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Internet Identity

Internet Identity (II) is the Internet Computer’s native authentication system. Users sign in with passkeys or OpenID accounts (Google, Apple, Microsoft) instead of passwords. Each user receives a unique principal per frontend origin, preventing cross-app tracking.

This guide covers setting up II authentication end-to-end: configuring your project, adding login to your frontend, and verifying callers in your backend.

How it works

When a user authenticates through Internet Identity, the following happens:

  1. Your frontend opens an II popup window.
  2. The user authenticates with a passkey or OpenID provider.
  3. II creates a delegation identity: a temporary key pair that can sign messages on behalf of the user’s master key.
  4. Your frontend receives this delegation and uses it to sign canister calls.
  5. The backend canister sees the user’s principal (derived from the delegation chain) as msg.caller.

Principal-per-app isolation: II derives a different principal for each frontend origin. A user logging into https://app-a.icp0.io gets a different principal than when logging into https://app-b.icp0.io, even with the same passkey. This prevents apps from correlating users across services.

Delegations expire. The frontend sets a maxTimeToLive when requesting the delegation (default recommendation: 8 hours). After expiry, the user must re-authenticate. The maximum allowed delegation lifetime is 30 days (2,592,000,000,000,000 nanoseconds).

Project setup

Configure icp.yaml for local Internet Identity

Add ii: true to your local network configuration. This tells icp-cli to deploy a local Internet Identity canister automatically:

networks:
  - name: local
    mode: managed
    ii: true

Install frontend packages

Terminal window
npm install @icp-sdk/auth @icp-sdk/core

Frontend integration

The AuthClient from @icp-sdk/auth handles the full login flow: opening the II popup, receiving the delegation, and managing session persistence.

Environment detection

Internet Identity runs at different URLs in local development versus mainnet. II uses a well-known frontend canister (uqzsh-gqaaa-aaaaq-qaada-cai) that you authenticate against. Detect the host to return the right URL:

import { AuthClient } from "@icp-sdk/auth/client";
import { HttpAgent, Actor } from "@icp-sdk/core/agent";
import { safeGetCanisterEnv } from "@icp-sdk/core/agent/canister-env";


// Read the ic_env cookie set by the asset canister or Vite dev server.
// Contains IC_ROOT_KEY and canister IDs: works in both local and production without
// environment branching. Available in browser contexts only; see note below for Node.js.
const canisterEnv = safeGetCanisterEnv();


function getIdentityProviderUrl() {
  const host = window.location.hostname;
  const isLocal =
    host === "localhost" ||
    host === "127.0.0.1" ||
    host.endsWith(".localhost");


  if (isLocal) {
    // icp-cli sets up a local alias: http://id.ai.localhost:8000
    return "http://id.ai.localhost:8000";
  }
  return "https://id.ai";
}

Login, logout, and session check

Create a single AuthClient instance on page load and reuse it for all operations:

// Create the auth client (once, on page load)
const authClient = await AuthClient.create();


// Check for existing session
const isAuthenticated = await authClient.isAuthenticated();
if (isAuthenticated) {
  const identity = authClient.getIdentity();
  // Restore session: create agent and actor with this identity
}


// Login
async function login() {
  return new Promise((resolve, reject) => {
    authClient.login({
      identityProvider: getIdentityProviderUrl(),
      maxTimeToLive: BigInt(8) * BigInt(3_600_000_000_000), // 8 hours
      onSuccess: () => {
        const identity = authClient.getIdentity();
        console.log("Logged in as:", identity.getPrincipal().toText());
        resolve(identity);
      },
      onError: (error) => {
        console.error("Login failed:", error);
        reject(error);
      },
    });
  });
}


// Logout
async function logout() {
  await authClient.logout();
  // Reset UI state or reload
}

Create an authenticated agent

After login, create an HttpAgent using the delegation identity. The agent signs all subsequent canister calls with the user’s delegated key:

async function createAuthenticatedActor(identity, canisterId, idlFactory) {
  const agent = await HttpAgent.create({
    identity,
    host: window.location.origin,
    rootKey: canisterEnv?.IC_ROOT_KEY,
  });


  return Actor.createActor(idlFactory, { agent, canisterId });
}

Node.js environments

safeGetCanisterEnv() reads the ic_env cookie set by the asset canister or Vite dev server (it only works in browser contexts. For Node.js scripts or tests connecting to a local replica, create the agent normally and call await agent.fetchRootKey() explicitly after creation. Never call fetchRootKey() against a mainnet endpoint) on mainnet the root key is pre-trusted, and fetching it at runtime exposes a man-in-the-middle risk.

Backend authentication

Your backend canister receives the caller’s principal automatically through the IC protocol. You do not pass the principal as a function argument: use msg.caller (Motoko) or ic_cdk::api::msg_caller() (Rust) to read it.

