Internet Identity Authentication

What This Is

Internet Identity (II) is the Internet Computer’s native authentication system. Users authenticate into II-powered apps either with passkeys stored in their devices or through OpenID accounts (e.g., Google, Apple, Microsoft) – no usernames or passwords required. Each user gets a unique principal per app, preventing cross-app tracking.

Prerequisites

• @icp-sdk/auth (>= 7.0.0), @icp-sdk/core (>= 5.3.0) (AttributesIdentity was added in core v5.3.0)
• For the Motoko backend example: mo:identity-attributes >= 0.4.0 (mops) — the mixin that injects the two sign-in methods and verifies the bundle for you. It pulls in mo:core >= 2.5.0 and requires moc >= 1.6.0 for the include mixin.

Canister IDs

Canister	ID	URL	Purpose
Internet Identity (backend)	rdmx6-jaaaa-aaaaa-aaadq-cai		Manages user keys and authentication logic
Internet Identity (frontend)	uqzsh-gqaaa-aaaaq-qaada-cai	https://id.ai	Serves the II web app; identity provider URL points here

Mistakes That Break Your Build

1. Using the wrong II URL for the environment. The identity provider URL must point to the frontend canister (uqzsh-gqaaa-aaaaq-qaada-cai), not the backend. Mainnet uses https://id.ai/authorize. Local-only II (when ii: true is set in icp.yaml) uses http://id.ai.localhost:8000/authorize. Both canister IDs are well-known and identical on mainnet and local replicas — hardcode them rather than doing a dynamic lookup.

2. Forgetting /authorize in the identityProvider URL. In @icp-sdk/auth 7.x the URL is used verbatim; the client does not append /authorize for you (it did in 5.x). Passing https://id.ai opens the II home page in the popup and never returns a delegation — the login button appears to do nothing. Always include the /authorize path.

3. Setting delegation expiry too long. Maximum delegation expiry is 30 days (2_592_000_000_000_000 nanoseconds). Longer values are silently clamped, which causes confusing session behavior. Use 8 hours for normal apps, 30 days maximum for “remember me” flows.

4. Not awaiting signIn() or skipping the try/catch. authClient.signIn() returns a promise that rejects when the user closes the popup or authentication fails. Without await and a catch, those failures are silently swallowed.

5. Using shouldFetchRootKey or fetchRootKey() instead of the ic_env cookie. The ic_env cookie (set by the frontend canister or the Vite dev server) already contains the root key as IC_ROOT_KEY. Pass it via the rootKey option to HttpAgent.create() — this works in both local and production environments without environment branching. See the icp-cli skill’s references/binding-generation.md for the pattern. Never call fetchRootKey() — it fetches the root key from the replica at runtime, which lets a man-in-the-middle substitute a fake key on mainnet.

6. Getting 2vxsx-fae as the principal after sign-in. That is the anonymous principal – it means authentication silently failed. Common causes: wrong identityProvider URL passed to the AuthClient constructor (especially missing /authorize), an unhandled rejection from signIn(), or reading getIdentity() before signIn() resolved.

7. Passing principal as string to backend. The AuthClient gives you an Identity object. Backend canister methods receive the caller principal automatically via the IC protocol – you do not pass it as a function argument. The caller principal is available on the backend via shared(msg) { msg.caller } in Motoko or ic_cdk::api::msg_caller() in Rust. For backend access control patterns, see the canister-security skill.

8. Adding derivationOrigin or ii-alternative-origins to handle the official gateway domains (ic0.app, icp0.io, icp.net). Internet Identity canonicalizes all three official canister gateway domains to one form during delegation (icp.net is the current default for new frontend canisters, replacing icp0.io), so a canister served at any of them produces the same principal. Do not add derivationOrigin or ii-alternative-origins configuration to handle this — it will break authentication. If a user reports getting a different principal, the cause is almost certainly a different passkey or device, not the domain. (A genuine second origin — a custom domain — is a different situation and does need this configuration: see “Serving an app at more than one origin”.)

