same as in Ethereum transactions themselves, a so-called nonce, which is the number of transactions sent by an account. -The smart contract will check if a nonce is used multiple times. +The smart contract checks if a nonce is used multiple times. -There is another type of replay attacks, it occurs when the -owner deploys a ``ReceiverPays`` smart contract, performs some payments, -and then destroy the contract. Later, she decides to deploy the -``RecipientPays`` smart contract again, but the new contract does not -know the nonces used in the previous deployment, so the attacker -can use the old messages again. +Another type of replay attack can occur when the owner deploys a ``ReceiverPays`` smart contract, makes some payments, and then destroys the contract. Later, they decide to deploy the ``RecipientPays`` smart contract again, but the new contract does not know the nonces used in the previous deployment, so the attacker can use the old messages again. -Alice can protect against it including -the contract's address in the message, and only -messages containing contract's address itself will be accepted. -This functionality can be found in the first two lines of the ``claimPayment()`` function in the full contract -at the end of this chapter. +Alice can protect against this attack by including the contract's address in the message, and only messages containing the contract's address itself will be accepted. You can find an example of this in the first two lines of the ``claimPayment()`` function of the full contract at the end of this section. Packing arguments ----------------- -Now that we have identified what information to include in the -signed message, we are ready to put the message together, hash it, -and sign it. For simplicity, we just concatenate the data. -The -`ethereumjs-abi <https://github.com/ethereumjs/ethereumjs-abi>`_ library provides -a function called ``soliditySHA3`` that mimics the behavior -of Solidity's ``keccak256`` function applied to arguments encoded -using ``abi.encodePacked``. -Putting it all together, here is a JavaScript function that -creates the proper signature for the ``ReceiverPays`` example: +Now that we have identified what information to include in the signed message, +we are ready to put the message together, hash it, and sign it. For simplicity, +we concatenate the data. The `ethereumjs-abi <https://github.com/ethereumjs/ethereumjs-abi>`_ +library provides a function called ``soliditySHA3`` that mimics the behaviour of +Solidity's ``keccak256`` function applied to arguments encoded using ``abi.encodePacked``. +Here is a JavaScript function that creates the proper signature for the ``ReceiverPays`` example: :: @@ -748,46 +729,30 @@ creates the proper signature for the ``ReceiverPays`` example: // nonce can be any unique number to prevent replay attacks // contractAddress is used to prevent cross-contract replay attacks function signPayment(recipient, amount, nonce, contractAddress, callback) { - var hash = "0x" + ethereumjs.ABI.soliditySHA3( + var hash = "0x" + abi.soliditySHA3( ["address", "uint256", "uint256", "address"], [recipient, amount, nonce, contractAddress] ).toString("hex"); - web3.personal.sign(hash, web3.eth.defaultAccount, callback); + web3.eth.personal.sign(hash, web3.eth.defaultAccount, callback); } Recovering the Message Signer in Solidity ----------------------------------------- -In general, ECDSA signatures consist of two parameters, ``r`` and ``s``. -Signatures in Ethereum include a third parameter called ``v``, that can be used -to recover which account's private key was used to sign in the message, -the transaction's sender. Solidity provides a built-in function -`ecrecover <mathematical-and-cryptographic-functions>`_ -that accepts a message along with the ``r``, ``s`` and ``v`` parameters and -returns the address that was used to sign the message. +In general, ECDSA signatures consist of two parameters, ``r`` and ``s``. Signatures in Ethereum include a third parameter called ``v``, that you can use to verify which account's private key was used to sign the message, and the transaction's sender. Solidity provides a built-in function `ecrecover <mathematical-and-cryptographic-functions>`_ that accepts a message along with the ``r``, ``s`` and ``v`` parameters and returns the address that was used to sign the message. Extracting the Signature Parameters ----------------------------------- -Signatures produced by web3.js are the concatenation of ``r``, ``s`` and ``v``, -so the first step is splitting those parameters back out. It can be done on the client, -but doing it inside the smart contract means only one signature parameter -needs to be sent rather than three. -Splitting apart a byte array into component parts is a little messy. -We will use `inline assembly <assembly>`_ to do the job -in the ``splitSignature`` function (the third function in the full contract -at the end of this chapter). +Signatures produced by web3.js are the concatenation of ``r``, ``s`` and ``v``, so the first step is to split these parameters apart. You can do this on the client-side, but doing it inside the smart contract means you only