typoを含む文章と複数の絵から構成されるCAPTCHAの提案と評価

著者らは，typo を含む 1 つの文章と複数の絵から構成される Multi-model CAPTCHA を提案する．これは人間がもつ 3 つの能力（typo を含む文章の意味を推測する能力，絵を理解する能力，文意と絵を紐づける能力）を利用した CAPTCHA であり「typo を含む文章が長いほど，機械は文意と絵の紐づけに時間がかかる（＝紐づけにかかる時間が短いほど人間である可能性が高い）」という仮説に基づく．実験の結果，構図が似た絵が複数あっても文意に合致する 1 つの絵を紐づけられること，なじみのある短い単語から構成された文章にタイポグリセミアが現れやすいこと，文章が長くなっても紐づけにかかる時間は指数関数的にならないこと（ユーザにとって難解にならないこと），が分かった．

We propose a Multi-model CAPTCHA that is composed of one sentence with typo and multiple pictures. This CAPTCHA utilizes human's three abilities (inferring the meaning of the sentence with typo, understanding the meaning of pictures, and linking the meaning of the sentence and corresponding picture), and it is based on a hypothesis, "the longer of the sentence with typo, the time could be longer for a machine to link the meaning of the sentence and corresponding picture (= the shorter of the time of linking, the higher of the probability of human)." We found three findings from our experiment: (1) Examinees were able to link the meaning of the sentence to corresponding picture even there were some pictures with similar composition, (2) Typoglycemia is likely to be appeared on a sentence constructed by short and familiar words, (3) The time for linking was not exponentially increased even the length of the sentence was getting long (= It was not getting difficult for users in this situation).