乘數加密解密

//功能：乘數加密 K=3 5 7 9 11 15 17 19 21 23 25

#include <stdio.h>
int str_len(char *str)//求字符串的長度
{
 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（必須是小寫能夠用函數改變） :");
 gets(str);
 printf("Please input the following number[3][5][9][11][15][17][19][21][23]:");//能夠求逆元的數
  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]==a[j])
    str[i] = b[(k*j)%26]//換
   else
    str[i] = str[i];//將空格等符號落下來
   }
  printf("%s\n",str);//打印
 return 0;
}

//解密

#include <stdio.h>

求字符串長度
int str_len(char *str)
{
 int len = 0;
 while(*(str++))
  len++;
 return len;
}

//求逆元
int mulny(int k)
{
 int t;
 for(int i = 0;i<26;i++)
 {
  t=k*i;
  if(t%26==1)
   return i;
 }
}

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 the following number[3][5][9][11][15][17][19][21][23]:");//26之內有逆元的數字
 scanf("%d",&k);
 int p = mulny(k);
 int len = str_len(str);
 for(int i = 0;i<len;i++)
  for(int j = 0;j<26;j++)
  {
   if(str[i]==a[j])
    str[i] = b[(p*j)%26];
   else
    str[i] = str[i];
  }
  printf("%s\n",str);
 return 0;
}

/*
會凱撒加密的這個應該沒啥大問題，可是剛剛玩，我調試仍是調試了一會的，我記得昨天都調好了，今天加密找不到了，存儲代碼的時候沒規律。哎哎，又改的幸虧不費勁。也許有錯的地方但願提出改正，我想進步哦！謝謝你們。

 

*/