Capcha or daily captcha?

If you are looking to use a CAPTCHA to protect an on-line resource you should choose your CAPTCHA carefully. There are many categories available these days but I would stick with a reading-based CAPTCHA. Personally, I would avoid the video, image-based (e.g Asirra - Machine Learning Attacks against the Asirra CAPTCHA) and audio categories, many of which have been defeated.

Many reading-based CAPTCHAs have also been defeated so I would choose carefully. The hardest part of defeating such a CAPTCHA is segmenting the individual characters. Once that is done classification of the characters is trivial (Computers beat humans at single character recognition). Therefore your choice should be a CAPTCHA that is hard to segment, whether that is by characters that touch their neighbours or overlap or by added adversarial clutter. The Caca Website will give you a good idea of what will offer (limited) protection and what won't.

HTH

Steak'n'Eggs

OMG, I never thought a simple question will raise such a complex answer. So far I have a form protected with a simple image-text verification Captcha made with http//www.123contactform.com. I noticed they have an option of "daily captcha" and I wanted to learn more about it.Based on your reply I think it won't matter that much, because without a stronger captcha system it's gonna be the same when it gets cracked.

Sorry to be the bearer of bad tidings but I've just had a quick look at http//www.123contactform.com and the CAPTCHA displayed on their screenshots would be about as useful as the proverbial chocolate teapot. Based on recent experience I would estimate that a couple of hours work would crack that with a success rate of around 99% (possibly higher!).