diff --git a/src/lib/modules/crypto.ts b/src/lib/modules/crypto.ts
index 8a476d1..da57744 100644
--- a/src/lib/modules/crypto.ts
+++ b/src/lib/modules/crypto.ts
@@ -76,6 +76,22 @@ export const signRSAMessage = async (message: BufferSource, privateKey: CryptoKe
   );
 };
 
+export const verifyRSASignature = async (
+  message: BufferSource,
+  signature: BufferSource,
+  publicKey: CryptoKey,
+) => {
+  return await window.crypto.subtle.verify(
+    {
+      name: "RSA-PSS",
+      saltLength: 32,
+    } satisfies RsaPssParams,
+    publicKey,
+    signature,
+    message,
+  );
+};
+
 export const generateAESKey = async () => {
   return await window.crypto.subtle.generateKey(
     {
@@ -136,3 +152,24 @@ export const signRequest = async <T>(data: T, privateKey: CryptoKey) => {
     signature: encodeToBase64(signature),
   });
 };
+
+export const signMasterKeyWrapped = async (
+  version: number,
+  masterKeyWrapped: ArrayBuffer,
+  privateKey: CryptoKey,
+) => {
+  const data = JSON.stringify({ version, key: encodeToBase64(masterKeyWrapped) });
+  const dataBuffer = new TextEncoder().encode(data);
+  return encodeToBase64(await signRSAMessage(dataBuffer, privateKey));
+};
+
+export const verifyMasterKeyWrappedSig = async (
+  version: number,
+  masterKeyWrappedBase64: string,
+  masterKeyWrappedSig: string,
+  publicKey: CryptoKey,
+) => {
+  const data = JSON.stringify({ version, key: masterKeyWrappedBase64 });
+  const dataBuffer = new TextEncoder().encode(data);
+  return await verifyRSASignature(dataBuffer, decodeFromBase64(masterKeyWrappedSig), publicKey);
+};
diff --git a/src/lib/server/db/mek.ts b/src/lib/server/db/mek.ts
index 3c0bfff..94e6553 100644
--- a/src/lib/server/db/mek.ts
+++ b/src/lib/server/db/mek.ts
@@ -1,8 +1,13 @@
-import { and, or, eq, lt, desc } from "drizzle-orm";
+import { and, or, eq } from "drizzle-orm";
 import db from "./drizzle";
-import { mek, clientMek, userClient } from "./schema";
+import { mek, clientMek } from "./schema";
 
-export const registerInitialMek = async (userId: number, createdBy: number, encMek: string) => {
+export const registerInitialMek = async (
+  userId: number,
+  createdBy: number,
+  encMek: string,
+  encMekSig: string,
+) => {
   await db.transaction(async (tx) => {
     await tx.insert(mek).values({
       userId,
@@ -16,70 +21,11 @@ export const registerInitialMek = async (userId: number, createdBy: number, encM
       clientId: createdBy,
       mekVersion: 1,
       encMek,
+      encMekSig,
     });
   });
 };
 
-export const registerActiveMek = async (
-  userId: number,
-  version: number,
-  createdBy: number,
-  clientMeks: {
-    clientId: number;
-    encMek: string;
-  }[],
-) => {
-  await db.transaction(async (tx) => {
-    // 1. Check if the clientMeks are valid
-    const userClients = await tx
-      .select()
-      .from(userClient)
-      .where(and(eq(userClient.userId, userId), eq(userClient.state, "active")));
-    if (
-      clientMeks.length !== userClients.length ||
-      !clientMeks.every((clientMek) =>
-        userClients.some((userClient) => userClient.clientId === clientMek.clientId),
-      )
-    ) {
-      throw new Error("Invalid key list");
-    }
-
-    // 2. Retire the old active MEK and insert the new one
-    await tx
-      .update(mek)
-      .set({
-        state: "retired",
-        retiredAt: new Date(),
-      })
-      .where(and(eq(mek.userId, userId), lt(mek.version, version), eq(mek.state, "active")));
-    await tx.insert(mek).values({
-      userId,
-      version,
-      createdBy,
-      createdAt: new Date(),
-      state: "active",
-    });
-
-    // 3. Insert the new client MEKs
-    await tx.insert(clientMek).values(
-      clientMeks.map(({ clientId, encMek }) => ({
-        userId,
-        clientId,
-        mekVersion: version,
-        encMek,
-      })),
-    );
-  });
-};
-
-export const getAllValidMeks = async (userId: number) => {
-  return await db
-    .select()
-    .from(mek)
-    .where(and(eq(mek.userId, userId), or(eq(mek.state, "active"), eq(mek.state, "retired"))))
-    .execute();
-};
-
 export const getInitialMek = async (userId: number) => {
   const meks = await db
     .select()
@@ -89,68 +35,6 @@ export const getInitialMek = async (userId: number) => {
   return meks[0] ?? null;
 };
 
