diff --git a/docs/systems/federation.md b/docs/systems/federation.md
index 6345eba4..3a5667a5 100644
--- a/docs/systems/federation.md
+++ b/docs/systems/federation.md
@@ -12,6 +12,7 @@ Source files:
   - `routes/federation/reconciliation.ts` -- DM federated-id reconciliation + dead-incarnation artifact sweeps (worker-facing maintenance)
   - `routes/federation/events/*.ts` -- Inbound relay event processors, grouped by domain: `dmMessages`, `membership`, `friends`, `calls`, `dmState` (presence/read-state/close/reopen/file-rejected), and `dispatch` (`processRelayEvents`, the fan-out entry point shared by the HTTP relay handler and the initial-sync worker)
   - `routes/federation/handlers/*.ts` -- Fastify route registrars, grouped by endpoint concern: `peerHandshake` (initiate/accept/ensure/rotate/denied), `peerAdmin` (peer list/CRUD/reset/recheck/rotate), `approvals` (approval queue + peering subscriptions/notifications + approve/deny helpers), `relay` (identity delete, relay, epoch, sync), `lookup` (user lookups), `attach` (verify-attach-proof, `/api/users/@me/reattach`)
+    - `routes/federation/handlers/s2sAuth.ts` -- `authenticateS2SPeer(request, reply, opts?)`: the shared inbound S2S-HMAC auth preamble (parse headers → resolve active peer → optional per-peer rate limit **before** signature → verify HMAC signature → nonce replay). Adopted by the six endpoints whose preamble is byte-identical: `DELETE /identity`, `POST /relay`, `POST /sync` (`relay.ts`), `POST /users/lookup`, `POST /users/by-home-id` (`lookup.ts`), and `POST /verify-attach-proof` (`attach.ts`). Returns `{ ok: true, peer, nonce }` or, having already sent the rejection reply, `{ ok: false }` (caller must `return`). **Intentional non-adopters** (each keeps a load-bearing gate the helper would flatten, documented in its own docstring/comment): `POST /epoch` (revoked-only gate for peer recovery, 400 on missing headers, no nonce check), `POST /peer/rotate` (active-only, no nonce check), `POST /peer/denied` (`awaiting_approval` gate, synthetic no-grace secret verify).
 - `packages/server/src/utils/federationAuth.ts` -- HMAC signing, verification, header parsing, `getOurOrigin()`
 - `packages/server/src/utils/federationOutbox.ts` -- Event queuing, coalescing, relay payload construction, mutation log, participant/target resolution
 - `packages/server/src/utils/federationLookup.ts` -- HMAC-signed remote-user lookups: `lookupRemoteUser` (by username) and `lookupRemoteUserByHomeId` (reverse lookup, used by stub backfill)
diff --git a/packages/server/src/routes/federation/handlers/attach.ts b/packages/server/src/routes/federation/handlers/attach.ts
index 8eb557d7..c67f0b3d 100644
--- a/packages/server/src/routes/federation/handlers/attach.ts
+++ b/packages/server/src/routes/federation/handlers/attach.ts
@@ -3,7 +3,6 @@ import { config } from '../../../config.js';
 import { getDb, getRawDb, schema } from '../../../db/index.js';
 import { authenticate } from '../../../utils/auth.js';
 import { fetchHomeProfileByHomeId, verifyAttachProofWithPeer } from '../../../utils/federationAttach.js';
-import { parseFederationHeaders, verifyPeerSignature } from '../../../utils/federationAuth.js';
 import { sendSignedJson } from './signedResponse.js';
 import { sanitizeUser } from '../../../utils/sanitize.js';
 import { collectProfileBroadcastTargetIds } from '../../../utils/userDeletion.js';
@@ -15,7 +14,8 @@ import type { FastifyInstance, FastifyReply } from 'fastify';
 import { buildDmChannelPayload } from '../dmChannels.js';
 import { extractDomain } from '../identity.js';
 import { downloadProfileAsset } from '../profile.js';
-import { isLookupRateLimited, isNonceDuplicate } from '../rateLimits.js';
+import { isLookupRateLimited } from '../rateLimits.js';
+import { authenticateS2SPeer } from './s2sAuth.js';
 import { reconcileDmChannelFederatedId } from '../reconciliation.js';
 import type { DmReconcileResult } from '../reconciliation.js';
 
@@ -37,39 +37,15 @@ export function registerAttachRoutes(app: FastifyInstance): void {
       const db = getDb();
       const rawDb = getRawDb();
 
-      // 1. Verify HMAC headers (mirror by-home-id / users-lookup).
-      const fedHeaders = parseFederationHeaders(request.headers as Record<string, string | string[] | undefined>);
-      if (!fedHeaders) {
-        return reply.code(401).send({ error: 'Missing or malformed federation headers', statusCode: 401 });
-      }
-
-      const peer = db
-        .select()
-        .from(schema.federationPeers)
-        .where(eq(schema.federationPeers.origin, fedHeaders.origin))
-        .get();
-
-      if (!peer || peer.status !== 'active') {
-        return reply.code(403).send({ error: 'Unknown or inactive peer', statusCode: 403 });
-      }
-
-      if (isLookupRateLimited(peer.origin)) {
-        return reply.code(429).header('Retry-After', '60').send({ error: 'Rate limit exceeded', statusCode: 429 });
-      }
-
-      const bodyString = JSON.stringify(request.body);
-      if (!verifyPeerSignature(bodyString, fedHeaders.signature, fedHeaders.timestamp, fedHeaders.nonce, peer)) {
-        return reply.code(401).send({ error: 'Invalid signature', statusCode: 401 });
-      }
-
-      // Replay protection
-      if (fedHeaders.nonce) {
-        if (isNonceDuplicate(peer.origin, fedHeaders.nonce)) {
-          return reply.code(409).send({ error: 'Duplicate nonce — possible replay', statusCode: 409 });
-        }
-      } else if (peer.nonceSupported) {
-        return reply.code(401).send({ error: 'Nonce required — peer previously supported nonces', statusCode: 401 });
-      }
+      // Shared inbound S2S-auth preamble: headers → active peer → rate limit →
+      // signature → nonce replay. Shares the lookup rate-limit bucket (60/min) by
+      // design (this is the same friend-request-originator flow as /users/lookup),
+      // running BEFORE signature with `Retry-After: 60`; no missing-nonce warning.
+      const auth = authenticateS2SPeer(request, reply, {
+        rateLimiter: { limited: isLookupRateLimited, retryAfterSeconds: 60 },
+      });
+      if (!auth.ok) return;
+      const { peer } = auth;
 
