apf_portal/apps/portal-bff/src/auth/auth.service.ts

import {
  ConfidentialClientApplication,
  CryptoProvider,
  type AuthenticationResult,
} from '@azure/msal-node';
import { Inject, Injectable } from '@nestjs/common';
import { PRE_AUTH_COOKIE_TTL_MS } from './auth.cookie';
import { AuthCodeFlowException } from './auth.errors';
import { ENTRA_CONFIG, type EntraConfig } from './entra-config.token';
import { MSAL_CLIENT } from './msal-client.token';

/**
 * Payload carried in the pre-auth cookie between `/auth/login` and
 * `/auth/callback`: the OIDC `state` (anti-CSRF nonce) and the PKCE
 * `codeVerifier` (secret the BFF sends back to Entra to prove it
 * was the one that asked for the code). `createdAt` lets the
 * callback reject cookies older than the flow's expected duration.
 */
export interface PreAuthPayload {
  readonly state: string;
  readonly codeVerifier: string;
  readonly createdAt: number;
}

export interface AuthCodeFlowStart {
  readonly authUrl: string;
  readonly preAuthPayload: PreAuthPayload;
}

/**
 * Identity resolved by `completeAuthCodeFlow`. Carries the claims
 * the rest of the BFF / SPA care about post-authentication:
 *   - `oid`: stable per-user object id inside the Entra tenant.
 *   - `tid`: tenant id the user authenticated against (for the
 *     dual-audience / multi-tenant logic to come).
 *   - `username` / `displayName`: surfaced in the UI.
 *   - `amr`: authentication methods reference — the array used by
 *     the BFF's MFA sanity-check (ADR-0011) and by `@RequireMfa`
 *     freshness checks once the session lands.
 *   - `roles`: Entra app roles assigned to this user on the BFF's
 *     app registration. Surfaced for the future `@RequireAdmin` /
 *     `@RequireRole(...)` guards (ADR-0020 admin module). Always
 *     present in the shape — an empty array means the user has no
 *     app role on this app registration, not that the claim was
 *     unparseable.
 */
export interface AuthenticatedUser {
  readonly oid: string;
  readonly tid: string;
  readonly username: string;
  readonly displayName: string;
  readonly amr: readonly string[];
  readonly roles: readonly string[];
}

/**
 * The minimum OIDC scopes — `openid` to get an ID token, `profile`
 * for the user's display name / preferred_username, `email` for the
 * email claim. Refresh tokens (`offline_access`) are deliberately
 * omitted in v1: sessions are short-lived (per ADR-0010) and the
 * user re-authenticates through Entra rather than the BFF refreshing
 * tokens behind their back.
 */
const SCOPES: readonly string[] = ['openid', 'profile', 'email'];

@Injectable()
export class AuthService {
  private readonly crypto = new CryptoProvider();

  constructor(
    @Inject(MSAL_CLIENT) private readonly msal: ConfidentialClientApplication,
    @Inject(ENTRA_CONFIG) private readonly config: EntraConfig,
  ) {}

  /**
   * First leg of the OIDC Authorization Code + PKCE flow. Builds
   * the URL the user gets redirected to (Entra's authorize
   * endpoint) and the pre-auth payload the controller stores in a
   * signed cookie so the callback can verify the round-trip later.
   *
   * MSAL Node owns the PKCE code generation (`CryptoProvider`) so
   * the verifier / challenge pair is canonical. The state nonce is
   * a fresh GUID per flow.
   *
   * `redirectUri` is passed by the caller (user-portal or
   * admin-portal controller) per ADR-0020 §"Sessions — distinct
   * from `portal-shell`". Same MSAL client, distinct redirect URI →
   * Entra routes the callback to the matching session.
   */
  async beginAuthCodeFlow(redirectUri: string): Promise<AuthCodeFlowStart> {
    const { verifier, challenge } = await this.crypto.generatePkceCodes();
    const state = this.crypto.createNewGuid();

    const authUrl = await this.msal.getAuthCodeUrl({
      redirectUri,
      scopes: [...SCOPES],
      state,
      codeChallenge: challenge,
      codeChallengeMethod: 'S256',
    });

    return {
      authUrl,
      preAuthPayload: {
        state,
        codeVerifier: verifier,
        createdAt: Date.now(),
      },
    };
  }

