Several people have said that they can't read these at all. That's pretty surprising to me and I'll probably reconsider the design.

There don't seem to be many good open source CAPTCHA scripts which aren't written in PHP or the like.

OCR keeps getting better and I've no reason to believe that this CAPTCHA is anything wonderful but, to my eyes at least, it looks pretty hard and it's sufficiently different to any of the broken CAPTCHAs that I have reason to hope that it'll be a while before this is broken.

To use it just set it up as a CGI script and construct a URL to it which looks like .../captcha.cgi/somekey

The string /somekey will be HMACed with the key defined below and the first 5 bytes of the result are used (mod 16) as an index into the list of valid letters: V12K4567H9AETXYK

FileSizeSHA1Type

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