Rainbow Table
A precomputed table of hash values used to reverse cryptographic hash functions, enabling rapid password cracking against unsalted hashes.
A rainbow table is a precomputed lookup table that maps hash values back to their original plaintext inputs. Instead of hashing every possible password during an attack (which is time-consuming), an attacker pre-calculates hashes for millions of common passwords and stores the results.
When they obtain a hashed password from a breach, they simply look it up in the table. If the hash matches, the password is instantly revealed. Rainbow tables can crack unsalted MD5 and SHA1 hashes in seconds.
The defense is salting: adding a random value to each password before hashing. Since the salt is unique per password, precomputed tables are useless: each combination would need its own table. Modern password hashing algorithms (bcrypt, Argon2) include salting by default.
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