//字符转换成ASCII 码数值
 char charA = 'a';
 int intA = charA; //char 强转为int 即得到对应的ASCII 码值，'a'的值为97

//ASCII 码值转成char
int intA = 97;//97 对应的ASCII 码'a'
char charA = (char) intA; //int 值强转为char 即得到对应的ASCII 字符，即'a'

 /**
 * 加密
 * @param input 数据源（需要加密的数据）
 * @param key 秘钥，即偏移量
 * @return 返回加密后的数据
 */
 public static String encrypt(String input, int key) {
 //得到字符串里的每一个字符
 char[] array = input.toCharArray();

 for (int i = 0; i < array.length; ++i) {
 //字符转换成ASCII 码值
 int ascii = array[i];
 //字符偏移，例如a->b
 ascii = ascii + key;
 //ASCII 码值转换为char
 char newChar = (char) ascii;
 //替换原有字符
 array[i] = newChar;

 //以上4 行代码可以简写为一行
 //array[i] = (char) (array[i] + key);
 }

 //字符数组转换成String
 return new String(array);
 }

 /**
 * 解密
 * @param input 数据源（被加密后的数据）
 * @param key 秘钥，即偏移量
 * @return 返回解密后的数据
 */
 public static String decrypt(String input, int key) {
 //得到字符串里的每一个字符
 char[] array = input.toCharArray();
 for (int i = 0; i < array.length; ++i) {
 //字符转换成ASCII 码值
 int ascii = array[i];
 //恢复字符偏移，例如b->a
 ascii = ascii - key;
 //ASCII 码值转换为char
 char newChar = (char) ascii;
 //替换原有字符
 array[i] = newChar;

 //以上4 行代码可以简写为一行
 //array[i] = (char) (array[i] - key);
 }

 //字符数组转换成String
 return new String(array);
 }

/**
 * 频率分析法破解凯撒密码
 */
public class FrequencyAnalysis {
 //英文里出现次数最多的字符
 private static final char MAGIC_CHAR = 'e';
 //破解生成的最大文件数
 private static final int DE_MAX_FILE = 4;

 public static void main(String[] args) throws Exception {
 //测试1，统计字符个数
 //printCharCount("article1_en.txt");

 //加密文件
 //int key = 3;
 //encryptFile("article1.txt", "article1_en.txt", key);

 //读取加密后的文件
 String artile = file2String("article1_en.txt");
 //解密（会生成多个备选文件）
 decryptCaesarCode(artile, "article1_de.txt");
 }

 public static void printCharCount(String path) throws IOException{
 String data = file2String(path);
 List<Entry<Character, Integer>> mapList = getMaxCountChar(data);
 for (Entry<Character, Integer> entry : mapList) {
 //输出前几位的统计信息
 System.out.println("字符'" + entry.getKey() + "'出现" + entry.getValue() + "次");
 }
 }

 public static void encryptFile(String srcFile, String destFile, int key) throws IOException {
 String artile = file2String(srcFile);
 //加密文件
 String encryptData = MyEncrypt.encrypt(artile, key);
 //保存加密后的文件
 string2File(encryptData, destFile);
 }

 /**
 * 破解凯撒密码
 * @param input 数据源
 * @return 返回解密后的数据
 */
 public static void decryptCaesarCode(String input, String destPath) {
 int deCount = 0;//当前解密生成的备选文件数
 //获取出现频率最高的字符信息（出现次数越多越靠前）
 List<Entry<Character, Integer>> mapList = getMaxCountChar(input);
 for (Entry<Character, Integer> entry : mapList) {
 //限制解密文件备选数
 if (deCount >= DE_MAX_FILE) {
 break;
 }

 //输出前几位的统计信息
 System.out.println("字符'" + entry.getKey() + "'出现" + entry.getValue() + "次");

 ++deCount;
 //出现次数最高的字符跟MAGIC_CHAR的偏移量即为秘钥
 int key = entry.getKey() - MAGIC_CHAR;
 System.out.println("猜测key = " + key + "， 解密生成第" + deCount + "个备选文件" + "\n");
 String decrypt = MyEncrypt.decrypt(input, key);

 String fileName = "de_" + deCount + destPath;
 string2File(decrypt, fileName);
 }
 }

 //统计String里出现最多的字符
 public static List<Entry<Character, Integer>> getMaxCountChar(String data) {
 Map<Character, Integer> map = new HashMap<Character, Integer>();
 char[] array = data.toCharArray();
 for (char c : array) {
 if(!map.containsKey(c)) {
 map.put(c, 1);
 }else{
 Integer count = map.get(c);
 map.put(c, count + 1);
 }
 }

