Date: 21 January 2009
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How hard is it to choose a good password? Most people believe that
choosing a good password is easy. After all, how is somebody going to guess my wife's maiden name?
In reality, people usually choose poor passwords. In 1990
[Klein 1990] an attempt
to crack a large password database revealed over three hundred passwords in
the first fifteen minutes! One fifth of all password were obtained
in the first week and
approximately one quarter were cracked by the end of the search.
More than half of the
cracked passwords were six characters or less and some accounts didn't
even have a password.
An intruder only needs one password!
Choosing a good password is a trade off between something that is difficult
to guess versus something that is easy to remember. While @G7x.m^l is
probably a good password, nobody will remember it and
it is certain to appear as a sticky note attached to a terminal.
Conversely, your first name is very easy to remember, but it is also trivial
Some simple rules of thumb
Some simple guidelines that will help you choose better passwords are:
- A password should be a minimum of eight characters long.
- Try to include some form of punctuation or digit.
- Use mixed case passwords if possible.
- Choose a phrase or a combination of words, that make the password easier
- Do not use a word that can be found in any dictionary (including
foreign language dictionaries).
- Do not use a keyboard pattern such as qwertyui or
oeuidhtn (look at a Dvorak keyboard).
- Do not repeat any character more than once in a row like zzzzzzzz.
- Do not use all punctuation, all digit or all alphabetic.
- Do not use things that can be easily determined such as:
- Phone numbers.
- Car registration.
- Friends' or relatives' names.
- Your name or employment details.
- Any Date.
- Never use your account name as its password.
- Use different passwords for each machine.
- Change the password regularly and do not reuse passwords.
- Do not append or prepend a digit or punctuation mark to a word.
- Do not reverse words.
- Do not replace letters with similar looking numbers.
For instance, all of the letters i should not be blindly
replaced replaced by the digit 1.
The principle behind password cracking is quite simple: take a large word
list, encrypt each word and check if the encrypted string matches the user's
password. Word lists that are used frequently include English and other
language dictionaries, common names, pet names, television and movie characters,
character patterns on keyboards (for example, qwerty)
and jargon or slang terms.
To allow for the case that the user has not chosen a word in your
word list, an intruder can and usually will apply a large number of simple
rules to each word in the word list and check if any of these encrypt to the
user's passwords. Typical rules include appending and prepending digits and other
punctuation characters to words, reversing words, capitalising words, converting words
to all upper or all lower case, substituting letters or digits for other letters
and naturally many combinations of these. Since computers are fast, applying these
rules and encrypting the resulting guess doesn't take much time and a lot of guesses
can be made in a very short time.
In addition, a CD based database is supposed
to have been produced that contains every word in a large dictionary plus many
rule based permutations of these words encrypted in every possible manner. This
reduces password cracking to a simple (and fast) database lookup.
How long is a good password?
The simple answer to this is that in general the longer the password
Assuming that you're using a reasonable selection of characters for your
password, say letters and numbers, then the following table presents the
number of passwords possible for the various choices of length. It also
includes an estimate of how much time would be required to crack the password
using a brute force attack.
The cracking time field is derived from a report in September 1993, that
claimed the record for the speed of cracking passwords. The claim was
that 6.4 million passwords per second could be tested. Given that computer
speeds are increasing continuously, the following times are almost certainly
over estimates of the actual time required.
Number of passwords for each length
|Length||Number of Passwords||Number of passwords||Cracking Time
|1||62||Not nearly enough
||Try this by hand
||Almost no time
||One quarter of a million
||Less than one second
||Almost one billion
||Two and a half minutes
||Fifty six billion||Two and a half hours
||Three and a half trillion||One week
||Two hundred trillion||One year
||Thirteen quadrillion||Seventy years
||Eight hundred and forty quadrillion
||Lots||A quarter of a million years
||Even more||Sixteen million years
Having said that longer is better, it is important to note that many
machines artificially restrict the length of the password usually by silently
truncating what you enter to their maximum length. Since this length is often
eight characters under Unix, the rest of this article will assume that an eight
character password is being used.
What characters should a good password contain?
The previous section assumed that passwords consisted of upper and
lower case letters and digits. What happens if this character set is
increased or decreased? The following table presents some of the options for
eight character passwords:
Number of eight character passwords
|Type of Password||Number of|
|7-bit ASCII ||128 ||72057594037927936
||Three hundred and fifty years
|Printable Characters ||95 ||6634204312890625
||Thirty three years
|Letters and Numbers ||62 ||218340105584896
|Letters only ||52 ||53459728531456
||Ninety six days
|Lowercase with one Uppercase
|Lowercase only ||26 ||208827064576
|English words: eight letters or longer ||special
||Less than one second
So clearly, the richer the character set being used, the harder it will
be to crack passwords. You should attempt to include as a minimum both upper
and lower case characters
and if possible, you should also include some digits, punctuation symbols and/or
control codes in your password.
Rarely used passwords and secure storage
There is one situation where writing down your password is a good idea - protecting something important that doesn't require credentials very often. For instance, the root password on a server probably doesn't need to be used every day.
In a case like this it is a good idea to create a long, very complex password that is hard to remember, write it down and store the password in some kind of secure storage (like a safe). On the rare occasion that the password is needed it can be retrieved from storage and used (and the password then returned to storage). The password should still be changed regularly.
Of course, situations vary. If you find that you (or your users) have a tendancy to forget passwords and start making simpler, less secure passwords it may be better to use a complex password and write it down.
Just remember that if anyone gets a hold of the written down version they have a free pass into the system. Any written down passwords should not be kept on or near your computer and preferable should not be kept near any information that identifies you. Store it securely - a locked drawer is much better than your wallet.
Examples of how to construct good passwords
So now that typical bad passwords have been discussed, how is a
good password constructed? Try combining two or more words together or taking
the first (or second or last) letter of each word in an easily remembered
phrase. Then mangle the result by adding capitals, digits and punctuation
characters. As an extra measure, control characters can also be introduced.
Some examples of using multiple words with punctuation
Here is a pair of good examples of using multiple words:
- gOt%L0st! - got lost!
- heLP4me$ - help for me (money)
And here is a bad one:
Some examples of using a phrase
Here are three good examples of using phrases:
- rsKf0myH - Raindrops keep falling on my head.
- wru2rxy? - Who are you to ask why.
- bWiIso3! - Beware the ides of March!
And here is a bad one:
- Aaaaaaaa - Always assert an ambiguous axiom and argue aggressively.
Passwords to never, EVER use
There is a very handy list of the worst 500 passwords over at What's My Pass?.
In addition to that, all the sample passwords listed in this article are now
known, and should not be used by anyone.