Signed requests: strictly increasing timestamps per key

Ed25519 signatures are deterministic, so two identical requests from one key in the same
millisecond (e.g. two devices subscribing to the same path, or parallel identical reads)
had the same signature, and the server rejected the second as a replay.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017Qxbkp4tA7G5SCWjzWuxmE

src/utils/request-auth.ts

@@ -54,9 +54,19 @@ export class RequestAuth {
     return new TextEncoder().encode(`${timestamp}:${path}:${body}`)
   }
 
-  /** Auth headers for a request to `path` with `body`. */
+  /** Last timestamp used per public key in this process (see `headers`). */
+  private static readonly lastTimestamps = new Map<string, number>()
+
+  /**
+   * Auth headers for a request to `path` with `body`. Timestamps per public key strictly increase
+   * within a process: Ed25519 signatures are deterministic, so two identical requests in the same
+   * millisecond would otherwise carry the same signature, and the second would be rejected as a
+   * replay.
+   */
   static async headers(signer: RequestSigner, path: string, body: string, now: number = Date.now()): Promise<Record<string, string>> {
-    const timestamp = String(now)
+    const time = Math.max(now, (RequestAuth.lastTimestamps.get(signer.publicKey) ?? 0) + 1)
+    RequestAuth.lastTimestamps.set(signer.publicKey, time)
+    const timestamp = String(time)
     const signature = await signer.sign(RequestAuth.payload(timestamp, path, body))
     return {
       [AUTH_HEADERS.publicKey]: signer.publicKey,