CAPTCHA项目是Completely Automated Public Turing Test to Tell Computers and Humans Apart (全自动区分计算机和人类的图灵测试)的简称。CAPTCHA的目的是区分计算机和人类的一种程序算法，是一种区分用户是计算机和人的计算程序，这种程序必须能生成并评价人类能很容易通过但计算机却通不过的测试。
http://baike.baidu.com/view/538168.htm
简单来说，我们在各个网站看到的验证码的算法就是CAPTCHA。
卡内基梅隆大学（Carnegie Mellon University，CMU）设计了一个名叫 reCAPTCHA 的强大系统，让他们的电脑去向人类求助。具体做法是：将OCR软件无法识别的文字扫描图传给世界各大网站，用以替换原来的验证码图片；那些网站的用户在正确识别出这些文字之后，其答案便会被传回CMU。
reCAPTCHA验证码系统在2009年的时候被Google收购，整体界面有点类似于CAPTCHA，但是安全性能更高，通常都是由两个单词组成，一个是机器可以识别的（用来测试你是否是由人来访问的），还有一个是机器不能读取的（用来帮助将书本中的文字数字化）。
http://baike.baidu.com/view/1944705.htm
http://jandan.net/2013/02/17/recaptcha.html
在危地马拉人发明这个验证码程序的时候，是为了防止电脑识别密码盗用，所以用机器难以识别的图片让人识别，精彩的是，谷歌图书馆用它来让人识别机器扫描无法识别的文字，当我们输入验证码的时候，居然是在为谷歌做文字识别工作。
这个验证码的演示地址：http://www.google.com/recaptcha/mailhide/
一些截图：
它的原理如下：
现在OCR的阅读效率实在是不高，下面这张图就可以说明问题。
它的情景是这样的，有一天，某台机器扫描了一本书，想把它转成电子版：
处理出来是这样子的：
The Hreckinridge’ and Lane Democrats, having taken courage at the recent eastern advises,
are [xxxxxxxxxx] energetically for the campaign: Several prominent Democrats who at first
favoredDonoLea, are coming out. for the other aide, apparently under the [xxxxxxxx] of Federal
[xxxxxxxxx]. An address to the National Democracy of ,1ifornia, urging the party to
supportHaeeslipslDas, has recently been published, which manifestlybss strengthened that aide of
the [xxxxxxxxx]: It is signed by 65 Democrats, many of whom occupy respectab e and prominent
positions in the party, 22 of them are Federal office-holders,[xxxxx] more are recipients of
Federal patronage, and the others represent a mass of politicians giving the document [xxxx][xxxxxx]
mTheDcu8las Democrats are also active The Irish and German vote will mostly go with ths# branch of
the party, but it is[xxxxxxxxx] to [xxxxxxxx] [xxxxx] [xxxx] [xx] the stronger. Thus far 17 IT
newspapers have declared for DonGres, 13 for Base$- IaaIDGS and 9 remain non-committal, with even
chances of going either way. Under these circumstances the Republicans entertain not unjustifiable
hopes that the Democratic divisions may be so equal,- ly balanced as to give the State [xx]
LIaCOLV.Same very [xxxxxxx] Bell and Everett meetings have been held in different parts of the State,
bat thus far that party does not exhibit much rank sad ale air en.
这个是原书质量比较差的：
看到这个，电脑就傻眼了，吐出来一堆这种东西：
‘ letz-1- rrk fit: 1′ . on its to Vc ,rt, cann into tlm yc H_ tcr,la, .n. ‘l l; , arc ti:( h of thc 1″,
ats that to ltc rc: ,;. , I; ., l: rel!;n. tani., , ./olio, IJuteilu, . 1!’i./_ ;lr”n. Iiam! Jr.r.
F’l,nr_.Z.._%i;;, ,, : rt-Irn: am/ tf.rri.:, t?m steamer as a tr nW r. Uu ,tin;t, c ac?1 1″,at firm/
a t;nn, accor.liu; to .t rn. ‘Cl.w r. wu ru lm:nui MistinW /y in u;th, -. ink ;:,k as to “what w ax
1111, :111(I vle:iR a of ;: (,am( into, mnr r-, tm if tlm wo r( uu.i n:’ of t?u : la?:Iv. ‘c : ol
in thc , ucr:atic , , Tlau :; will h:aw tu-li.r . ’1′Im yap?tts Il ,,n an,/ I, ,rr:l. r, (,t tf,
is r:ity, start witli it, with lu:rtic: ol 1- e:l.k.
看得懂吗？反正我是看不懂。reCAPTCHA验证码的出现，就是为了改变这种情况的。下面这张图可以很好的解释它的工作原理：
上面的步骤就是：
1、我们首先扫描了一本有一大堆单词的书
2、我们找出不能被电脑识别的单词，这些单词有可能是不同的字体。当然了，损坏程度也与书籍年代以及扫描质量有关。
3、为了让这些验证码更安全，我们会让它更扭曲并且在上面加入横线。
4、一个验证码是有两个这样的单词组成的。
有了reCAPTCHA验证码的帮助，第二张图片上面的文字就会变得清楚多了（不过还是有一点小错误）：
The New-York State yacht Squadron, on its annual cruise to Newport came into the harbor
yesterday afternoon. The following are the names of the boats that came to anchor here:
Jessie, gera loliv erelun Annie, Mannering, Julia, Bonita, Magic wut, Rambler, floumblie,
Henrietta, Sea-Drift and Maria, with the steamer America as a tender. On anchoring each
boat fired a gun, according to custom. The reports were heard distinctly in the city,
causing considerable inquiry as to “what was up,” and quite a number of sanguine individuals came
into our office to inquire if the guns were not annunciatory signals of the successful laying of the
Atlantic Cable. We invariably replied in the negative. The squadron will leave to-day for Newport.
The yachts Washington and buub r of this city, start with it, with parties of New Haven people.
有的人可能要问了，既然机器都看不明白那他怎么判断你输对了还是错了呢？
Google是这样解释的：
两个验证码里面有一个是正确的，被人审核过的，而另一个是不正确的，机器读不出来的。当你把那个正确的输对以后我们就会默认另外一个也是对的，这样，你每输入一次验证码，就为人类的知识宝库里增加了一个单词。
下图就是对这个原理过程的一个描述:
了解和申请 reCAPTCHA ：www.google.com/recaptcha
一些开源的验证码相关内容：
http://www.oschina.net/project/tag/248/captcha