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<iostream>
#include<string.h>
using namespace std;
int main()
{
	char k[100]={0},c[1000]={0},m[1000]={0};
	while(cin>>k>>c)
	{
		for(int i=0,j=0;i<strlen(c);i++,j++)
		{
			if(c[i]<='z'&&c[i]>='a'&&k[j]<='z'&&k[j]>='a')
			{
				m[i]=c[i]-(k[j]-97);
				if(m[i]>122) m[i]=m[i]-26;
				if(m[i]<97) m[i]=m[i]+26;
			}
			if(c[i]<='Z'&&c[i]>='A'&&k[j]<='Z'&&k[j]>='A')
			{
				m[i]=c[i]-(k[j]-65);
				if(m[i]>90) m[i]=m[i]-26;
				if(m[i]<65) m[i]=m[i]+26;
			}
			if(c[i]<='Z'&&c[i]>='A'&&k[j]<='z'&&k[j]>='a')
			{
				m[i]=(c[i]+32)-(k[j]-97)-32;
				if(m[i]>90) m[i]=m[i]-26;
				if(m[i]<65) m[i]=m[i]+26;
			}
			if(c[i]<='z'&&c[i]>='a'&&k[j]<='Z'&&k[j]>='A')
			{
				m[i]=c[i]+-(k[j]-97+32);
				if(m[i]>122) m[i]=m[i]-26;
				if(m[i]<97) m[i]=m[i]+26;
			}
			if(j==strlen(k)-1) j=-1;
		}
		cout<<m<<endl;
	}
}

Java :

import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		while (scanner.hasNextLine()) {
			String key = scanner.nextLine(), encoded = scanner.nextLine();
			System.out.println(decode(encoded, key));
		}
	}

	public static String decode(String encoded, String key) {
		key = key.toLowerCase();
		String decoded = "";
		for (int i = 0; i < encoded.length(); i++) {
			char c = encoded.charAt(i);
			int shift = key.charAt(i % key.length()) - 'a';
			if (c >= 'A' && c <= 'Z') {
				int offset = c - 'A';
				c = (char)('A' + (offset + 26 - shift) % 26);
			} else if (c >= 'a' && c <= 'z') {
				int offset = c - 'a';
				c = (char)('a' + (offset + 26 - shift) % 26);
			}
			decoded += c;
		}
		return decoded;
	}
}