       // 2. Sign every downstream response with the peer's shared secret so the
       // caller can trust the identity (or the fail-closed verdict) it carries.
diff --git a/packages/server/src/routes/federation/handlers/lookup.ts b/packages/server/src/routes/federation/handlers/lookup.ts
index 10fc5127..6188d36c 100644
--- a/packages/server/src/routes/federation/handlers/lookup.ts
+++ b/packages/server/src/routes/federation/handlers/lookup.ts
@@ -1,8 +1,8 @@
 import { getDb, schema } from '../../../db/index.js';
-import { parseFederationHeaders, verifyPeerSignature } from '../../../utils/federationAuth.js';
 import { and, eq, isNull, or } from 'drizzle-orm';
 import type { FastifyInstance } from 'fastify';
-import { isLookupRateLimited, isNonceDuplicate } from '../rateLimits.js';
+import { isLookupRateLimited } from '../rateLimits.js';
+import { authenticateS2SPeer } from './s2sAuth.js';
 
 export function registerLookupRoutes(app: FastifyInstance): void {
   // ─── POST /api/federation/users/lookup ─────────────────────────────────────
@@ -19,39 +19,14 @@ export function registerLookupRoutes(app: FastifyInstance): void {
     async (request, reply) => {
       const db = getDb();
 
-      // 1. Verify HMAC (mirror relay endpoint)
-      const fedHeaders = parseFederationHeaders(request.headers as Record<string, string | string[] | undefined>);
-      if (!fedHeaders) {
-        return reply.code(401).send({ error: 'Missing or malformed federation headers', statusCode: 401 });
-      }
-
-      const peer = db
-        .select()
-        .from(schema.federationPeers)
-        .where(eq(schema.federationPeers.origin, fedHeaders.origin))
-        .get();
-
-      if (!peer || peer.status !== 'active') {
-        return reply.code(403).send({ error: 'Unknown or inactive peer', statusCode: 403 });
-      }
-
-      if (isLookupRateLimited(peer.origin)) {
-        return reply.code(429).header('Retry-After', '60').send({ error: 'Rate limit exceeded', statusCode: 429 });
-      }
-
-      const bodyString = JSON.stringify(request.body);
-      if (!verifyPeerSignature(bodyString, fedHeaders.signature, fedHeaders.timestamp, fedHeaders.nonce, peer)) {
-        return reply.code(401).send({ error: 'Invalid signature', statusCode: 401 });
-      }
-
-      // 1b. Nonce-based replay protection
-      if (fedHeaders.nonce) {
-        if (isNonceDuplicate(peer.origin, fedHeaders.nonce)) {
-          return reply.code(409).send({ error: 'Duplicate nonce — possible replay', statusCode: 409 });
-        }
-      } else if (peer.nonceSupported) {
-        return reply.code(401).send({ error: 'Nonce required — peer previously supported nonces', statusCode: 401 });
-      }
+      // Shared inbound S2S-auth preamble: headers → active peer → rate limit →
+      // signature → nonce replay. The per-peer lookup rate limiter (60/min) runs
+      // BEFORE signature verification and sends `Retry-After: 60`. This endpoint
+      // never logged on a missing nonce (logMissingNonce omitted).
+      const auth = authenticateS2SPeer(request, reply, {
+        rateLimiter: { limited: isLookupRateLimited, retryAfterSeconds: 60 },
+      });
+      if (!auth.ok) return;
 
       // 2. Validate body
       const rawUsername = (request.body as { username?: unknown } | null)?.username;
@@ -109,39 +84,14 @@ export function registerLookupRoutes(app: FastifyInstance): void {
     async (request, reply) => {
       const db = getDb();
 
-      // 1. Verify HMAC headers
-      const fedHeaders = parseFederationHeaders(request.headers as Record<string, string | string[] | undefined>);
-      if (!fedHeaders) {
-        return reply.code(401).send({ error: 'Missing or malformed federation headers', statusCode: 401 });
-      }
-
-      const peer = db
-        .select()
-        .from(schema.federationPeers)
-        .where(eq(schema.federationPeers.origin, fedHeaders.origin))
-        .get();
-
-      if (!peer || peer.status !== 'active') {
-        return reply.code(403).send({ error: 'Unknown or inactive peer', statusCode: 403 });
-      }
-
-      if (isLookupRateLimited(peer.origin)) {
-        return reply.code(429).header('Retry-After', '60').send({ error: 'Rate limit exceeded', statusCode: 429 });
-      }
-
-      const bodyString = JSON.stringify(request.body);
-      if (!verifyPeerSignature(bodyString, fedHeaders.signature, fedHeaders.timestamp, fedHeaders.nonce, peer)) {
-        return reply.code(401).send({ error: 'Invalid signature', statusCode: 401 });
-      }
-
-      // Replay protection
-      if (fedHeaders.nonce) {
-        if (isNonceDuplicate(peer.origin, fedHeaders.nonce)) {
-          return reply.code(409).send({ error: 'Duplicate nonce — possible replay', statusCode: 409 });
-        }
-      } else if (peer.nonceSupported) {
-        return reply.code(401).send({ error: 'Nonce required — peer previously supported nonces', statusCode: 401 });
-      }
+      // Shared inbound S2S-auth preamble: headers → active peer → rate limit →
+      // signature → nonce replay. Same shape as /users/lookup: per-peer lookup
+      // rate limiter (60/min) BEFORE signature, `Retry-After: 60`, no
+      // missing-nonce warning.
+      const auth = authenticateS2SPeer(request, reply, {
+        rateLimiter: { limited: isLookupRateLimited, retryAfterSeconds: 60 },
+      });
+      if (!auth.ok) return;
 
