Thursday, May 21, 2009

Choosing and Protecting Passwords

Cyber Security Tip ST04-002


Passwords are a common form of authentication and are often the only barrier
between a user and your personal information. There are several programs
attackers can use to help guess or "crack" passwords, but by choosing good
passwords and keeping them confidential, you can make it more difficult for
an unauthorized person to access your information.

Why do you need a password?

Think about the number of personal identification numbers (PINs), passwords,
or passphrases you use every day: getting money from the ATM or using your
debit card in a store, logging on to your computer or email, signing in to
an online bank account or shopping cart...the list seems to just keep
getting longer. Keeping track of all of the number, letter, and word
combinations may be frustrating at times, and maybe you've wondered if all
of the fuss is worth it. After all, what attacker cares about your personal
email account, right? Or why would someone bother with your practically
empty bank account when there are others with much more money? Often, an
attack is not specifically about your account but about using the access to
your information to launch a larger attack. And while having someone gain
access to your personal email might not seem like much more than an
inconvenience and threat to your privacy, think of the implications of an
attacker gaining access to your social security number or your medical

One of the best ways to protect information or physical property is to
ensure that only authorized people have access to it. Verifying that someone
is the person they claim to be is the next step, and this authentication
process is even more important, and more difficult, in the cyber world.
Passwords are the most common means of authentication, but if you don't
choose good passwords or keep them confidential, they're almost as
ineffective as not having any password at all. Many systems and services
have been successfully broken into due to the use of insecure and inadequate
passwords, and some viruses and worms have exploited systems by guessing
weak passwords.

How do you choose a good password?

Most people use passwords that are based on personal information and are
easy to remember. However, that also makes it easier for an attacker to
guess or "crack" them. Consider a four-digit PIN number. Is yours a
combination of the month, day, or year of your birthday? Or the last four
digits of your social security number? Or your address or phone number?
Think about how easily it is to find this information out about somebody.
What about your email password—is it a word that can be found in the
dictionary? If so, it may be susceptible to "dictionary" attacks, which
attempt to guess passwords based on words in the dictionary.

Although intentionally misspelling a word ("daytt" instead of "date") may
offer some protection against dictionary attacks, an even better method is
to rely on a series of words and use memory techniques, or mnemonics, to
help you remember how to decode it. For example, instead of the password
"hoops," use "IlTpbb" for "[I] [l]ike [T]o [p]lay [b]asket[b]all." Using
both lowercase and capital letters adds another layer of obscurity. Your
best defense, though, is to use a combination of numbers, special
characters, and both lowercase and capital letters. Change the same example
we used above to "Il!2pBb." and see how much more complicated it has become
just by adding numbers and special characters.

Longer passwords are more secure than shorter ones because there are more
characters to guess, so consider using passphrases when you can. For
example, "This passwd is 4 my email!" would be a strong password because it
has many characters and includes lowercase and capital letters, numbers, and
special characters. You may need to try different variations of a
passphrase—many applications limit the length of passwords, and some do not
accept spaces. Avoid common phrases, famous quotations, and song lyrics.

Don't assume that now that you've developed a strong password you should use
it for every system or program you log into. If an attacker does guess it,
he would have access to all of your accounts. You should use these
techniques to develop unique passwords for each of your accounts.

Here is a review of tactics to use when choosing a password:
* Don't use passwords that are based on personal information that can be
easily accessed or guessed.
* Don't use words that can be found in any dictionary of any language.
* Develop a mnemonic for remembering complex passwords.
* Use both lowercase and capital letters.
* Use a combination of letters, numbers, and special characters.
* Use passphrases when you can.
* Use different passwords on different systems.

How can you protect your password?

Now that you've chosen a password that's difficult to guess, you have to
make sure not to leave it someplace for people to find. Writing it down and
leaving it in your desk, next to your computer, or, worse, taped to your
computer, is just making it easy for someone who has physical access to your
office. Don't tell anyone your passwords, and watch for attackers trying to
trick you through phone calls or email messages requesting that you reveal
your passwords (see Avoiding Social Engineering and Phishing Attacks for
more information).

If your internet service provider (ISP) offers choices of authentication
systems, look for ones that use Kerberos, challenge/response, or public key
encryption rather than simple passwords (see Understanding ISPs and
Supplementing Passwords for more information). Consider challenging service
providers that only use passwords to adopt more secure methods.

Also, many programs offer the option of "remembering" your password, but
these programs have varying degrees of security protecting that information.
Some programs, such as email clients, store the information in clear text in
a file on your computer. This means that anyone with access to your computer
can discover all of your passwords and can gain access to your information.
For this reason, always remember to log out when you are using a public
computer (at the library, an internet cafe, or even a shared computer at
your office). Other programs, such as Apple's Keychain and Palm's Secure
Desktop, use strong encryption to protect the information. These types of
programs may be viable options for managing your passwords if you find you
have too many to remember.

There's no guarantee that these techniques will prevent an attacker from
learning your password, but they will make it more difficult.

Authors: Mindi McDowell, Jason Rafail, Shawn Hernan

Produced 2004 by US-CERT, a government organization.


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