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
from tkinter import *
import os

def register_user():
    username_info  = username.get()
    password_info = password.get()

    file = open(username_info, "w")
    file.write(username_info+"\n")
    file.write(password_info)
    file.close()

    #clear the fields
    username_entry.delete(0, END)
    password_entry.delete(0, END)
    Label(screen1, text = "Registration Success", fg = "green", font = ("calibri, 11")).pack()

def register():
    global screen1
    screen1 = Toplevel(screen)
    screen1.title("Register")
    screen1.geometry("300x250")

    global username
    global password
    global username_entry
    global password_entry

    username = StringVar()
    password = StringVar()

    Label(screen1, text = "Please enter details below").pack()
    Label(screen1, text = "").pack()
    Label(screen1, text = "Username * ").pack()
    username_entry = Entry(screen1, textvariable = username)
    username_entry.pack()
    Label(screen1, text = "").pack()
    Label(screen1, text = "Password * ").pack()    
    password_entry = Entry(screen1, textvariable = password)
    password_entry.pack()
    Label(screen1, text = "").pack()
    Button(screen1, text = "Register", width = "10", height = "1", command = register_user).pack()

def delete3():
    screen3.destroy()

def delete4():
    screen4.destroy()

def delete5():
    screen5.destroy()

def logout():
    screen7.destroy()

def saved():
    screen10 = Toplevel(screen)
    screen10.title("Saved")
    screen10.geometry("100x100")
    Label(screen10, text = "").pack()
    Label(screen10, text = "Saved").pack()

def save():

    filename = raw_filename.get()
    notes = raw_notes.get()

    data = open(filename, "w")
    data.write(notes)
    data.close

    # save screeen
    saved()

def create_notes():
    global raw_filename
    global raw_notes
    raw_filename = StringVar()
    raw_notes = StringVar()

    screen9 = Toplevel(screen)
    screen9.title("Info")
    screen9.geometry("300x250")
    Label(screen9, text = "Please enter a filename for the note below : ").pack()
    Entry(screen9, textvariable = raw_filename).pack()
    Label(screen9, text = "Please enter the notes for the file below : ").pack()
    Entry(screen9, textvariable = raw_notes).pack()
    Button(screen9, text = "Save", command = save).pack()

def view_notes1():
    filename1 = raw_filename1.get()
    data = open(filename1, "r")
    data1 = data.read()
    screen12 = Toplevel(screen)
    screen12.title("Notes")
    screen12.geometry("400x400")
    Label(screen12, text = data1).pack()

def view_notes():
    screen11 = Toplevel(screen)
    screen11.title("Info")
    screen11.geometry("250x250")
    all_files = os.listdir()
    Label(screen11, text = "Please use one of the filenames below : ").pack()
    Label(screen11, text = all_files).pack()
    global raw_filename1
    raw_filename1 = StringVar()
    Entry(screen11, textvariable=raw_filename1).pack()
    Button(screen11, command=view_notes1, text="OK").pack()

def delete_note1():
    filename3 = raw_filename2.get()
    os.remove(filename3)
    screen14 = Toplevel(screen)
    screen14.title("Notes")
    screen14.geometry("400x400")
    Label(screen14, text = filename3+" removed").pack()

def delete_note():
    screen13 = Toplevel(screen)
    screen13.title("Info")
    screen13.geometry("250x250")
    all_files = os.listdir()
    Label(screen13, text = "Please use one of the filenames below : ").pack()
    Label(screen13, text = all_files).pack()
    global raw_filename2
    raw_filename2 = StringVar()
    Entry(screen13, textvariable=raw_filename2).pack()
    Button(screen13, command=delete_note1, text="OK").pack()

def session():
    screen8 = Toplevel(screen)
    screen8.title("Dashboard")
    screen8.geometry("400x400")
    Label(screen8, text = "Welcome to the dashboard").pack()
    Button(screen8, text = "Create Note", command = create_notes).pack()
    Button(screen8, text = "View Note", command = view_notes).pack()
    Button(screen8, text = "Delete Note", command = delete_note).pack()

def login_success():
    session()
    # global screen3
    # screen3 = Toplevel(screen)
    # screen3.title("Success")
    # screen3.geometry("150x100")
    # Label(screen3, text = "").pack()
    # Label(screen3, text = "Login Success").pack()
    # Button(screen3, text = "OK", command = delete3).pack()


def password_not_recognized():
    global screen4
    screen4 = Toplevel(screen)
    screen4.title("Password Not Recognized")
    screen4.geometry("150x100")
    Label(screen4, text = "").pack()
    Label(screen4, text = "Password Error").pack()
    Button(screen4, text = "OK", command = delete4).pack()


def user_not_found():
    global screen5
    screen5 = Toplevel(screen)
    Label(screen5, text = "").pack()
    screen5.title("User Not Found")
    screen5.geometry("150x100")
    Label(screen5, text = "User Not Found Error").pack()
    Button(screen5, text = "OK", command = delete5).pack()



def login_verify():
    username1 = username_verify.get()
    password1 = password_verify.get()

    username_entry2.delete(0, END)
    password_entry2.delete(0, END)

    list_of_files = os.listdir()
    if username1 in list_of_files: 
        file1 = open(username1, "r")
        verify = file1.read().splitlines()
        if password1 in verify:
            # print("login success!")
            login_success()
        else:
            # print("password has not been recognized")
            password_not_recognized()
    else:
        # print("User not found!")
        user_not_found()


def login():
    global screen2
    screen2 = Toplevel(screen)
    screen2.title("Login")
    screen2.geometry("300x250")

    global username_verify
    global password_verify

    username_verify = StringVar()
    password_verify = StringVar()

    global username_entry2  #Login infor
    global password_entry2  #Login infor

    Label(screen2, text = "Please enter details below to login").pack()
    Label(screen2, text = "").pack()
    Label(screen2, text = "Username * ").pack()
    username_entry2 = Entry(screen2, textvariable = username_verify)
    username_entry2.pack()
    Label(screen2, text = "").pack()
    username_entry2.pack()
    Label(screen2, text = "Password * ").pack()    
    password_entry2 = Entry(screen2, textvariable = password_verify)
    password_entry2.pack()
    Label(screen2, text = "").pack()
    Button(screen2, text = "Login", width = 10, height = 1, command = login_verify).pack()


def main_screen():
    global screen
    screen = Tk()
    screen.geometry("300x250")
    screen.title("Notes 1.0")
    Label(text = "Notes 1.0", bg = "grey", width = "300", height="2", font = ("Calibri", 13)).pack()
    Label(text = "").pack()
    Button(text = "Login", height = "2", width="30", command = login).pack()
    Label(text = "").pack()
    Button(text = "Register", height = "2", width="30", command = register).pack()
    Label(text = "").pack()

    screen.mainloop()

main_screen()

```

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