A rainbow table attack is a precomputation technique for reversing cryptographic hash functions. It uses massive lookup tables that map hash values back to their original plaintext passwords. Instead of computing hashes in real time, the attacker looks up a captured hash in the table to instantly find the corresponding password. Rainbow tables represent a time-memory tradeoff, trading storage space for dramatically faster cracking speed.
Rainbow tables are generated in advance by computing hashes for a defined keyspace of possible passwords and storing the results in a compressed chain format. When the attacker obtains a password hash from a compromised database, they search the rainbow table for that hash value. If found, the table reveals the original plaintext password. The chain structure uses reduction functions to compress what would be an enormous lookup table into a manageable size. Tools like RainbowCrack and ophcrack automate both table generation and lookup operations. A rainbow table for all alphanumeric passwords up to eight characters can crack any matching hash in seconds, compared to hours or days for real-time computation.
Rainbow table attacks are the reason modern systems use salted hashing. A salt is a random value added to each password before hashing, ensuring that identical passwords produce different hashes. This makes precomputed tables useless because the attacker would need a separate table for every possible salt value. Organizations must ensure all password storage uses salted adaptive hashing like bcrypt, scrypt, or Argon2. Legacy systems using unsalted MD5 or SHA-1 hashes are immediately vulnerable to rainbow table attacks and should be migrated to modern hashing algorithms.