       // 2. Validate body
       const rawId = (request.body as { homeUserId?: unknown } | null)?.homeUserId;
diff --git a/packages/server/src/routes/federation/handlers/peerHandshake.ts b/packages/server/src/routes/federation/handlers/peerHandshake.ts
index 56818e4e..8c287035 100644
--- a/packages/server/src/routes/federation/handlers/peerHandshake.ts
+++ b/packages/server/src/routes/federation/handlers/peerHandshake.ts
@@ -625,6 +625,9 @@ export function registerPeerHandshakeRoutes(app: FastifyInstance): void {
   // ─── POST /api/federation/peer/rotate ───────────────────────────────────────
   // Server-to-server: accept a secret rotation request from a peer instance.
   // Authenticated via HMAC-SHA256 signature (current secret), NOT JWT.
+  // NON-ADOPTER of authenticateS2SPeer (deliberate): active-only like the helper
+  // but runs NO nonce replay check (the rotation body is its own replay unit);
+  // sharing the helper would add a nonce gate this endpoint never had.
   app.post<{ Body: { newSecret: string } }>(
     '/api/federation/peer/rotate',
     async (request, reply) => {
@@ -684,6 +687,10 @@ export function registerPeerHandshakeRoutes(app: FastifyInstance): void {
   // Server-to-server: receive a denial notification from a remote instance.
   // Authenticated via HMAC-SHA256 signature (the secret we sent in our original
   // peer/accept request, which the remote stored in their approval queue).
+  // NON-ADOPTER of authenticateS2SPeer (deliberate): gates on 'awaiting_approval'
+  // (404 on no peer row, 409 on wrong status — not the helper's active-only 403),
+  // verifies against a SYNTHETIC no-grace secret object, and runs no nonce check.
+  // Entirely different control flow.
   app.post<{ Body: { origin: string; reason: 'denied_by_admin' | 'expired'; message?: string } }>(
     '/api/federation/peer/denied',
     async (request, reply) => {
diff --git a/packages/server/src/routes/federation/handlers/relay.ts b/packages/server/src/routes/federation/handlers/relay.ts
index d093a0b5..bf297d98 100644
--- a/packages/server/src/routes/federation/handlers/relay.ts
+++ b/packages/server/src/routes/federation/handlers/relay.ts
@@ -15,7 +15,8 @@ import type { FastifyInstance } from 'fastify';
 import { processRelayEvents } from '../events/dispatch.js';
 import { extractDomain } from '../identity.js';
 import { resolveLocalOrigin } from '../origin.js';
-import { isNonceDuplicate, isRelayRateLimited } from '../rateLimits.js';
+import { isRelayRateLimited } from '../rateLimits.js';
+import { authenticateS2SPeer } from './s2sAuth.js';
 
 export function registerRelayRoutes(app: FastifyInstance): void {
   // ─── DELETE /api/federation/identity ──────────────────────────────────────
@@ -26,37 +27,11 @@ export function registerRelayRoutes(app: FastifyInstance): void {
     async (request, reply) => {
       const db = getDb();
 
-      // 1. Verify HMAC signature (same pattern as relay endpoint)
-      const fedHeaders = parseFederationHeaders(request.headers as Record<string, string | string[] | undefined>);
-      if (!fedHeaders) {
-        return reply.code(401).send({ error: 'Missing or malformed federation headers', statusCode: 401 });
-      }
-
-      const peer = db
-        .select()
-        .from(schema.federationPeers)
-        .where(eq(schema.federationPeers.origin, fedHeaders.origin))
-        .get();
-
-      if (!peer || peer.status !== 'active') {
-        return reply.code(403).send({ error: 'Unknown or inactive peer', statusCode: 403 });
-      }
-
-      const bodyString = JSON.stringify(request.body);
-      if (!verifyPeerSignature(bodyString, fedHeaders.signature, fedHeaders.timestamp, fedHeaders.nonce, peer)) {
-        return reply.code(401).send({ error: 'Invalid signature', statusCode: 401 });
-      }
-
-      // Nonce-based replay protection
-      if (fedHeaders.nonce) {
-        if (isNonceDuplicate(peer.origin, fedHeaders.nonce)) {
-          return reply.code(409).send({ error: 'Duplicate nonce — possible replay', statusCode: 409 });
-        }
-      } else if (peer.nonceSupported) {
-        return reply.code(401).send({ error: 'Nonce required — peer previously supported nonces', statusCode: 401 });
-      } else {
-        console.warn(`[federation] Peer ${peer.origin} does not support replay protection (no nonce)`);
-      }
+      // Shared inbound S2S-auth preamble: headers → active peer → signature →
+      // nonce replay. No rate limiter; warns on a legacy peer's missing nonce.
+      const auth = authenticateS2SPeer(request, reply, { logMissingNonce: true });
+      if (!auth.ok) return;
+      const { peer } = auth;
 
       // 2. Validate body
       const { homeUserId, homeInstance, mode } = request.body;
@@ -77,7 +52,7 @@ export function registerRelayRoutes(app: FastifyInstance): void {
       }
 
       // 4. Attribution guard: only the user's home instance can delete them
-      if (!user.homeInstance || extractDomain(user.homeInstance) !== extractDomain(fedHeaders.origin)) {
+      if (!user.homeInstance || extractDomain(user.homeInstance) !== extractDomain(peer.origin)) {
         return reply.code(403).send({ error: 'Attribution mismatch: you can only delete users from your own instance', statusCode: 403 });
       }
 
