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* Signing in to Dropbox, with a code the reader copies off Dropbox’s own page.
*
* A Dropbox folder is the one remote place in this application that cannot be
* read without a sign-in at all. A bucket has public objects, Drive has public
* files behind an API key, and a share link on `dl.dropboxusercontent.com` is
* one file somebody already decided to publish (§24.5). *Browsing* a Dropbox is
* none of those: the listing API takes a bearer token and nothing else, so the
* reader signs in to their own account, in their own browser, and this module is
* how.
*
* ## Why the code is pasted rather than redirected back
*
* Every other OAuth flow in the world ends at a **redirect URI** — the
* authorization server sends the browser back to an address registered in
* advance, with the code in the query. This application has no address it can
* register that works: the published site has one origin, a fork has another,
* the single-file build (§19) has an *opaque* origin, and the desktop build
* serves itself from `app://viewer` (`electron/protocol.cts`). A redirect flow
* would therefore be a feature of one deployment, and a lab that forked the site
* would have to register a second URI before its readers could sign in.
*
* Dropbox supports the one shape that needs no address at all: an authorize
* request **with no `redirect_uri`** makes Dropbox display the authorization
* code on its own page, for the reader to copy into this one. It is two
* gestures instead of one, and it is the only arrangement that works
* identically from a website, a fork, a laptop’s desktop app and a file on a
* USB stick — which is the property §19 and §21 both need and which no amount of
* redirect-URI bookkeeping would buy.
*
* ## PKCE, and the secret that is therefore not needed
*
* A code is worth something to whoever holds it, and the reason a public client
* may exchange one without a client secret is **PKCE** (RFC 7636): this page
* makes a fresh random *verifier*, sends only its SHA-256 *challenge* to the
* authorize endpoint, and proves it was the one that asked by presenting the
* verifier at the token endpoint. The verifier is held in memory here between
* the two halves and nowhere else. A client secret would be the alternative, and
* a secret shipped in a page that anybody can read is not a secret — which is
* exactly the case PKCE was written for.
*
* `token_access_type=online`, so the exchange yields **no refresh token**.
* A refresh token is a long-lived credential, §24.8 refuses to store one, and a
* long-lived credential held in memory and lost at every reload is a thing that
* costs a reader an extra sign-in per tab in exchange for nothing. Asking for
* the short-lived token is asking for exactly what this application can honestly
* keep.
*
* ## What is kept, and for how long
*
* Nothing in storage. The **app key** — the site’s registration of a Dropbox
* app, public by construction (it is in the authorize URL, which the reader
* can read in their own address bar) and authorising nothing on its own — is
* in the page, taken by the build from the `DROPBOX_APP_KEY` variable
* (`config/siteDefaults.ts`, docs/deploy.md); no reader is asked for one. The
* **token**, in memory only and never written (§24.8), held by
* `s3/credentials.ts`’s bearer registry for the two API hosts, which is a
* module-level `Map` a reload empties.
*
* Nothing here asks Dropbox who the reader is: there is no `account_info.read`
* in the permissions this needs, so the app never learns the address behind the
* account. What it can say is *signed in, until 14:32*, which is the whole of
* what a reader needs in order to read the next `401`.
*
* ## Where this is absent
*
* The **standalone build** only (§19): its origin is opaque, so every remote
* request fails before a byte moves, and a sign-in that authorises requests
* nothing can make is §8.5’s control that cannot keep its promise. The
* **desktop** build keeps it, unlike the Google sign-in next door — nothing here
* is registered against an origin, so `app://viewer` sends these requests as
* well as any other page does.
*/
import { clearBearer, setBearer } from '../s3/credentials'
import { IS_STANDALONE } from '../version'
import { SITE_DEFAULTS } from '../config/siteDefaults'
/**
* The two hosts one sign-in is registered for.
*
* Exact hosts rather than a `*.dropboxapi.com` glob, for the reason
* `auth/google.ts` states about its own pair: a bearer token is a credential and
* every host it is sent to receives it. `api.dropboxapi.com` takes the listing
* and the temporary-link calls; `content.dropboxapi.com` is where a download
* would go if this viewer ever made one directly. `dl.dropboxusercontent.com`,
* which is where a temporary link actually points, is deliberately **not** here:
* that link carries its own signature in the URL and needs no header, and
* putting the account’s token on an unauthenticated content host would publish
* it to one.
