diff --git a/keyserver/src/creators/account-creator.js b/keyserver/src/creators/account-creator.js
--- a/keyserver/src/creators/account-creator.js
+++ b/keyserver/src/creators/account-creator.js
@@ -16,6 +16,7 @@
   RegisterResponse,
   RegisterRequest,
 } from 'lib/types/account-types.js';
+import type { SignedIdentityKeysBlob } from 'lib/types/crypto-types.js';
 import type {
   PlatformDetails,
   DeviceTokenUpdateRequest,
@@ -116,6 +117,7 @@
     createNewUserCookie(id, {
       platformDetails: request.platformDetails,
       deviceToken,
+      signedIdentityKeysBlob: request.signedIdentityKeysBlob,
     }),
     deleteCookie(viewer.cookieID),
     dbQuery(newUserQuery),
@@ -206,6 +208,7 @@
   +deviceTokenUpdateRequest?: ?DeviceTokenUpdateRequest,
   +platformDetails: PlatformDetails,
   +socialProof: SIWESocialProof,
+  +signedIdentityKeysBlob?: ?SignedIdentityKeysBlob,
 };
 // Note: `processSIWEAccountCreation(...)` assumes that the validity of
 //       `ProcessSIWEAccountCreationRequest` was checked at call site.
@@ -213,7 +216,7 @@
   viewer: Viewer,
   request: ProcessSIWEAccountCreationRequest,
 ): Promise<string> {
-  const { calendarQuery } = request;
+  const { calendarQuery, signedIdentityKeysBlob } = request;
   await verifyCalendarQueryThreadIDs(calendarQuery);
 
   const time = Date.now();
@@ -231,6 +234,7 @@
       platformDetails: request.platformDetails,
       deviceToken,
       socialProof: request.socialProof,
+      signedIdentityKeysBlob,
     }),
     deleteCookie(viewer.cookieID),
     dbQuery(newUserQuery),
diff --git a/keyserver/src/responders/user-responders.js b/keyserver/src/responders/user-responders.js
--- a/keyserver/src/responders/user-responders.js
+++ b/keyserver/src/responders/user-responders.js
@@ -217,6 +217,23 @@
 ): Promise<RegisterResponse> {
   const request: RegisterRequest = input;
   await validateInput(viewer, registerRequestInputValidator, request);
+  const { signedIdentityKeysBlob } = request;
+  if (signedIdentityKeysBlob) {
+    const identityKeys: IdentityKeysBlob = JSON.parse(
+      signedIdentityKeysBlob.payload,
+    );
+
+    const olmUtil: OLMUtility = getOLMUtility();
+    try {
+      olmUtil.ed25519_verify(
+        identityKeys.primaryIdentityPublicKeys.ed25519,
+        signedIdentityKeysBlob.payload,
+        signedIdentityKeysBlob.signature,
+      );
+    } catch (e) {
+      throw new ServerError('invalid_signature');
+    }
+  }
   return await createAccount(viewer, request);
 }
 
diff --git a/keyserver/src/session/cookies.js b/keyserver/src/session/cookies.js
--- a/keyserver/src/session/cookies.js
+++ b/keyserver/src/session/cookies.js
@@ -648,10 +648,10 @@
 }
 
 type UserCookieCreationParams = {
-  platformDetails: PlatformDetails,
-  deviceToken?: ?string,
-  socialProof?: ?SIWESocialProof,
-  signedIdentityKeysBlob?: ?SignedIdentityKeysBlob,
+  +platformDetails: PlatformDetails,
+  +deviceToken?: ?string,
+  +socialProof?: ?SIWESocialProof,
+  +signedIdentityKeysBlob?: ?SignedIdentityKeysBlob,
 };
 
 // The result of this function should never be passed directly to the Viewer
diff --git a/lib/types/account-types.js b/lib/types/account-types.js
--- a/lib/types/account-types.js
+++ b/lib/types/account-types.js
@@ -52,6 +52,7 @@
   +calendarQuery?: ?CalendarQuery,
   +deviceTokenUpdateRequest?: ?DeviceTokenUpdateRequest,
   +platformDetails: PlatformDetails,
+  +signedIdentityKeysBlob?: SignedIdentityKeysBlob,
 };
 
 export type RegisterResponse = {