Blazor Authentication

SIWE Authentication in Blazor

Implement Sign-In with Ethereum (EIP-4361) authentication in Blazor. Supports Blazor Server (direct signing) and Blazor WASM (REST API + JWT).

NuGet: Nethereum.Siwe, Nethereum.Blazor

A complete starter template is available:

dotnet new install Nethereum.Templates.Pack
dotnet new nethereum-siwe

Template repo: https://github.com/Nethereum/Nethereum.Templates.Pack (templates/Nethereum.Templates.Siwe)

SIWE Flow

1. Server creates a SiweMessage with a random nonce
2. Client signs the message with their wallet
3. Server verifies signature matches the claimed address
4. Server issues a JWT (WASM) or stores session (Server)
5. Blazor <AuthorizeView> checks claims

Blazor Server (Simplest)

Everything runs in one process. No REST API needed.

Setup (Program.cs)

builder.Services.AddSingleton<SiweMessageService>();
builder.Services.AddSingleton<NethereumSiweAuthenticatorService>();
builder.Services.AddScoped<IAccessTokenService, ProtectedSessionStorageAccessTokenService>();
builder.Services.AddScoped<AuthenticationStateProvider, SiweAuthenticationServerStateProvider>();

Authentication Flow

The SiweAuthenticationServerStateProvider orchestrates the full flow:

public async Task AuthenticateAsync(string address = null)
{
    if (string.IsNullOrEmpty(address))
        address = await EthereumHostProvider.GetProviderSelectedAccountAsync();

    var siweMessage = new DefaultSiweMessage();
    siweMessage.Address = address.ConvertToEthereumChecksumAddress();
    siweMessage.SetExpirationTime(DateTime.UtcNow.AddMinutes(10));
    siweMessage.SetNotBefore(DateTime.UtcNow);

    var fullMessage = await nethereumSiweAuthenticatorService.AuthenticateAsync(siweMessage);
    await _accessTokenService.SetAccessTokenAsync(SiweMessageStringBuilder.BuildMessage(fullMessage));
    await MarkUserAsAuthenticated();
}

The NethereumSiweAuthenticatorService (from Nethereum.UI) handles:

• Building the message string with nonce
• Prompting the wallet to sign via IEthereumHostProvider
• Verifying the signature

Blazor WASM + REST API

REST API: Generate SIWE Message

[AllowAnonymous]
[HttpPost("newsiwemessage")]
public IActionResult GenerateNewSiweMessage([FromBody] string address)
{
    var message = new DefaultSiweMessage();
    message.SetExpirationTime(DateTime.UtcNow.AddMinutes(10));
    message.SetNotBefore(DateTime.UtcNow);
    message.Address = address.ConvertToEthereumChecksumAddress();
    return Ok(_siweMessageService.BuildMessageToSign(message));
}

REST API: Authenticate and Issue JWT

[AllowAnonymous]
[HttpPost("authenticate")]
public async Task<IActionResult> Authenticate(AuthenticateRequest request)
{
    var siweMessage = SiweMessageParser.Parse(request.SiweEncodedMessage);

    if (!await _siweMessageService.IsUserAddressRegistered(siweMessage))
        return Unauthorized("Invalid User");

    if (!await _siweMessageService.IsMessageSignatureValid(siweMessage, request.Signature))
        return Unauthorized("Invalid Signature");

    if (!_siweMessageService.IsMessageTheSameAsSessionStored(siweMessage))
        return Unauthorized("Nonce mismatch");

    if (!_siweMessageService.HasMessageDateStartedAndNotExpired(siweMessage))
        return Unauthorized("Expired");

    var token = _siweJwtAuthorisationService.GenerateToken(siweMessage, request.Signature);
    return Ok(new AuthenticateResponse { Address = siweMessage.Address, Jwt = token });
}

WASM Client: Sign and Authenticate

public async Task AuthenticateAsync(string address)
{
    var siweMessage = await _siweUserLoginService.GenerateNewSiweMessage(address);
    var signedMessage = await EthereumHostProvider.SignMessageAsync(siweMessage);
    var response = await _siweUserLoginService.Authenticate(
        SiweMessageParser.Parse(siweMessage), signedMessage);

    if (response.Jwt != null)
    {
        await _accessTokenService.SetAccessTokenAsync(response.Jwt);
        await MarkUserAsAuthenticated();
    }
}

Using AuthorizeView

Two roles are available:

<AuthorizeView Roles="EthereumConnected">
    <Authorized>
        <p>Wallet connected: @context.User.Identity?.Name</p>
        <AuthorizeView Roles="SiweAuthenticated">
            <NotAuthorized>
                <button @onclick="LoginAsync">Sign In</button>
            </NotAuthorized>
        </AuthorizeView>
    </Authorized>
    <NotAuthorized>
        <button @onclick="ConnectWalletAsync">Connect Wallet</button>
    </NotAuthorized>
</AuthorizeView>

<AuthorizeView Roles="SiweAuthenticated">
    <Authorized>
        <p>Authenticated as @context.User.FindFirst(ClaimTypes.NameIdentifier)?.Value</p>
        <MyProtectedComponent />
    </Authorized>
</AuthorizeView>

NFT-Gated Access

Use ERC721BalanceEthereumUserService to require users hold a specific NFT:

var userService = new ERC721BalanceEthereumUserService(web3, nftContractAddress);
var siweService = new SiweMessageService(
    new InMemorySessionNonceStorage(),
    userService);

IsUserAddressRegistered will check the user's NFT balance before allowing authentication.

Smart Contract Wallets (ERC-1271 / ERC-6492)

Pass a Web3 instance to SiweMessageService for smart contract wallet signature verification:

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

IsMessageSignatureValid automatically falls back to ERC-1271 on-chain verification, and supports ERC-6492 for counterfactual (not-yet-deployed) smart wallets.

SiweMessageService API

Claims Structure

new Claim(ClaimTypes.Name, userName);
new Claim(ClaimTypes.NameIdentifier, ethereumAddress);
new Claim(ClaimTypes.Role, "EthereumConnected");
new Claim(ClaimTypes.Role, "SiweAuthenticated");

For full documentation, see: https://docs.nethereum.com/docs/blazor-dapp-integration/guide-blazor-authentication