*/
export const DROPBOX_HOSTS = ['api.dropboxapi.com', 'content.dropboxapi.com'] as const
/** Where the reader is sent, and where the code they bring back is exchanged. */
export const DROPBOX_AUTHORIZE_URL = 'https://www.dropbox.com/oauth2/authorize'
export const DROPBOX_TOKEN_URL = 'https://api.dropboxapi.com/oauth2/token'
export const DROPBOX_REVOKE_URL = 'https://api.dropboxapi.com/2/auth/token/revoke'
/**
* The two permissions the app is registered with, named wherever Dropbox
* refuses for want of one.
*
* They are set on the **app**, in Dropbox’s App Console, rather than asked for
* per sign-in — which is why the authorize URL below carries no `scope`
* parameter and why a missing permission is a thing the lab fixes once rather
* than something a reader can grant from here. `docs/hosting.md` has the steps.
*/
export const DROPBOX_PERMISSIONS = ['files.metadata.read', 'files.content.read'] as const
/**
* Whether this build can sign in at all.
*
* See the header: only the standalone build cannot, and it cannot because it
* has no network rather than because of anything about Dropbox.
*/
export const DROPBOX_SIGN_IN_AVAILABLE = !IS_STANDALONE
/** What the Dropbox section and the folder backend both draw themselves from. */
export interface DropboxState {
/** This site was built with an app key, so signing in is a thing that could work. */
configured: boolean
/** A token is held **and** has not expired. */
signedIn: boolean
/**
* Epoch milliseconds, and it **outlives the token** — `auth/google.ts`’s
* `GoogleState.expiresAt` says why: *this page had a token and it ran
* out* is a different sentence from *this page has never signed in*, and only
* the first is answered by "sign in again".
*/
expiresAt: number | null
}
// ---------------------------------------------------------------------------
// The app key, which is the site's
// ---------------------------------------------------------------------------
/**
* The app key a sign-in names: the one this site was built with
* (`config/siteDefaults.ts`), or `null` on a site built without one.
*
* Nothing a reader can enter and nothing this browser keeps, for the reason
* `auth/google.ts` gives for the client id: it names the deployment's
* registration, and the dialog offers the sign-in rather than a box.
*/
export function readAppKey(): string | null {
return SITE_DEFAULTS.dropboxAppKey
}
// ---------------------------------------------------------------------------
// The token, which is not stored
// ---------------------------------------------------------------------------
let token: string | null = null
let expiresAt: number | null = null
/**
* The PKCE verifier for the sign-in in progress, and nothing longer-lived.
*
* In memory between {@link beginSignIn} and {@link finishSignIn}, which is the
* whole of its useful life: it is the proof that the page exchanging the code is
* the page that asked for it, so a copy of it surviving in storage would be a
* copy of half a credential lying about.
*/
let verifier: string | null = null
const listeners = new Set<() => void>()
function announce(): void {
for (const listener of listeners) listener()
}
/**
* Drop a token that has run out, and say whether one was dropped.
*
* Checked when the state is asked for rather than on a timer, for `bearerFor`’s
* reason: a timer is a thing to cancel when the page goes away, and the only
* moment the answer matters is the moment somebody asks.
*/
function expire(): boolean {
if (token === null || expiresAt === null || expiresAt > Date.now()) return false
token = null
for (const host of DROPBOX_HOSTS) clearBearer(host)
return true
}
/**
* What this page can say about Dropbox right now.
*
* Recomputed on every call rather than cached, because the answer changes
* without anybody touching this module: the token expires on its own. The app
* key does not — it is the one the site was built with.
*/
export function dropboxState(): DropboxState {
// Announced from a *getter*, for the reason `googleState` documents: the
// listeners this wakes call back in, find the token already dropped and
// return false, so it is one notification and no recursion — and without it a
// dialog left open across an expiry would go on displaying a time that has
// passed.
if (expire()) announce()
return { configured: readAppKey() !== null, signedIn: token !== null, expiresAt }
}
/** Hear about a sign-in, a sign-out or an expiry. */
export function subscribeDropbox(listener: () => void): () => void {
listeners.add(listener)
return () => {
listeners.delete(listener)
}
}
// ---------------------------------------------------------------------------
// PKCE
// ---------------------------------------------------------------------------
/** `+/=` out, `-_` in, padding gone: RFC 4648 §5, which is what PKCE wants. */
function base64url(bytes: Uint8Array): string {
let binary = ''
for (const byte of bytes) binary += String.fromCharCode(byte)
return btoa(binary).replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '')
}
/**
* A fresh code verifier: 86 characters from the unreserved set.