@@ -86,7 +61,7 @@ export function registerRelayRoutes(app: FastifyInstance): void {
       // homeUserIds. Idempotent 200: from the caller's perspective this
       // identity does not exist here.
       if (user.federationHomeOrphaned === 1) {
-        console.log(`[federation] Ignoring S2S identity delete for detached account ${user.id} from ${fedHeaders.origin}`);
+        console.log(`[federation] Ignoring S2S identity delete for detached account ${user.id} from ${peer.origin}`);
         return reply.code(200).send({ success: true });
       }
 
@@ -130,7 +105,7 @@ export function registerRelayRoutes(app: FastifyInstance): void {
       // 11. Force-disconnect WS if somehow still connected (unlikely but safe)
       connectionManager.forceDisconnectUser(user.id);
 
-      console.log(`[federation] Identity deleted for user ${user.id} (${user.username}) via S2S from ${fedHeaders.origin}, mode=${mode}`);
+      console.log(`[federation] Identity deleted for user ${user.id} (${user.username}) via S2S from ${peer.origin}, mode=${mode}`);
 
       return reply.code(200).send({ success: true });
     },
@@ -145,41 +120,25 @@ export function registerRelayRoutes(app: FastifyInstance): void {
     async (request, reply) => {
       const db = getDb();
 
-      // 1. Verify HMAC signature
-      const fedHeaders = parseFederationHeaders(request.headers as Record<string, string | string[] | undefined>);
-      if (!fedHeaders) {
-        return reply.code(401).send({ error: 'Missing or malformed federation headers', statusCode: 401 });
-      }
+      // Shared inbound S2S-auth preamble: headers → active peer → rate limit →
+      // signature → nonce replay. The per-peer relay rate limiter runs BEFORE
+      // signature verification (avoid HMAC work on a flood); warns on a legacy
+      // peer's missing nonce.
+      const auth = authenticateS2SPeer(request, reply, {
+        rateLimiter: { limited: isRelayRateLimited },
+        logMissingNonce: true,
+      });
+      if (!auth.ok) return;
+      const { peer } = auth;
 
-      const peer = db
-        .select()
-        .from(schema.federationPeers)
-        .where(eq(schema.federationPeers.origin, fedHeaders.origin))
-        .get();
-
-      if (!peer || peer.status !== 'active') {
-        return reply.code(403).send({ error: 'Unknown or inactive peer', statusCode: 403 });
-      }
-
-      // 1b. Per-peer rate limiting (before expensive HMAC verification)
-      if (isRelayRateLimited(peer.origin)) {
-        return reply.code(429).send({ error: 'Rate limit exceeded', statusCode: 429 });
-      }
-
-      // Serialize body back to JSON for HMAC verification (we control both sides)
-      const bodyString = JSON.stringify(request.body);
-      if (!verifyPeerSignature(bodyString, fedHeaders.signature, fedHeaders.timestamp, fedHeaders.nonce, peer)) {
-        return reply.code(401).send({ error: 'Invalid signature', statusCode: 401 });
-      }
-
-      // 1b-epoch. Fast-path baseline population (design §3.2). The signature just
-      // verified proves the peer holds the current shared secret, so the epoch it
-      // carries in `sourceInstanceId` is authentic. Populate-if-null ONLY: a valid
-      // relay can never carry an epoch differing from a non-null baseline (a
-      // different incarnation implies a different secret that fails HMAC), so we
-      // only ever fill a NULL — never overwrite. This is independent of per-event
-      // processing and does not affect relay accept/reject in any way. Old peers
-      // omit the field → skip (backward-compatible no-op).
+      // 1b-epoch. Fast-path baseline population (design §3.2). The signature the
+      // preamble verified proves the peer holds the current shared secret, so the
+      // epoch it carries in `sourceInstanceId` is authentic. Populate-if-null
+      // ONLY: a valid relay can never carry an epoch differing from a non-null
+      // baseline (a different incarnation implies a different secret that fails
+      // HMAC), so we only ever fill a NULL — never overwrite. Independent of
+      // per-event processing; does not affect relay accept/reject in any way. Old
+      // peers omit the field → skip (backward-compatible no-op).
       const claimedEpoch = request.body.sourceInstanceId;
       if (claimedEpoch && !peer.peerInstanceId) {
         db.update(schema.federationPeers)
@@ -191,18 +150,6 @@ export function registerRelayRoutes(app: FastifyInstance): void {
           .run();
       }
 
-      // 1c. Nonce-based replay protection
-      if (fedHeaders.nonce) {
-        if (isNonceDuplicate(peer.origin, fedHeaders.nonce)) {
-          return reply.code(409).send({ error: 'Duplicate nonce — possible replay', statusCode: 409 });
-        }
-      } else if (peer.nonceSupported) {
-        // Peer previously sent nonces but this request doesn't have one — reject
-        return reply.code(401).send({ error: 'Nonce required — peer previously supported nonces', statusCode: 401 });
-      } else {
-        console.warn(`[federation] Peer ${peer.origin} does not support replay protection (no nonce)`);
-      }
-
       // 2. Validate request body shape
       const body = request.body;
       if (!body || body.version !== 1 || !Array.isArray(body.events)) {
@@ -226,7 +173,7 @@ export function registerRelayRoutes(app: FastifyInstance): void {
         .set({
           lastSeenAt: Date.now(),
           consecutiveFailures: 0,
-          ...(fedHeaders.nonce && !peer.nonceSupported ? { nonceSupported: 1 } : {}),
+          ...(auth.nonce && !peer.nonceSupported ? { nonceSupported: 1 } : {}),
         })
         .where(eq(schema.federationPeers.id, peer.id))
         .run();
@@ -257,6 +204,12 @@ export function registerRelayRoutes(app: FastifyInstance): void {
   // writing it as the peer's baseline (design §3.2 / §9). The value itself
   // (instanceId) is already public via /instance/info; signing is for
   // baseline-integrity, not confidentiality.
+  //
+  // NON-ADOPTER of authenticateS2SPeer (deliberate): gates on status !== 'revoked'
+  // (ANY non-revoked peer must answer so a needs_attention/unreachable peer can
+  // drive RECOVERY via this signed round-trip), returns 400 (not 401) on missing
+  // headers, and runs NO nonce check. Folding it into the helper would flatten the
+  // recovery gate and the status code.
   app.post(
     '/api/federation/epoch',
     { bodyLimit: 4 * 1024 },
@@ -301,40 +254,15 @@ export function registerRelayRoutes(app: FastifyInstance): void {
       const db = getDb();
       const rawDb = getRawDb();
 