-export const getNextActiveMekVersion = async (userId: number) => {
-  const meks = await db
-    .select({ version: mek.version })
-    .from(mek)
-    .where(eq(mek.userId, userId))
-    .orderBy(desc(mek.version))
-    .limit(1)
-    .execute();
-  if (!meks[0]) {
-    throw new Error("No MEK found");
-  }
-  return meks[0].version + 1;
-};
-
-export const registerClientMeks = async (
-  userId: number,
-  clientId: number,
-  clientMeks: {
-    version: number;
-    encMek: string;
-  }[],
-) => {
-  await db.transaction(async (tx) => {
-    // 1. Check if the client is valid
-    const userClients = await tx
-      .select()
-      .from(userClient)
-      .where(
-        and(
-          eq(userClient.userId, userId),
-          eq(userClient.clientId, clientId),
-          eq(userClient.state, "active"),
-        ),
-      );
-    if (userClients.length === 0) {
-      throw new Error("Invalid client");
-    }
-
-    // 2. Check if the clientMeks are valid
-    const meks = await tx
-      .select()
-      .from(mek)
-      .where(and(eq(mek.userId, userId), or(eq(mek.state, "active"), eq(mek.state, "retired"))));
-    if (
-      clientMeks.length !== meks.length ||
-      !clientMeks.every((clientMek) => meks.some((mek) => mek.version === clientMek.version))
-    ) {
-      throw new Error("Invalid key list");
-    }
-
-    // 3. Insert the client MEKs
-    await tx.insert(clientMek).values(
-      clientMeks.map(({ version, encMek }) => ({
-        userId,
-        clientId,
-        mekVersion: version,
-        encMek,
-      })),
-    );
-  });
-};
-
 export const getAllValidClientMeks = async (userId: number, clientId: number) => {
   return await db
     .select()
diff --git a/src/lib/server/db/schema/mek.ts b/src/lib/server/db/schema/mek.ts
index f3f8b6b..8ab6fb8 100644
--- a/src/lib/server/db/schema/mek.ts
+++ b/src/lib/server/db/schema/mek.ts
@@ -30,8 +30,9 @@ export const clientMek = sqliteTable(
     clientId: integer("client_id")
       .notNull()
       .references(() => client.id),
-    mekVersion: integer("master_encryption_key_version").notNull(),
-    encMek: text("encrypted_master_encryption_key").notNull(),
+    mekVersion: integer("version").notNull(),
+    encMek: text("encrypted_key").notNull(), // Base64
+    encMekSig: text("encrypted_key_signature").notNull(), // Base64
   },
   (t) => ({
     pk: primaryKey({ columns: [t.userId, t.clientId, t.mekVersion] }),
diff --git a/src/lib/server/modules/mek.ts b/src/lib/server/modules/mek.ts
index 940774a..0019ce0 100644
--- a/src/lib/server/modules/mek.ts
+++ b/src/lib/server/modules/mek.ts
@@ -1,6 +1,25 @@
+import { error } from "@sveltejs/kit";
+import { getUserClientWithDetails } from "$lib/server/db/client";
 import { getInitialMek } from "$lib/server/db/mek";
+import { verifySignature } from "$lib/server/modules/crypto";
 
