Question

用C语言实现任意字符串的加密，其中，字母用凯撒加密方法加密，非字母不变

Answer 1

我尽量用注释阐述了思路，希望可以帮到你！！

#include<stdio.h>
#include<string.h>
#define N 80 //可加密字符串最大长度

char plaintext[N]={0}; //明文，输入时输入字符，参与运算时强制转换成整数
int ciphertext[N]={0}; //密文，保存成整数，输出时强制转换成字符
int k; //后(右)移位数，相当于密钥

void getPlainText() //获得明文字符串
{
printf("请输入明文:");
scanf("%s",plaintext);
printf("\n");
}

void getLength() //获取后(右)移位数(密钥)
{
printf("请输入后移的位数:");
scanf("%d",&k);
k%=26; //因为字母只有26个，所以超过26相当于重复
}

void Caesar_cipher() //凯撒加密，本程序采用的是字母循环后(右)移
{
unsigned int i;

for(i=0;i<strlen(plaintext);i++)
{
//两个bool类型的变量是为了判断字符是否是字母(包括大写和小写)
bool flag1=plaintext[i]>='a'&&plaintext[i]<='z';
bool flag2=plaintext[i]>='A'&&plaintext[i]<='Z';

if(flag1||flag2){ //如果是字母，加密
ciphertext[i]=(int)plaintext[i]+k; //字母在字母表中后(右)移K位
if(ciphertext[i]>(int)'z'){ //保证是循环后(右)移
ciphertext[i]-=26;
}
}
else //非字母字符，不做处理，原样保存
ciphertext[i]=(int)plaintext[i];

}

}

void printCipherText() //输出加密后的密文
{
unsigned int i;
printf("\n加密后的密文是:");
for(i=0;i<strlen(plaintext);i++) //把参与计算后是整数强制转换成对应的字符
printf("%c",(char)ciphertext[i]);
printf("\n");

}

void main()
{
getPlainText(); //明文
getLength(); //后(右)移位数
Caesar_cipher(); //凯撒加密
printCipherText(); //密文

}

Answer 2

（2）kaiser加密算法
具体程序：
#include<stdio.h>
#include<conio.h>
char encrypt(char ch,int n)/*加密函数，把字符向右循环移位n*/
{
while(ch>='A'&&ch<='Z')
{
return ('A'+(ch-'A'+n)%26);
}
while(ch>='a'&&ch<='z')
{
return ('a'+(ch-'a'+n)%26);
}
return ch;
}
void menu()/*菜单，1.加密，2.解密，3.暴力破解，密码只能是数字*/
{
clrscr();
printf("\n=========================================================");
printf("\n1.Encrypt the file");
printf("\n2.Decrypt the file");
printf("\n3.Force decrypt file");
printf("\n4.Quit\n");
printf("=========================================================\n");
printf("Please select a item:");
return;
}

main()
{
int i,n;
char ch0,ch1;
FILE *in,*out;
char infile[20],outfile[20];
textbackground(BLACK);
textcolor(LIGHTGREEN);
clrscr();

sleep(3);/*等待3秒*/
menu();
ch0=getch();
while(ch0!='4')
{
if(ch0=='1')
{
clrscr();
printf("\nPlease input the infile:");
scanf("%s",infile);/*输入需要加密的文件名*/
if((in=fopen(infile,"r"))==NULL)
{
printf("Can not open the infile!\n");
printf("Press any key to exit!\n");
getch();
exit(0);
}
printf("Please input the key:");
scanf("%d",&n);/*输入加密密码*/
printf("Please input the outfile:");
scanf("%s",outfile);/*输入加密后文件的文件名*/
if((out=fopen(outfile,"w"))==NULL)
{
printf("Can not open the outfile!\n");
printf("Press any key to exit!\n");
fclose(in);
getch();
exit(0);
}
while(!feof(in))/*加密*/
{
fputc(encrypt(fgetc(in),n),out);
}
printf("\nEncrypt is over!\n");
fclose(in);
fclose(out);
sleep(1);
}
if(ch0=='2')
{
clrscr();
printf("\nPlease input the infile:");
scanf("%s",infile);/*输入需要解密的文件名*/
if((in=fopen(infile,"r"))==NULL)
{
printf("Can not open the infile!\n");
printf("Press any key to exit!\n");
getch();
exit(0);
}
printf("Please input the key:");
scanf("%d",&n);/*输入解密密码（可以为加密时候的密码）*/
n=26-n;
printf("Please input the outfile:");
scanf("%s",outfile);/*输入解密后文件的文件名*/
if((out=fopen(outfile,"w"))==NULL)
{
printf("Can not open the outfile!\n");
printf("Press any key to exit!\n");
fclose(in);
getch();
exit(0);
}
while(!feof(in))
{
fputc(encrypt(fgetc(in),n),out);
}
printf("\nDecrypt is over!\n");
fclose(in);
fclose(out);
sleep(1);
}
if(ch0=='3')
{
clrscr();
printf("\nPlease input the infile:");
scanf("%s",infile);/*输入需要解密的文件名*/
if((in=fopen(infile,"r"))==NULL)
{
printf("Can not open the infile!\n");
printf("Press any key to exit!\n");
getch();
exit(0);
}
printf("Please input the outfile:");
scanf("%s",outfile);/*输入解密后文件的文件名*/
if((out=fopen(outfile,"w"))==NULL)
{
printf("Can not open the outfile!\n");
printf("Press any key to exit!\n");
fclose(in);
getch();
exit(0);
}
for(i=1;i<=25;i++)/*暴力破解过程，在察看信息正确后，可以按'Q'或者'q'退出*/
{
rewind(in);
rewind(out);
clrscr();
printf("==========================================================\n");
printf("The outfile is:\n");
printf("==========================================================\n");
while(!feof(in))
{
ch1=encrypt(fgetc(in),26-i);
putch(ch1);
fputc(ch1,out);
}
printf("\n========================================================\n");
printf("The current key is: %d \n",i);/*显示当前破解所用密码*/
printf("Press 'Q' to quit and other key to continue......\n");
printf("==========================================================\n");
ch1=getch();
if(ch1=='q'||ch1=='Q')/*按'Q'或者'q'时退出*/
{
clrscr();
printf("\nGood Bye!\n");
fclose(in);
fclose(out);
sleep(3);
exit(0);
}
}
printf("\nForce decrypt is over!\n");
fclose(in);
fclose(out);
sleep(1);
}
menu();
ch0=getch();
}
clrscr();

printf("\nGood Bye!\n");
sleep(3);
}
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