-      // 1. Verify HMAC signature
-      const fedHeaders = parseFederationHeaders(request.headers as Record<string, string | string[] | undefined>);
-      if (!fedHeaders) {
-        return reply.code(401).send({ error: 'Missing or malformed federation headers', statusCode: 401 });
-      }
-
-      const peer = db
-        .select()
-        .from(schema.federationPeers)
-        .where(eq(schema.federationPeers.origin, fedHeaders.origin))
-        .get();
-
-      if (!peer || peer.status !== 'active') {
-        return reply.code(403).send({ error: 'Unknown or inactive peer', statusCode: 403 });
-      }
-
-      const bodyString = JSON.stringify(request.body);
-      if (!verifyPeerSignature(bodyString, fedHeaders.signature, fedHeaders.timestamp, fedHeaders.nonce, peer)) {
-        return reply.code(401).send({ error: 'Invalid signature', statusCode: 401 });
-      }
-
-      // 1b. Nonce-based replay protection
-      if (fedHeaders.nonce) {
-        if (isNonceDuplicate(peer.origin, fedHeaders.nonce)) {
-          return reply.code(409).send({ error: 'Duplicate nonce — possible replay', statusCode: 409 });
-        }
-      } else if (peer.nonceSupported) {
-        return reply.code(401).send({ error: 'Nonce required — peer previously supported nonces', statusCode: 401 });
-      } else {
-        console.warn(`[federation] Peer ${peer.origin} does not support replay protection (no nonce) [sync]`);
-      }
+      // Shared inbound S2S-auth preamble: headers → active peer → signature →
+      // nonce replay. No rate limiter; warns (with the ` [sync]` tag) on a legacy
+      // peer's missing nonce.
+      const auth = authenticateS2SPeer(request, reply, { logMissingNonce: true, logContext: 'sync' });
+      if (!auth.ok) return;
+      const { peer } = auth;
 
