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As table salt adds zest to popcorn, the salt that Morris and Thompson sprinkled into the DES algorithm added a little more spice and variety. The DES salt is a 12-bit number, between 0 and 4095, which slightly changes the result of the DES function. Each of the 4096 different salts makes a password encrypt a different way.
When you change your password, the /bin/passwd program selects a salt based on the time of day. The salt is converted into a two-character string and is stored in the /etc/passwd file along with the encrypted "password."[10] In this manner, when you type your password at login time, the same salt is used again. UNIX stores the salt as the first two characters of the encrypted password.
[10] By now, you know that what is stored in the /etc/passwd file is not really the encrypted password. However, everyone calls it that, and we will do the same from here on. Otherwise, we'll need to keep typing "the superencrypted block of zeros that is used to verify the user's password" everywhere in the book, filling many extra pages and contributing to the premature demise of yet more trees.
Table 8.2 shows how a few different words encrypt with different salts.
Regardless, tons of references out there.