lab1

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileWriter;
import java.io.IOException;
import java.math.BigInteger;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Scanner;

public class MainClass {

    public static void main(String[] args) throws IOException{
        System.out.println("Type 1 for registration or 2 for entering the system");
        Scanner in = new Scanner(System.in);
        int a = in.nextInt();
        //in.close();
        if (a == 1) {
            reg();
        }
        else if (a == 2){
            ent();
        }
        else{
            System.out.println("Error\n Type 1 for registration or 2 for entering the system");
        }
    }

    public static void reg(){
         String filename = "C:\\Users\\nikishkin\\Documents\\Labs\\SKEY\\logins.txt";
         String filename2 = "C:\\Users\\nikishkin\\Documents\\Labs\\SKEY\\passwords.txt";

         System.out.println("Create username");
         Scanner in = new Scanner(System.in);
         String userName = in.nextLine();
         //in.close();

         String userdata = "C:\\Users\\nikishkin\\Documents\\Labs\\SKEY\\" + userName + ".txt";

         System.out.println("Введите любую строку");
         //Scanner in = new Scanner(System.in);
         String line = in.nextLine();
         System.out.println(line);
         //in.close();

         //добавляем логин в файл логинов
         try{
             FileWriter sw = new FileWriter(filename,true);
             sw.write(userName);
             sw.append('\n');
             sw.flush();
             sw.close();
         }catch(Exception e){
             System.out.print(e.getMessage());
         }

         try{
             FileWriter fw = new FileWriter(userdata, true);
             for (int i = 0; i < 10; i++){
                 String hashLine = md5Custom(line);
                 fw.write(hashLine + "\n");
                 line = hashLine;
             }
             fw.close();
         }catch(Exception e){
             System.out.print(e.getMessage());
         }

         //добавляем хэш в файл хэшей
         try{
             FileWriter sw = new FileWriter(filename2,true);
             sw.write(md5Custom(line) + "\n");
             sw.close();
         }catch(Exception e){
             System.out.print(e.getMessage());
         }
    }

    public static void ent() throws IOException{
        System.out.println("Type your username");
        Scanner in = new Scanner(System.in);
        String userName = in.nextLine();
        //in.close();

        System.out.println("Type your last password");
        Scanner in2 = new Scanner(System.in);
        String password = in2.nextLine();
        in2.close();

        String hashLine = md5Custom(password);

        int loginNumber = -1;

        FileInputStream fis = new FileInputStream(new File("C:\\Users\\nikishkin\\Documents\\Labs\\SKEY\\logins.txt"));
        byte[] content = new byte[fis.available()];
        fis.read(content);
        fis.close();
        String[] lines = new String(content, "Cp1251").split("\n");
        int i = 1;
        for (String line : lines) {
            String[] words = line.split(" ");
            int j = 1;
            for (String word : words) {
                if (word.equalsIgnoreCase(userName)) {
                    loginNumber = i;
                }
                j++;
            }
            i++;
        }

        if (loginNumber != -1){
            int stringNumber = 1;
            Scanner scanner = new Scanner (new File ("C:\\Users\\nikishkin\\Documents\\Labs\\SKEY\\passwords.txt"));
            while (scanner.hasNextLine()) {
                String numline = scanner.nextLine();
                if(stringNumber == loginNumber){
                    if(hashLine.equals(numline)){
                        System.out.println("OK");
                        Charset charset = StandardCharsets.UTF_8;
                        Path path = Paths.get("C:\\Users\\nikishkin\\Documents\\Labs\\SKEY\\passwords.txt");
                        Files.write(path,
                            new String(Files.readAllBytes(path), charset).replace(hashLine, password)
                                .getBytes(charset));
                    }
                    else{
                        System.out.println("NOT OK");
                    }
                    break;
                }
                stringNumber++;
            }
            scanner.close();
        }
    }

    public static String md5Custom(String st) {
        MessageDigest messageDigest = null;
        byte[] digest = new byte[0];

        try {
            messageDigest = MessageDigest.getInstance("MD5");
            messageDigest.reset();
            messageDigest.update(st.getBytes());
            digest = messageDigest.digest();
        } catch (NoSuchAlgorithmException e) {
            // тут можно обработать ошибку
            // возникает она если в передаваемый алгоритм в getInstance(,,,) не существует
            e.printStackTrace();
        }

        BigInteger bigInt = new BigInteger(1, digest);
        String md5Hex = bigInt.toString(16);

        while( md5Hex.length() < 32 ){
            md5Hex = "0" + md5Hex;
        }

        return md5Hex;
    }
}