凱撒（Caesar）密碼加密解密c語言

我想用英語寫註釋，麻痹，寫不明白！
編譯器是vc++6.0

輸入：omnia gallia est
密鑰：3
加密：RPQLD JDOOLD HVW

#include <stdio.h>
int str_len(char *str)//計算字符串長度
{
 int len = 0;            //設置長度初始值爲0
 while(*(str++))      //用指針遞增指向下一個內容直到空爲止
   len++;                //記數
 return len;             //返回記錄的數
}

int main(int argc, char *argv[])
{

//不懂阿斯科馬 就把字母存放在數組中了
 char a[26] = {'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};
 char b[26] = {'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};
 char str[50];//定義一個接收輸入字符串的數組
 int k;
 printf("Please input the words :");
 gets(str);// 得用gets 有空格 好使
 printf("Please input your number:");
  scanf("%d",&k);//輸入密鑰
 int len = str_len(str);
/*代碼主體兩個循環，一層循環控制輸入字符，一個一個取出來
與字母表對照，找到位置，再用密鑰找出對應的大寫字母*/
 for(int i = 0;i<len;i++)
  for(int j = 0;j<26;j++)
  {
   if(str[i]==a[j])
    str[i] = b[(j+k)%26];
   else
    str[i] = str[i];
  }
  printf("%s\n",str);//輸出字符串
 return 0;
}

//解密

#include <stdio.h>
int str_len(char *str)
{
 int len = 0;
 while(*(str++))
  len++;
 return len;
}

int main(int argc, char *argv[])
{
 char a[26] = {'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};
 char b[26] = {'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};
 char str[50];
 int k;
 printf("Please input the words :");
 gets(str);
 printf("Please input your number:");
  scanf("%d",&k);
 int len = str_len(str);
 for(int i = 0;i<len;i++)
  for(int j = 0;j<26;j++)
  {
   if(str[i]==b[j])
    str[i] = a[(j+26-k)%26];
   else
    str[i] = str[i];
  }
  printf("%s\n",str);
 return 0;
}