 export const isInitialMekNeeded = async (userId: number) => {
   const initialMek = await getInitialMek(userId);
   return !initialMek;
 };
+
+export const verifyClientEncMekSig = async (
+  userId: number,
+  clientId: number,
+  version: number,
+  encMek: string,
+  encMekSig: string,
+) => {
+  const userClient = await getUserClientWithDetails(userId, clientId);
+  if (!userClient) {
+    error(500, "Invalid access token");
+  }
+
+  const data = JSON.stringify({ version, key: encMek });
+  return verifySignature(Buffer.from(data), encMekSig, userClient.client.sigPubKey);
+};
diff --git a/src/lib/server/services/mek.ts b/src/lib/server/services/mek.ts
index a3c8449..9fc0452 100644
--- a/src/lib/server/services/mek.ts
+++ b/src/lib/server/services/mek.ts
@@ -1,14 +1,7 @@
 import { error } from "@sveltejs/kit";
-import { getAllUserClients, setUserClientStateToActive } from "$lib/server/db/client";
-import {
-  registerInitialMek,
-  registerActiveMek,
-  getAllValidMeks,
-  getNextActiveMekVersion,
-  registerClientMeks,
-  getAllValidClientMeks,
-} from "$lib/server/db/mek";
-import { isInitialMekNeeded } from "$lib/server/modules/mek";
+import { setUserClientStateToActive } from "$lib/server/db/client";
+import { registerInitialMek, getAllValidClientMeks } from "$lib/server/db/mek";
+import { isInitialMekNeeded, verifyClientEncMekSig } from "$lib/server/modules/mek";
 
 export const getClientMekList = async (userId: number, clientId: number) => {
   const clientMeks = await getAllValidClientMeks(userId, clientId);
@@ -17,6 +10,7 @@ export const getClientMekList = async (userId: number, clientId: number) => {
       version: clientMek.master_encryption_key.version,
       state: clientMek.master_encryption_key.state,
       mek: clientMek.client_master_encryption_key.encMek,
+      mekSig: clientMek.client_master_encryption_key.encMekSig,
     })),
   };
 };
@@ -25,53 +19,14 @@ export const registerInitialActiveMek = async (
   userId: number,
   createdBy: number,
   encMek: string,
+  encMekSig: string,
 ) => {
   if (!(await isInitialMekNeeded(userId))) {
     error(409, "Initial MEK already registered");
+  } else if (!(await verifyClientEncMekSig(userId, createdBy, 1, encMek, encMekSig))) {
+    error(400, "Invalid signature");
   }
 
-  await registerInitialMek(userId, createdBy, encMek);
+  await registerInitialMek(userId, createdBy, encMek, encMekSig);
   await setUserClientStateToActive(userId, createdBy);
 };
-
-export const registerNewActiveMek = async (
-  userId: number,
-  createdBy: number,
-  clientMeks: {
-    clientId: number;
-    encMek: string;
-  }[],
-) => {
-  const userClients = await getAllUserClients(userId);
-  const activeUserClients = userClients.filter(({ state }) => state === "active");
-  if (
-    clientMeks.length !== activeUserClients.length ||
-    !clientMeks.every((clientMek) =>
-      activeUserClients.some((userClient) => userClient.clientId === clientMek.clientId),
-    )
-  ) {
-    error(400, "Invalid key list");
-  }
-
-  const newMekVersion = await getNextActiveMekVersion(userId);
-  await registerActiveMek(userId, newMekVersion, createdBy, clientMeks);
-};
-
-export const shareMeksForNewClient = async (
-  userId: number,
-  targetClientId: number,
-  clientMeks: {
-    version: number;
-    encMek: string;
-  }[],
-) => {
-  const meks = await getAllValidMeks(userId);
-  if (
-    clientMeks.length !== meks.length ||
-    !clientMeks.every((clientMek) => meks.some((mek) => mek.version === clientMek.version))
-  ) {
-    error(400, "Invalid key list");
-  }
-
-  await registerClientMeks(userId, targetClientId, clientMeks);
-};
diff --git a/src/lib/services/key.ts b/src/lib/services/key.ts
index d55cb93..2eba920 100644
--- a/src/lib/services/key.ts
+++ b/src/lib/services/key.ts
@@ -7,6 +7,7 @@ import {
   signRSAMessage,
   makeAESKeyNonextractable,
   unwrapAESKeyUsingRSA,
+  verifyMasterKeyWrappedSig,
 } from "$lib/modules/crypto";
 import { masterKeyStore } from "$lib/stores";
 
