diff --git a/src/__tests__/storageKeys.test.ts b/src/__tests__/storageKeys.test.ts
new file mode 100644
index 0000000..4cd01fe
--- /dev/null
+++ b/src/__tests__/storageKeys.test.ts
@@ -0,0 +1,22 @@
+import { fingerprint, expiryNotifiedStorageKey } from '../lib/storageKeys';
+
+const WALLET_A = 'GBRPYHIL2CI3FNQ4BXLFMNDLFJUNPU2HY3ZMFSHONUCEOASW7QC7OX2H';
+const WALLET_B = 'GCKFBEIYTKP5RDBQMTVVALONAOPBXICILMAFL6ANEHMIYX7XFBZ7ZMGT';
+
+describe('storageKeys (#548)', () => {
+  it('does not include the raw wallet address in the expiry storage key', () => {
+    const key = expiryNotifiedStorageKey(WALLET_A);
+    expect(key.startsWith('ps_expiry_notified_')).toBe(true);
+    expect(key).not.toContain(WALLET_A);
+    expect(key).not.toContain(WALLET_A.slice(0, 8));
+  });
+
+  it('is deterministic per wallet so notifications persist across renders', () => {
+    expect(expiryNotifiedStorageKey(WALLET_A)).toBe(expiryNotifiedStorageKey(WALLET_A));
+  });
+
+  it('gives different wallets different keys', () => {
+    expect(expiryNotifiedStorageKey(WALLET_A)).not.toBe(expiryNotifiedStorageKey(WALLET_B));
+    expect(fingerprint('a')).not.toBe(fingerprint('b'));
+  });
+});
diff --git a/src/lib/storageKeys.ts b/src/lib/storageKeys.ts
new file mode 100644
index 0000000..6f33e75
--- /dev/null
+++ b/src/lib/storageKeys.ts
@@ -0,0 +1,24 @@
+/**
+ * Non-reversible, deterministic fingerprint of a string (cyrb53, 53-bit).
+ * Used to scope browser-storage keys per wallet without writing the raw
+ * address into the key name, where it is visible to anyone inspecting
+ * DevTools > Application > Storage (#548). Not a cryptographic hash -- it only
+ * needs to be stable and collision-resistant across a handful of wallets.
+ */
+export function fingerprint(value: string): string {
+  let h1 = 0xdeadbeef;
+  let h2 = 0x41c6ce57;
+  for (let i = 0; i < value.length; i++) {
+    const ch = value.charCodeAt(i);
+    h1 = Math.imul(h1 ^ ch, 2654435761);
+    h2 = Math.imul(h2 ^ ch, 1597334677);
+  }
+  h1 = Math.imul(h1 ^ (h1 >>> 16), 2246822507) ^ Math.imul(h2 ^ (h2 >>> 13), 3266489909);
+  h2 = Math.imul(h2 ^ (h2 >>> 16), 2246822507) ^ Math.imul(h1 ^ (h1 >>> 13), 3266489909);
+  return (4294967296 * (2097151 & h2) + (h1 >>> 0)).toString(36);
+}
+
+/** sessionStorage key for the policy IDs already warned about for a wallet. */
+export function expiryNotifiedStorageKey(address: string): string {
+  return `ps_expiry_notified_${fingerprint(address)}`;
+}