9. Generating the attribute nonce on the frontend. The nonce passed to requestAttributes MUST come from a backend canister call. A frontend-generated nonce defeats replay protection: the canister cannot verify that the bundle’s implicit:nonce is one it actually issued. Have the backend mint and return the nonce from _internet_identity_sign_in_start (the mo:identity-attributes mixin provides it in Motoko; you write it in Rust), and check it against the bundle’s implicit fields when the user calls _internet_identity_sign_in_finish.

10. Reading attribute data without verifying the signer. The IC verifies the signature, not the identity of the signer — any canister can produce a valid bundle. The trusted signer is rdmx6-jaaaa-aaaaa-aaadq-cai (Internet Identity). The check looks different per language:

    • Motoko: use the mo:identity-attributes mixin. include IdentityAttributes({ onVerified }) verifies the signer, origin, nonce, and freshness for you and runs onVerified only on a bundle that passes — configure trusted_attribute_signers and frontend_origins in icp.yaml (see “Backend: Reading Identity Attributes”). Don’t hand-roll the ICRC-3 decode or the signer check on top of mo:core/CallerAttributes unless you need behavior the library doesn’t cover.
    • Rust: there is no CDK wrapper yet. Always check msg_caller_info_signer() against the trusted issuer principal before reading msg_caller_info_data(). Skipping this lets an attacker canister forge attributes like email = "admin@you.com".

11. Substituting {tid} in the Microsoft scoped-key prefix. The microsoft OpenID provider URL is the literal string https://login.microsoftonline.com/{tid}/v2.0 — {tid} is part of the URL, not a tenant-ID placeholder you fill in. Bundle keys returned by scopedKeys({ openIdProvider: 'microsoft' }) look like openid:https://login.microsoftonline.com/{tid}/v2.0:email exactly, and the backend must look up that literal key. Replacing {tid} with a tenant GUID will silently miss every attribute lookup.

12. Treating email as verified. email and verified_email are distinct keys.

    • email is the raw email string from the user’s II-linked account. II does not check it. Treat it as user-supplied input.
    • verified_email is the same email as email, but only present when the source OpenID provider (e.g., Google) marked it as verified and II surfaced that signal through. Use verified_email for any access gating (admin allowlists, capability checks). Use email only for soft uses like contact info or mailing lists. Request both for fallback behaviour: both are returned with the same value when the source provider marked the email as verified, only email when it didn’t.

13. Serving /.well-known/ii-app-metadata on the wrong origin, or without CORS. II reads app metadata from the origin identities are derived for — your validated derivationOrigin when the request sets one, the request’s own origin otherwise. A document published only on the alternative origin the user visits is never fetched. The document and the logo it points at are both read cross-origin, and they fail differently: without Access-Control-Allow-Origin on the document none of your metadata is used (II falls back to its curated entry if it ships one for your app, and to your origin alone otherwise), while an unreadable logo costs you the logo alone — the name and description still render. See “Showing your app’s name, description, and logo on the sign-in screen”.

14. Assuming a bad field in ii-app-metadata is just dropped, or confusing a rejected logo with a rejected document. One field that fails validation invalidates the whole document: none of your metadata is applied, not just the offending field (II then falls back to its curated entry if it ships one for your app, and to your origin alone otherwise). name is capped at 40 Unicode code points and description at 120, counted on the value as served. logo straddles the two failure modes — a URL that is not on the same origin as the document fails document validation and takes the whole document down with it, and that includes your own canister on a sibling gateway domain, since II may fetch the document from any of ic0.app, icp0.io, or icp.net (write the URL relative) — while an SVG (image/svg+xml is not accepted; serve a raster copy), an oversized image, or one that cannot be fetched or decoded costs you the logo alone.

Using II during local development

Default: use mainnet II from your local network. Starting with icp-cli >= 0.2.4, the local network (pocket-ic, launched by icp-cli-network-launcher) is configured to trust the mainnet subnet’s BLS signatures. Delegations signed by https://id.ai are accepted by your local replica, so both the sign-in flow and authenticated calls to a locally-deployed backend just work — no extra config in icp.yaml, no local II canister to manage, and the UI is the real one your users will see.

