diff --git a/keyserver/src/responders/landing-handler.js b/keyserver/src/responders/landing-handler.js
--- a/keyserver/src/responders/landing-handler.js
+++ b/keyserver/src/responders/landing-handler.js
@@ -13,6 +13,7 @@
   isValidPrimaryIdentityPublicKey,
   isValidSIWENonce,
   isValidSIWEMessageType,
+  isValidSIWEMessageString,
 } from 'lib/utils/siwe-utils.js';
 
 import { getMessageForException } from './utils.js';
@@ -152,6 +153,25 @@
     return;
   }
   const siweMessageType = ((siweMessageTypeRawString: any): SIWEMessageType);
+  const siweMessageToSign = req.header('siwe-message-to-sign');
+
+  if (
+    siweMessageToSign !== null &&
+    siweMessageToSign !== undefined &&
+    !isValidSIWEMessageString(decodeURIComponent(siweMessageToSign))
+  ) {
+    res.status(400).send({
+      message: 'Invalid siwe message to sign.',
+    });
+    return;
+  }
+
+  if (siweMessageToSign && (siweNonce || siwePrimaryIdentityPublicKey)) {
+    res.status(400).send({
+      message:
+        'Nonce and complete message to sign cannot be provided at the same time.',
+    });
+  }
 
   const [{ jsURL, fontURLs, cssInclude }, LandingSSR] = await Promise.all([
     getAssetInfo(),
@@ -215,6 +235,7 @@
       siweNonce={siweNonce}
       siwePrimaryIdentityPublicKey={siwePrimaryIdentityPublicKey}
       siweMessageType={siweMessageType}
+      siweMessageToSign={siweMessageToSign}
     />,
   );
   reactStream.pipe(res, { end: false });
@@ -227,12 +248,16 @@
   const siweMessageTypeString = siweMessageType
     ? `"${siweMessageType}"`
     : 'null';
+  const siweMessageToSignString = siweMessageToSign
+    ? `"${siweMessageToSign}"`
+    : 'null';
   // prettier-ignore
   res.end(html`</div>
         <script>var routerBasename = "${routerBasename}";</script>
         <script>var siweNonce = ${siweNonceString};</script>
         <script>var siwePrimaryIdentityPublicKey = ${siwePrimaryIdentityPublicKeyString};</script>
         <script>var siweMessageType = ${siweMessageTypeString};</script>
+        <script>var siweMessageToSign = ${siweMessageToSignString};</script>
         <script src="${jsURL}"></script>
       </body>
     </html>
diff --git a/landing/landing-ssr.react.js b/landing/landing-ssr.react.js
--- a/landing/landing-ssr.react.js
+++ b/landing/landing-ssr.react.js
@@ -14,6 +14,7 @@
   +siweNonce: ?string,
   +siwePrimaryIdentityPublicKey: ?string,
   +siweMessageType: ?SIWEMessageType,
+  +siweMessageToSign: ?string,
 };
 function LandingSSR(props: LandingSSRProps): React.Node {
   const {
@@ -22,6 +23,7 @@
     siweNonce,
     siwePrimaryIdentityPublicKey,
     siweMessageType,
+    siweMessageToSign,
   } = props;
 
   const siweContextValue = React.useMemo(
@@ -29,8 +31,14 @@
       siweNonce,
       siwePrimaryIdentityPublicKey,
       siweMessageType,
+      siweMessageToSign,
     }),
-    [siweNonce, siwePrimaryIdentityPublicKey, siweMessageType],
+    [
+      siweNonce,
+      siwePrimaryIdentityPublicKey,
+      siweMessageType,
+      siweMessageToSign,
+    ],
   );
   const routerContext = React.useMemo(() => ({}), []);
   return (
diff --git a/landing/root.js b/landing/root.js
--- a/landing/root.js
+++ b/landing/root.js
@@ -12,6 +12,7 @@
 declare var siweNonce: ?string;
 declare var siwePrimaryIdentityPublicKey: ?string;
 declare var siweMessageType: ?SIWEMessageType;
+declare var siweMessageToSign: ?string;
 
 function RootComponent(): React.Node {
   const siweContextValue = React.useMemo(
@@ -19,6 +20,7 @@
       siweNonce,
       siwePrimaryIdentityPublicKey,
       siweMessageType,
+      siweMessageToSign,
     }),
     [],
   );
diff --git a/landing/siwe-context.js b/landing/siwe-context.js
--- a/landing/siwe-context.js
+++ b/landing/siwe-context.js
@@ -8,12 +8,14 @@
   +siweNonce: ?string,
   +siwePrimaryIdentityPublicKey: ?string,
   +siweMessageType: ?SIWEMessageType,
+  +siweMessageToSign: ?string,
 };
 
 const SIWEContext: React.Context<SIWEContextType> = React.createContext({
   siweNonce: null,
   siwePrimaryIdentityPublicKey: null,
   siweMessageType: null,
+  siweMessageToSign: null,
 });
 
 export { SIWEContext };
diff --git a/landing/siwe.react.js b/landing/siwe.react.js
--- a/landing/siwe.react.js
+++ b/landing/siwe.react.js
@@ -63,40 +63,72 @@
   });
 }
 
