创建一个控制台程序，复制代码运行即可

核心代码转载于：http://blog.csdn.net/JarvisChu/article/details/5785870

#include "stdafx.h"
#include<stdio.h>  
#include<stdlib.h>  
#include<string.h>   

void encfile(char *in_filename,char *pwd,char *out_filename);  
void decryptfile(char* in_filename,char *pwd,char *out_filename);  

int main(int argc,char *argv[])  
{  
 int option;  
 char in_filename[30];  
 char out_filename[30];  
 char pwd[8];  

 printf("                   thank you for using this program....../n"); 
 printf("1. Encrypt a file         2. Decrypt a file/n");     
 printf("chose your option....."); 

 scanf("%d",&option); 
 getchar(); 
 if(argc!=4){  
  printf("/nPlease input In-filename:/n");  
  gets(in_filename);  
  printf("Please input your Password:/n");  
  gets(pwd);  
  printf("Please input Out-filename:/n");  
  gets(out_filename);  

 }  
 else{  
  strcpy(in_filename,argv[1]);  
  strcpy(pwd,argv[2]);  
  strcpy(out_filename,argv[3]);  
 }  

 switch(option){ 
 case 1: //加密      
  encfile(in_filename,pwd,out_filename);  
  break; 
 case 2://解密  
  decryptfile(in_filename,pwd,out_filename); 
  break; 
 default: 
  break; 
 }    

 system("pause");  
 return 0;  
}  

  
void encfile(char *in_filename,char *pwd,char *out_file)  
{  
 FILE *fp1,*fp2;  
 register char ch;  
 int j=0;  
 int j0=0;  
 fp1=fopen(in_filename,"r");  
 if(fp1==NULL){  
  printf("cannot open in-file./n");  
  exit(1);  
 }  
 fp2=fopen(out_file,"w");  
 if(fp2==NULL){  
  printf("cannot open or create out-file./n");  
  exit(1);  
 }  

 while(pwd[++j0]);   

 ch=fgetc(fp1);  

   
 while(!feof(fp1)){  
  if(j0 >7) 
   j0 = 0; 
  ch += pwd[j0++]; 
  fputc(ch,fp2);  
  ch=fgetc(fp1);  
 }  
 fclose(fp1);  
 fclose(fp2);  
}  

  
void decryptfile(char *in_filename,char *pwd,char *out_file)  
{  
 FILE *fp1,*fp2;  
 register char ch;  
 int j=0;  
 int j0=0;  
 fp1=fopen(in_filename,"r");  
 if(fp1==NULL){  
  printf("cannot open in-file./n");  
  exit(1);  
 }  
 fp2=fopen(out_file,"w");  
 if(fp2==NULL){  
  printf("cannot open or create out-file./n");  
  exit(1);  
 }  

 while(pwd[++j0]);    
 ch=fgetc(fp1);  
   
 while(!feof(fp1)){  
  if(j0 >7) 
   j0 = 0; 
  ch -= pwd[j0++]; 
  fputc(ch,fp2); 
  ch=fgetc(fp1);  
 }  
 fclose(fp1);  
 fclose(fp2);  
}