Point your frontend at https://id.ai/authorize unconditionally and you’re done.

Fallback: deploy II locally

Only use this if you need fully-offline dev or want to test against a specific II build. Add ii: true to the local network in your icp.yaml:

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

This deploys the II canisters automatically when the local network is started. The II frontend will be available at http://id.ai.localhost:8000, and the identityProvider URL becomes http://id.ai.localhost:8000/authorize. No canister entry is needed in your project — II is not part of your project’s canisters. For the full icp.yaml canister configuration, see the icp-cli and static-site skills.

Frontend: Vanilla JavaScript/TypeScript Sign-In Flow

This is framework-agnostic. Adapt the DOM manipulation to your framework.

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 frontend canister or Vite dev server).
// Contains the root key and canister IDs — works in both local and production.
const canisterEnv = safeGetCanisterEnv();

// Construct once — identityProvider (and optionally derivationOrigin or
// openIdProvider for one-click sign-in: 'google' | 'apple' | 'microsoft')
// are configured at construction time, not per sign-in. Always include the
// `/authorize` path — the client uses the URL verbatim in 7.x.
//
// Use mainnet II even from local dev: pocket-ic (icp-cli >= 0.2.4) trusts
// mainnet subnet signatures. Override to http://id.ai.localhost:8000/authorize
// only if you have `ii: true` in icp.yaml and want fully-offline dev.
const authClient = new AuthClient({
  identityProvider: "https://id.ai/authorize",
});

// Sign in: signIn() returns the new Identity directly and rejects if the user
// closes the popup or authentication fails.
async function signIn() {
  try {
    const identity = await authClient.signIn({
      maxTimeToLive: BigInt(8) * BigInt(3_600_000_000_000), // 8 hours in nanoseconds
    });
    console.log("Signed in as:", identity.getPrincipal().toText());
    return identity;
  } catch (error) {
    console.error("Sign-in failed:", error);
    throw error;
  }
}

// Sign out
async function signOut() {
  await authClient.signOut();
  // Optionally reload or reset UI state
}

// Create an authenticated agent and actor.
// Uses rootKey from the ic_env cookie — no shouldFetchRootKey or environment branching needed.
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 });
}

// Initialization — wraps async setup in a function so this code works with
// any bundler target (Vite defaults to es2020 which lacks top-level await).
async function init() {
  // isAuthenticated() is sync; getIdentity() is async.
  if (authClient.isAuthenticated()) {
    const identity = await authClient.getIdentity();
    const actor = await createAuthenticatedActor(identity, canisterId, idlFactory);
    // Use actor to call backend methods
  }
}

init();

Serving an app at more than one origin

II derives a principal per origin, so https://<canister-id>.icp.net and https://shop.example.com are two different users to the same person. To keep one account per person, pick one origin as the derivation origin and list the others as alternative origins.

Pick the canister address as the derivation origin. Custom domains can be changed or dropped; the canister address cannot.

1. The alternative origin passes derivationOrigin. The primary origin does not — it is only set on the other origins.

const authClient = new AuthClient({
  identityProvider: "https://id.ai/authorize",
  derivationOrigin: "https://<canister-id>.icp.net",
});

2. The derivation origin’s canister serves the list. Put the file at dir/.well-known/ii-alternative-origins:

{ "alternativeOrigins": ["https://shop.example.com"] }

A maximum of 10 alternative origins can be listed. Entries are origins — no trailing slashes and no paths.

3. With @dfinity/static-site, add a _headers block. .well-known/ is uploaded automatically, but this file has no extension, so its media type is not application/json, and the certified-assets canister sets no CORS header by default. II needs both:

/.well-known/ii-alternative-origins
  Content-Type: application/json
  Access-Control-Allow-Origin: *

Do not reach for .ic-assets.json5 — that is the legacy asset canister’s config file, and the static-site recipe does not read or even upload it, so the headers would silently never apply. See the static-site skill.

