[ISN] Security UPDATE--Mathematical Strength of
Passphrases--November 3, 2004
isn at c4i.org
Thu Nov 4 03:13:30 EST 2004
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1. In Focus: Mathematical Strength of Passphrases
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==== 1. In Focus: Mathematical Strength of Passphrases ====
by Mark Joseph Edwards, News Editor, mark at ntsecurity / net
Last week, I wrote about why passphrases might be a better idea than
passwords. In essence, passphrases are longer and stronger, easier to
remember, and more resistant to the assaults of many of the more
popular password crackers.
In previous editions of this newsletter, I've mentioned articles by
Jesper Johansson, Microsoft security program manager. Recently,
Johansson published part 2 of the three-part series "The Great
Debates: Pass Phrases vs. Passwords," which compares passphrases and
passwords. In part 1 (at the first URL below), Johansson covers the
fundamentals, including how passwords are stored. In part 2 (at the
second URL below), he looks at the strength of each approach, and in
part 3, due out later this month, if I understand correctly, he will
offer guidance on how to select stronger passwords and configure
Part 2 of the series is very interesting because Johansson offers
insight into why "longer is stronger" in many cases. Some
password-cracking tools attempt to precompute all possible hashes and
store them on disk in order to quicken computation time when trying to
crack a given password. Johannson points out that precomputing for LAN
Manager (LM) hashes is feasible because storing all possible hashes
for a 14-character password, for example, based on a 76-character set
(the number of characters on a standard American English keyboard when
you include lower- and uppercase letters, numbers, punctuation, and
special characters) would require about 310TB of storage. Granted,
that's a huge amount of data, but storing it is feasible given the
file systems available today. On the other hand, trying to store all
the possible NT hashes given the same 14-character password and
76-character set wouldn't be feasible because NT's hash algorithm
produces longer hashes that would require 5,652,897,009 exabytes (EB)
of storage, which according to Johannson, "exceeds the capacity of any
file system today." So you can see that using at least 14 characters
for passwords and NT hashes makes cracking take much longer than using
shorter passwords and LM hashes because all the possible NT hashes
can't be precomputed and stored to disk to save processing time.
If all the characters in a password are alphanumeric, and especially
if all the letters are the same case, then cracking doesn't take as
long as if some nonalphanumeric characters and mixed-case letters are
used. As you might know, cracking programs check first for common
words using techniques such as dictionary attacks. And if you use only
upper- or lowercase letters, the alphanumeric characters add up to
only 26 letters and 10 digits, or 36 characters. But if you use the
entire set of 76 characters, you greatly increase password strength
because you increase the amount of time required to crack your
Essentially, the strength of a password (or passphrase) is a function
of the size of the character set, the number and randomness of
characters used from that set, and the computing power of the platform
used to attempt to break the password. Because you can't precisely
determine which platform crackers might have at their disposal, you
could assume the worst-case scenario--that they have the power of a
distributed computing network and massive amounts of storage and will
therefore be able to crack your password much more quickly than if
they worked alone or with a few associates. That means you should
consider using password policies that defend against such threats as
much as possible by requiring passwords longer than 14 characters,
requiring some nonalphanumeric characters, defending your network at
all levels against sniffing, and so on.
If you're interested in more information about password strength or
need some logical reasoning to justify new password policies for your
network, be sure to read Johannson's articles. He goes into a lot of
detail (which isn't over the head of a typical network administrator)
and offers several anecdotes and cases studies that I think you'll
find interesting. Also, please take a moment to visit our Security Hot
Topic Web page and answer our latest Instant Poll question: "What
password length do you enforce on your network?" I'm interested to
know whether you agree that longer passwords are stronger passwords.
On another note, we're happy to announce the IT Prolympics--a contest
designed to recognize the most proficient Active Directory (AD)
experts in the nation. The gold medal winner will get an
all-expenses-paid trip to TechEd 2005. Plus, we'll feature photos and
test scores of gold, silver, and bronze winners in the January issue
of Windows IT Pro magazine. Learn more about IT Prolympics and enter
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==== 4. Instant Poll ====
Results of Previous Poll:
Do you use Mac OS X on your network?
The voting has closed in this Windows IT Pro Security Hot Topic
nonscientific Instant Poll. Here are the results from the 46 votes.
- 33% Yes
- 7% No, but we intend to
- 61% No
- 0% I'm not sure
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What password length do you enforce on your network?
Go to the Security Hot Topic and submit your vote for
- 14 or fewer characters
- 15 to 24 characters
- 25 to 34 characters
- 35 to 44 characters
- 45 or more characters
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