       // Ratchet: mark peer as nonce-supporting if this is the first nonce we've seen
-      if (fedHeaders.nonce && !peer.nonceSupported) {
+      if (auth.nonce && !peer.nonceSupported) {
         db.update(schema.federationPeers)
           .set({ nonceSupported: 1 })
           .where(eq(schema.federationPeers.id, peer.id))
diff --git a/packages/server/src/routes/federation/handlers/s2sAuth.test.ts b/packages/server/src/routes/federation/handlers/s2sAuth.test.ts
new file mode 100644
index 00000000..6b6e7b41
--- /dev/null
+++ b/packages/server/src/routes/federation/handlers/s2sAuth.test.ts
@@ -0,0 +1,275 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
+import Fastify, { type FastifyInstance } from 'fastify';
+import Database from 'better-sqlite3';
+import { drizzle } from 'drizzle-orm/better-sqlite3';
+import fs from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { randomUUID } from 'node:crypto';
+import * as schema from '../../../db/schema.js';
+import { setWorkerId } from '../../../utils/snowflake.js';
+import { signRequest } from '../../../utils/federationAuth.js';
+import type { S2SAuthOptions } from './s2sAuth.js';
+
+setWorkerId(1);
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+
+// Module-level mutable state. Each beforeEach reassigns sqlite/testDb;
+// the getDb getter in the mock closes over the current binding.
+type TestDb = ReturnType<typeof drizzle<typeof schema>>;
+let sqlite: Database.Database;
+let testDb: TestDb;
+
+const PEER_ORIGIN = 'https://orbit.test';
+const PEER_SECRET = 'a'.repeat(64);
+
+vi.mock('../../../db/index.js', () => ({
+  getDb: () => testDb,
+  getRawDb: () => sqlite,
+  schema,
+}));
+
+// Wrap verifyPeerSignature in a passthrough spy so ordering (rate-limit BEFORE
+// signature) can be asserted by call count while real HMAC verification still runs.
+const { verifySpy } = vi.hoisted(() => ({ verifySpy: vi.fn() }));
+vi.mock('../../../utils/federationAuth.js', async (importActual) => {
+  const actual = await importActual<typeof import('../../../utils/federationAuth.js')>();
+  verifySpy.mockImplementation(actual.verifyPeerSignature);
+  return {
+    ...actual,
+    verifyPeerSignature: (...args: Parameters<typeof actual.verifyPeerSignature>) => verifySpy(...args),
+  };
+});
+
+function applyMigrations(db: Database.Database): void {
+  const dir = path.resolve(__dirname, '../../../../drizzle');
+  for (const f of fs.readdirSync(dir).filter(f => f.endsWith('.sql')).sort()) {
+    const sqlText = fs.readFileSync(path.join(dir, f), 'utf8');
+    for (const stmt of sqlText.split(/-->\s*statement-breakpoint/)) {
+      const clean = stmt.trim();
+      if (clean) db.exec(clean);
+    }
+  }
+}
+
+function seedPeer(status = 'active', nonceSupported = 0): void {
+  testDb.insert(schema.federationPeers).values({
+    id: 'peer-1',
+    origin: PEER_ORIGIN,
+    hmacSecret: PEER_SECRET,
+    status,
+    nonceSupported,
+    createdAt: Date.now(),
+    lastSeenAt: Date.now(),
+    consecutiveFailures: 0,
+    consecutiveAuthFailures: 0,
+  } as typeof schema.federationPeers.$inferInsert).run();
+}
+
+// Build a tiny app whose sole route drives authenticateS2SPeer and echoes the
+// result. `authOpts` is injected verbatim so the rate-limiter (an injectable
+// plain object) and log flags can be controlled per test.
+async function buildApp(authOpts: S2SAuthOptions = {}): Promise<FastifyInstance> {
+  const app = Fastify({ logger: false });
+  const { authenticateS2SPeer } = await import('./s2sAuth.js');
+  app.post('/probe', async (request, reply) => {
+    const result = authenticateS2SPeer(request, reply, authOpts);
+    if (!result.ok) return; // a reply was already sent
+    return reply.code(200).send({
+      ok: true,
+      peerOrigin: result.peer.origin,
+      nonce: result.nonce,
+    });
+  });
+  await app.ready();
+  return app;
+}
+
+/** Signed headers WITH a nonce (default valid path). */
+function signedHeaders(body: string, nonce: string = randomUUID()): Record<string, string> {
+  const timestamp = Date.now();
+  const sig = signRequest(body, PEER_SECRET, timestamp, nonce);
+  return {
+    'X-Federation-Origin': PEER_ORIGIN,
+    'X-Federation-Timestamp': String(timestamp),
+    'X-Federation-Nonce': nonce,
+    'X-Federation-Signature': `sha256=${sig}`,
+    'Content-Type': 'application/json',
+  };
+}
+
+/** Signed headers WITHOUT a nonce (legacy peer form: sign `${ts}.${body}`). */
+function signedHeadersNoNonce(body: string): Record<string, string> {
+  const timestamp = Date.now();
+  const sig = signRequest(body, PEER_SECRET, timestamp, null);
+  return {
+    'X-Federation-Origin': PEER_ORIGIN,
+    'X-Federation-Timestamp': String(timestamp),
+    'X-Federation-Signature': `sha256=${sig}`,
+    'Content-Type': 'application/json',
+  };
+}
+
+async function probe(app: FastifyInstance, headers: Record<string, string>, body: object = {}) {
+  return app.inject({ method: 'POST', url: '/probe', headers, payload: JSON.stringify(body) });
+}
+
+describe('authenticateS2SPeer', () => {
+  let warnSpy: ReturnType<typeof vi.spyOn>;
+
+  beforeEach(() => {
+    sqlite = new Database(':memory:');
+    testDb = drizzle(sqlite, { schema });
+    applyMigrations(sqlite);
+    verifySpy.mockClear();
+    warnSpy = vi.spyOn(console, 'warn').mockImplementation(() => {});
+  });
+
+  afterEach(() => {
+    warnSpy.mockRestore();
+    sqlite.close();
+  });
+
+  it('missing/invalid federation headers → 401 (helper never emits /epoch\'s 400)', async () => {
+    seedPeer('active');
+    const app = await buildApp();
+    const res = await probe(app, { 'Content-Type': 'application/json' }, { hello: 'world' });
+    expect(res.statusCode).toBe(401);
+    expect(JSON.parse(res.body)).toEqual({ error: 'Missing or malformed federation headers', statusCode: 401 });
+    // Guard against the non-adopter /epoch's 400: the shared helper is 401-only here.
+    expect(res.statusCode).not.toBe(400);
+  });
+
+  it('peer not found → 403 with exact body', async () => {
+    // No peer seeded.
+    const app = await buildApp();
+    const body = { hello: 'world' };
+    const res = await probe(app, signedHeaders(JSON.stringify(body)), body);
+    expect(res.statusCode).toBe(403);
+    expect(JSON.parse(res.body)).toEqual({ error: 'Unknown or inactive peer', statusCode: 403 });
