Siwe

SIWE Authentication with Nethereum

When to Use

• User wants to authenticate users via their Ethereum wallet
• Building a dApp that needs sign-in functionality
• Implementing NFT-gated or token-gated access
• Need to verify a user controls a specific Ethereum address
• Building REST API authentication with Ethereum wallets
• Need fine-grained capability authorization (RECAP/EIP-5573)

Required Packages

dotnet add package Nethereum.Siwe

This includes Nethereum.Siwe.Core (message model, parser) automatically.

Basic SIWE Flow

using Nethereum.Siwe;
using Nethereum.Siwe.Core;

// 1. Create the service
var siweService = new SiweMessageService();

// 2. Build the message
var message = new SiweMessage
{
    Domain = "myapp.com",
    Address = userAddress.ConvertToEthereumChecksumAddress(),
    Statement = "Sign in to MyApp",
    Uri = "https://myapp.com",
    Version = "1",
    ChainId = "1"
};
message.SetIssuedAtNow();
message.SetExpirationTime(DateTime.UtcNow.AddMinutes(30));

// 3. Get the text for the wallet to sign (nonce auto-generated)
string messageToSign = siweService.BuildMessageToSign(message);

// 4. After user signs, verify
var parsed = SiweMessageParser.Parse(messageToSign);
bool valid = await siweService.IsValidMessage(parsed, signature);
// Checks: signature + dates + session nonce + user registration

NFT-Gated Access

var nftService = new ERC721BalanceEthereumUserService(
    nftContractAddress, rpcUrl);

var siweService = new SiweMessageService(
    new InMemorySessionNonceStorage(),
    nftService);

// IsValidMessage now also checks NFT ownership
bool valid = await siweService.IsValidMessage(parsed, signature);

Smart Contract Wallet Support (ERC-1271)

var siweService = new SiweMessageService(
    new InMemorySessionNonceStorage(),
    ethereumUserService: null,
    web3ForERC1271Validation: web3);

// Falls back to ERC-1271/ERC-6492 for smart contract wallets
bool valid = await siweService.IsMessageSignatureValid(parsed, signature);

RECAP Capabilities (EIP-5573)

using Nethereum.Siwe.Core.Recap;

var recapMessage = SiweRecapMsgBuilder.Init(message)
    .AddDefaultActions(new SiweNamespace("eip155"), new HashSet<string> { "sign" })
    .AddTargetActions(new SiweNamespace("https"), "https://api.myapp.com",
        new HashSet<string> { "read", "write" })
    .Build();

// Check permissions after verification
bool canWrite = recapMessage.HasPermissions(
    new SiweNamespace("https"), "https://api.myapp.com", "write");

Custom Session Storage

public class RedisSessionStorage : ISessionStorage
{
    public void AddOrUpdate(SiweMessage msg) { /* store by nonce */ }
    public SiweMessage GetSiweMessage(SiweMessage msg) { /* lookup by nonce */ }
    public void Remove(SiweMessage msg) { /* delete */ }
    public void Remove(string nonce) { /* delete */ }
}

var siweService = new SiweMessageService(new RedisSessionStorage());

Custom User Validation

public class MyUserService : IEthereumUserService
{
    public Task<bool> IsUserAddressRegistered(string address)
    {
        // Check database, allowlist, token balance, etc.
        return Task.FromResult(true);
    }
}

REST API Authentication (.NET 5+)

using Nethereum.Siwe.Authentication;

var loginService = new SiweApiUserLoginService<User>(httpClient);
string messageToSign = await loginService.GenerateNewSiweMessage(address);
var response = await loginService.Authenticate(parsedMessage, signature);
string jwt = response.Jwt;
var user = await loginService.GetUser(jwt);
await loginService.Logout(jwt);

Key Decision Points

Important Notes

• Always use UTC for SetExpirationTime and SetNotBefore
• BuildMessageToSign generates a fresh nonce each call — don't reuse
• InMemorySessionNonceStorage doesn't survive restarts — use persistent storage in production
• Set ChainId to match the user's connected chain

For full documentation, see: https://docs.nethereum.com/docs/protocols/guide-siwe