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
@@ -22,6 +22,7 @@
 } from 'lib/types/device-types.js';
 import type { CalendarQuery } from 'lib/types/entry-types.js';
 import { messageTypes } from 'lib/types/message-types';
+import type { SIWESocialProof } from 'lib/types/siwe-types.js';
 import { threadTypes } from 'lib/types/thread-types';
 import { ServerError } from 'lib/utils/errors';
 import { values } from 'lib/utils/objects';
@@ -208,6 +209,7 @@
   +deviceTokenUpdateRequest?: ?DeviceTokenUpdateRequest,
   +platformDetails: PlatformDetails,
   +primaryIdentityPublicKey: ?string,
+  +socialProof: SIWESocialProof,
 };
 // Note: `processSIWEAccountCreation(...)` assumes that the validity of
 //       `ProcessSIWEAccountCreationRequest` was checked at call site.
@@ -237,6 +239,7 @@
       platformDetails: request.platformDetails,
       deviceToken,
       primaryIdentityPublicKey: request.primaryIdentityPublicKey,
+      socialProof: request.socialProof,
     }),
     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
@@ -26,7 +26,11 @@
 } from 'lib/types/account-types';
 import type { CalendarQuery } from 'lib/types/entry-types.js';
 import { defaultNumberPerThread } from 'lib/types/message-types';
-import type { SIWEAuthRequest, SIWEMessage } from 'lib/types/siwe-types.js';
+import type {
+  SIWEAuthRequest,
+  SIWEMessage,
+  SIWESocialProof,
+} from 'lib/types/siwe-types.js';
 import type {
   SubscriptionUpdateRequest,
   SubscriptionUpdateResponse,
@@ -204,6 +208,7 @@
   userID: string,
   calendarQuery: ?CalendarQuery,
   primaryIdentityPublicKey?: ?string,
+  socialProof?: ?SIWESocialProof,
 ): Promise<LogInResponse> {
   const request: LogInRequest = input;
   const newServerTime = Date.now();
@@ -215,6 +220,7 @@
       platformDetails: request.platformDetails,
       deviceToken,
       primaryIdentityPublicKey,
+      socialProof,
     }),
     deleteCookie(viewer.cookieID),
   ]);
@@ -383,7 +389,14 @@
       ? getPublicKeyFromSIWEStatement(statement)
       : null;
 
-  // 5. Create account with call to `processSIWEAccountCreation(...)`
+  // 5. Construct `SIWESocialProof` object with the stringified
+  //    SIWEMessage and the corresponding signature.
+  const socialProof: SIWESocialProof = {
+    siweMessage: siweMessage.toMessage(),
+    siweMessageSignature: signature,
+  };
+
+  // 6. Create account with call to `processSIWEAccountCreation(...)`
   //    if address does not correspond to an existing user.
   let userID = await fetchUserIDForEthereumAddress(siweMessage.address);
   if (!userID) {
@@ -392,6 +405,7 @@
       deviceTokenUpdateRequest: deviceTokenUpdateRequest,
       platformDetails,
       primaryIdentityPublicKey: primaryIdentityPublicKey,
+      socialProof: socialProof,
     };
     userID = await processSIWEAccountCreation(
       viewer,
@@ -399,13 +413,14 @@
     );
   }
 
-  // 6. Complete login with call to `processSuccessfulLogin(...)`.
+  // 7. Complete login with call to `processSuccessfulLogin(...)`.
   return await processSuccessfulLogin(
     viewer,
     input,
     userID,
     calendarQuery,
     primaryIdentityPublicKey,
+    socialProof,
   );
 }
 
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
@@ -19,6 +19,7 @@
   sessionIdentifierTypes,
   type SessionIdentifierType,
 } from 'lib/types/session-types';
+import type { SIWESocialProof } from 'lib/types/siwe-types.js';
 import type { InitialClientSocketMessage } from 'lib/types/socket-types';
 import type { UserInfo } from 'lib/types/user-types';
 import { values } from 'lib/utils/objects';
@@ -645,6 +646,7 @@
   platformDetails: PlatformDetails,
   deviceToken?: ?string,
   primaryIdentityPublicKey?: ?string,
+  socialProof?: ?SIWESocialProof,
 };
 
 // The result of this function should never be passed directly to the Viewer
@@ -658,7 +660,12 @@
   userID: string,
   params: UserCookieCreationParams,
 ): Promise<UserViewerData> {
-  const { platformDetails, deviceToken, primaryIdentityPublicKey } = params;
+  const {
+    platformDetails,
+    deviceToken,
+    primaryIdentityPublicKey,
+    socialProof,
+  } = params;
   const { platform, ...versions } = platformDetails || defaultPlatformDetails;
   const versionsString =
     Object.keys(versions).length > 0 ? JSON.stringify(versions) : null;
@@ -681,10 +688,11 @@
     deviceToken,
     versionsString,
     primaryIdentityPublicKey,
+    JSON.stringify(socialProof),
   ];
   const query = SQL`
     INSERT INTO cookies(id, hash, user, platform, creation_time, last_used,
-      device_token, versions, public_key)
+      device_token, versions, public_key, social_proof)
     VALUES ${[cookieRow]}
   `;
   await dbQuery(query);
diff --git a/lib/types/siwe-types.js b/lib/types/siwe-types.js
--- a/lib/types/siwe-types.js
+++ b/lib/types/siwe-types.js
@@ -26,6 +26,11 @@
   ...LogInExtraInfo,
 };
 
+export type SIWESocialProof = {
+  +siweMessage: string,
+  +siweMessageSignature: string,
+};
+
 // This is a message that the rendered webpage (landing/siwe.react.js) uses to
 // communicate back to the React Native WebView that is rendering it
 // (native/account/siwe-panel.react.js)
@@ -106,4 +111,5 @@
   // message version.
   +type?: 'Personal signature',
   +validate: (signature: string, provider?: any) => Promise<SIWEMessage>,
+  +toMessage: () => string,
 };