diff --git a/AGENTS.md b/AGENTS.md
index ae1f930..283fcbd 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -53,7 +53,7 @@ Guidelines for AI coding agents working on the official Micro Club website.
 - Rate limits are counted in MongoDB, shared by every server instance: the Server Actions call `isRateLimited()` from `src/lib/rate-limit.ts` (`rate_limits` collection, keyed by `clientIp()`), Better Auth uses `rateLimit.storage: "database"` (`rateLimit` collection, on in production only). Both read the client IP from `x-real-ip`, then `x-forwarded-for`.
 - `pnpm db:seed-registrations` fills the local database with fake registrations (emails `@seed.test`, replaced on each run; refuses a non-local `MONGODB_URI`). Their student IDs do not exist, so their page shows "File unavailable".
 - Emails go through `sendEmail()` in `src/lib/email.ts` (Resend, `RESEND_API_KEY`) with the shared branded layout (`renderEmail`). Without a key, development prints them to the server console.
-- Student IDs are uploaded by the browser straight to Cloudinary with a signature from `src/actions/public/sign-student-id-upload.ts` (`authenticated` assets, no public URL), then `src/actions/public/submit-registration.ts` reads the upload back and saves the registration. Needs the `CLOUDINARY_*` variables.
+- Student IDs are uploaded by the browser straight to Cloudinary with a signature from `src/actions/public/sign-student-id-upload.ts` (`authenticated` assets, no public URL), then `src/actions/public/submit-registration.ts` checks Cloudinary's signature of the upload response (no Admin API call: the free plan allows 500 per hour) and saves the registration. Rate limits there are per email, with a loose per-IP backstop (campus networks and CGNAT share one IP). Needs the `CLOUDINARY_*` variables.
 - Registrations open and close with `REGISTRATIONS_WINDOW` in `src/config/registrations.ts` (`opensAt` / `closesAt`, ISO dates or `null`), read through `registrationsPhase()` in `src/lib/registrations/window.ts`. Outside the window `/registrations` is a 404 and the actions refuse submissions; the menu bar shows "Opening soon" or "Register now" with a countdown (`RegistrationsButton` in `RegistrationsCorner`), or nothing when closed.
 
 ## Admin area and auth
diff --git a/src/actions/public/sign-student-id-upload.ts b/src/actions/public/sign-student-id-upload.ts
index 38cce53..bfb4ad7 100644
--- a/src/actions/public/sign-student-id-upload.ts
+++ b/src/actions/public/sign-student-id-upload.ts
@@ -11,11 +11,12 @@ export type SignatureResult =
   | { status: "rate-limited" }
   | { status: "error" }
 
-const RATE_LIMIT = { limit: 10, windowMs: 60 * 60 * 1000 }
+/** Per IP, loose: many students share one address (campus network, CGNAT), and retries re-sign. */
+const RATE_LIMIT = { limit: 60, windowMs: 60 * 60 * 1000 }
 
 /**
  * Hands the browser a signed, short-lived permission to upload one student ID to Cloudinary.
- * Each IP gets 10 per hour, so the signature cannot be used to fill the storage.
+ * Each IP gets 60 per hour, so the signature cannot be used to fill the storage.
  */
 export async function createStudentIdUploadSignature(): Promise<SignatureResult> {
   if (!areRegistrationsOpen()) return { status: "error" }
diff --git a/src/actions/public/submit-registration.ts b/src/actions/public/submit-registration.ts
index 9a480f3..630c358 100644
--- a/src/actions/public/submit-registration.ts
+++ b/src/actions/public/submit-registration.ts
@@ -3,7 +3,7 @@
 import { headers } from "next/headers"
 
 import { PHONE_COUNTRIES, STUDENT_ID_RULES } from "@/const/registrations"
-import { deleteStudentId, getStudentIdUpload, type UploadedStudentId } from "@/lib/cloudinary"
+import { deleteStudentId, verifyStudentIdUpload, type UploadedStudentId } from "@/lib/cloudinary"
 import { connectToDatabase } from "@/lib/mongodb"
 import { clientIp, isRateLimited } from "@/lib/rate-limit"
 import { RegistrationModel } from "@/lib/registrations/model"
@@ -35,7 +35,12 @@ export type RegistrationResult =
   | { status: "rate-limited" }
   | { status: "error" }
 
