relay: add support for NIP42 authentication

nip42.ts

@@ -0,0 +1,42 @@
+import {EventTemplate, Event, Kind} from './event'
+import {Relay} from './relay'
+
+/**
+ * Authenticate via NIP-42 flow.
+ *
+ * @example
+ * const sign = window.nostr.signEvent
+ * relay.on('auth', challenge =>
+ *   authenticate({ relay, sign, challenge })
+ * )
+ */
+export const authenticate = async ({
+  challenge,
+  relay,
+  sign
+}: {
+  challenge: string
+  relay: Relay
+  sign: (e: EventTemplate) => Promise<Event>
+}): Promise<void> => {
+  const e: EventTemplate = {
+    kind: Kind.ClientAuth,
+    created_at: Math.floor(Date.now() / 1000),
+    tags: [
+      ['relay', relay.url],
+      ['challenge', challenge]
+    ],
+    content: ''
+  }
+  const sub = relay.publish(await sign(e), 'AUTH')
+  return new Promise((resolve, reject) => {
+    sub.on('ok', function ok() {
+      sub.off('ok', ok)
+      resolve()
+    })
+    sub.on('failed', function fail(reason: string) {
+      sub.off('failed', fail)
+      reject(reason)
+    })
+  })
+}