  /**
   * Second leg. Verifies the state round-trip, refuses cookies
   * older than the flow TTL, asks MSAL to exchange the auth code
   * for tokens using the stored PKCE verifier, then runs the
   * BFF-side sanity-checks ADR-0009 mandates (state, expiry, `amr`).
   *
   * MSAL Node performs the heavy ID-token validation itself
   * (signature against Entra's JWKS, issuer + audience, exp, nbf).
   * What this method adds on top is the application-layer policy:
   *   - anti-CSRF state binding,
   *   - flow-TTL replay protection,
   *   - MFA presence (`amr`) per ADR-0011.
   */
  async completeAuthCodeFlow(
    code: string,
    state: string,
    preAuth: PreAuthPayload,
    redirectUri: string,
    now: number = Date.now(),
  ): Promise<AuthenticatedUser> {
    if (state !== preAuth.state) {
      throw new AuthCodeFlowException({ kind: 'state-mismatch' });
    }
    if (now - preAuth.createdAt > PRE_AUTH_COOKIE_TTL_MS) {
      throw new AuthCodeFlowException({ kind: 'flow-expired' });
    }

    let result: AuthenticationResult | null;
    try {
      result = await this.msal.acquireTokenByCode({
        code,
        codeVerifier: preAuth.codeVerifier,
        redirectUri,
        scopes: [...SCOPES],
      });
    } catch (err) {
      throw new AuthCodeFlowException({
        kind: 'token-exchange-failed',
        cause: errorCause(err),
      });
    }

    if (!result) {
      throw new AuthCodeFlowException({
        kind: 'token-exchange-failed',
        cause: 'msal returned null result',
      });
    }

    return this.toAuthenticatedUser(result);
  }

  /**
   * Build the Entra RP-initiated logout URL. Hits the v2.0 logout
   * endpoint on the configured authority and passes the
   * `post_logout_redirect_uri` so the browser lands back on the SPA
   * after Entra finishes its side of the sign-out.
   *
   * We deliberately do *not* pass `id_token_hint` (which would skip
   * the account-picker on multi-account devices) because v1 doesn't
   * persist the id_token in the session yet — token persistence
   * lands with downstream API support per ADR-0014. The account
   * picker is the safer default in the interim.
   */
  buildLogoutUrl(postLogoutRedirectUri: string): string {
    const url = new URL(`${this.config.authority}/oauth2/v2.0/logout`);
    url.searchParams.set('post_logout_redirect_uri', postLogoutRedirectUri);
    return url.toString();
  }

  private toAuthenticatedUser(result: AuthenticationResult): AuthenticatedUser {
    const claims = result.idTokenClaims as Record<string, unknown>;

    // `amr` is an optional claim. Entra includes it when it has
    // explicit auth-method tracking to report (fresh interactive
    // sign-in with MFA, etc.) and may omit it for SSO / refresh
    // flows or when no Conditional Access policy requires the
    // method to be surfaced. The BFF surfaces whatever is there in
    // the audit log / future `@RequireMfa` guard — but it does NOT
    // reject the token on empty / missing `amr`. Per ADR-0011, MFA
    // enforcement is org-side via Conditional Access, and the
    // BFF's "sanity check" is that MSAL accepted the token at all
    // (signature + issuer + audience + exp validated). Sensitive
    // routes that need a fresh MFA event will assert it explicitly
    // through the `@RequireMfa({ freshness: 600 })` decorator once
    // it lands.
    const amr = Array.isArray(claims['amr'])
      ? (claims['amr'] as unknown[]).filter((v): v is string => typeof v === 'string')
      : [];

    // `roles` is an optional claim. Entra includes it when the user
    // has at least one app role assigned on the BFF's app
    // registration. The value is an array of the role `value`
    // strings declared in the registration manifest (e.g. `admin`).
    // No role assignment → claim is omitted entirely; we normalise
    // to an empty array so consumers can call `.includes('admin')`
    // unconditionally. Per ADR-0020, role-based authorisation is the
    // single source of authority for admin access; the AdminRoleGuard
    // will read this field on every `/api/admin/*` request.
    const roles = Array.isArray(claims['roles'])
      ? (claims['roles'] as unknown[]).filter((v): v is string => typeof v === 'string')
      : [];

    return {
      oid: requireString(claims['oid'], 'oid'),
      tid: requireString(claims['tid'], 'tid'),
      username:
        typeof claims['preferred_username'] === 'string'
          ? (claims['preferred_username'] as string)
          : (result.account?.username ?? ''),
      displayName:
        typeof claims['name'] === 'string'
          ? (claims['name'] as string)
          : (result.account?.name ?? ''),
      amr,
      roles,
    };
  }
}

function requireString(value: unknown, claim: string): string {
  if (typeof value !== 'string' || value === '') {
    throw new AuthCodeFlowException({
      kind: 'token-exchange-failed',
      cause: `id token missing required string claim: ${claim}`,
    });
  }
  return value;
}

function errorCause(err: unknown): string {
  if (err instanceof Error) {
    return err.message;
  }
  return String(err);
}