*
* RFC 7636 §4.1 allows 43 to 128 characters and asks for cryptographic
* randomness. 64 random bytes base64url-encode to 86 of them, which is the
* middle of the permitted range and spends nothing to be there — and encoding
* the bytes rather than drawing characters from the alphabet by index is what
* keeps the distribution flat, since 256 does not divide by the 66 unreserved
* characters and the naive loop would favour the first 58 of them.
*/
export function makeVerifier(): string {
const bytes = new Uint8Array(64)
crypto.getRandomValues(bytes)
return base64url(bytes)
}
/**
* The S256 challenge for a verifier: `BASE64URL(SHA256(ASCII(verifier)))`.
*
* Exported because it is the one piece of this flow with a **published test
* vector** behind it (RFC 7636 appendix B), and a challenge computed slightly
* wrong — the padding left on, `+` and `/` not swapped, the hash taken over
* UTF-16 — produces an authorize page that looks perfect and a token exchange
* that answers `invalid_grant` two gestures later, from a host no test
* environment can reach.
*/
export async function challengeOf(verifier: string): Promise<string> {
const digest = await crypto.subtle.digest('SHA-256', new TextEncoder().encode(verifier))
return base64url(new Uint8Array(digest))
}
// ---------------------------------------------------------------------------
// The flow
// ---------------------------------------------------------------------------
let fetchImpl: typeof fetch | null = null
/**
* Use `impl` instead of the global `fetch`, or `null` to put it back.
*
* The seam the tests drive the token exchange through, and the counterpart of
* `auth/google.ts`’s `setTokenClientFactory`: no Dropbox host is reachable from
* a test environment (nor from the sandbox this was written in), so the choice
* was a stub or no assertions at all about the one module here that obtains a
* credential. A module-level setter rather than a parameter, because the callers
* are two buttons and threading a `fetch` from each of them would put a test’s
* concern into the production call sites.
*/
export function setDropboxFetch(impl: typeof fetch | null): void {
fetchImpl = impl
}
function doFetch(url: string, init: RequestInit): Promise<Response> {
return (fetchImpl ?? globalThis.fetch)(url, init)
}
/**
* An hour, used only when the token response carries no readable `expires_in`.
*
* Deliberately short rather than optimistic. The fallback matters more than it
* looks — an unparseable value would become a `NaN` expiry, and `NaN > now` is
* false in one direction while `NaN <= now` is false in the other, which is a
* token that neither expires nor is ever usable depending on which comparison
* asked — and of the two finite wrong answers, the short one fails the way every
* expiry already fails (an offer to sign in again) while the long one would have
* the dialog claim a token is good hours after Dropbox stopped honouring it.
*/
const DEFAULT_LIFETIME_SECONDS = 3600
/**
* Start a sign-in: a fresh verifier, and the URL the reader opens.
*
* Answers the URL rather than opening it, because opening a tab is a thing a
* **click handler** may do and this is not one — a popup opened from a promise
* callback is blocked by every browser — and because the section wants the URL
* for the link it shows beside the button, for the reader whose pop-up blocker
* ate the tab.
*
* Nothing is fetched here. The authorize URL is a page the *reader* visits;
* this page contacts Dropbox for the first time in {@link finishSignIn}, which
* is the gesture §11 asks a third-party request to sit behind.
*/
export async function beginSignIn(): Promise<{ url: string }> {
if (!DROPBOX_SIGN_IN_AVAILABLE) {
throw new Error(
'This build cannot sign in to Dropbox: the single-file build has an opaque origin, so ' +
'every request to Dropbox fails before a byte moves. Open a Dropbox file by its share ' +
'link in the web or desktop build, or download it and drop it here.',
)
}
const appKey = readAppKey()
if (appKey === null) {
throw new Error(
'Signing in to Dropbox is not set up on this site: it needs a Dropbox app key, which the ' +
'site’s maintainer provides as DROPBOX_APP_KEY (docs/deploy.md).',
)
}
const fresh = makeVerifier()
const challenge = await challengeOf(fresh)
// Kept only once the challenge is computed, so a failed digest leaves the
// previous attempt's verifier alone rather than replacing it with one no
// authorize page was ever shown for.
verifier = fresh
// Built with `URLSearchParams` rather than by concatenation, so an app key
// with a character that needs escaping is escaped once and correctly. There
// is **no `redirect_uri`**, and that absence is the feature: it is what makes
// Dropbox display the code instead of sending it somewhere (see the header).
const query = new URLSearchParams({
client_id: appKey,
response_type: 'code',
code_challenge: challenge,
code_challenge_method: 'S256',
token_access_type: 'online',
})
return { url: `${DROPBOX_AUTHORIZE_URL}?${query.toString()}` }
}
/**
* Exchange the code the reader pasted for a token, and hold it for both hosts.
