C :

#include<stdio.h>
#include<string.h>
int main()
{
    int i,j,t,k;
    char m[1005],c[105];
    while(scanf("%s%s",c,m)==2)
    {
        t=strlen(c);
        for(i=0;i<t;i++)
            if(c[i]<='Z')c[i]-='A';else c[i]-='a';
        for(i=0,j=0;i<strlen(m);i++)
        {
            k=m[i]-c[j++];
            if(m[i]>='a')if(k>='a')printf("%c",k);else printf("%c",k+26);
            else if(k>='A')printf("%c",k);else printf("%c",k+26);
            if(j==t)j=0;
        }
        printf("\n");
    }
    return 0;
}

C++ :

#include<stdio.h>
#include<string.h>
int main()
{
	char a[101];
	char b[1002];
	while(scanf("%s",a)!=EOF)
	{
		getchar();
		scanf("%s",b);getchar();
		int bb[1002]={0};
		int n=strlen(b);
		int nn=strlen(a);
		for (int i=0;i<n;i++)
			if (b[i]>='a'&&b[i]<='z') {b[i]=b[i]-'a'+'A';bb[i]=1;}
			for (int i=0;i<nn;i++)
				if (a[i]>='a'&&a[i]<='z') a[i]=a[i]-'a'+'A';
			for (int i=0;i<n;i++)
			{
				if (b[i]-'A'>=a[i%nn]-'A') b[i]=b[i]+'A'-a[i%nn];
				else b[i]=b[i]+26+'A'-a[i%nn];
			}
			for (int i=0;i<n;i++)
			{
				if (bb[i])
					printf("%c",b[i]-'A'+'a');
				else
					printf("%c",b[i]);
			}
			printf("\n");
	}
	return 0;
}