+async function signCompleteMessageWithEthereum(
+  address: string,
+  message: string,
+  signer: Signer,
+) {
+  const decodedMessage = decodeURIComponent(message);
+  const signature = await signer.signMessage({ message: decodedMessage });
+  postMessageToNativeWebView({
+    type: 'siwe_success',
+    address,
+    message: decodedMessage,
+    signature,
+  });
+}
+
 const queryClient = new QueryClient();
 
 function SIWE(): React.Node {
   const { address } = useAccount();
   const { data: signer } = useWalletClient();
-  const { siweNonce, siwePrimaryIdentityPublicKey, siweMessageType } =
-    React.useContext(SIWEContext);
+  const {
+    siweNonce,
+    siwePrimaryIdentityPublicKey,
+    siweMessageType,
+    siweMessageToSign,
+  } = React.useContext(SIWEContext);
+
   const onClick = React.useCallback(() => {
-    invariant(siweNonce, 'nonce must be present during SIWE attempt');
     invariant(siweMessageType, 'message type must be set during SIWE attempt');
     invariant(
-      siwePrimaryIdentityPublicKey,
-      'primaryIdentityPublicKey must be present during SIWE attempt',
+      siweMessageToSign || (siwePrimaryIdentityPublicKey && siweNonce),
+      'nonce with primaryIdentityPublicKey or complete message to sign must be ' +
+        'present during SIWE attempt',
     );
-    const statement = getSIWEStatementForPublicKey(
-      siwePrimaryIdentityPublicKey,
-      siweMessageType,
-    );
-    void signInWithEthereum(address, signer, siweNonce, statement);
+
+    if (siweMessageToSign) {
+      void signCompleteMessageWithEthereum(address, siweMessageToSign, signer);
+      return;
+    }
+
+    if (siweNonce && siwePrimaryIdentityPublicKey) {
+      const statement = getSIWEStatementForPublicKey(
+        siwePrimaryIdentityPublicKey,
+        siweMessageType,
+      );
+      void signInWithEthereum(address, signer, siweNonce, statement);
+    }
   }, [
     address,
     signer,
     siweNonce,
     siwePrimaryIdentityPublicKey,
     siweMessageType,
+    siweMessageToSign,
   ]);
 
   const { openConnectModal, connectModalOpen } = useConnectModal();
-  const hasNonce = siweNonce !== null && siweNonce !== undefined;
+  const hasNonceOrCompleteMessage =
+    (siweNonce !== null && siweNonce !== undefined) ||
+    (siweMessageToSign !== null && siweMessageToSign !== undefined);
+
   React.useEffect(() => {
-    if (hasNonce && openConnectModal) {
+    if (hasNonceOrCompleteMessage && openConnectModal) {
       openConnectModal();
     }
-  }, [hasNonce, openConnectModal]);
+  }, [hasNonceOrCompleteMessage, openConnectModal]);
 
   const [wcModalOpen, setWCModalOpen] = React.useState(false);
 
@@ -135,10 +167,12 @@
       </div>
     );
   }
-  if (!hasNonce) {
+  if (!hasNonceOrCompleteMessage) {
     return (
       <div className={css.wrapper}>
-        <h1 className={css.h1}>Unable to proceed: nonce not found.</h1>
+        <h1 className={css.h1}>
+          Unable to proceed: neither nonce nor complete message found.
+        </h1>
       </div>
     );
   } else if (!signer) {
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
@@ -141,10 +141,18 @@
 export const SIWEMessageTypes = Object.freeze({
   MSG_AUTH: 'msg_auth',
   MSG_BACKUP: 'msg_backup',
+  MSG_BACKUP_RESTORE: 'msg_backup_restore',
 });
 
 export type SIWEMessageType = $Values<typeof SIWEMessageTypes>;
 
+export type SIWESignatureRequestData =
+  | { +messageType: SIWEMessageType }
+  | {
+      +messageType: 'msg_backup_restore',
+      +messageToSign: string,
+    };
+
 export type SIWEBackupSecrets = {
   +message: string,
   +signature: string,
diff --git a/lib/utils/siwe-utils.js b/lib/utils/siwe-utils.js
--- a/lib/utils/siwe-utils.js
+++ b/lib/utils/siwe-utils.js
@@ -82,6 +82,11 @@
   );
 }
 
+function isValidSIWEMessageString(candidate: string): boolean {
+  const candidateMessage = new SiweMessage(candidate);
+  return isValidSIWEMessage(candidateMessage);
+}
+
 function getSIWEStatementForPublicKey(
   publicKey: string,
   messageType: SIWEMessageType,
@@ -134,7 +139,12 @@
   `Your signature on this message will be used to derive ` +
   `a secret key that will encrypt your Comm backup.`;
 
+const siweBackupRestoreMessageSigningExplanationStatements: string =
+  `Your signature on this message will be used to derive ` +
+  `a secret key that will decrypt you Comm backup.`;
+
 const siweBackupMessageSigningButtonStatement = 'Create a backup key';
+const siweBackupRestoreMessageSigningButtonStatement = 'Retrieve a backup key';
 
 const userTextsForSIWEMessageTypes: {
   +[signatureRequestType: string]: {
@@ -153,6 +163,11 @@
     showTermsAgreement: false,
     buttonStatement: siweBackupMessageSigningButtonStatement,
   },
+  [SIWEMessageTypes.MSG_BACKUP_RESTORE]: {
+    explanationStatement: siweBackupRestoreMessageSigningExplanationStatements,
+    showTermsAgreement: false,
+    buttonStatement: siweBackupRestoreMessageSigningButtonStatement,
+  },
 };
 
 export {
@@ -163,6 +178,7 @@
   isValidPrimaryIdentityPublicKey,
   createSIWEMessage,
   isValidSIWEMessage,
+  isValidSIWEMessageString,
   getSIWEStatementForPublicKey,
   isValidSIWEStatementWithPublicKey,
   getPublicKeyFromSIWEStatement,