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A rainbow table is a one-way hash cracking method created by Philippe Oechslin that uses a faster time-memory trade-off technique . Lists of hashed passwords are pre-generated into files with a utility (rtgen) that allows for hashes to be cracked in a short period of time, generally less than a minute. Tables are hash specific, e.g. MD5 tables can only crack MD5 hashes.
Defense against Rainbow Tables
A rainbow table is essentially worthless against one-way hashes that include tokens (or salts, in the case of 56-bit DES). For example, if a password hash is generated using the following function (where "." is the concatenation operator):
hash = md5sum(password . token)
...a password cracker would have to generate both every possible token for every possible password -- a rainbow table would not give any benefit. However, if a password hash is generated using the following function:
hash = md5sum(password)
...a password cracker may benefit from a rainbow table.
Nearly all distributions and variations of Unix, Linux, and BSD use one-way hashes and salts, though many PHP web applications use regular MD5.
External Links
- Plain-Text Team - Largest Distributed Rainbow Table Cracking System (over 600gb of tables)
- Project RainbowCrack - Official site.
- RainbowCrack Tutorial - Tutorial on using Rainbow Tables.
- winrtgen - GUI for rtgen.
- PassCracking.com - MD5 online cracking.
- rainbowtables.shmoo.com - Free LanMan rainbow tables.
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