Proposal and Evaluation of Multi-model CAPTCHA with Typoglycemia

Abstract

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).