system: You are an expert security code reviewer. You will be shown a source code file. Your task is to name the single function or class that contains the specified CWE vulnerability.

OUTPUT FORMAT: Respond with the exact function or class name (no parentheses, no qualifiers, no module prefixes), wrapped in \boxed{...}. Example: `\boxed{salted_hmac}`. Output the boxed name ONLY -- no explanation, no other text.

user: ```python
#!/usr/bin/env python3.6
import password
import user

password_obj=password.Password()

def sign_up():
    '''
    Function that allows a new user to sign up.
    '''
    
    username_signup=" "
    password_signup=" " 

    print(" ")
    print("-----Sign up here-----")
    username_valid=True
    while username_valid:
        username_signup=input("Username (at least 5 chars): ")
        if len(username_signup)<5:
            username_valid=True
            print("**Username too short. Try again.")
        else:
            username_valid=False 
        
    want_password_valid=True
    while want_password_valid:        
        want_sys_password=input("Want system generated password? (Y/n): ")        
        if want_sys_password=="Y":
            want_password_valid=False
            password_signup=password_obj.gen_password()            
            print("Your password: "+password_signup+" (copied to clipboard)")
            print("Signup successful!")
        elif want_sys_password=="n":
            password_signup=input("Password (at least 5 chars): ")
            password_confirm=input("Confirm password: ")
            if len(password_signup)<5:
                want_password_valid=True
                print("**Password too short. Try again.")
            elif password_confirm==password_signup:
                print("Signup successful!")                
                want_password_valid=False
            else:
                print("**Passwords did not match. Try again.")
                want_password_valid=True
        else:
            print("**Invalid choice. Choose Y/n")
            want_password_valid=True

    new_user=user.User(username_signup,password_signup)
    new_user.add_user(new_user)
    login()

        

def login():
    '''
    Function that allows a user with an existing account to login.
    '''
    is_login=True
    while is_login:
        print(" ")
        print("-----Log in here-----")
        username_login=input("Username: ")
        password_login=input("Password: ")
        login_valid=user.User.check_login(username_login, password_login)
        if login_valid:
            print("Login successful!")
            is_login=False
            user_obj=user.User.return_user(username_login, password_login)
            account_menu(username_login, user_obj)
        else:
            print("**Login unsuccessful. Try again.")
            is_login=True


def account_menu(this_user_name, this_user_object):
    '''
    Function that displays a navigation menu in a user's account.
    '''
    print(" ")
    print("*"*45)    
    print(f'WELCOME TO YOUR ACCOUNT, {this_user_name.upper()}')
    print("Options menu")
    print("1. Add existing credential  - press 1")
    print("2. Create new credential    - press 2")
    print("3. View saved credentials   - press 3")
    print("4. Copy username & password - press 4")
    print("5. Delete saved credential  - press 5")
    print("6. Log out                  - press 6")    
        
    is_selected=True
    while is_selected:
        print(" ")
        selected=input("What do you want to do? Press option: ")
        if selected=="1":
            is_selected=True
            this_user_object.credential.add_credential()
        elif selected=="2":
            is_selected=True
            this_user_object.credential.create_credential()
        elif selected=="3":
            is_selected=True
            this_user_object.credential.view_credentials()
        elif selected=="4":
            is_selected=True
            this_user_object.credential.copy_credential()
        elif selected=="5":
            is_selected=True
            this_user_object.credential.delete_credential()
        elif selected=="6":
            is_selected=False 
            print("LOGGED OUT.")
            print(" ")       
        else:
            print("**Invalid option. Try again.")
            is_selected=True
            


def main():
    print("PASSWORD LOCKER")
    print("="*15)
    proceed="1"
    to_proceed=True
    while to_proceed:        
        proceed=input("Press 1 to login or 0 to exit: ")
        if proceed=="1":
            to_proceed=True

            has_account_valid=True    
            while has_account_valid:                
                has_account=input("Have an account? (y/N): ")
                if has_account=="y":            
                    login()
                    has_account_valid=False
                elif has_account=="N":            
                    sign_up()
                    has_account_valid=False
                else:
                    print("**Invalid choice. Choose y/N")
                    has_account_valid=True
            
        elif proceed=="0":
            to_proceed=False
        else:
            print("**Invalid option.")
            to_proceed=True
    print("---BYE---")    


if __name__ == "__main__":
    main ()
```

Name the function or class in this code that contains a `CWE-312: Cleartext Storage of Sensitive Information` vulnerability.
Answer: sign_up