diff --git a/keyserver/src/responders/siwe-nonce-responders.js b/keyserver/src/responders/siwe-nonce-responders.js
--- a/keyserver/src/responders/siwe-nonce-responders.js
+++ b/keyserver/src/responders/siwe-nonce-responders.js
@@ -1,7 +1,11 @@
 // @flow
 
-async function siweNonceResponder(): Promise<string> {
-  return 'UNIMPLEMENTED';
+import { generateNonce } from 'siwe';
+
+import type { SIWENonceResponse } from 'lib/types/siwe-types.js';
+
+async function siweNonceResponder(): Promise<SIWENonceResponse> {
+  return { nonce: generateNonce() };
 }
 
 export { siweNonceResponder };
diff --git a/lib/types/siwe-types.js b/lib/types/siwe-types.js
new file mode 100644
--- /dev/null
+++ b/lib/types/siwe-types.js
@@ -0,0 +1,5 @@
+// @flow
+
+export type SIWENonceResponse = {
+  +nonce: string,
+};