Another technique that is becoming more popular is email and web-based challenge-response systems which generate a blurry graphic that is (presumably) only readable by humans. [....] This scheme does have a number of downsides because it requires a graphical browser to post messages and may be unusable by the visually impaired.
The second accessibility problem might be fixable by adding a similar method that replaces the image with a link to a sound file reciting a short sequence of random numbers (composed from snippets each saying a single numeral). Visually impaired people are likely to use an audio-capable browser. To make it harder to break this by computer analysis, each site should use files composed of numerals spoken by different people, and maybe also add some intentional backround noise.
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