-const RATE_LIMIT = { limit: 5, windowMs: 60 * 60 * 1000 }
+/**
+ * Per IP, a loose backstop only: a campus network or a mobile carrier (CGNAT) puts many students
+ * behind one address. The real limit is per email, the one key a registration has.
+ */
+const IP_RATE_LIMIT = { limit: 30, windowMs: 60 * 60 * 1000 }
+const EMAIL_RATE_LIMIT = { limit: 5, windowMs: 60 * 60 * 1000 }
 const MONGO_DUPLICATE_KEY = 11000
 
 function readUploadReference(raw: FormDataEntryValue | null): StudentIdUpload | null {
@@ -44,7 +49,11 @@ function readUploadReference(raw: FormDataEntryValue | null): StudentIdUpload |
     const valid =
       typeof upload.publicId === "string" &&
       STUDENT_ID_PUBLIC_ID.test(upload.publicId) &&
-      (upload.resourceType === "image" || upload.resourceType === "raw")
+      (upload.resourceType === "image" || upload.resourceType === "raw") &&
+      typeof upload.version === "number" &&
+      typeof upload.signature === "string" &&
+      typeof upload.format === "string" &&
+      typeof upload.bytes === "number"
     return valid ? (upload as StudentIdUpload) : null
   } catch {
     return null
@@ -63,8 +72,8 @@ function uploadProblem(stored: UploadedStudentId) {
  * Saves a registration: the form values (`values`, JSON), a reference to the student ID the browser
  * already uploaded to Cloudinary (`upload`, JSON) and the honeypot, as one `FormData`.
  *
- * Order: validate -> rate limit -> duplicate email -> read the upload back from Cloudinary ->
- * save in MongoDB. A rejected registration (invalid file, duplicate) deletes its upload; after a
+ * Order: validate -> rate limit (per IP and per email) -> duplicate email -> check Cloudinary's
+ * signature of the upload -> save in MongoDB. A rejected registration (invalid file, duplicate) deletes its upload; after a
  * failure that may pass (database down) the upload stays, so the retry reuses it. A filled
  * honeypot gets a fake success (bots learn nothing). Needs `MONGODB_URI` and the `CLOUDINARY_*`
  * variables (`.env.example`).
@@ -108,8 +117,12 @@ export async function submitRegistration(formData: FormData): Promise<Registrati
 
   const requestHeaders = await headers()
   const ip = clientIp(requestHeaders)
-  if (await isRateLimited(`registration:${ip}`, RATE_LIMIT.limit, RATE_LIMIT.windowMs))
-    return { status: "rate-limited" }
+  const email = values.email.toLowerCase()
+  const [ipLimited, emailLimited] = await Promise.all([
+    isRateLimited(`registration:${ip}`, IP_RATE_LIMIT.limit, IP_RATE_LIMIT.windowMs),
+    isRateLimited(`registration-email:${email}`, EMAIL_RATE_LIMIT.limit, EMAIL_RATE_LIMIT.windowMs),
+  ])
+  if (ipLimited || emailLimited) return { status: "rate-limited" }
 
   const discard = () =>
     deleteStudentId(upload).catch((error) =>
@@ -119,13 +132,13 @@ export async function submitRegistration(formData: FormData): Promise<Registrati
   try {
     await connectToDatabase()
 
-    const email = values.email.toLowerCase()
     if (await RegistrationModel.exists({ email })) {
       await discard()
       return { status: "duplicate" }
     }
 
