import javax.crypto.Cipher;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.util.Base64;

public class RSAExample {
    public static void main(String[] args) throws Exception {
        // Генерация ключей
        KeyPairGenerator keyGen = KeyPairGenerator.getInstance("RSA");
        keyGen.initialize(2048); // 2048-bit key
        KeyPair keyPair = keyGen.generateKeyPair();

        // Шифрование
        Cipher cipher = Cipher.getInstance("RSA");
        cipher.init(Cipher.ENCRYPT_MODE, keyPair.getPublic());
        String plainText = "Hello, World!";
        byte[] encryptedBytes = cipher.doFinal(plainText.getBytes());
        String encryptedText = Base64.getEncoder().encodeToString(encryptedBytes);
        System.out.println("Encrypted: " + encryptedText);

        // Дешифрование
        cipher.init(Cipher.DECRYPT_MODE, keyPair.getPrivate());
        byte[] decryptedBytes = cipher.doFinal(Base64.getDecoder().decode(encryptedText));
        String decryptedText = new String(decryptedBytes);
        System.out.println("Decrypted: " + decryptedText);
    }
}

import java.io.FileWriter;
import java.io.IOException;

public class Task200225 {
    public static void main(String[] args) {
        try (FileWriter writer = new FileWriter("output.txt")) {
            writer.write("Hello");
            writer.flush();
            writer.write(" World");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

import javax.crypto.Cipher;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.util.Base64;

public class RSAExample {
    public static void main(String[] args) throws Exception {
        // Генерация ключей
        KeyPairGenerator keyGen = KeyPairGenerator.getInstance("RSA");
        keyGen.initialize(2048); // 2048-bit key
        KeyPair keyPair = keyGen.generateKeyPair();

        // Шифрование
        Cipher cipher = Cipher.getInstance("RSA");
        cipher.init(Cipher.ENCRYPT_MODE, keyPair.getPublic());
        String plainText = "Hello, World!";
        byte[] encryptedBytes = cipher.doFinal(plainText.getBytes());
        String encryptedText = Base64.getEncoder().encodeToString(encryptedBytes);
        System.out.println("Encrypted: " + encryptedText);

        // Дешифрование
        cipher.init(Cipher.DECRYPT_MODE, keyPair.getPrivate());
        byte[] decryptedBytes = cipher.doFinal(Base64.getDecoder().decode(encryptedText));
        String decryptedText = new String(decryptedBytes);
        System.out.println("Decrypted: " + decryptedText);
    }
}

import java.security.MessageDigest;

public class MD5Example {
    public static void main(String[] args) throws Exception {
        String input = "Hello, world!";
        MessageDigest md = MessageDigest.getInstance("MD5");
        byte[] hash = md.digest(input.getBytes());

        StringBuilder hexString = new StringBuilder();
        for (byte b : hash) {
            hexString.append(String.format("%02x", b));
        }
        System.out.println("MD5 Hash: " + hexString.toString());
    }
}

import java.security.MessageDigest;

public class SHA256Example {
    public static void main(String[] args) throws Exception {
        String input = "Hello, world!";
        MessageDigest sha = MessageDigest.getInstance("SHA-256");
        byte[] hash = sha.digest(input.getBytes());

        StringBuilder hexString = new StringBuilder();
        for (byte b : hash) {
            hexString.append(String.format("%02x", b));
        }
        System.out.println("SHA-256 Hash: " + hexString.toString());
    }
}

import java.security.*;

public class DigitalSignatureExample {
    public static void main(String[] args) throws Exception {
        // Генерация ключей
        KeyPairGenerator keyGen = KeyPairGenerator.getInstance("RSA");
        keyGen.initialize(2048);
        KeyPair pair = keyGen.generateKeyPair();

        Signature signature = Signature.getInstance("SHA256withRSA");
        signature.initSign(pair.getPrivate());

        String data = "Important message";
        signature.update(data.getBytes());
        byte[] signedData = signature.sign();

        // Проверка подписи
        signature.initVerify(pair.getPublic());
        signature.update(data.getBytes());
        boolean isValid = signature.verify(signedData);

        System.out.println("Подпись действительна? " + isValid);
    }
}

class Alfa {
    void print(Alfa obj) {
        System.out.println("A");
    }
}

class Beta extends Alfa {
    void print(Beta obj) {
        System.out.println("B");
    }
}

public class Task210225 {
    public static void main(String[] args) {
        Alfa obj1 = new Beta();
        Alfa obj2 = new Alfa();
        obj1.print(obj2);
    }
}

import java.security.SecureRandom;
import java.util.Base64;

public class SaltExample {
    public static void main(String[] args) {
        byte[] salt = new byte[16]; // 16 байт соли
        new SecureRandom().nextBytes(salt);
        System.out.println("Сгенерированная соль: " + Base64.getEncoder().encodeToString(salt));
    }
}

import java.security.SecureRandom;
import java.util.Base64;

public class IVExample {
    public static void main(String[] args) {
        byte[] iv = new byte[16]; // IV должен быть 16 байт для AES
        new SecureRandom().nextBytes(iv);
        System.out.println("Сгенерированный IV: " + Base64.getEncoder().encodeToString(iv));
    }
}

import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import java.util.Base64;

public class ECBExample {
    public static void main(String[] args) throws Exception {
        String plaintext = "AAAAAAAAAAAAAAAAAAAAAAAA"; // 24 байта - два одинаковых блока

        KeyGenerator keyGen = KeyGenerator.getInstance("AES");
        keyGen.init(128);
        SecretKey key = keyGen.generateKey();

        Cipher cipher = Cipher.getInstance("AES/ECB/PKCS5Padding");
        cipher.init(Cipher.ENCRYPT_MODE, key);
        byte[] encrypted = cipher.doFinal(plaintext.getBytes());

        System.out.println("Зашифрованный текст (ECB): " + Base64.getEncoder().encodeToString(encrypted));
    }
}
Использовать ECB не рекомендуется!

import javax.crypto.*;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;
import java.util.Base64;

public class CBCExample {
    public static void main(String[] args) throws Exception {
        String plaintext = "Hello, Secure World!";
        byte[] key = new byte[16]; // 128-битный ключ
        byte[] iv = new byte[16];  // IV

        Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
        SecretKeySpec keySpec = new SecretKeySpec(key, "AES");
        IvParameterSpec ivSpec = new IvParameterSpec(iv);

        cipher.init(Cipher.ENCRYPT_MODE, keySpec, ivSpec);
        byte[] encrypted = cipher.doFinal(plaintext.getBytes());

        System.out.println("Зашифрованные данные (CBC): " + Base64.getEncoder().encodeToString(encrypted));
    }
}

import javax.crypto.*;
import javax.crypto.spec.GCMParameterSpec;
import javax.crypto.spec.SecretKeySpec;
import java.util.Base64;

public class AESGCMExample {
    public static void main(String[] args) throws Exception {
        String plaintext = "Hello, Secure World!";
        byte[] key = new byte[16];
        byte[] iv = new byte[12];

        Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
        SecretKeySpec keySpec = new SecretKeySpec(key, "AES");
        GCMParameterSpec gcmSpec = new GCMParameterSpec(128, iv);

        cipher.init(Cipher.ENCRYPT_MODE, keySpec, gcmSpec);
        byte[] encrypted = cipher.doFinal(plaintext.getBytes());

        System.out.println("Зашифрованные данные (GCM): " + Base64.getEncoder().encodeToString(encrypted));
    }
}
GCM – самый защищённый вариант!