@@ -45,7 +46,7 @@ export const requestClientRegistration = async (
   return res.ok;
 };
 
-export const requestMasterKeyDownload = async (decryptKey: CryptoKey) => {
+export const requestMasterKeyDownload = async (decryptKey: CryptoKey, verfiyKey: CryptoKey) => {
   const res = await callAPI("/api/mek/list", { method: "GET" });
   if (!res.ok) return false;
 
@@ -55,17 +56,28 @@ export const requestMasterKeyDownload = async (decryptKey: CryptoKey) => {
       version: number;
       state: "active" | "retired";
       mek: string;
+      mekSig: string;
     }[];
   };
+
   const masterKeys = await Promise.all(
-    masterKeysWrapped.map(async ({ version, state, mek: masterKeyWrapped }) => ({
-      version,
-      state,
-      masterKey: await makeAESKeyNonextractable(
-        await unwrapAESKeyUsingRSA(decodeFromBase64(masterKeyWrapped), decryptKey),
-      ),
-    })),
+    masterKeysWrapped.map(
+      async ({ version, state, mek: masterKeyWrapped, mekSig: masterKeyWrappedSig }) => ({
+        version,
+        state,
+        masterKey: await makeAESKeyNonextractable(
+          await unwrapAESKeyUsingRSA(decodeFromBase64(masterKeyWrapped), decryptKey),
+        ),
+        isValid: await verifyMasterKeyWrappedSig(
+          version,
+          masterKeyWrapped,
+          masterKeyWrappedSig,
+          verfiyKey,
+        ),
+      }),
+    ),
   );
+  if (!masterKeys.every(({ isValid }) => isValid)) return false;
 
   await storeMasterKeys(
     masterKeys.map(({ version, state, masterKey }) => ({ version, state, key: masterKey })),
diff --git a/src/routes/(fullscreen)/auth/login/+page.svelte b/src/routes/(fullscreen)/auth/login/+page.svelte
index 776bb6d..299a523 100644
--- a/src/routes/(fullscreen)/auth/login/+page.svelte
+++ b/src/routes/(fullscreen)/auth/login/+page.svelte
@@ -29,7 +29,10 @@
 
       // TODO: Multi-user support
 
-      if ($masterKeyStore || (await requestMasterKeyDownload($clientKeyStore.decryptKey))) {
+      if (
+        $masterKeyStore ||
+        (await requestMasterKeyDownload($clientKeyStore.decryptKey, $clientKeyStore.verifyKey))
+      ) {
         await goto(data.redirectPath);
       } else {
         await redirect("/client/pending");
diff --git a/src/routes/(fullscreen)/client/pending/+page.svelte b/src/routes/(fullscreen)/client/pending/+page.svelte
index f362bd5..3d000b2 100644
--- a/src/routes/(fullscreen)/client/pending/+page.svelte
+++ b/src/routes/(fullscreen)/client/pending/+page.svelte
@@ -16,11 +16,13 @@
     if ($masterKeyStore) {
       goto(data.redirectPath);
     } else if ($clientKeyStore) {
-      requestMasterKeyDownload($clientKeyStore.decryptKey).then(async (ok) => {
-        if (ok) {
-          return await goto(data.redirectPath);
-        }
-      });
+      requestMasterKeyDownload($clientKeyStore.decryptKey, $clientKeyStore.verifyKey).then(
+        async (ok) => {
+          if (ok) {
+            return await goto(data.redirectPath);
+          }
+        },
+      );
     }
   });
 </script>
diff --git a/src/routes/(fullscreen)/key/export/service.ts b/src/routes/(fullscreen)/key/export/service.ts
index 68c4f60..cc8e7be 100644
--- a/src/routes/(fullscreen)/key/export/service.ts
+++ b/src/routes/(fullscreen)/key/export/service.ts
@@ -1,6 +1,6 @@
 import { callAPI } from "$lib/hooks";
 import { storeClientKey } from "$lib/indexedDB";
-import { encodeToBase64, signRequest } from "$lib/modules/crypto";
+import { encodeToBase64, signRequest, signMasterKeyWrapped } from "$lib/modules/crypto";
 import type { ClientKeys } from "$lib/stores";
 
