캡챠 방식 정리

* 구글 recapcha

https://www.google.com/recaptcha/intro/index.html

http://stove99.tistory.com/28

http://sourceforge.net/projects/simplecaptcha/files/simplecaptcha-1.2.1.jar/download

* simple capcha

http://zero-gravity.tistory.com/243

* 지엠스팸프리

http://www.casternet.com/spamfree/

* Securimage

https://www.phpcaptcha.org/

http://musasin84.blog.me/220285554667