- (void)download: (NSURLDownload *)download didReceiveAuthenticationChallenge: (NSURLAuthenticationChallenge *)challenge
{
    [[challenge sender] useCredential:newCredential forAuthenticationChallenge:challenge];
}

- (void)useCredential: (NSURLCredential *)credential forAuthenticationChallenge: (NSURLAuthenticationChallenge *)challenge
{
    NSURLCredential *newCredential = [NSURLCredential credentialWithUser:@"user" password:@"pass" persistence:NSURLCredentialPersistenceNone];
}