Order matters. Pin the derivation origin before an origin has users. Repointing an origin that has already collected sign-ins orphans every account made under it.

Showing your app’s name, description, and logo on the sign-in screen

By default the II sign-in screens identify your app by its origin alone. To have II also show a name, a short tagline, and a logo, serve a JSON document at /.well-known/ii-app-metadata. This is permissionless — there is no list to join and no approval step — and it supersedes the curated entry II still ships for a small set of known apps.

{
  "name": "Example App",
  "description": "A short tagline shown on the sign-in screen",
  "logo": "/logo.png"
}

Publish it on the derivation origin, not on every origin. II fetches the document from the origin identities are derived for: your derivationOrigin once validated when the auth request sets one, and the request’s own origin otherwise. Publish it once on the derivation origin and every alternative origin that origin certifies is presented with the same name, description, and logo — there is nothing to keep in sync. A copy served only on the alternative origin the user actually visits is never read.

All three fields are optional and unknown fields are ignored, so a document stays valid as II adds fields. The rules that decide whether yours is used:

• name is at most 40 characters and description at most 120, counted in Unicode code points on the value as served — before whitespace collapsing, which is display-only and never rescues an over-long value.
• Each field must contain at least one visible character. A field holding only whitespace or invisible characters is rejected, not treated as absent.
• Control characters (other than the ASCII whitespace \t, \n, \v, \f, \r), U+FEFF, and the bidirectional embeddings and overrides U+202A–U+202E are rejected: they can make rendered text read differently from what it contains. The characters mixed-direction and non-Latin names legitimately need are accepted — the marks U+200E, U+200F, U+061C, the isolates U+2066–U+2069, and the zero-width characters U+200B–U+200D — but isolates must be balanced: a field must close every isolate it opens and close none it did not open.
• One bad field invalidates the whole document, which is then ignored; the offending field is not dropped on its own. None of your document is applied rather than half of it; II then falls back to its curated entry if it ships one for your app, and to your origin alone otherwise. II logs which field is at fault to the browser console — check the console on the sign-in screen when metadata does not appear. A document carrying no field II recognises is likewise ignored; a valid one replaces the curated fallback entry wholesale.
• logo must be a raster image URL on the same origin as the document (relative URLs resolve against it), served as image/png, image/jpeg, image/webp, image/gif, or image/avif, at most 1 MiB, and at most 4096 pixels per axis. image/svg+xml is rejected — serve a rasterised copy of a vector logo. II downloads the image (it is never hotlinked), redraws it at up to 512 pixels on its longest side (an animated image is flattened to its first frame), and renders that copy, so a roughly square PNG or WebP of about 512 pixels is the right thing to ship. Write the URL relative (/logo.png): II normalizes a canister gateway origin onto ic0.app and tries its icp0.io and icp.net twins in turn, so the document may be fetched from a sibling gateway domain of the same canister — and the same-origin check below runs against whichever one answered. An absolute URL pinned to one of them is cross-origin at the other two.
• Only the shape of logo — a non-empty URL on the document’s own origin — is part of the validation above. Once it passes, a logo that cannot be fetched or decoded, or that breaks the content-type, size, or dimension rules, costs you the logo alone: the name and description still render.
• The document must not exceed 8 KiB, must be answered with 200, and must not redirect (II follows redirects for neither the document nor the logo). II requests it without credentials and gives up after 10 seconds.

Both the document and the logo are read cross-origin, so both need CORS headers. With @dfinity/static-site, extend the same _headers file used for alternative origins:

/.well-known/ii-app-metadata
  Content-Type: application/json
  Access-Control-Allow-Origin: *

/logo.png
  Access-Control-Allow-Origin: *

.well-known/ is uploaded automatically, but this file has no extension either, so set its media type with the bare Content-Type: form. As above, .ic-assets.json5 is the legacy asset canister’s config and is not read by this recipe.

Missing, unreachable, or invalid metadata never blocks sign-in: the screen falls back to the curated entry, and to your origin alone otherwise. And because the file is under the sole control of the origin serving it, publishing it verifies nothing about your app — II keeps displaying the origin alongside whatever you provide, since the origin is the part users can actually check.

