Index: P3-UseCaseModel/UseCase0002.md
===================================================================
--- P3-UseCaseModel/UseCase0002.md	(revision d8ce4e224bde01e8f87558691226298db515617e)
+++ P3-UseCaseModel/UseCase0002.md	(revision d8ce4e224bde01e8f87558691226298db515617e)
@@ -0,0 +1,34 @@
+# Use-case 0002 — Log in
+
+**Initiating actor:** Visitor
+
+**Other actors:** —
+
+A registered user authenticates so the system can treat subsequent actions as a Trader.
+
+## Scenario
+
+1. Visitor chooses "Login" from the anonymous menu.
+2. System prompts for username and password.
+3. Visitor enters values.
+4. System looks up the user:
+
+   ```sql
+   SELECT id, password_hash
+     FROM project.users
+    WHERE username = $1;
+   ```
+5. If no row is returned, system responds "Invalid credentials." and scenario ends.
+6. If a row is returned, system compares the stored hash against sha256 of the entered password. On mismatch it responds "Invalid credentials." and ends.
+7. On match, system records the returned `id` and username in the session and displays the authenticated menu.
+
+### Alternate flow 4a — authenticated lookup with live balance
+
+The system may combine identity lookup with live balance in a single query, for use cases that need both:
+
+```sql
+SELECT id, available_balance, invested_balance
+  FROM project.users
+ WHERE username = $1
+   AND password_hash = encode(digest($2, 'sha256'), 'hex');
+```