In order to make it difficult for computer programs to identify the characters, various types of distractions are played over them, such as random white noise, loud noises between characters, other voices. Some audio captchas use purposely low-quality recordings.
White noise is relatively easy to filter out, but competing voices and sounds that present sound patterns similar to letters and numbers are the most difficult for Decaptcha to discern, the researchers say. These are called symantic distractions and require human intelligence to sort them out with a high degree of accuracy.
Working in favor of Decaptcha is that the creators of audio captchas have to make them simple enough for humans to figure out the letters and numbers the vast majority of the time. The balance between simple enough for humans to distinguish and difficult enough for computers to miss is tricky, the researchers say.
The researchers recommend tightening up security of audio captchas through use of more symantic noise.
The researchers say they are working on ways to break audio captchas that use entire words rather than just characters to see whether they are more safe.
They also want to analyze the differences in the ways humans make mistakes decoding captchas and the ways computers make mistakes. That way captchas can be designed to make it more difficult and costly to device programs that defeat them, they say.
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