+  });
+
+  it('peer present but non-active status → 403 with exact body', async () => {
+    seedPeer('needs_attention');
+    const app = await buildApp();
+    const body = { hello: 'world' };
+    const res = await probe(app, signedHeaders(JSON.stringify(body)), body);
+    expect(res.statusCode).toBe(403);
+    expect(JSON.parse(res.body)).toEqual({ error: 'Unknown or inactive peer', statusCode: 403 });
+  });
+
+  it('rate-limited WITH retryAfterSeconds → 429 + Retry-After header', async () => {
+    seedPeer('active');
+    const app = await buildApp({ rateLimiter: { limited: () => true, retryAfterSeconds: 60 } });
+    const body = { hello: 'world' };
+    const res = await probe(app, signedHeaders(JSON.stringify(body)), body);
+    expect(res.statusCode).toBe(429);
+    expect(res.headers['retry-after']).toBe('60');
+    expect(JSON.parse(res.body)).toEqual({ error: 'Rate limit exceeded', statusCode: 429 });
+  });
+
+  it('rate-limited WITHOUT retryAfterSeconds → 429, no Retry-After header', async () => {
+    seedPeer('active');
+    const app = await buildApp({ rateLimiter: { limited: () => true } });
+    const body = { hello: 'world' };
+    const res = await probe(app, signedHeaders(JSON.stringify(body)), body);
+    expect(res.statusCode).toBe(429);
+    expect(res.headers['retry-after']).toBeUndefined();
+    expect(JSON.parse(res.body)).toEqual({ error: 'Rate limit exceeded', statusCode: 429 });
+  });
+
+  it('rate-limit fires BEFORE signature verification (verifyPeerSignature not reached)', async () => {
+    seedPeer('active');
+    const app = await buildApp({ rateLimiter: { limited: () => true, retryAfterSeconds: 60 } });
+    // Deliberately BAD signature: if signature ran first we would see 401, not 429.
+    const headers = signedHeaders(JSON.stringify({ hello: 'world' }));
+    headers['X-Federation-Signature'] = 'sha256=' + 'f'.repeat(64);
+    const res = await probe(app, headers, { hello: 'world' });
+    expect(res.statusCode).toBe(429);
+    expect(verifySpy).not.toHaveBeenCalled();
+  });
+
+  it('bad signature → 401 with exact body', async () => {
+    seedPeer('active');
+    const app = await buildApp();
+    const headers = signedHeaders(JSON.stringify({ hello: 'world' }));
+    headers['X-Federation-Signature'] = 'sha256=' + 'f'.repeat(64);
+    const res = await probe(app, headers, { hello: 'world' });
+    expect(res.statusCode).toBe(401);
+    expect(JSON.parse(res.body)).toEqual({ error: 'Invalid signature', statusCode: 401 });
+  });
+
+  it('duplicate nonce → 409 with exact body', async () => {
+    seedPeer('active');
+    const app = await buildApp();
+    const body = { hello: 'world' };
+    const nonce = 'dup-nonce-fixed-1';
+    // First request records the nonce and passes.
+    const first = await probe(app, signedHeaders(JSON.stringify(body), nonce), body);
+    expect(first.statusCode).toBe(200);
+    // Second request with the SAME nonce is a replay.
+    const second = await probe(app, signedHeaders(JSON.stringify(body), nonce), body);
+    expect(second.statusCode).toBe(409);
+    expect(JSON.parse(second.body)).toEqual({ error: 'Duplicate nonce — possible replay', statusCode: 409 });
+  });
+
+  it('nonce missing + peer SUPPORTS nonce → 401 with exact body', async () => {
+    seedPeer('active', 1); // nonceSupported = 1
+    const app = await buildApp();
+    const body = { hello: 'world' };
+    const res = await probe(app, signedHeadersNoNonce(JSON.stringify(body)), body);
+    expect(res.statusCode).toBe(401);
+    expect(JSON.parse(res.body)).toEqual({ error: 'Nonce required — peer previously supported nonces', statusCode: 401 });
+  });
+
+  it('nonce missing + peer does NOT support nonce → passes; logMissingNonce=true warns', async () => {
+    seedPeer('active', 0);
+    const app = await buildApp({ logMissingNonce: true });
+    const body = { hello: 'world' };
+    const res = await probe(app, signedHeadersNoNonce(JSON.stringify(body)), body);
+    expect(res.statusCode).toBe(200);
+    expect(JSON.parse(res.body)).toEqual({ ok: true, peerOrigin: PEER_ORIGIN, nonce: null });
+    expect(warnSpy).toHaveBeenCalledWith(
+      `[federation] Peer ${PEER_ORIGIN} does not support replay protection (no nonce)`,
+    );
+  });
+
+  it('nonce missing + peer does NOT support nonce → passes; logMissingNonce=false stays silent', async () => {
+    seedPeer('active', 0);
+    const app = await buildApp({ logMissingNonce: false });
+    const body = { hello: 'world' };
+    const res = await probe(app, signedHeadersNoNonce(JSON.stringify(body)), body);
+    expect(res.statusCode).toBe(200);
+    expect(warnSpy).not.toHaveBeenCalled();
+  });
+
+  it('logContext appends the endpoint suffix to the missing-nonce warn (sync parity)', async () => {
+    seedPeer('active', 0);
+    const app = await buildApp({ logMissingNonce: true, logContext: 'sync' });
+    const body = { hello: 'world' };
+    const res = await probe(app, signedHeadersNoNonce(JSON.stringify(body)), body);
+    expect(res.statusCode).toBe(200);
+    expect(warnSpy).toHaveBeenCalledWith(
+      `[federation] Peer ${PEER_ORIGIN} does not support replay protection (no nonce) [sync]`,
+    );
+  });
+
+  it('success → { ok:true, peer, nonce } with the parsed nonce', async () => {
+    seedPeer('active');
+    const app = await buildApp();
+    const body = { hello: 'world' };
+    const nonce = 'success-nonce-1';
+    const res = await probe(app, signedHeaders(JSON.stringify(body), nonce), body);
+    expect(res.statusCode).toBe(200);
+    expect(JSON.parse(res.body)).toEqual({ ok: true, peerOrigin: PEER_ORIGIN, nonce });
+  });
+
+  it('success with a rate-limiter that is under the cap → passes through', async () => {
+    seedPeer('active');
+    const app = await buildApp({ rateLimiter: { limited: () => false, retryAfterSeconds: 60 } });
+    const body = { hello: 'world' };
+    const res = await probe(app, signedHeaders(JSON.stringify(body)), body);
+    expect(res.statusCode).toBe(200);
+    expect(JSON.parse(res.body).ok).toBe(true);
+  });
+});
diff --git a/packages/server/src/routes/federation/handlers/s2sAuth.ts b/packages/server/src/routes/federation/handlers/s2sAuth.ts
new file mode 100644
index 00000000..deebda3d
