密码学实验： Virginia加解密算法C语言实现
1.实验步骤
- 用户输入密钥，对明文文件进行加密
- 使用唯密文攻击破解密钥
- 根据所得密钥进行解密
2.实验测试
（1）明文文本
（2）密钥：humin
（3）密文
（4）程序运行结果
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代码如下：
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<math.h>
#define MAX 10000
#define LEN 50
/*Virginia加密算法*/
void encrypt()
{
FILE *fin,*fout;
char ch,cc;
char filename1[LEN]="F:\\cryptology\\plaintext.txt";
char filename2[LEN]="F:\\cryptology\\cipertext.txt";
char key[MAX];
int keylen,i=0;
printf("请输入加密密钥:\n") ;
scanf("%s",key);
keylen=strlen(key);
fin=fopen(filename1,"r");
if(fin==NULL)
{
printf("Fail to open the file1!\n");
}
fout=fopen(filename2,"w");
if(fout==NULL)
{
printf("Fail to open the file2!\n");
}
printf("加密后的密文：\n");
while((ch=fgetc(fin))!=EOF)
{
cc=(ch+(key[i%keylen]-'a'))<='z'?(ch+(key[i%keylen]-'a')):(ch+(key[i%keylen]-'a'))-26;
fprintf(fout,"%c",cc);
i++;
printf("%c",cc);
}
printf("\n");
fclose(fin);//关闭输入流
fclose(fout);
}
/*唯密文破解密钥*/
void getKey(char filename1[],char key[])
{
FILE *fin,*fout;
char sub[100]="";
char try[100]="";
int alphabat[26]={0};//用于统计字母出现的频率
double assese[100]={0}; //统计每个字串的IC'值
int Count=1;//密钥长度
int i,j;
double avg=0;
double p[30]={0.0805,0.0162,0.0320,0.0365,0.1231,0.0228,0.0161,0.0514,0.0718,0.0010,
0.0520,0.0403,0.0225,0.0719,0.0794,0.0229,0.0020,0.0603,0.0659,0.0959,
0.0310,0.0093,0.0203,0.0020,0.0188,0.0009};//字母出现的统计频率
int keyIndex[MAX];//密钥的位置
// step1：估算Virginia多表代换加密的秘钥长度
while((int)(fabs(avg-0.065)*1000)>1)
{
char ch;
Count++;
avg=0.0;
//将密文分成Count个子串，然后计算其IC’的平均值
memset(assese,0,sizeof(assese));
printf("分成%d个子串\n",Count);
for(i=0;i<Count;i++)
{
fin=fopen(filename1,"r");
if(fin==NULL)
{
printf("Fail to open the cipertextfile!\n");
exit(0);
}else{
sprintf(sub,"F:\\cryptology\\sub%d.txt",i+1);//创建Count个文本文件
fout=fopen(sub,"w");
if(fout== NULL )
{
printf("Fail to open subFile.");
break ;
}else
{
int len=0;
memset(alphabat,0,sizeof(alphabat));//初始化字母出现的频率
for(j=0;j<i;j++)//第i个字串的开始点
{
if((ch=fgetc(fin))!=EOF) continue;
}
while((ch=fgetc(fin))!=EOF)
{
fprintf(fout,"%c",ch);
alphabat[ch-'a']++;
len++;
printf("%c",ch);
for(j=0;j<Count-1;j++)//同一个字串内的字母相距 Count-1
{
if((ch=fgetc(fin))!=EOF)
{
continue;
}
}
}
printf("\n");
//计算IC'
for(j=0;j<26;j++)
{
if(alphabat[j]>1)
{
assese[i+1]+=((double)alphabat[j]/len)*((double)(alphabat[j]-1)/(len-1));
}
}
avg+=assese[i+1];
}
fclose(fout);
fclose(fin);//关闭输入流
}
}
avg=avg/Count;//求IC'的平均值
if((int)(fabs(avg-0.065)*1000)<=1)//IC'的平均值接近于0.065
{
printf("第%d次的IC'=%f\n",Count,avg);
break;
}
printf("第%d次的IC'=%f\n",Count,avg);
}
//step2：再计算秘钥中的每个字符
for(i=0;i<Count;i++)
{
double index[27]={0.0};//字串每一次移位的拟重合指数
int fre[26]={0};
double max=0.0;
sprintf(sub,"F:\\cryptology\\sub%d.txt",i+1);//打开字串文件
sprintf(try,"F:\\cryptology\\try%d.txt",i+1);
for(j=0;j<26;j++)
{
char ch,cc;
int k;
int n=0;//字串的长度
memset(fre,0,sizeof(fre));
fin=fopen(sub,"r");
fout=fopen(try,"w");
while((ch=fgetc(fin))!=EOF)
{
cc=(ch-j)>='a'?(ch-j):(ch-j)+26;//左移j位
fprintf(fout,"%c",cc);
fre[cc-'a']++;//统计移位后每个 字母出现的频率
n++;
}
//计算拟重合指数
for(k=0;k<26;k++)
{
index[j-1]+=(p[k]*fre[k])/n;
}
}
//求最大的拟重合指数
max=index[0];
for(j=1;j<26;j++)
{
if(index[j]>max)
{
max=index[j];
keyIndex[i]=j;//保存当前字串的密钥
}
}
fclose(fin);
fclose(fout);
}
printf("所求密钥为：\n");
for(i=0;i<Count;i++)//输出密钥
{
printf("%c",keyIndex[i]+'a'+1);
key[i]= keyIndex[i]+'a'+1;//保存密钥
}
printf("\n");
}
/*Virginia已知密钥解密算法*/
void decrypt()
{
FILE *fin,*fout;
char ch,cc;
char key[MAX];
char filename1[LEN]="F:\\cryptology\\cipertext.txt";
char filename2[LEN]="F:\\cryptology\\decryptiontext.txt";
int keylen,i=0;
getKey(filename1,key);
keylen=strlen(key);
fin=fopen(filename1,"r");
fout=fopen(filename2,"w");
if(fout==NULL)
{
printf("fail\n");
}
printf("解密得到的明文:\n");
while((ch=fgetc(fin))!=EOF)
{
cc=(ch-(key[i%keylen]-'a'))>='a'?(ch-(key[i%keylen]-'a')):(ch-(key[i%keylen]-'a'))+26;
fprintf(fout,"%c",cc);
i++;
printf("%c",cc);
}
printf("\n");
fclose(fin);//关闭输入流
fclose(fout);
}
int main()
{
int choice;
while(1)
{
printf("----------------------------------------\n");
printf("\t\t1.加密\n");
printf("\t\t2.解密\n");
printf("\t\t3.退出\n");
printf("----------------------------------------\n");
printf("请输入你的选择：\n") ;
scanf("%d",&choice);
switch(choice)
{
case 1:
encrypt();
break;
case 2:
decrypt();
break;
case 3:
exit(0);
break;
}
}
return 0;
}