diff --git a/src/admin/web/manage.ts b/src/admin/web/manage.ts
index dd031a7..62421ec 100644
--- a/src/admin/web/manage.ts
+++ b/src/admin/web/manage.ts
@@ -17,7 +17,7 @@ import {
 } from "../../shared/users/schema";
 import { getLastNImages } from "../../shared/file-storage/image-history";
 import { blacklists, parseCidrs, whitelists } from "../../shared/cidr";
-import { invalidatePowHmacKey } from "../../user/web/pow-captcha";
+import { invalidatePowChallenges } from "../../user/web/pow-captcha";
 
 const router = Router();
 
@@ -323,7 +323,7 @@ router.post("/maintenance", (req, res) => {
         user.disabledReason = "Admin forced expiration.";
         userStore.upsertUser(user);
       });
-      invalidatePowHmacKey();
+      invalidatePowChallenges();
       flash.type = "success";
       flash.message = `${temps.length} temporary users marked for expiration.`;
       break;
@@ -348,6 +348,7 @@ router.post("/maintenance", (req, res) => {
         throw new HttpError(400, "Invalid difficulty" + selected);
       }
       config.powDifficultyLevel = selected;
+      invalidatePowChallenges();
       break;
     }
     case "generateTempIpReport": {
diff --git a/src/config.ts b/src/config.ts
index d4e4899..ad59f1d 100644
--- a/src/config.ts
+++ b/src/config.ts
@@ -519,7 +519,7 @@ function generateSigningKey() {
 }
 
 const signingKey = generateSigningKey();
-export const COOKIE_SECRET = signingKey;
+export const SECRET_SIGNING_KEY = signingKey;
 
 export async function assertConfigIsValid() {
   if (process.env.MODEL_RATE_LIMIT !== undefined) {
diff --git a/src/shared/hmac-signing.ts b/src/shared/hmac-signing.ts
new file mode 100644
index 0000000..6216857
--- /dev/null
+++ b/src/shared/hmac-signing.ts
@@ -0,0 +1,18 @@
+/** Module for generating and verifying HMAC signatures. */
+
+import crypto from "crypto";
+import { SECRET_SIGNING_KEY } from "../config";
+
+/**
+ * Generates a HMAC signature for the given message. Optionally salts the
+ * key with a provided string.
+ */
+export function signMessage(msg: any, salt: string = ""): string {
+  const hmac = crypto.createHmac("sha256", SECRET_SIGNING_KEY + salt);
+  if (typeof msg === "object") {
+    hmac.update(JSON.stringify(msg));
+  } else {
+    hmac.update(msg);
+  }
+  return hmac.digest("hex");
+}
diff --git a/src/shared/inject-csrf.ts b/src/shared/inject-csrf.ts
index b438740..24cb051 100644
--- a/src/shared/inject-csrf.ts
+++ b/src/shared/inject-csrf.ts
@@ -1,9 +1,9 @@
 import { doubleCsrf } from "csrf-csrf";
 import express from "express";
-import { config, COOKIE_SECRET } from "../config";
+import { config, SECRET_SIGNING_KEY } from "../config";
 
 const { generateToken, doubleCsrfProtection } = doubleCsrf({
-  getSecret: () => COOKIE_SECRET,
+  getSecret: () => SECRET_SIGNING_KEY,
   cookieName: "csrf",
   cookieOptions: {
     sameSite: "strict",
diff --git a/src/shared/with-session.ts b/src/shared/with-session.ts
index 25448f2..89693ff 100644
--- a/src/shared/with-session.ts
+++ b/src/shared/with-session.ts
@@ -1,14 +1,14 @@
 import cookieParser from "cookie-parser";
 import expressSession from "express-session";
 import MemoryStore from "memorystore";
-import { config, COOKIE_SECRET } from "../config";
+import { config, SECRET_SIGNING_KEY } from "../config";
 
 const ONE_WEEK = 1000 * 60 * 60 * 24 * 7;
 
-const cookieParserMiddleware = cookieParser(COOKIE_SECRET);
+const cookieParserMiddleware = cookieParser(SECRET_SIGNING_KEY);
 
 const sessionMiddleware = expressSession({
-  secret: COOKIE_SECRET,
+  secret: SECRET_SIGNING_KEY,
   resave: false,
   saveUninitialized: false,
   store: new (MemoryStore(expressSession))({ checkPeriod: ONE_WEEK }),
diff --git a/src/user/web/pow-captcha.ts b/src/user/web/pow-captcha.ts
index 8e7e9fc..b57b393 100644
--- a/src/user/web/pow-captcha.ts
+++ b/src/user/web/pow-captcha.ts
@@ -2,6 +2,7 @@ import crypto from "crypto";
 import express from "express";
 import argon2 from "@node-rs/argon2";
 import { z } from "zod";
+import { signMessage } from "../../shared/hmac-signing";
 import {
   authenticate,
   createUser,
@@ -13,15 +14,13 @@ import { config } from "../../config";
 /** Lockout time after verification in milliseconds */
 const LOCKOUT_TIME = 1000 * 60; // 60 seconds
 
-/** HMAC key for signing challenges; regenerated on startup */
-let hmacSecret = crypto.randomBytes(32).toString("hex");
+let powKeySalt = crypto.randomBytes(32).toString("hex");
 
 /**
- * Regenerate the HMAC key used for signing challenges. Calling this function
- * will invalidate all existing challenges.
+ * Invalidates any outstanding unsolved challenges.
  */
-export function invalidatePowHmacKey() {
-  hmacSecret = crypto.randomBytes(32).toString("hex");
+export function invalidatePowChallenges() {
+  powKeySalt = crypto.randomBytes(32).toString("hex");
 }
 
 const argon2Params = {
@@ -141,16 +140,6 @@ function generateChallenge(clientIp?: string, token?: string): Challenge {
   };
 }
 
-function signMessage(msg: any): string {
-  const hmac = crypto.createHmac("sha256", hmacSecret);
-  if (typeof msg === "object") {
-    hmac.update(JSON.stringify(msg));
-  } else {
-    hmac.update(msg);
-  }
-  return hmac.digest("hex");
-}
-
 async function verifySolution(
   challenge: Challenge,
   solution: string,
@@ -225,11 +214,11 @@ router.post("/challenge", (req, res) => {
       return;
     }
     const challenge = generateChallenge(req.ip, refreshToken);
-    const signature = signMessage(challenge);
+    const signature = signMessage(challenge, powKeySalt);
     res.json({ challenge, signature });
   } else {
     const challenge = generateChallenge(req.ip);
-    const signature = signMessage(challenge);
+    const signature = signMessage(challenge, powKeySalt);
     res.json({ challenge, signature });
   }
 });
@@ -253,7 +242,7 @@ router.post("/verify", async (req, res) => {
   }
 
   const { challenge, signature, solution } = result.data;
-  if (signMessage(challenge) !== signature) {
+  if (signMessage(challenge, powKeySalt) !== signature) {
     res.status(400).json({
       error:
         "Invalid signature; server may have restarted since challenge was issued. Please request a new challenge.",