/**
 * Signs and verifies the server's four kinds of token (session, media,
 * password reset and unsubscribe), all HS256 with the one key. Only a token's signature and
 * expiry are checked here: `lib/auth.ts` checks a session or media token
 * against its user.
 *
 * It also signs and verifies the tokens instances exchange for federated
 * sign-in (identity tokens and events), EdDSA with the instance's Ed25519 key,
 * whose public half other instances fetch: `instance-key-service.ts` keeps
 * the key, and the federation services pass it in.
 *
 * Importing this module throws when `GERBEUR_JWT_SECRET` is unset.
 *
 * @module
 */

import {
  createLocalJWKSet,
  exportJWK,
  generateKeyPair,
  importJWK,
  type JSONWebKeySet,
  type JWK,
  type JWTPayload,
  jwtVerify,
  SignJWT,
} from "@panva/jose";

import {
  type AuthPayload,
  isAuthPayload,
  isPasswordResetPayload,
  type PasswordResetPayload,
} from "../model/auth.ts";
import { JWT_SECRET } from "../config.ts";
import { type DigestCadence, isDigestCadence } from "../../shared/settings.ts";
import type { AuthTokens, Role } from "../../shared/user.ts";

if (!JWT_SECRET) {
  throw new Error(
    "GERBEUR_JWT_SECRET environment variable is required. Generate one with: openssl rand -hex 32",
  );
}
const JWT_KEY = new TextEncoder().encode(JWT_SECRET);

/**
 * Signs a password reset token for `userId`, valid for an hour. The token
 * isn't single-use by itself: `requestPasswordReset` stores it, and
 * `resetPassword` marks it used.
 */
export async function createPasswordResetToken(
  userId: string,
): Promise<string> {
  return await new SignJWT({ purpose: "password-reset", userId })
    .setProtectedHeader({ alg: "HS256" })
    // Two requests in the same second would otherwise sign the same token,
    // which is the primary key of `password_reset_tokens`.
    .setJti(crypto.randomUUID())
    .setExpirationTime("1h")
    .sign(JWT_KEY);
}

/**
 * Returns the payload of a password reset token, or null when it is invalid,
 * expired or another kind of token. Whether it was used is the caller's check.
 */
export async function verifyPasswordResetToken(
  token: string,
): Promise<PasswordResetPayload | null> {
  try {
    const { payload } = await jwtVerify(token, JWT_KEY);
    if (!isPasswordResetPayload(payload)) return null;
    return payload as PasswordResetPayload;
  } catch {
    return null;
  }
}

/**
 * Signs a session token carrying `payload`, valid for 7 days.
 * {@link issueTokens} issues it on sign-up, sign-in and password change, and
 * the client sends it as a bearer token.
 */
export async function createJWT(
  payload: Omit<AuthPayload, "exp">,
): Promise<string> {
  return await new SignJWT(payload)
    .setProtectedHeader({ alg: "HS256" })
    // A password change revokes tokens by comparing `iat` with
    // `users.tokens_valid_after`.
    .setIssuedAt()
    .setExpirationTime("7d")
    .sign(JWT_KEY);
}

/**
 * Returns a new session token and media token for `user`, as sign-up
 * (`POST /api/users`), sign-in (`POST /api/sessions`) and a password change
 * (`PUT /api/me/password`) answer them.
 */
export async function issueTokens(
  user: { id: string; username: string; role: Role },
): Promise<AuthTokens> {
  return {
    token: await createJWT({
      userId: user.id,
      username: user.username,
      role: user.role,
    }),
    mediaToken: await createMediaToken(user.id),
  };
}

/**
 * Signs a media token for `userId`, valid for 7 days. Media elements can't send
 * an `Authorization` header, so the URLs of private files, thumbnails and
 * playlist covers carry this token as `?token=`, where it lands in logs and
 * browser history. It only reads media, so a leaked URL doesn't expose the
 * account.
 */
export async function createMediaToken(userId: string): Promise<string> {
  return await new SignJWT({ purpose: "media", userId })
    .setProtectedHeader({ alg: "HS256" })
    .setIssuedAt()
    .setExpirationTime("7d")
    .sign(JWT_KEY);
}

/**
 * Returns the user id and issue time of a media token, or null when it is
 * invalid, expired or another kind of token.
 */
export async function verifyMediaToken(
  token: string,
): Promise<{ userId: string; iat?: number } | null> {
  try {
    const { payload } = await jwtVerify(token, JWT_KEY);
    if (payload.purpose !== "media" || typeof payload.userId !== "string") {
      return null;
    }
    return { userId: payload.userId, iat: payload.iat };
  } catch {
    return null;
  }
}

