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
@@ -24,7 +24,10 @@
   notificationTypeValues,
   logInActionSources,
 } from 'lib/types/account-types.js';
-import type { IdentityKeysBlob } from 'lib/types/crypto-types.js';
+import type {
+  IdentityKeysBlob,
+  SignedIdentityKeysBlob,
+} from 'lib/types/crypto-types.js';
 import type { CalendarQuery } from 'lib/types/entry-types.js';
 import { defaultNumberPerThread } from 'lib/types/message-types.js';
 import type {
@@ -217,12 +220,20 @@
   +userID: string,
   +calendarQuery: ?CalendarQuery,
   +socialProof?: ?SIWESocialProof,
+  +signedIdentityKeysBlob?: ?SignedIdentityKeysBlob,
 };
 
 async function processSuccessfulLogin(
   params: ProcessSuccessfulLoginParams,
 ): Promise<LogInResponse> {
-  const { viewer, input, userID, calendarQuery, socialProof } = params;
+  const {
+    viewer,
+    input,
+    userID,
+    calendarQuery,
+    socialProof,
+    signedIdentityKeysBlob,
+  } = params;
 
   const request: LogInRequest = input;
   const newServerTime = Date.now();
@@ -234,6 +245,7 @@
       platformDetails: request.platformDetails,
       deviceToken,
       socialProof,
+      signedIdentityKeysBlob,
     }),
     fetchNotAcknowledgedPolicies(userID, baseLegalPolicies),
     deleteCookie(viewer.cookieID),
@@ -396,6 +408,7 @@
     input,
     userID: id,
     calendarQuery,
+    signedIdentityKeysBlob,
   });
 }
 
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
@@ -8,6 +8,7 @@
 
 import { hasMinCodeVersion } from 'lib/shared/version-utils.js';
 import type { Shape } from 'lib/types/core.js';
+import type { SignedIdentityKeysBlob } from 'lib/types/crypto-types.js';
 import type { Platform, PlatformDetails } from 'lib/types/device-types.js';
 import type { CalendarQuery } from 'lib/types/entry-types.js';
 import {
@@ -649,6 +650,7 @@
   platformDetails: PlatformDetails,
   deviceToken?: ?string,
   socialProof?: ?SIWESocialProof,
+  signedIdentityKeysBlob?: ?SignedIdentityKeysBlob,
 };
 
 // The result of this function should never be passed directly to the Viewer
@@ -662,7 +664,8 @@
   userID: string,
   params: UserCookieCreationParams,
 ): Promise<UserViewerData> {
-  const { platformDetails, deviceToken, socialProof } = params;
+  const { platformDetails, deviceToken, socialProof, signedIdentityKeysBlob } =
+    params;
   const { platform, ...versions } = platformDetails || defaultPlatformDetails;
   const versionsString =
     Object.keys(versions).length > 0 ? JSON.stringify(versions) : null;
@@ -685,10 +688,11 @@
     deviceToken,
     versionsString,
     JSON.stringify(socialProof),
+    signedIdentityKeysBlob ? JSON.stringify(signedIdentityKeysBlob) : null,
   ];
   const query = SQL`
     INSERT INTO cookies(id, hash, user, platform, creation_time, last_used,
-      device_token, versions, social_proof)
+      device_token, versions, social_proof, signed_identity_keys)
     VALUES ${[cookieRow]}
   `;
   await dbQuery(query);