The normative rules, including a JSON Schema you can validate the document against in CI, are in the App metadata section of the Internet Identity specification. The schema expresses every rule above except isolate balancing.

Frontend: Requesting Identity Attributes

When the backend needs more than the user’s principal (e.g., a verified email), Internet Identity can return signed attributes alongside the delegation. The flow is a two-method handshake on the backend: _internet_identity_sign_in_start mints a nonce, and _internet_identity_sign_in_finish verifies the bundle. In Motoko the mo:identity-attributes mixin provides both methods; in Rust you implement them by hand (see “Backend: Reading Identity Attributes”). The frontend below is identical against either backend.

Available attribute keys

requestAttributes({ keys, nonce }) requires both keys and nonce: there is no default key set, you must pass an explicit list. The keys II currently accepts are:

Key	What it IS	When to use
name	The user’s display name from the II-linked account.	Personalisation in the UI.
email	The raw email string from the user’s II-linked account. II does not check it. Treat as user-supplied input.	Mailing-list signups, contact email, anything where you don’t gate access on the email.
verified_email	The same email as email, but only present when the source OpenID provider (e.g., Google) marked it as verified and II surfaced that signal. The provider’s verification is what makes it trustworthy.	Access gating (e.g. an admin allowlist by email). Treat this as the only trustworthy email for authorisation.

Request both email and verified_email if you want fallback behaviour: when the source provider marked the email as verified, both keys are present with the same value; when it didn’t, only email is returned.

scopedKeys({ openIdProvider, keys? }) rewrites the keys above into provider-scoped keys of the form openid:<provider-url>:<key>, so II returns the values from the linked OpenID account directly (with implicit consent, no extra prompt). Provider URLs:

Provider	URL prefix in the bundle keys
'google'	openid:https://accounts.google.com:
'apple'	openid:https://appleid.apple.com:
'microsoft'	openid:https://login.microsoftonline.com/{tid}/v2.0: (the {tid} part is literal: do not substitute a tenant ID into it)

The keys argument to scopedKeys is optional and defaults to ['name', 'email', 'verified_email']. (requestAttributes itself has no default; the scopedKeys helper just builds the array you then pass to it.) Examples:

• scopedKeys({ openIdProvider: 'google' }) → ['openid:https://accounts.google.com:name', 'openid:https://accounts.google.com:email', 'openid:https://accounts.google.com:verified_email']
• scopedKeys({ openIdProvider: 'google', keys: ['email'] }) → ['openid:https://accounts.google.com:email']

The same email vs verified_email rule applies to scoped keys: use the verified variant when the email gates access.

import { AuthClient } from "@icp-sdk/auth/client";
import { AttributesIdentity } from "@icp-sdk/core/identity";
import { HttpAgent, Actor } from "@icp-sdk/core/agent";
import { Principal } from "@icp-sdk/core/principal";

const II_PRINCIPAL = "rdmx6-jaaaa-aaaaa-aaadq-cai";

// `idl` and `canisterId` are your backend's interface factory and ID. The
// backend exposes _internet_identity_sign_in_start / _internet_identity_sign_in_finish.
async function signInWithAttributes(authClient, canisterId, idl) {
  // Anonymous handle, used only to mint the nonce.
  const anonymousAgent = await HttpAgent.create();
  const anonymousActor = Actor.createActor(idl, { agent: anonymousAgent, canisterId });

  // Mint the nonce, sign in, and request attributes in parallel. Passing the
  // nonce as a promise lets requestAttributes start before it resolves, so the
  // user still sees a single Internet Identity interaction. A frontend-generated
  // nonce would defeat replay protection — see Mistake #9.
  const noncePromise = anonymousActor._internet_identity_sign_in_start();
  const signInPromise = authClient.signIn({
    maxTimeToLive: BigInt(8) * BigInt(3_600_000_000_000), // 8 hours in nanoseconds
  });
  const attributesPromise = authClient.requestAttributes({
    keys: ["name", "verified_email"], // library reads verified_email for its email field
    nonce: noncePromise,
  });

  const identity = await signInPromise;
  const attributes = await attributesPromise;

