As we covered in the previous section, Auth makes use of the sign-in with wallet standard to allow users to prove their identity and login to your application. In this section, we'll cover the details of how this process works.

We want to allow the user to use their wallet as their identity to login into our applications. So we need a way for them to prove that they are in fact the owner of their wallet address, so that we can trust their identity.

The way we do this is by asking the user to sign a message with their wallet. Because of the way that digital signature algorithms work, when a user signs a message with their wallet, we can be sure that the message was signed by the owner of the wallet. This is because the signature can only be is generated using the private key of the wallet, which only the owner of the wallet knows.

This is the exact insight that motivated the Sign-in with Ethereum (EIP4361) and Sign-in with Wallet (CAIP122) standards. These standards define a uniform structure of the message that wallets should sign in order to login to any applications, taking into account a number of security concerns which we will go over here. Auth makes use of these standards to allow users to login to your application (specifically CAIP122, which is a superset of EIP4361 that works for any blockchain).

Each sign-in with wallet message contains a number of fields that are used to specify the details of the login request. These fields are used to prevent phishing attacks, add security, and to ensure that the user is signing in to the correct application. We'll go over the most important fields in more detail below, but here is the full configuration of a sign-in with wallet request:

Using this configuration, we can create a sign-in with wallet message by combining the fields together into the following structure, as specified by EIP4361 and CAIP122

${domain} wants you to sign in with your ${type} account:
${address}
${statement}
URI: ${uri}
Version: ${version}
Chain ID: ${chain-id}
Nonce: ${nonce}
Issued At: ${issued-at}
Expiration Time: ${expiration-time}
Not Before: ${not-before}
Resources:
- ${resources[0]}
- ${resources[1]}
...
- ${resources[n]}

Let's take a look at some of the most important fields here and understand what they are used for.

domain is one of the most important fields on the sign-in with wallet message. It is used to prevent phishing attacks by specifying the domain of the application that the user is logging into. This should match the domain of the application that the user is on while making a login request, and should be enforced by the server receiving the login request.

This helps to mitigate the possibility that a user could be tricked into signing a login message to an application by a malicious third-party website, allowing the the third-party to authenticate as the user.

address is the wallet address that the user is attempting to login with. This is used to help the user ensure that they are signing in with the correct wallet, as they can see the login message when they are signing it.

This is an arbitrary statement displayed as the user is logging in, an is often used to display a link to an applications terms of service. By default, Auth uses this line to further prevent phishing attacks as it prompts the user to double-check that the domain matches the current URL of the app that they are on: Please ensure that the domain above matches the URL of the current website.

This field specifies the chainId of the wallet that's logging in, which is especially important for users logging in with smart contract wallets. Since smart contract wallets exist on only on chain, it's important that the user specifies which chain their wallet exists on, so that the server then knows where to interact with the smart contract wallet to verify the signature.

The nonce is used to prevent replay attacks by adding a unique ID to each login message. This way, servers can keep track of login messages that have already been used, and reject login attempts with old payloads.

expirationTime is used to increase security my making login payloads invalid after a certain duration. By default Auth sets login payloads to expire after 5 minutes. This is because anyone who obtains a login payload from another user can use it to login as that user, so it's important that login payloads expire quickly.

Once a user signs a sign-in with wallet message and sends it to the server, the server then needs to verify that the signature is valid. This involves checking that the message was signed by the correct wallet address, as well as by checking the validity of the individual fields on the login payload. Let's take a look at all the key steps involved in verifying a login request.

First, we ensure that all the static values on the login payload are correct. This includes checking that the domain, type, statement, uri, version, and resources all match the static values that the server is expecting, as these should be the same across all login requests.

Next, we ensure that the payload hasn't already been used for another login, as that would indicate that there could be a replay attack occurring. This can be accomplished using any data-storage scheme by checking that the nonce is not already present in our storage, and if not, we add it to our storage for later validation.

Next, we check that the payload is currently valid. This includes checking that the issuedAt and notBefore fields are before the current time, and that the expirationTime is after the current time. This ensures that the payload is valid at the time that the server is verifying it.

Finally, we check that the user attempting to login actually signed the login payload. This ensures that the user is the one who is attempting to login, and not a malicious third-party.

However, there is one nuance at this step. If a user is signing in with a smart contract wallet (EIP1271), then we need to make a smart contract call to the isValidSignature function to check if the signature is valid - and this requires that the chainId on the login payload is correct. This is because smart contract wallets exist on only on chain, so we need to know which chain the wallet exists on in order to interact with the smart contract.