*
* Rejects rather than reporting quietly, because the caller is a button: the
* section shows the message under the box. The messages name the fix, as §10
* requires, and the two that can happen before Dropbox is ever contacted — no
* verifier, an empty box — are worth distinguishing precisely because both look
* like "finish did nothing".
*/
export async function finishSignIn(code: string): Promise<void> {
const text = code.trim()
if (text === '') {
throw new Error(
'Paste the code Dropbox showed on its own page, then press finish. It is a long string ' +
'on the page that opened when you pressed sign in.',
)
}
const appKey = readAppKey()
if (appKey === null) {
throw new Error(
'The app key was forgotten while this sign-in was open. Save it again under ' +
'Connect → Dropbox and start the sign-in over; the code on the other tab is no longer ' +
'good for anything.',
)
}
const held = verifier
if (held === null) {
throw new Error(
'This page has no record of starting that sign-in, so it cannot prove the code is its ' +
'own. That happens when the tab was reloaded in between. Press sign in to Dropbox ' +
'again and use the new code.',
)
}
// Form-encoded, which is what the token endpoint takes — and no client
// secret, because PKCE is what stands in for one (see the header). The
// verifier goes here and travels nowhere else.
const body = new URLSearchParams({
code: text,
grant_type: 'authorization_code',
client_id: appKey,
code_verifier: held,
})
let response: Response
try {
response = await doFetch(DROPBOX_TOKEN_URL, {
method: 'POST',
headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
body: body.toString(),
})
} catch (cause) {
throw new Error(
`Could not reach ${DROPBOX_TOKEN_URL} to exchange the code. A network that blocks ` +
'Dropbox blocks this feature entirely. Underlying error: ' +
(cause instanceof Error ? cause.message : String(cause)),
)
}
if (!response.ok) {
const detail = await response.text().catch(() => '')
// A code is good once and for a few minutes. Saying so is the difference
// between a reader pressing finish again with the same string forever and a
// reader pressing sign in again, which is the fix.
throw new Error(
`Dropbox refused the code (${response.status}${detail === '' ? '' : `: ${detail}`}). An ` +
'authorization code may be exchanged once and expires within minutes, so the usual ' +
'cause is a code that has already been used or has gone stale — press sign in to ' +
'Dropbox again and paste the new one.',
)
}
let payload: unknown
try {
payload = await response.json()
} catch {
throw new Error(
'Dropbox answered the code exchange with something that is not JSON, so there is no token ' +
'in it. Trying the sign-in again is the only thing to do about it.',
)
}
const granted = (payload as Record<string, unknown>)?.['access_token']
if (typeof granted !== 'string' || granted === '') {
throw new Error(
'Dropbox answered the code exchange without a token, so nothing was granted. Trying the ' +
'sign-in again is the only thing to do about it.',
)
}
const seconds = Number((payload as Record<string, unknown>)['expires_in'])
const lifetime = Number.isFinite(seconds) && seconds > 0 ? seconds : DEFAULT_LIFETIME_SECONDS
const until = Date.now() + lifetime * 1000
// Both hosts from one response, because one sign-in produced one token that
// covers both. Registering them one at a time is the arrangement where
// somebody forgets the second and a download fails after a sign-in the reader
// watched succeed.
for (const host of DROPBOX_HOSTS) setBearer(host, granted, until)
token = granted
expiresAt = until
// The code has been spent; keeping the verifier would leave a second `finish`
// able to send a request that can only fail.
verifier = null
announce()
}
/**
* Drop the token, and tell Dropbox to forget it.
*
* The local half is what matters and happens first: the bearers go, so the next
* listing is refused rather than sent with a credential the reader has just
* disclaimed. The revoke is best effort — it is a network call on a page that
* may be closing — and nothing here waits for it or reports its failure,
* because a reader who pressed *sign out* has already got what they asked for
* from this browser’s side. It is sent at all because the alternative is a token
* that goes on being valid at Dropbox for its remaining hours; the header is
* written out here rather than read back from the registry, which by then is
* empty.
*/
export function signOut(): void {
const held = token
token = null
expiresAt = null
verifier = null
for (const host of DROPBOX_HOSTS) clearBearer(host)
if (held !== null) {
void doFetch(DROPBOX_REVOKE_URL, {
method: 'POST',
headers: { Authorization: `Bearer ${held}` },
}).catch(() => {
/* best effort: the grant is Dropbox's record, the token is already gone here */
})
}
announce()
}
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