  // Wrap the identity so the signed bundle travels as sender_info on each call.
  const verifiedAgent = await HttpAgent.create({
    identity: new AttributesIdentity({
      inner: identity,
      attributes,
      // The Internet Identity backend canister is the trusted attribute signer.
      signer: { canisterId: Principal.fromText(II_PRINCIPAL) },
    }),
  });
  const verifiedActor = Actor.createActor(idl, { agent: verifiedAgent, canisterId });

  // The backend verifies signer, origin, nonce, and freshness, then runs its
  // onVerified logic. Returns { ok } on success, { err } otherwise.
  const result = await verifiedActor._internet_identity_sign_in_finish();
  if ("err" in result) {
    throw new Error(`Attribute verification failed: ${JSON.stringify(result.err)}`);
  }
  return identity;
}

Each signed bundle carries three implicit fields the backend MUST verify:

• implicit:nonce — matches a single-use nonce the canister issued and consumes on sign-in, so a captured bundle cannot be replayed.
• implicit:origin — the frontend origin, preventing a malicious dapp from forwarding bundles to a different backend.
• implicit:issued_at_timestamp_ns — issuance time, letting the canister reject stale bundles even when the nonce is still valid.

For OpenID one-click sign-in, scope the attributes to the provider with the scopedKeys helper: authentication and attribute sharing happen in a single step (no extra prompt). Construct the client with openIdProvider, then swap the keys for the scoped forms. The rest of signInWithAttributes above is unchanged.

import { AuthClient, scopedKeys } from "@icp-sdk/auth/client";

const authClient = new AuthClient({
  identityProvider: "https://id.ai/authorize",
  openIdProvider: "google",
});

// In signInWithAttributes, request the Google-scoped keys instead. They arrive
// in the bundle as e.g. "openid:https://accounts.google.com:verified_email",
// and the mo:identity-attributes library maps them onto the same name/email fields.
const attributesPromise = authClient.requestAttributes({
  keys: scopedKeys({ openIdProvider: "google", keys: ["name", "verified_email"] }),
  nonce: noncePromise,
});

Backend: Reading Identity Attributes

The backend exposes two methods the frontend calls: _internet_identity_sign_in_start (mints a nonce) and _internet_identity_sign_in_finish (verifies the wrapped bundle and runs your logic). The checks are the same in both languages — the bundle must be signed by a trusted signer, its implicit:origin must be one you allow, its implicit:issued_at_timestamp_ns must be fresh, and its implicit:nonce must be one you issued and have not consumed — but Motoko gets them from a library and Rust does them by hand.

Always verify the signer. The IC checks that the bundle is signed; it does not check who signed it. Any canister can produce a valid bundle. The trusted signer for II is rdmx6-jaaaa-aaaaa-aaadq-cai.

Motoko: the mo:identity-attributes mixin

Add the library to mops.toml:

[dependencies]
identity-attributes = "0.4.1"
core                = "2.5.0"

[toolchain]
moc = "1.6.0"

include IdentityAttributes({ onVerified }) injects both sign-in methods and runs your onVerified callback only on a bundle that passes every check. It resolves the bundle to { name : ?Text; email : ?Text; sso : ?Text } — email comes from the verified_email key (or its openid: / sso: scoped form), which is why the frontend requests verified_email. sso is the matched trusted domain when name/email came from sso: keys, otherwise null.

import IdentityAttributes "mo:identity-attributes";
import Map "mo:core/Map";
import Principal "mo:core/Principal";

persistent actor {
  type Profile = { name : ?Text; email : ?Text; sso : ?Text };

  let profiles = Map.empty<Principal, Profile>();

