This is because it has to perform some calculations from its hash "chains" to produce the values that it does not directly store. The downside of a rainbow table is that it requires more computational power to use than a pure hash table. A pure hash table would contain only about 4 billion hashes. By contrast, a plaintext dictionary of the same size would contain about 6.5 billion passwords. For comparison, a rainbow table that is 64 GB in size can calculate over 70 trillion hashes. For example, 2.5 million hashes could be stored in a text file of 25 entries. It uses a special reduction function to dramatically reduce the size of the dictionary. A rainbow table is a compromise between a plaintext table and a pure hash table. The size of your dictionary could become unwieldy. The trade-off, however, is that most hashes are considerably larger than the original password. If your dictionary contained pre-computed hashes instead of plaintext passwords, your password cracker could simply compare the password hashes to the dictionary hashes until a match is found. If you manage to steal the credentials file that contains the users' hashed passwords, you could conduct an offline attack on that file. If there is a match, the user is assumed to have supplied the correct password and is permitted entry. It then compares the result to the hash in its credentials file. When a user logs on, the system takes the submitted password and hashes it. Instead, they store hashes of the passwords. Most operating systems do not store user credentials in cleartext. The cracker does not have to spend time computing the hash of every password it tries. This makes offline password cracking attacks faster. A rainbow table attack is a type of dictionary attack in which the passwords in the wordlist have been pre-computed into their corresponding hash values, and then compressed in a highly efficient manner.
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