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through game's server API). In our case, we could not do the entire game logic
at our server, because we were dependent on the external search engines, whose
API we could not queried from one (server) machine (because of queries-per-
day limits) but from multiple client machines. Therefore, the client side told our
server about the search result count and this opened the possibility to spoof the
communication to our server (resp. the communication from the search engine to
our clients) and provide false results. Since we have not very wide audience for
the game, we experienced this type of attack only once. Nevertheless, we have
to take some measures that ultimately led us to the creation of the a posteriori
cheating detection.
Concerning the TermBlaster modification of the LSG, the stopword threat was solved
by the gamemechanics, the set of words allowed to usewas defined by the game itself.
The game danger of interfering with the game's implementation was also ruled-out:
the search engine was now part of the game's implementation on the server side and
thus, we were able to handle all of the logic in the safe environment with no options
for “result spoofing”.
8.1.2.1 Term Banning
The “stopword deviation” is a downside effect of the approximative artifact validation
method. It comes with the probability that player realizes the possibility of exploiting
the scoring rules. In this case, the scoring mechanism was simple and known to the
players, so some of them realized the possibility of using a stopword as a “ultimate
word”. Technically, by entering stopwords, player is not violating the game rules,
but due to it's negative effects, we needed to restrict this.
We restricted the stopwords by explicitly banning them (
prevention by restrictive
rules
). We did so by imposing several rules:
All negative search terms used by players must be contained within a dictionary
(we used WordNet) as either nouns, adjectives, verbs or adverbs (this effectively
excludes most words that carry no semantics, though as a downside it also prevents
usage of perfectly valid terms outside of the dictionary).
The player cannot use the same term as within the task query (including its morphs,
checked by a dictionary).
The most frequent words of the language cannot be used. We used 200 most
frequent English words from Wikipedia.
The player cannot use words, which are commonly acting as HTML tags or which
are commonly used for creating website structure (e.g. “menu”). We have prepared
this set manually.
When players attempted to use these, they received a message explaining why they
cannot do so. The ban-list initially helped during the gameplay, and kept many players
from experimenting with stopwords. However, some still continued in finding words
working like stopwords which were at the same time not yet banned. To identify
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