 //输出统计信息
 /*for (Entry<Character, Integer> entry : map.entrySet()) {
 System.out.println(entry.getKey() + "出现" + entry.getValue() + "次");
 }*/

 //获取获取最大值
 int maxCount = 0;
 for (Entry<Character, Integer> entry : map.entrySet()) {
 //不统计空格
 if (/*entry.getKey() != ' ' && */entry.getValue() > maxCount) { 
 maxCount = entry.getValue();
 }
 }

 //map转换成list便于排序
 List<Entry<Character, Integer>> mapList = new ArrayList<Map.Entry<Character,Integer>>(map.entrySet());
 //根据字符出现次数排序
 Collections.sort(mapList, new Comparator<Entry<Character, Integer>>(){
 @Override
 public int compare(Entry<Character, Integer> o1,
 Entry<Character, Integer> o2) {
 return o2.getValue().compareTo(o1.getValue());
 }
 });
 return mapList;
 }

 public static String file2String(String path) throws IOException {
 FileReader reader = new FileReader(new File(path));
 char[] buffer = new char[1024];
 int len = -1;
 StringBuffer sb = new StringBuffer();
 while ((len = reader.read(buffer)) != -1) {
 sb.append(buffer, 0, len);
 }
 return sb.toString();
 }

 public static void string2File(String data, String path){
 FileWriter writer = null;
 try {
 writer = new FileWriter(new File(path));
 writer.write(data);
 } catch (Exception e) {
 e.printStackTrace();
 }finally {
 if (writer != null) {
 try {
 writer.close();
 } catch (IOException e) {
 e.printStackTrace();
 }
 }
 }

 }
}

 int i = 97;
 String bit = Integer.toBinaryString(i);
 //输出：97 对应的二进制数据为： 1100001
 System.out.println(i + "对应的二进制数据为： " + bit);

//byte 的取值范围：-128 到127
System.out.println(Byte.MIN_VALUE + "到" + Byte.MAX_VALUE);

//1,得到cipher 对象（可翻译为密码器或密码系统）
 Cipher cipher = Cipher.getInstance("DES");
 //2，创建秘钥
 SecretKey key = KeyGenerator.getInstance("DES").generateKey();
 //3，设置操作模式（加密/解密）
 cipher.init(Cipher.ENCRYPT_MODE, key);
 //4，执行操作
 byte[] result = cipher.doFinal("黑马".getBytes());

//生成随机秘钥
SecretKey secretKey = KeyGenerator.getInstance("AES").generateKey();
//序列化秘钥到磁盘上
FileOutputStream fos = new FileOutputStream(new File("heima.key"));

ObjectOutputStream oos = new ObjectOutputStream(fos);
oos.writeObject(secretKey);

//从磁盘里读取秘钥
FileInputStream fis = new FileInputStream(new File("heima.key"));
ObjectInputStream ois = new ObjectInputStream(fis);
Key key = (Key) ois.readObject();

//创建密钥写法1
KeySpec keySpec = new DESKeySpec(key.getBytes());
SecretKey secretKey = SecretKeyFactory.getInstance(ALGORITHM).
generateSecret(keySpec);

//创建密钥写法2
//SecretKey secretKey = new SecretKeySpec(key.getBytes(), KEY_ALGORITHM);

Cipher cipher = Cipher.getInstance(CIPHER_ALGORITHM);
cipher.init(Cipher.DECRYPT_MODE, secretKey);
//得到key 后，后续代码就是Cipher 的写法，此处省略...

//秘钥算法
private static final String KEY_ALGORITHM = "DES";
//加密算法：algorithm/mode/padding 算法/工作模式/填充模式
private static final String CIPHER_ALGORITHM = "DES/ECB/PKCS5Padding";
//秘钥
private static final String KEY = "12345678";//DES 秘钥长度必须是8 位或以上
//private static final String KEY = "1234567890123456";//AES 秘钥长度必须是16 位

//初始化秘钥
SecretKey secretKey = new SecretKeySpec(KEY.getBytes(), KEY_ALGORITHM);
Cipher cipher = Cipher.getInstance(CIPHER_ALGORITHM);

//加密
cipher.init(Cipher.ENCRYPT_MODE, secretKey);
byte[] result = cipher.doFinal(input.getBytes());

//AES、DES 在CBC 操作模式下需要iv 参数
IvParameterSpec iv = new IvParameterSpec(key.getBytes());

//加密
cipher.init(Cipher.ENCRYPT_MODE, secretKey, iv);