Rainbow table

A rainbow table is a precomputed data structure that maps hashes back to the passwords that produced them. Rather than guessing and hashing in real time, an attacker builds (or downloads) the table once and then looks up a stolen digest almost instantly. Clever chaining keeps the file size manageable while still covering enormous numbers of candidate passwords.

Rainbow tables are devastating against unsalted, fast hashes such as MD5 and SHA-1, where a single table can crack millions of credentials. Their fatal weakness is the salt: because a salt makes every hash unique, an attacker would need a fresh table for each salt value, which is computationally hopeless.

This is exactly why modern password storage always salts and uses slow key derivation functions — a combination that renders rainbow tables obsolete.