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A rainbow table is a technological resource for “cracking” cryptographic hash functions and discovering plaintext passwords in an authentication database. Rainbow tables use a specific algorithmic matching function to essentially look up the information needed to crack password hashes.
The idea of using rainbow tables or other hash cracking methods comes from the fundamental use of password hashes in authentication databases. Any system that has password authentication has to store all those passwords. However, storing the plaintext password leaves the system extremely vulnerable to unauthorized access by hackers. In response, many system administrators started storing the passwords as hash values.
A rainbow table is one way to get the plaintext passwords, regardless of the protective hash value strategy mentioned above. Other methods include a brute force attack or a dictionary method. By storing more information, rainbow tables can accomplish the same results as a brute force attack with less computer processing effort.