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Re: What's the idea of different salts in crypt()?

by cacharbe (Curate)
on Sep 20, 2001 at 16:36 UTC ( #113573=note: print w/replies, xml ) Need Help??

in reply to What's the idea of different salts in crypt()?

This is directly from The Handbook of Applied Cryptography
Alfred J. Menezes, Paul C. van Oorschot and Scott A. Vanstone
CRC Press
ISBN: 0-8493-8523-7

Chapter 10, page 390
(v) Salting passwords
To make dictionary attacks less effective, each password, upon initial entry, may be aug-mented with a t-bit random string called a salt (it alters the “flavor” of the password; cf. x10.2.3) before applying the one-way function. Both the hashed password and the salt are recorded in the password file. When the user subsequently enters a password, the system looks up the salt, and applies the one-way function to the entered password, as altered or augmented by the salt. The difficulty of exhaustive search on any particular user’s pass-word is unchanged by salting (since the salt is given in cleartext in the password file); how-ever, salting increases the complexity of a dictionary attack against a large set of passwords simultaneously, by requiring the dictionary to contain 2 t variations of each trial password, implying a larger memory requirement for storing an encrypted dictionary, and correspondingly more time for its preparation. Note that with salting, two users who choose the same password have different entries in the system password file. In some systems, it may be appropriate to use an entity’s userid itself as salt.

Although the entire book is available in PDF at the link I provided above, I highly suggest that anyone interested in crypto add it to their paper library, right next to Bruce Schneier's book.


  • Comment on Re: What's the idea of different salts in crypt()?

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Bruce Schneier (I little OT)
by Rhose (Priest) on Sep 20, 2001 at 19:57 UTC
      Although a little pedantic at times, I feel that it should be the Number One Read (tm) for any and all CIOs and CSOs. I see people everyday who are quelled into thinking that security can be had with a product, and they are duped into leaving the humans, and the process alone.

      Security is a process, not a product, and I think Bruce hits the nail on the head (a bunch of times). I didn't mention Secrets and Lies in my post because it deals with the whole process, and I wanted text book product investigation.

      Yes, read "Secrets and Lies". Even if you aren't interested in how encryption works on a mathematical level, how to properly implement the process should be foremost on everyone's minds.


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