  // Injects _internet_identity_sign_in_start / _internet_identity_sign_in_finish.
  // onVerified runs only on a bundle that passed the signer, origin, nonce, and
  // freshness checks.
  include IdentityAttributes({
    onVerified = func(caller, attrs) {
      profiles.add(caller, attrs);
    };
  });

  public query func getProfile(caller : Principal) : async ?Profile {
    profiles.get(caller)
  };
};

Configure the env vars in icp.yaml so icp deploy sets them on the canister:

canisters:
  - name: backend
    settings:
      environment_variables:
        # II backend principal (required). List your local II principal too if tests run against it.
        trusted_attribute_signers: "rdmx6-jaaaa-aaaaa-aaadq-cai"
        # Allowed frontend origins, comma-separated (required).
        frontend_origins: "https://your-app.icp.net"
        # Trusted SSO domains, comma-separated (optional; omit to reject all sso:* keys).
        trusted_sso_domains: "your-org.com"

If trusted_attribute_signers is unset the bundle is rejected as untrusted; if frontend_origins is unset _internet_identity_sign_in_finish returns #err(#FrontendOriginsNotConfigured). Both are the right behavior: an unconfigured canister must not trust attribute bundles. The method returns Result<(), IdentityAttributesError>; the error variants (#NoAttributes, #MalformedCandid, #FrontendOriginMismatch, #Stale, #UnknownNonce, #AmbiguousAttribute, #UntrustedSsoSource, #MixedSsoSources) tell the frontend whether to retry with a fresh nonce or surface a bug.

Rust: implement the same two methods by hand

There is no CDK wrapper yet (ic-cdk >= 0.20.1), so write the two methods yourself. _internet_identity_sign_in_start mints a nonce and stores it; _internet_identity_sign_in_finish checks the signer with msg_caller_info_signer(), decodes the ICRC-3 Value::Map from msg_caller_info_data(), and verifies origin, freshness, and the nonce before reading attributes. This mirrors what the Motoko library does internally. The bundle’s entries are:

• implicit:nonce (Blob) — must match a nonce this canister minted and not yet consumed.
• implicit:origin (Text) — must match a trusted frontend origin.
• implicit:issued_at_timestamp_ns (Nat) — reject if outside your freshness window.
• The attribute keys you requested (e.g. "verified_email", or the openid: / sso: scoped form).

use candid::{decode_one, CandidType, Deserialize, Principal};
use ic_cdk::api::{msg_caller, msg_caller_info_data, msg_caller_info_signer, time};
use ic_cdk::update;
use std::cell::RefCell;
use std::collections::HashSet;

const II_PRINCIPAL: &str = "rdmx6-jaaaa-aaaaa-aaadq-cai";
const TRUSTED_ORIGIN: &str = "https://your-app.icp.net";
const FRESHNESS_NS: u64 = 300_000_000_000; // 5 minutes

thread_local! {
    // Nonces issued by sign_in_start and consumed by sign_in_finish.
    static PENDING_NONCES: RefCell<HashSet<Vec<u8>>> = RefCell::new(HashSet::new());
}

// Mirrors the mo:identity-attributes Result so the frontend's `"err" in result`
// check works against either backend.
#[derive(CandidType)]
enum SignInResult {
    #[serde(rename = "ok")]
    Ok,
    #[serde(rename = "err")]
    Err(String),
}

#[derive(CandidType, Deserialize)]
enum Icrc3Value {
    Nat(candid::Nat),
    Int(candid::Int),
    Blob(Vec<u8>),
    Text(String),
    Array(Vec<Icrc3Value>),
    Map(Vec<(String, Icrc3Value)>),
}

fn lookup_text<'a>(entries: &'a [(String, Icrc3Value)], key: &str) -> Option<&'a str> {
    entries.iter().find_map(|(k, v)| match v {
        Icrc3Value::Text(s) if k == key => Some(s.as_str()),
        _ => None,
    })
}

fn lookup_blob<'a>(entries: &'a [(String, Icrc3Value)], key: &str) -> Option<&'a [u8]> {
    entries.iter().find_map(|(k, v)| match v {
        Icrc3Value::Blob(b) if k == key => Some(b.as_slice()),
        _ => None,
    })
}

fn lookup_nat<'a>(entries: &'a [(String, Icrc3Value)], key: &str) -> Option<&'a candid::Nat> {
    entries.iter().find_map(|(k, v)| match v {
        Icrc3Value::Nat(n) if k == key => Some(n),
        _ => None,
    })
}