/**
 * Returns the payload of a session token whose signature and expiry are valid,
 * or null. Call `authenticate` (`lib/auth.ts`) instead, which also checks the
 * token against its user.
 */
export async function verifyJWT(token: string): Promise<AuthPayload | null> {
  try {
    const { payload } = await jwtVerify(token, JWT_KEY);

    if (!isAuthPayload(payload)) {
      return null;
    }

    return payload;
  } catch (err) {
    console.error("JWT verification failed:", err);
    return null;
  }
}

/**
 * Signs the token of a digest's unsubscribe link, which turns off `cadence`
 * for `userId` without signing in. It never expires, so the link in an old
 * email still works; turning the digest back on doesn't revoke it, which is
 * harmless since all it can do is turn it off again.
 */
export async function createUnsubscribeToken(
  userId: string,
  cadence: DigestCadence,
): Promise<string> {
  return await new SignJWT({ purpose: "unsubscribe", userId, cadence })
    .setProtectedHeader({ alg: "HS256" })
    .sign(JWT_KEY);
}

/**
 * Returns the user and cadence of an unsubscribe token, or null when it is
 * invalid or another kind of token.
 */
export async function verifyUnsubscribeToken(
  token: string,
): Promise<{ userId: string; cadence: DigestCadence } | null> {
  try {
    const { payload } = await jwtVerify(token, JWT_KEY);
    if (
      payload.purpose !== "unsubscribe" || typeof payload.userId !== "string" ||
      !isDigestCadence(payload.cadence)
    ) {
      return null;
    }
    return { userId: payload.userId, cadence: payload.cadence };
  } catch {
    return null;
  }
}

/**
 * The algorithm of the tokens instances exchange: Ed25519 signatures, short
 * and fast, with no parameters to get wrong.
 */
const INSTANCE_ALG = "EdDSA";

/**
 * An instance's key, ready to sign, from {@link importInstanceKey}. Its
 * `kid` names the public half in the instance's JWKS.
 */
export interface InstanceSigningKey {
  kid: string;
  privateKey: CryptoKey;
}

/**
 * Returns a new Ed25519 key pair as JWKs, with a random `kid`, for
 * `instance-key-service.ts` to store when the instance has none. The public
 * JWK carries the `kid`, `alg` and `use` a JWKS entry needs.
 */
export async function generateInstanceKey(): Promise<
  { kid: string; privateJwk: JWK; publicJwk: JWK }
> {
  const { privateKey, publicKey } = await generateKeyPair(INSTANCE_ALG, {
    crv: "Ed25519",
    extractable: true,
  });
  const kid = crypto.randomUUID();
  return {
    kid,
    privateJwk: await exportJWK(privateKey),
    publicJwk: {
      ...await exportJWK(publicKey),
      kid,
      alg: INSTANCE_ALG,
      use: "sig",
    },
  };
}

/** Returns the stored private JWK `privateJwk` as a key to sign with. */
export async function importInstanceKey(
  kid: string,
  privateJwk: JWK,
): Promise<InstanceSigningKey> {
  const privateKey = await importJWK(privateJwk, INSTANCE_ALG);
  if (!(privateKey instanceof CryptoKey)) {
    throw new Error("The instance key is not an asymmetric key");
  }
  return { kid, privateKey };
}

/**
 * Signs `payload` with the instance's key, from `issuer` (this instance) to
 * `audience` (the instance it is for), valid for `expiresIn`, with a random
 * `jti`. The identity tokens of `POST /api/oauth/tokens` and the events of
 * the outbox are signed with it.
 */
export async function signInstanceToken(
  payload: JWTPayload,
  key: InstanceSigningKey,
  claims: { issuer: string; audience: string; subject: string },
  expiresIn: string,
): Promise<string> {
  return await new SignJWT(payload)
    .setProtectedHeader({ alg: INSTANCE_ALG, kid: key.kid })
    .setIssuer(claims.issuer)
    .setAudience(claims.audience)
    .setSubject(claims.subject)
    .setIssuedAt()
    .setJti(crypto.randomUUID())
    .setExpirationTime(expiresIn)
    .sign(key.privateKey);
}

/**
 * Returns the payload of `token` once its signature checks against `jwks`,
 * the keys of `issuer`, and it is addressed to `audience` and unexpired.
 *
 * @throws {Error} jose's error when it doesn't verify; its `code` is
 * `ERR_JWKS_NO_MATCHING_KEY` when no key of `jwks` has its `kid`, which
 * tells the caller to refetch the keys, the issuer having rotated them.
 */
export async function verifyInstanceToken(
  token: string,
  jwks: JSONWebKeySet,
  claims: { issuer: string; audience: string },
): Promise<JWTPayload> {
  const { payload } = await jwtVerify(token, createLocalJWKSet(jwks), {
    issuer: claims.issuer,
    audience: claims.audience,
    algorithms: [INSTANCE_ALG],
  });
  return payload;
}