need to send one signature parameter rather than three. Splitting apart a byte array into component parts is a messy, so we use `inline assembly <assembly>`_ to do the job in the ``splitSignature`` function (the third function in the full contract at the end of this section). Computing the Message Hash -------------------------- -The smart contract needs to know exactly what parameters were signed, -and so it must recreate the message from the parameters and use that -for signature verification. The functions ``prefixed`` and -``recoverSigner`` do this and their use can be found in the -``claimPayment`` function. - +The smart contract needs to know exactly what parameters were signed, and so it +must recreate the message from the parameters and use that for signature verification. +The functions ``prefixed`` and ``recoverSigner`` do this in the ``claimPayment`` function. The full contract ----------------- @@ -861,41 +826,26 @@ The full contract Writing a Simple Payment Channel ================================ -Alice will now build a simple but complete implementation of a payment channel. -Payment channels use cryptographic signatures to make repeated transfers -of Ether securely, instantaneously, and without transaction fees. +Alice now builds a simple but complete implementation of a payment channel. Payment channels use cryptographic signatures to make repeated transfers of Ether securely, instantaneously, and without transaction fees. What is a Payment Channel? -------------------------- -Payment channels allow participants to make repeated transfers of Ether without -using transactions. This means that the delays and fees associated with transactions -can be avoided. We are going to explore a simple unidirectional payment channel between -two parties (Alice and Bob). Using it involves three steps: +Payment channels allow participants to make repeated transfers of Ether without using transactions. This means that you can avoid the delays and fees associated with transactions. We are going to explore a simple unidirectional payment channel between two parties (Alice and Bob). It involves three steps: 1. Alice funds a smart contract with Ether. This "opens" the payment channel. 2. Alice signs messages that specify how much of that Ether is owed to the recipient. This step is repeated for each payment. 3. Bob "closes" the payment channel, withdrawing their portion of the Ether and sending the remainder back to the sender. -Note that only steps 1 and 3 require Ethereum transactions, step 2 means that -the sender transmits a cryptographically signed message to the recipient via off chain ways (e.g. email). -This means only two transactions are required to support any number of transfers. +.. note:: + Only steps 1 and 3 require Ethereum transactions, step 2 means that the sender transmits a cryptographically signed message to the recipient via off chain methods (e.g. email). This means only two transactions are required to support any number of transfers. -Bob is guaranteed to receive their funds because the smart contract escrows -the Ether and honors a valid signed message. The smart contract also enforces a timeout, -so Alice is guaranteed to eventually recover their funds even if the recipient refuses -to close the channel. -It is up to the participants in a payment channel to decide how long to keep it open. -For a short-lived transaction, such as paying an internet cafe for each minute of network access, -or for a longer relationship, such as paying an employee an hourly wage, a payment could last for months or years. +Bob is guaranteed to receive their funds because the smart contract escrows the Ether and honours a valid signed message. The smart contract also enforces a timeout, so Alice is guaranteed to eventually recover their funds even if the recipient refuses to close the channel. It is up to the participants in a payment channel to decide how long to keep it open. For a short-lived transaction, such as paying an internet café for each minute of network access, or for a longer relationship, such as paying an employee an hourly wage, a payment could last for months or years. Opening the Payment Channel --------------------------- -To open the payment channel, Alice deploys the smart contract, -attaching the Ether to be escrowed and specifying the intendend recipient -and a maximum duration for the channel to exist. It is the function -``SimplePaymentChannel`` in the contract, that is at the end of this chapter. +To open the payment channel, Alice deploys the smart contract, attaching the Ether to be escrowed and specifying the intended recipient and a maximum duration for the channel to exist. This is the function ``SimplePaymentChannel`` in the contract, at the end of this section. Making Payments --------------- @@ -910,27 +860,26 @@ Each message includes the following information: * The total amount of Ether that is owed the recipient so far. A payment channel is closed just once, at the end of a series of transfers. -Because of this, only one of the messages sent will be redeemed. This is why +Because of this, only one of the messages sent is redeemed. This is why