diff --git a/frontend/src/logic/sdk.ts b/frontend/src/logic/sdk.ts
new file mode 100644
index 0000000..dfaf8eb
--- /dev/null
+++ b/frontend/src/logic/sdk.ts
@@ -0,0 +1,35 @@
+
+// db.users: Signup table map<username, password>
+// db.user: Current logged-in user
+const db = localStorage
+
+export function signup(username: string, password: string)
+{
+  if (!db.users)
+    db.users = JSON.stringify({})
+
+  const users = JSON.parse(db.users)
+  if (users[username])
+    throw new Error('User already exists')
+
+  users[username] = password
+}
+
+export function login(username: string, password: string)
+{
+  const users = JSON.parse(db.users)
+  if (users[username] !== password)
+    throw new Error('Invalid credentials')
+
+  db.user = username
+}
+
+export function logout()
+{
+  delete db.user
+}
+
+export function isLoggedIn()
+{
+  return !!db.user
+}