Caution: With tools such as these, we do not condone their use for anything 
but testing networks for which you have the authority and for implementing defensive measures. Have fun!
Rainbow tables reduce the difficulty in brute force cracking a single password by creating a large pre-generated data set of hashes from nearly every possible password. Rainbow Tables and RainbowCrack come from the work and subsequent paper by Philippe Oechslin [1]. The method, known as the Faster Time-Memory Trade-Off Technique, is based on research by Martin Hellman &Ronald Rivest done in the early 1980’s on the performance trade-offs between processing time and the memory needed for cryptanalysis. In his paper published in 2003, Oechslin refined the techniques and showed that the attack could reduce the time to attack 99.9% of Microsoft’s LAN Manager passwords (alpha characters only) to 13.6 seconds from 101 seconds. Further algorithm refinements also reduced the number of false positives produced by the system.
The main benefit of Rainbow Tables is that while the actual creation of the rainbow tables takes much more time than cracking a single hash, after they are generated you can use the tables over and over again. Additionally, once you have generated the Rainbow Tables, RainbowCrack is faster than brute force attacks and needs less memory than full dictionary attacks.
Rainbow Tables are popular with a particularly weak password algorithm known as Microsoft LM hash. LM stands for LAN Manager, this password algorithm was used in earlier days of Windows and still lives on only for compatibility reasons. By default Windows XP or even Windows Server 2003 keeps the LM hash of your passwords in addition to a more secure hash (NTLM or NTLMv2). This allows for the benefit of backwards compatibility with older operating systems on your network but unfortunately makes the job of password cracking easier if you can obtain the LM hashes instead of the NTLM hashes. The Ethical Hacker Network - Tutorial: Rainbow Tables and RainbowCrack
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