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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 (PDF). 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.

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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.
• PassCracking.ru - MD5 online cracking + database.
• rainbowtables.shmoo.com - Free LanMan rainbow tables.
• http://rainbowcrack.com/ - Community based distributed rainbow table generation

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