C :

#include "stdio.h"
#include "string.h"


char m[1000], c[1000], k[1000];

char change(char ch,int i)
{
	char number,x,a,b;
	b = k[i];
	if ((ch >= 'a') && (ch <= 'z'))
	{
		a = 'a';
		if ((b >= 'A') && (b <= 'Z')) b = b - 'A' + 'a';
	}
	else a='A';


	number = ch - a;
	x = b - a;
	if (number < x) number = number + 26;
	number = number - x + a;
	return (number);
}

void work()
{
	int lengthc, lengthk, i, j,tag;
	lengthk = strlen(k);
	lengthc = strlen(c);
	tag = 0;
	for (i = 0; i <= lengthc; i++)
	{
		m[i] = change(c[i], tag);
		if (tag == lengthk-1) tag = 0;
		else tag++;
	}
	m[lengthc] = '\0';
	printf("%s\n", m);

}

void main()
{

	while (scanf("%s", &k) != EOF)
	{
		scanf("%s", &c);
		work();
	}

}

C++ :

#include<stdio.h>
#include<string.h>
int main(){
	char c[1001],k[101];
	while(scanf("%s",k)!=EOF){
		  int all;
		  all=strlen(k);
		  scanf("%s",c);
		  int sum=0,t=0,ju=0;
		  while(c[t]!=NULL){
			  if(sum==all) sum=0;
			  if(c[t]<97) 
			  {ju=1;
			   c[t]=c[t]+32;
			  }//大写
			  else ju=0;//小写
			  for(int i=97;i<=122;i++){
				  if(k[sum]<97) k[sum]=k[sum]+32;  
				  if((i-97+k[sum]-97)%26+97==c[t]){
					  if(ju==0) printf("%c",i);
					  else printf("%c",i-32);
				  }  
			  }
			  t++;
			  sum++;
		  }
		  printf("\n");
	}
	return 0;
}