// Mint a fresh nonce. The frontend calls this anonymously before sign-in.
#[update]
async fn _internet_identity_sign_in_start() -> Vec<u8> {
    let nonce = ic_cdk::management_canister::raw_rand()
        .await
        .expect("raw_rand failed");
    PENDING_NONCES.with_borrow_mut(|n| n.insert(nonce.clone()));
    nonce
}

// Runs every check the mo:identity-attributes mixin runs internally.
fn verified_attributes() -> Result<Vec<(String, Icrc3Value)>, String> {
    // 1. Trusted signer: the IC checks the signature, not who signed it.
    let trusted = Principal::from_text(II_PRINCIPAL).unwrap();
    if msg_caller_info_signer() != Some(trusted) {
        return Err("Untrusted attribute signer".to_string());
    }

    // 2. Decode the bundle as an ICRC-3 Value::Map.
    let value: Icrc3Value =
        decode_one(&msg_caller_info_data()).map_err(|_| "Malformed attribute bundle".to_string())?;
    let Icrc3Value::Map(entries) = value else {
        return Err("Expected attribute map".to_string());
    };

    // 3. Origin must be one we allow.
    let origin = lookup_text(&entries, "implicit:origin").ok_or("Missing origin")?;
    if origin != TRUSTED_ORIGIN {
        return Err(format!("Untrusted frontend origin: {origin}"));
    }

    // 4. Bundle must be fresh.
    let issued_at: u64 = lookup_nat(&entries, "implicit:issued_at_timestamp_ns")
        .ok_or("Missing timestamp")?
        .0
        .clone()
        .try_into()
        .map_err(|_| "Timestamp out of range".to_string())?;
    if time() > issued_at + FRESHNESS_NS {
        return Err("Bundle too old".to_string());
    }

    // 5. Nonce must be one we issued and have not consumed yet.
    let nonce = lookup_blob(&entries, "implicit:nonce").ok_or("Missing nonce")?;
    if !PENDING_NONCES.with_borrow_mut(|n| n.remove(nonce)) {
        return Err("Unknown or already-consumed nonce".to_string());
    }

    Ok(entries)
}

#[update]
fn _internet_identity_sign_in_finish() -> SignInResult {
    let entries = match verified_attributes() {
        Ok(entries) => entries,
        Err(e) => return SignInResult::Err(e),
    };

    // Your app logic. verified_email gates access — see Mistake #12.
    let Some(email) = lookup_text(&entries, "verified_email") else {
        return SignInResult::Err("Missing verified_email".to_string());
    };
    let caller = msg_caller();
    let name = lookup_text(&entries, "name");
    // e.g. persist a profile keyed by `caller` here.
    let _ = (caller, email, name);

    SignInResult::Ok
}

Backend: Access Control

Backend access control (anonymous principal rejection, role guards, caller binding in async functions) is not II-specific — the same patterns apply regardless of authentication method. See the canister-security skill for complete Motoko and Rust examples.

5.x API notes

If you are pinned to @icp-sdk/auth 5.x, the same flow uses a different (callback-based) API:

• await AuthClient.create({...}) instead of new AuthClient({...})
• identityProvider passed per-call to login({...}) rather than at construction
• authClient.login({ onSuccess, onError }) — promise wrapper required around it
• authClient.logout() instead of authClient.signOut()
• await authClient.isAuthenticated() (async) instead of sync
• authClient.getIdentity() (sync) instead of async
• 5.x auto-appends /authorize to the identityProvider URL, so you can pass just https://id.ai. In 7.x the path is required.
• No requestAttributes / AttributesIdentity support — the identity-attributes flow above requires 7.x.

Upgrade to 7.x when you can — the promise-based API is harder to misuse and the callback variant has been removed.