Reject anonymous callers

Any unauthenticated request uses the anonymous principal (2vxsx-fae). Reject it in protected endpoints:

import Principal "mo:core/Principal";
import Runtime "mo:core/Runtime";


persistent actor {
  func requireAuth(caller : Principal) : () {
    if (Principal.isAnonymous(caller)) {
      Runtime.trap("Anonymous principal not allowed.");
    };
  };


  public shared query ({ caller }) func whoAmI() : async Text {
    if (Principal.isAnonymous(caller)) {
      "anonymous"
    } else {
      Principal.toText(caller)
    };
  };


  public shared ({ caller }) func protectedAction() : async Text {
    requireAuth(caller);
    "Action performed by " # Principal.toText(caller)
  };
};

Rust: capture caller before await

In async update functions, bind the caller at the top of the function before any .await points. The current ic-cdk executor preserves the caller across await points, but capturing it early is a defensive practice that guards against future executor changes:

#[update]
async fn protected_async_action() -> String {
    let caller = require_auth(); // Capture before any await
    // Replace with your actual async canister call, e.g.:
    // ic_cdk::call::<_, (String,)>(some_canister_id, "some_method", ()).await
    format!("Action completed by {}", caller)
}

Local development

Start the local network and deploy. With ii: true in your icp.yaml, icp-cli deploys a local Internet Identity canister automatically:

Terminal window
icp network start
icp deploy

icp-cli pulls the mainnet II Wasm when deploying locally and registers a local alias so the II frontend is reachable at http://id.ai.localhost:8000. Use the getIdentityProviderUrl helper (shown in the environment detection section above) to point to this URL in local development.

To test authentication from the command line:

Terminal window
# Test as the default identity (authenticated)
icp canister call backend whoAmI


# Test as anonymous using --identity to avoid changing your global default
icp canister call backend protectedAction --identity anonymous
# Expected: Error containing "Anonymous principal not allowed"

For mainnet deployment, Internet Identity is already running: backend canister rdmx6-jaaaa-aaaaa-aaadq-cai and frontend canister uqzsh-gqaaa-aaaaq-qaada-cai (served at https://id.ai). Both IDs are identical on local replicas when ii: true is configured. Deploy only your own canisters:

Terminal window
icp deploy -e ic

Alternative origins

By default, each frontend origin produces a different user principal. If you serve your app from multiple domains (for example, migrating from <canister-id>.icp0.io to a custom domain), users would get different principals on each domain.

II now automatically handles the icp0.io vs ic0.app domain difference: you do not need to use derivationOrigin or ii-alternative-origins for that case. Use alternative origins only when you have two genuinely distinct custom domains that should share the same user principal.

To keep principals consistent across your own custom domains, configure alternative origins:

  1. On the primary origin (A): Create a file at .well-known/ii-alternative-origins listing the alternative domains:

    {
      "alternativeOrigins": ["https://www.yourcustomdomain.com"]
    }

    A maximum of 10 alternative origins can be listed. No trailing slashes or paths.

  2. Configure the asset canister to serve the .well-known directory. Add an .ic-assets.json5 in your frontend source:

    [
      {
        "match": ".well-known",
        "ignore": false
      },
      {
        "match": ".well-known/ii-alternative-origins",
        "headers": {
          "Access-Control-Allow-Origin": "*",
          "Content-Type": "application/json"
        },
        "ignore": false
      }
    ]

  3. On the alternative origin (B): Set the derivationOrigin in your login call to point back to the primary origin:

    authClient.login({
      identityProvider: "https://id.ai",
      derivationOrigin: "https://xxxxx.icp0.io", // primary origin A
      onSuccess: () => { /* ... */ },
    });

    The primary origin (A) does not need derivationOrigin: it is only required on alternative origins.

For full details, see the Internet Identity specification.

Common mistakes

  • Using the wrong II URL per environment: local development must point to http://id.ai.localhost:8000, mainnet to https://id.ai. Use the getIdentityProviderUrl helper (shown above) to switch based on hostname.
  • fetch “Illegal invocation” in bundled builds: always pass fetch: window.fetch.bind(window) to HttpAgent.create(). Without explicit binding, bundlers (Vite, webpack) extract fetch from window and call it without the correct this context.
  • Missing onSuccess/onError callbacks: authClient.login() requires both. Without them, login failures are silently swallowed.
  • Delegation expiry too long: the maximum is 30 days. Values above this are silently clamped, causing confusing session behavior. Use 8 hours for typical apps.
  • Passing principal as a string argument: the backend reads the caller automatically from the IC protocol. Do not pass it as a function parameter.
  • Using shouldFetchRootKey: true in browser code: pass rootKey: canisterEnv?.IC_ROOT_KEY from safeGetCanisterEnv() instead. shouldFetchRootKey: true fetches the root key from the replica at runtime, which lets a man-in-the-middle substitute a fake key on mainnet. For Node.js scripts targeting a local replica only, await agent.fetchRootKey() is acceptable: but never on mainnet.
  • Creating multiple AuthClient instances: create one on page load and reuse it. Multiple instances cause race conditions with session storage.

Next steps