each message specifies a cumulative total amount of Ether owed, rather than the amount of the individual micropayment. The recipient will naturally choose to redeem the most recent message because that is the one with the highest total. -The nonce per-message is not needed anymore, because the smart contract will -only honor a single message. The address of the smart contract is still used +The nonce per-message is not needed anymore, because the smart contract only honors a single message. The address of the smart contract is still used to prevent a message intended for one payment channel from being used for a different channel. -Here is the modified javascript code to cryptographically sign a message from the previous chapter: +Here is the modified JavaScript code to cryptographically sign a message from the previous section: :: function constructPaymentMessage(contractAddress, amount) { - return ethereumjs.ABI.soliditySHA3( + return abi.soliditySHA3( ["address", "uint256"], [contractAddress, amount] ); } function signMessage(message, callback) { - web3.personal.sign( + web3.eth.personal.sign( "0x" + message.toString("hex"), web3.eth.defaultAccount, callback @@ -951,18 +900,15 @@ Closing the Payment Channel When Bob is ready to receive their funds, it is time to close the payment channel by calling a ``close`` function on the smart contract. -Closing the channel pays the recipient the Ether they are owed and destroys the contract, -sending any remaining Ether back to Alice. -To close the channel, Bob needs to provide a message signed by Alice. +Closing the channel pays the recipient the Ether they are owed and destroys the contract, sending any remaining Ether back to Alice. To close the channel, Bob needs to provide a message signed by Alice. The smart contract must verify that the message contains a valid signature from the sender. The process for doing this verification is the same as the process the recipient uses. The Solidity functions ``isValidSignature`` and ``recoverSigner`` work just like their -JavaScript counterparts in the previous section. The latter is borrowed from the -``ReceiverPays`` contract in the previous chapter. +JavaScript counterparts in the previous section, with the latter function borrowed from the ``ReceiverPays`` contract. -The ``close`` function can only be called by the payment channel recipient, -who will naturally pass the most recent payment message because that message +Only the payment channel recipient can call the ``close`` function, +who naturally passes the most recent payment message because that message carries the highest total owed. If the sender were allowed to call this function, they could provide a message with a lower amount and cheat the recipient out of what they are owed. @@ -977,13 +923,11 @@ Channel Expiration Bob can close the payment channel at any time, but if they fail to do so, Alice needs a way to recover their escrowed funds. An *expiration* time was set at the time of contract deployment. Once that time is reached, Alice can call -``claimTimeout`` to recover their funds. You can see the ``claimTimeout`` function in the -full contract. +``claimTimeout`` to recover their funds. You can see the ``claimTimeout`` function in the full contract. After this function is called, Bob can no longer receive any Ether, so it is important that Bob closes the channel before the expiration is reached. - The full contract ----------------- @@ -1081,16 +1025,15 @@ The full contract } -Note: The function ``splitSignature`` is very simple and does not use all security checks. -A real implementation should use a more rigorously tested library, such as -openzepplin's `version <https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/contracts/ECRecovery.sol>`_ of this code. - - +.. note:: + The function ``splitSignature`` does not use all security + checks. A real implementation should use a more rigorously tested library, + such as openzepplin's `version <https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/contracts/ECRecovery.sol>`_ of this code. Verifying Payments ------------------ -Unlike in our previous chapter, messages in a payment channel aren't +Unlike in the previous section, messages in a payment channel aren't redeemed right away. The recipient keeps track of the latest message and redeems it when it's time to close the payment channel. This means it's critical that the recipient perform their own verification of each message. @@ -1105,10 +1048,8 @@ The recipient should verify each message using the following process: 4. Verify that the signature is valid and comes from the payment channel sender. We'll use the `ethereumjs-util <https://github.com/ethereumjs/ethereumjs-util>`_ -library to write this verifications. The final step can be done a number of ways, -but if it's being done in **JavaScript**. -The following code borrows the `constructMessage` function from the signing **JavaScript code** -above: +library to write this verification. The final step can be done a number of ways, +and we use JavaScript. The following code borrows the `constructMessage` function from the signing **JavaScript code** above: :: |