请求服务器上面的一个文件，需要用户名和密码才能访问

(void)connection:(NSURLConnection *)connectiondidReceiveAuthenticationChallenge:(NSURLAuthenticationChallenge*)challenge
{
NSString* username =@"Private";
NSString* password =@"aaa";

NSURLCredential * cre =[[NSURLCredential alloc] initWithUser:usernamepassword:password
             persistence:NSURLCredentialPersistenceNone];
[[challenge sender]useCredential:creforAuthenticationChallenge:challenge];
}
如果用户名或密码输入错误如下：
想判断是否失败就

if ([challengefailureResponse])
{
    NSLog(@"fail");
}