Index: docs/P3-UseCaseModel/UseCase0001.md
===================================================================
--- docs/P3-UseCaseModel/UseCase0001.md	(revision b715712c7d5d3ee27c932a8a9d8e0a2bc6e85127)
+++ docs/P3-UseCaseModel/UseCase0001.md	(revision b715712c7d5d3ee27c932a8a9d8e0a2bc6e85127)
@@ -0,0 +1,40 @@
+# Use-case 0001 — Register new account
+
+**Initiating actor:** Visitor
+
+**Other actors:** —
+
+A new person creates an account on EduBerza so they can later log in as a Trader. The system validates input, refuses duplicates, and stores a hashed password.
+
+## Scenario
+
+1. Visitor chooses "Register" from the anonymous menu.
+2. System prompts for username, email, full name and password.
+3. Visitor enters values.
+4. System validates:
+   - username, email, password are non-empty.
+   - email contains `@`.
+   - password is at least 6 characters.
+5. System checks whether the chosen username or email already exists:
+
+   ```sql
+   SELECT EXISTS (
+       SELECT 1 FROM project.users
+        WHERE username = $1 OR email = $2
+   );
+   ```
+6. If the row exists, system informs the user and the scenario ends. Otherwise it creates the account:
+
+   ```sql
+   INSERT INTO project.users (username, email, full_name, password_hash, available_balance)
+   VALUES ($1, $2, $3, encode(digest($4, 'sha256'), 'hex'), 0);
+   ```
+7. System confirms success and returns to the anonymous menu; Visitor can then proceed to UC0002.
+
+### Alternate flow 3a — invalid email
+
+If step 4 fails email validation, system shows "Invalid email." and scenario returns to step 2.
+
+### Alternate flow 5a — duplicate
+
+If step 5 returns `true`, system shows "Username or email already taken." and scenario ends.