 export { requestTokenUpgrade } from "$lib/services/auth";
@@ -53,6 +53,7 @@ export const requestInitialMasterKeyRegistration = async (
     body: await signRequest(
       {
         mek: encodeToBase64(masterKeyWrapped),
+        mekSig: await signMasterKeyWrapped(1, masterKeyWrapped, signKey),
       },
       signKey,
     ),
diff --git a/src/routes/api/mek/register/+server.ts b/src/routes/api/mek/register/+server.ts
deleted file mode 100644
index ab454a7..0000000
--- a/src/routes/api/mek/register/+server.ts
+++ /dev/null
@@ -1,32 +0,0 @@
-import { text } from "@sveltejs/kit";
-import { z } from "zod";
-import { authorize } from "$lib/server/modules/auth";
-import { parseSignedRequest } from "$lib/server/modules/crypto";
-import { registerNewActiveMek } from "$lib/server/services/mek";
-import type { RequestHandler } from "@sveltejs/kit";
-
-export const POST: RequestHandler = async ({ request, cookies }) => {
-  const { userId, clientId } = await authorize(cookies, "activeClient");
-  const { meks } = await parseSignedRequest(
-    clientId,
-    await request.json(),
-    z.object({
-      meks: z.array(
-        z.object({
-          clientId: z.number().int().positive(),
-          mek: z.string().base64().nonempty(),
-        }),
-      ),
-    }),
-  );
-
-  await registerNewActiveMek(
-    userId,
-    clientId,
-    meks.map(({ clientId, mek }) => ({
-      clientId,
-      encMek: mek,
-    })),
-  );
-  return text("MEK registered", { headers: { "Content-Type": "text/plain" } });
-};
diff --git a/src/routes/api/mek/register/initial/+server.ts b/src/routes/api/mek/register/initial/+server.ts
index 0f495ff..a5aa6d9 100644
--- a/src/routes/api/mek/register/initial/+server.ts
+++ b/src/routes/api/mek/register/initial/+server.ts
@@ -11,14 +11,15 @@ export const POST: RequestHandler = async ({ request, cookies }) => {
     error(403, "Forbidden");
   }
 
-  const { mek } = await parseSignedRequest(
+  const { mek, mekSig } = await parseSignedRequest(
     clientId,
     await request.json(),
     z.object({
       mek: z.string().base64().nonempty(),
+      mekSig: z.string().base64().nonempty(),
     }),
   );
 
-  await registerInitialActiveMek(userId, clientId, mek);
+  await registerInitialActiveMek(userId, clientId, mek, mekSig);
   return text("MEK registered", { headers: { "Content-Type": "text/plain" } });
 };
diff --git a/src/routes/api/mek/share/+server.ts b/src/routes/api/mek/share/+server.ts
deleted file mode 100644
index 122e659..0000000
--- a/src/routes/api/mek/share/+server.ts
+++ /dev/null
@@ -1,30 +0,0 @@
-import { text } from "@sveltejs/kit";
-import { z } from "zod";
-import { authorize } from "$lib/server/modules/auth";
-import { parseSignedRequest } from "$lib/server/modules/crypto";
-import { shareMeksForNewClient } from "$lib/server/services/mek";
-import type { RequestHandler } from "./$types";
-
-export const POST: RequestHandler = async ({ request, cookies }) => {
-  const { userId, clientId } = await authorize(cookies, "activeClient");
-  const { targetId, meks } = await parseSignedRequest(
-    clientId,
-    await request.json(),
-    z.object({
-      targetId: z.number().int().positive(),
-      meks: z.array(
-        z.object({
-          version: z.number().int().positive(),
-          mek: z.string().base64().nonempty(),
-        }),
-      ),
-    }),
-  );
-
-  await shareMeksForNewClient(
-    userId,
-    targetId,
-    meks.map(({ version, mek }) => ({ version, encMek: mek })),
-  );
-  return text("MEK shared", { headers: { "Content-Type": "text/plain" } });
-};