维吉尼亚密码解密

利用重合法根据密文串找到**长度，找到**从而找到**，我也不知道自己理解的对不对，仅供参考
https://wenku.baidu.com/view/8f9d144deef9aef8941ea76e58fafab069dc44a7.html 这个网址是有关维吉尼亚密码的和重合指数法解密的基本知识

import java.util.Scanner;
/*
 * 利用重合指数法解密，划分子串，计算每个子串与C=0.065的差值
 * 若差值小于0.01说明此次划分的子串个数就是**字长度,然后根据
 * **字长度划分字符再结合公式得到**，从而找到明文
 */
public class Virginia {

	public Virginia() {
		// TODO Auto-generated constructor stub
	}

	public static void main(String[] args) {
		System.out.println("输入密文：");
		Scanner str=new Scanner(System.in);
		Virginia vir=new Virginia();
		String s=str.nextLine();
		int branchLength=0;//**长度
		//确定**长度
		for(int i=1;i<s.length();i++) {
			if(vir.suitable(s, i)) {
				//说明此次划分的每个子串都离0.065很近
				branchLength=i;
				break;
			}
		}
		//确定**
		int[] key=vir.getKeys(s, branchLength);
		//得到明文
		String plainCode=vir.deCode(s,key);
		System.out.println("明文是：\n"+plainCode);
	}
	
	//根据密文s和**keys得到明文 
	public String deCode(String s,int[] keys) {
		String text="";
		for(int i=0,j=0;i<s.length();i++,j++) {
			if(j==keys.length)   
				j=0;
			int result=0;
			if((s.charAt(i)-'A'-keys[j])<0) {
				result=(s.charAt(i)-'A'-keys[j])+26+'A';
			}else {
				result=(s.charAt(i)-'A'-keys[j])+'A';
			}
			text+=(char)result;
		}
		return text;
	}
	
	//根据密文串s和**长度m确定**
	public int[] getKeys(String s,int m) {
		float[] FRE= {0.082f,0.015f,0.028f,0.043f,0.127f,0.022f,0.020f,0.061f,
				0.070f,0.002f,0.008f,0.040f,0.024f,0.067f,0.075f,0.019f,0.001f,
				0.060f,0.063f,0.091f,0.028f,0.010f,0.023f,0.001f,0.020f,0.001f};
		float a=0.0f;
		String[] bs=branches(s,5);
		int[] keys=new int[bs.length];
		for(int t=0;t<bs.length;t++) {
			for(int k=0;k<26;k++) {
				for(int i=0;i<26;i++) {
					a+=FRE[i]*pinlv(bs[t],(char)((i+k)%26+65));
				}
	
				if(Math.abs(0.065-a)<0.01) {
					keys[t]=k;
				}
				a=0;
			}
		}
		return keys;
	}
	
	//把字符串str划分成m个子串
	public String[] branches(String str,int m) {
		String[] branches=new String[m];//子串数组
		for(int i=0;i<str.length();) {
			for(int j=0;j<m;j++,i++) {
				if(branches[j]==null) {
					branches[j]=""+str.charAt(i);
				}else if(i<str.length()) {
					branches[j]=branches[j]+str.charAt(i);
				}
			}
		}
		return branches;
	}
	
	//计算子串str中某一字符c出现的频率
	public float pinlv(String str,char c) {
		float sum=0.0f;
		
		//统计字符c出现的次数
		for(int i=0;i<str.length();i++) {
			if(str.charAt(i)==c) {
				sum++;
			}
		}
		return sum/(float)str.length();
	}
	
	//判断每个子串的重合指数是否都接近0.065
	public boolean suitable(String str,int m) {
		final float C=0.065f;
		String[] bs=branches(str,m);//子串数组
		
		//若每个子串的重合指数和0.065的差值的绝对值小于0.01,则此次划分合理，返回true
		for(int j=0;j<m;j++) {
			if(Math.abs(C-overlapIndex(bs[j]))>0.01) {
				return false;
			}
		}
		return true;
	}
	
	//计算字符串str重合指数
	public float overlapIndex(String str) {
		float index=0;
		float sum=0;
		float frequency=0;//字母频度
		float length=str.length();//文本长度
		
		for(int i=0;i<str.length();i++) {
			char c=str.charAt(0);
			for(int j=0;j<str.length();j++) {
				if(str.charAt(j)==c) {
					frequency++;
				}
			}
			sum+=frequency*(frequency-1);
			str=str.replaceAll(str.substring(0, 1), "");
			frequency=0;
		}
		index=sum/(length*(length-1));
		return index;
	}
}