MD5 cannot be reversed. Professor Wang's collision method takes advantage of a feature of MD5 or SHA 1 algorithm. According to the features of MD5 and SHA 1 hash algorithm, a string of arbitrary length becomes a summary information of fixed length. Then there may be a problem here, that is, different strings may theoretically produce the same summary information. Professor Wang's so-called collision method is to collide those strings with the same abstract information produced by different strings. So it is named collision method. The collision is reflected here. Nothing is more vivid than this. According to SHA 1, MD5 and other hash algorithms, at the time of design, people who design this algorithm think that the possibility of different strings producing the same summary information is almost zero. Professor Wang proved that the abstract information rules generated by SHA 1 and MD5 can be cracked in a short time. This destroys the integrity of the original data. The so-called cracking is also reflected here. As for the cracking tools, the following addresses provide some free cracking services, which can crack some simple passwords, in fact, they are all cracked through dictionaries or violence. Www.cmd5.com www.xmd5.org I can sometimes crack some commonly used passwords with a certain success rate.