信息安全之程序实现简单替换加密，并用字母频率统计进行破解

package gh;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.Reader;
import java.util.HashMap;
import java.util.Iterator;
/**
 * 信息安全作业
 * @author ganhang
 *
 */
public class encrypt {
    private static HashMap<String, Integer> h=new HashMap<String, Integer>();
    public static void main(String[] args) {
//        System.out.println(readfile("2.txt"));
//        System.out.println(encode(readfile("2.txt"),3));
//        System.out.println(find(readfile("2.txt")));
        
        String encodeString =encode(readfile("2.txt"),3);//加密后的字符串
        String decodeString =decode(encodeString);//解密后的字符串
        System.out.println(decodeString);
    }
    /**
     * 解密
     * @param s
     */
    public static String decode(String s){
        String maxString =find(s);//找出出现最多的字母
        //System.out.println(maxString);
        char maxchar=‘e‘;//自然状态下出现最多的字母
        char max =maxString.charAt(0);//转字母
        int shift=max-maxchar;//获得偏移量(这里是+3)
        return encode(s,0-shift);//(解密只需要偏移-3)
    }
    /**
     * 找出出现频率最高的字母
     * @param s
     * @return
     */
    public static String find(String s){
        //HashMap<String, Integer> h=new HashMap<String, Integer>();
        char c[]=s.toCharArray();
        int count=0;
        for(int i=0;i<c.length;i++){
            if (c[i] <= ‘Z‘ && c[i] >= ‘A‘ || c[i] <= ‘z‘ && c[i] >= ‘a‘) {
                if(h.get(""+c[i])!=null)count=h.get(""+c[i]);
                h.put(c[i]+"",++count);
            }
        }
        int max=-1;
        String maxString=null;
        for (String str : h.keySet()) {
            if (h.get(str) > max) {
                max = h.get(str);
                maxString = str;
            }
        }
        return maxString;
    }
    /**
     * 读文件
     * @param url
     * @return
     */
    public static String readfile(String url){
        StringBuilder sb = new StringBuilder();
        try {
            Reader in=new InputStreamReader(new FileInputStream(url));
            char []cbuf=new char[1024];
            int len;
            while((len=in.read(cbuf))!=-1){
                sb.append(cbuf);
            }
            in.close();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return sb.toString();
    }
    /**
     * 移位加密
     * @param s
     * @param key
     * @return
     */
    public static String encode(String s, int key) {
        StringBuilder sb = new StringBuilder();
        char[] c = s.toCharArray();
        for (int i = 0; i < c.length; i++) {
            if (c[i] <= ‘Z‘ && c[i] >= ‘A‘ || c[i] <= ‘z‘ && c[i] >= ‘a‘) {
                char ch = (char) (c[i] + key);
                sb.append(ch);
            }else
                sb.append(c[i]);
        }
        return sb.toString();
    }
}