applied cryptography - protocols, algorithms, and source code in c

The first time alice wants to log in she types her

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Unformatted text preview: nough, this practically eliminates a dictionary attack against commonly used passwords because Mallory has to generate the one-way hash for each possible salt value. This is a simple attempt at an initialization vector (see Section 9.3). The point here is to make sure that Mallory has to do a trial encryption of each password in his dictionary every time he tries to break another person’s password, rather than just doing one massive precomputation for all possible passwords. A lot of salt is needed. Most UNIX systems use only 12 bits of salt. Even with that, Daniel Klein developed a password-guessing program that often cracks 40 percent of the passwords on a given host system within a week [847,848] (see Section 8.1). David Feldmeier and Philip Karn compiled a list of about 732,000 common passwords concatenated with each of 4096 possible salt values. They estimate that 30 percent of passwords on any given host can be broken with this list [561]. Salt isn’t a panacea; increasing the number of salt bits won’t solve everything. Salt only protects against general dictionary attacks on a password file, not against a concerted attack on a single password. It protects people who have the same password on multiple machines, but doesn’t make poorly chosen passwords any better. SKEY SKEY is an authentication program that relies on a one-way function for its security. It’s easy to explain. To set up the system, Alice enters a random number, R. The computer computes f(R), f(f(R)), f(f(f(R))), and so on, about a hundred times. Call these numbers x1, x2, x3,..., x100. The computer prints out this list of numbers, and Alice puts it in her pocket for safekeeping. The computer also stores x101, in the clear, in a login database next to Alice’s name. The first time Alice wants to log in, she types her name and x100. The computer calculates f(x100) and compares it with x101; if they match, Alice is authenticated. Then, the computer replaces x101 with x100 in the database. Alice crosses...
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This note was uploaded on 10/18/2010 for the course MATH CS 301 taught by Professor Aliulger during the Fall '10 term at Koç University.

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