//字符轉(zhuǎn)換成ASCII 碼數(shù)值
 char charA = 'a';
 int intA = charA; //char 強(qiáng)轉(zhuǎn)為int 即得到對應(yīng)的ASCII 碼值，'a'的值為97

//ASCII 碼值轉(zhuǎn)成char
int intA = 97;//97 對應(yīng)的ASCII 碼'a'
char charA = (char) intA; //int 值強(qiáng)轉(zhuǎn)為char 即得到對應(yīng)的ASCII 字符，即'a'

 /**
 * 加密
 * @param input 數(shù)據(jù)源（需要加密的數(shù)據(jù)）
 * @param key 秘鑰，即偏移量
 * @return 返回加密后的數(shù)據(jù)
 */
 public static String encrypt(String input, int key) {
 //得到字符串里的每一個字符
 char[] array = input.toCharArray();

 for (int i = 0; i < array.length; ++i) {
 //字符轉(zhuǎn)換成ASCII 碼值
 int ascii = array[i];
 //字符偏移，例如a->b
 ascii = ascii + key;
 //ASCII 碼值轉(zhuǎn)換為char
 char newChar = (char) ascii;
 //替換原有字符
 array[i] = newChar;

 //以上4 行代碼可以簡寫為一行
 //array[i] = (char) (array[i] + key);
 }

 //字符數(shù)組轉(zhuǎn)換成String
 return new String(array);
 }

 /**
 * 解密
 * @param input 數(shù)據(jù)源（被加密后的數(shù)據(jù)）
 * @param key 秘鑰，即偏移量
 * @return 返回解密后的數(shù)據(jù)
 */
 public static String decrypt(String input, int key) {
 //得到字符串里的每一個字符
 char[] array = input.toCharArray();
 for (int i = 0; i < array.length; ++i) {
 //字符轉(zhuǎn)換成ASCII 碼值
 int ascii = array[i];
 //恢復(fù)字符偏移，例如b->a
 ascii = ascii - key;
 //ASCII 碼值轉(zhuǎn)換為char
 char newChar = (char) ascii;
 //替換原有字符
 array[i] = newChar;

 //以上4 行代碼可以簡寫為一行
 //array[i] = (char) (array[i] - key);
 }

 //字符數(shù)組轉(zhuǎn)換成String
 return new String(array);
 }

/**
 * 頻率分析法破解凱撒密碼
 */
public class FrequencyAnalysis {
 //英文里出現(xiàn)次數(shù)最多的字符
 private static final char MAGIC_CHAR = 'e';
 //破解生成的最大文件數(shù)
 private static final int DE_MAX_FILE = 4;

 public static void main(String[] args) throws Exception {
 //測試1，統(tǒng)計字符個數(shù)
 //printCharCount("article1_en.txt");

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

 //讀取加密后的文件
 String artile = file2String("article1_en.txt");
 //解密（會生成多個備選文件）
 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) {
 //輸出前幾位的統(tǒng)計信息
 System.out.println("字符'" + entry.getKey() + "'出現(xiàn)" + 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);
 }

 /**
 * 破解凱撒密碼
 * @param input 數(shù)據(jù)源
 * @return 返回解密后的數(shù)據(jù)
 */
 public static void decryptCaesarCode(String input, String destPath) {
 int deCount = 0;//當(dāng)前解密生成的備選文件數(shù)
 //獲取出現(xiàn)頻率最高的字符信息（出現(xiàn)次數(shù)越多越靠前）
 List<Entry<Character, Integer>> mapList = getMaxCountChar(input);
 for (Entry<Character, Integer> entry : mapList) {
 //限制解密文件備選數(shù)
 if (deCount >= DE_MAX_FILE) {
 break;
 }

 //輸出前幾位的統(tǒng)計信息
 System.out.println("字符'" + entry.getKey() + "'出現(xiàn)" + entry.getValue() + "次");

 ++deCount;
 //出現(xiàn)次數(shù)最高的字符跟MAGIC_CHAR的偏移量即為秘鑰
 int key = entry.getKey() - MAGIC_CHAR;
 System.out.println("猜測key = " + key + "， 解密生成第" + deCount + "個備選文件" + "\n");
 String decrypt = MyEncrypt.decrypt(input, key);

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

 //統(tǒng)計String里出現(xiàn)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);
 }
 }

 //輸出統(tǒng)計信息
 /*for (Entry<Character, Integer> entry : map.entrySet()) {
 System.out.println(entry.getKey() + "出現(xiàn)" + entry.getValue() + "次");
 }*/

 //獲取獲取最大值
 int maxCount = 0;
 for (Entry<Character, Integer> entry : map.entrySet()) {
 //不統(tǒng)計空格
 if (/*entry.getKey() != ' ' && */entry.getValue() > maxCount) { 
 maxCount = entry.getValue();
 }
 }

 //map轉(zhuǎn)換成list便于排序
 List<Entry<Character, Integer>> mapList = new ArrayList<Map.Entry<Character,Integer>>(map.entrySet());
 //根據(jù)字符出現(xiàn)次數(shù)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);
 //輸出：97 對應(yīng)的二進(jìn)制數(shù)據(jù)為： 1100001
 System.out.println(i + "對應(yīng)的二進(jìn)制數(shù)據(jù)為： " + bit);

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

//1,得到cipher 對象（可翻譯為密碼器或密碼系統(tǒng)）
 Cipher cipher = Cipher.getInstance("DES");
 //2，創(chuàng)建秘鑰
 SecretKey key = KeyGenerator.getInstance("DES").generateKey();
 //3，設(shè)置操作模式（加密/解密）
 cipher.init(Cipher.ENCRYPT_MODE, key);
 //4，執(zhí)行操作
 byte[] result = cipher.doFinal("黑馬".getBytes());

//生成隨機(jī)秘鑰
SecretKey secretKey = KeyGenerator.getInstance("AES").generateKey();
//序列化秘鑰到磁盤上
FileOutputStream fos = new FileOutputStream(new File("heima.key"));

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

//從磁盤里讀取秘鑰
FileInputStream fis = new FileInputStream(new File("heima.key"));
ObjectInputStream ois = new ObjectInputStream(fis);
Key key = (Key) ois.readObject();

//創(chuàng)建密鑰寫法1
KeySpec keySpec = new DESKeySpec(key.getBytes());
SecretKey secretKey = SecretKeyFactory.getInstance(ALGORITHM).
generateSecret(keySpec);

//創(chuàng)建密鑰寫法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，后續(xù)代碼就是Cipher 的寫法，此處省略...

//秘鑰算法
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";
//秘鑰
private static final String KEY = "12345678";//DES 秘鑰長度必須是8 位或以上
//private static final String KEY = "1234567890123456";//AES 秘鑰長度必須是16 位

//初始化秘鑰
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 參數(shù)
IvParameterSpec iv = new IvParameterSpec(key.getBytes());

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