#include <cstdio>
#include <cstring>
#include <list>
#include <algorithm>
using namespace std;
const int MAX=1e6;
const int MAXP=(1e5)+1;
char message[MAX+1], encrypted[MAX+1];
int key[MAXP], both[MAX], known[MAX], nkn;
list<int> tofind;
bool feasible(int n){
	memset(key, -1, sizeof(int)*n);
	for(int it=0; it<nkn; it++){
		int pos=known[it];
		if(key[pos%n]==-1) key[pos%n]=both[pos];
		else if(key[pos%n]!=both[pos])return 0;
	}
	return 1;
}
int main(){
	int t; scanf("%d", &t);
	while(t--){
		int m; scanf("%d", &m);
		scanf("%s %s", message, encrypted);
		tofind.clear();
		nkn=0;
		int ntofind = 0;
		for(int i=0; message[i]!=0; i++)
			if(message[i]!='*'&&encrypted[i]!='*'){
					known[nkn++]=i;
					both[i]=(26+encrypted[i]-message[i])%26;
			}
			else if(message[i]=='*'&&encrypted[i]!='*')
					tofind.push_back(i), ntofind++;
		list<int>::iterator it;
		m = min(m, (int)strlen(message));
		for(int n=m/2 +1; n<=m; n++)
			if(feasible(n)){
				it=tofind.begin();
				while(it!=tofind.end())
					if(key[(*it)%n]==-1){
						message[*it]='*';
						it=tofind.erase(it);
					}
					else if(message[*it]=='*'){
						message[*it]=(encrypted[*it]-'A'+26-key[(*it)%n])%26 + 'A';
						it++;
					}
					else if(message[*it] != (encrypted[*it]-'A'+26-key[(*it)%n])%26 + 'A'){
						message[*it]='*';
						it=tofind.erase(it);
					}
					else it++;
			}
		printf("%s\n", message);
	}
}