67 lines
2.3 KiB
TypeScript
67 lines
2.3 KiB
TypeScript
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/**
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* Signed URLs for the generated share card (`/api/og`).
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*
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* The card prints whatever title and subtitle its URL carries, on the site's
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* own domain and in its colours. Unsigned, anyone could have the site render
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* text of their choosing — a fake notice passed around as the site's own
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* image — and every new string cost a fresh satori render. The pages that
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* emit the URL sign the text they put in it; the route renders only text that
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* carries a valid signature and falls back to the plain site card otherwise.
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*/
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import crypto from 'crypto';
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import { getConfig } from './config';
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export const OG_TITLE_MAX = 200;
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export const OG_SUBTITLE_MAX = 100;
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/** Truncates by code point, so a cut never leaves half a surrogate pair. */
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function clip(text: string, max: number): string {
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const chars = Array.from(text);
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return chars.length > max ? chars.slice(0, max).join('') : text;
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}
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function sign(secret: string, title: string, subtitle: string): string {
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return crypto
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.createHmac('sha256', secret)
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.update(`og-card\0${title}\0${subtitle}`)
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.digest('base64url')
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.slice(0, 22);
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}
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/** `/api/og` URL for a card showing `title` (and `subtitle`), signed. */
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export function ogImageUrl(title: string, subtitle = ''): string {
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let secret: string;
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try {
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secret = getConfig().authSecret;
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} catch {
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// No secret to sign with — the route then shows the site title anyway.
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return '/api/og';
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}
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const t = clip(title, OG_TITLE_MAX);
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const s = clip(subtitle, OG_SUBTITLE_MAX);
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const params = new URLSearchParams({ title: t });
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if (s) params.set('subtitle', s);
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params.set('sig', sign(secret, t, s));
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return `/api/og?${params.toString()}`;
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}
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/**
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* The title and subtitle of a card URL, or null unless the URL was produced
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* by `ogImageUrl()` with this secret.
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*/
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export function verifiedOgText(
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params: URLSearchParams,
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secret: string,
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): { title: string; subtitle: string } | null {
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const title = params.get('title');
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const subtitle = params.get('subtitle') ?? '';
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const sig = params.get('sig');
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if (!title || !sig || !secret) return null;
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const expected = Buffer.from(sign(secret, title, subtitle));
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const given = Buffer.from(sig);
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if (given.length !== expected.length || !crypto.timingSafeEqual(given, expected)) return null;
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return { title, subtitle };
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}
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