-    const stored = await getStudentIdUpload(upload.publicId, upload.resourceType)
+    // A forged reference is refused without touching the file: it may not be this visitor's.
+    const stored = verifyStudentIdUpload(upload)
     const problem = stored ? uploadProblem(stored) : "Please upload your ID again."
     if (!stored || problem) {
       if (stored) await discard()
diff --git a/src/lib/cloudinary.ts b/src/lib/cloudinary.ts
index 0b9fa8b..236451e 100644
--- a/src/lib/cloudinary.ts
+++ b/src/lib/cloudinary.ts
@@ -63,30 +63,44 @@ export function signStudentIdUpload(): StudentIdUploadSignature {
   }
 }
 
+/** The SDK's check of an upload response's signature, missing from its type definitions. */
+const verifyApiResponseSignature = (
+  cloudinary.utils as unknown as {
+    verify_api_response_signature: (publicId: string, version: number, signature: string) => boolean
+  }
+).verify_api_response_signature
+
 /**
- * Reads an uploaded student ID back from Cloudinary, so the size and format that get stored are
- * the real ones, not what the browser claims. `null` when there is no such upload.
+ * Checks an upload the browser reports against Cloudinary's signature of its upload response
+ * (`public_id` and `version`, signed with the API secret), so no Admin API call is needed: the
+ * free plan allows 500 of those per hour, which would cap the registrations. The format was already
+ * enforced by Cloudinary (`allowed_formats` is part of the signed upload), the size by the plan's
+ * file limit; the stored URL is signed here. `null` when the signature does not match.
  */
-export async function getStudentIdUpload(
-  publicId: string,
-  resourceType: string,
-): Promise<UploadedStudentId | null> {
+export function verifyStudentIdUpload(upload: {
+  publicId: string
+  resourceType: string
+  version: number
+  signature: string
+  format: string
+  bytes: number
+}): UploadedStudentId | null {
   configure()
-  try {
-    const resource = await cloudinary.api.resource(publicId, {
+  if (!verifyApiResponseSignature(upload.publicId, upload.version, upload.signature)) return null
+
+  return {
+    url: cloudinary.url(upload.publicId, {
       type: "authenticated",
-      resource_type: resourceType,
-    })
-    return {
-      url: resource.secure_url,
-      publicId: resource.public_id,
-      resourceType: resource.resource_type,
-      format: resource.format,
-      bytes: resource.bytes,
-    }
-  } catch (error) {
-    if ((error as { error?: { http_code?: number } }).error?.http_code === 404) return null
-    throw error
+      resource_type: upload.resourceType,
+      version: upload.version,
+      format: upload.format,
+      sign_url: true,
+      secure: true,
+    }),
+    publicId: upload.publicId,
+    resourceType: upload.resourceType,
+    format: upload.format,
+    bytes: upload.bytes,
   }
 }
 
diff --git a/src/lib/registrations/upload-student-id.ts b/src/lib/registrations/upload-student-id.ts
index 68ec3be..b4e4e97 100644
--- a/src/lib/registrations/upload-student-id.ts
+++ b/src/lib/registrations/upload-student-id.ts
@@ -1,9 +1,16 @@
 import type { StudentIdUploadSignature } from "@/lib/cloudinary"
 
-/** Reference to a student ID already stored in Cloudinary, sent with the registration. */
+/**
+ * Reference to a student ID already stored in Cloudinary, sent with the registration: Cloudinary's
+ * upload response, whose `signature` (over `publicId` and `version`) the server checks.
+ */
 export type StudentIdUpload = {
   publicId: string
   resourceType: string
+  version: number
+  signature: string
+  format: string
+  bytes: number
 }
 
 /** Public ids the server accepts: a file in the student ID folder, no path tricks. */
@@ -34,5 +41,12 @@ export async function uploadStudentId(
   if (!response.ok || !result?.public_id)
     throw new Error(result?.error?.message ?? `Upload failed (${response.status})`)
 
-  return { publicId: result.public_id, resourceType: result.resource_type }
+  return {
+    publicId: result.public_id,
+    resourceType: result.resource_type,
+    version: result.version,
+    signature: result.signature,
+    format: result.format,
+    bytes: result.bytes,
+  }
 }