I need to implement a CAPTCHA for a Chinese (Simplified) website. Does anyone know how CAPTCHAs are normally presented on Chinese sites? I have seen some which just display standard decimal numbers but since there's less variation these are not as secure as I'd like. Is it acceptable to use a CAPTCHA containing letters from the standard English alphabet?
Use numbers only. I've seen Russian, Chinese, and other non-Roman language sites take this approach. Presenting Roman captures is unwise, as it's not safe to assume that mangled alien characters will be legible to non-native speakers. (I find them hard to decipher at the best of times.)