--- /dev/null
+++ b/packages/server/src/routes/federation/handlers/s2sAuth.ts
@@ -0,0 +1,138 @@
+import { eq } from 'drizzle-orm';
+import type { FastifyReply, FastifyRequest } from 'fastify';
+import { getDb, schema } from '../../../db/index.js';
+import { parseFederationHeaders, verifyPeerSignature } from '../../../utils/federationAuth.js';
+import { isNonceDuplicate } from '../rateLimits.js';
+
+/** A `federation_peers` row, as returned by a `select().from(...).get()`. */
+type FederationPeerRow = typeof schema.federationPeers.$inferSelect;
+
+/**
+ * A rate limiter for the auth preamble. `limited(key)` returns true once the key
+ * (always `peer.origin` here) is at capacity. When `retryAfterSeconds` is set, a
+ * `Retry-After` header carrying that value is added to the 429 response.
+ */
+export interface S2SRateLimiter {
+  limited: (key: string) => boolean;
+  retryAfterSeconds?: number;
+}
+
+export interface S2SAuthOptions {
+  /** Run this limiter (keyed on `peer.origin`) BEFORE signature verification. */
+  rateLimiter?: S2SRateLimiter;
+  /**
+   * When a request omits a nonce AND the peer has never advertised nonce
+   * support, emit the legacy `console.warn`. Endpoints that historically logged
+   * this pass `true`; those that stayed silent pass `false`/omit.
+   */
+  logMissingNonce?: boolean;
+  /**
+   * Optional suffix for the missing-nonce warning, appended as ` [${logContext}]`.
+   * Preserves the per-endpoint log tag (`/sync` logged a ` [sync]` suffix; the
+   * `/identity` and `/relay` handlers logged no suffix).
+   */
+  logContext?: string;
+}
+
+/**
+ * Result of {@link authenticateS2SPeer}. On `ok: false` a reply has ALREADY been
+ * sent — the caller MUST `return` immediately without touching `reply` again.
+ */
+export type S2SAuthResult =
+  | { ok: true; peer: FederationPeerRow; nonce: string | null }
+  | { ok: false };
+
+/**
+ * Shared inbound S2S-auth preamble for HMAC-signed federation endpoints.
+ *
+ * Runs, IN THIS EXACT ORDER, the boilerplate that six endpoints share verbatim:
+ *   1. Parse federation headers — missing/malformed → 401.
+ *   2. Resolve the peer by origin; require `status === 'active'` → else 403.
+ *   3. (optional) Rate-limit on `peer.origin` — 429 (+ `Retry-After` when
+ *      configured). Deliberately BEFORE signature verification so a flooded peer
+ *      never costs an HMAC computation.
+ *   4. Verify the HMAC signature (honours rotation grace) → 401 on failure.
+ *   5. Nonce replay protection: present + duplicate → 409; absent while the peer
+ *      advertises nonce support → 401; absent otherwise → pass (optionally warn).
+ *
+ * On success returns `{ ok: true, peer, nonce }`; the caller resumes with its
+ * own body validation and side effects. On any rejection the reply is sent and
+ * `{ ok: false }` is returned — the caller must `return` at once.
+ *
+ * ── INTENTIONAL NON-ADOPTERS (do NOT fold these into this helper) ─────────────
+ * Three S2S endpoints deliberately keep bespoke auth because a load-bearing gate
+ * differs; sharing this helper would silently flatten it:
+ *   • `POST /api/federation/epoch` — gates on `status !== 'revoked'` (ANY
+ *     non-revoked peer answers, so a needs_attention/unreachable peer can drive
+ *     RECOVERY via the signed epoch round-trip), returns **400** (not 401) on
+ *     missing headers, and runs **no** nonce check.
+ *   • `POST /api/federation/peer/rotate` — active-only but runs **no** nonce
+ *     check (a lone shape; the rotation body is the replay unit).
+ *   • `POST /api/federation/peer/denied` — gates on `awaiting_approval` (404 on
+ *     no peer row, 409 on wrong status) and verifies against a SYNTHETIC
+ *     no-grace secret object; entirely different control flow.
+ * Also out of scope: `/peer/accept`, `/peer/initiate`, `/peer/ensure`
+ * (first-contact / JWT, not S2S-HMAC).
+ */
+export function authenticateS2SPeer(
+  request: FastifyRequest,
+  reply: FastifyReply,
+  opts: S2SAuthOptions = {},
+): S2SAuthResult {
+  const db = getDb();
+
+  // 1. Parse and require federation headers.
+  const fedHeaders = parseFederationHeaders(
+    request.headers as Record<string, string | string[] | undefined>,
+  );
+  if (!fedHeaders) {
+    reply.code(401).send({ error: 'Missing or malformed federation headers', statusCode: 401 });
+    return { ok: false };
+  }
+
+  // 2. Resolve the peer by origin; require an active relationship.
+  const peer = db
+    .select()
+    .from(schema.federationPeers)
+    .where(eq(schema.federationPeers.origin, fedHeaders.origin))
+    .get();
+
+  if (!peer || peer.status !== 'active') {
+    reply.code(403).send({ error: 'Unknown or inactive peer', statusCode: 403 });
+    return { ok: false };
+  }
+
+  // 3. Rate-limit BEFORE signature verification (avoid HMAC work on a flood).
+  if (opts.rateLimiter && opts.rateLimiter.limited(peer.origin)) {
+    reply.code(429);
+    if (opts.rateLimiter.retryAfterSeconds !== undefined) {
+      reply.header('Retry-After', String(opts.rateLimiter.retryAfterSeconds));
+    }
+    reply.send({ error: 'Rate limit exceeded', statusCode: 429 });
+    return { ok: false };
+  }
+
+  // 4. Verify the HMAC signature over the exact serialized body.
+  const bodyString = JSON.stringify(request.body);
+  if (!verifyPeerSignature(bodyString, fedHeaders.signature, fedHeaders.timestamp, fedHeaders.nonce, peer)) {
+    reply.code(401).send({ error: 'Invalid signature', statusCode: 401 });
+    return { ok: false };
+  }
+
+  // 5. Nonce-based replay protection.
+  if (fedHeaders.nonce) {
+    if (isNonceDuplicate(peer.origin, fedHeaders.nonce)) {
+      reply.code(409).send({ error: 'Duplicate nonce — possible replay', statusCode: 409 });
+      return { ok: false };
+    }
+  } else if (peer.nonceSupported) {
+    // Peer previously proved nonce support but this request omits one — reject.
+    reply.code(401).send({ error: 'Nonce required — peer previously supported nonces', statusCode: 401 });
+    return { ok: false };
+  } else if (opts.logMissingNonce) {
+    const suffix = opts.logContext ? ` [${opts.logContext}]` : '';
+    console.warn(`[federation] Peer ${peer.origin} does not support replay protection (no nonce)${suffix}`);
+  }
+
+  return { ok: true, peer, nonce: fedHeaders.nonce };
+}