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2009年2月3日 (二) 13:06的版本

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https://help.ubuntu.com/community/StrongPasswords

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1. title Strong Passwords

Introduction

Many security mechanisms present in Ubuntu and elsewhere require a password. Passwords are the most commonly used security tactic in computing, and are frequently used to protect sensitive information, such as your e-mail account, or your Ubuntu user account. Choosing a strong password for any application, or service which may require one is extremely important. If the password you choose is based on common information such as a dictionary word, an attacker may use a so-called "brute-force" method of determining your password, and subsequently compromise the account, or data protected with that password. This guide demonstrates the generation of strong passwords with applications available to Ubuntu. This guide does not recommend a formal password policy, including rotation/change scheduling, re-use prevention strategies, and the like. Such principles are beyond the scope of this guide, and the reader is requested to see the resources presented at the end of this guide for further information on passwords, and formal password policies.

介绍

在Ubuntu和其它系统上存在的许多安全机制都要求一个密码。密码是在计算和保护敏感信息中被普遍采用的安全策略，比如你的e-mail帐户或者Ubuntu用户帐户。为任何一个应用程序或服务选择一个强有力的密码是极其重要的。如何你选择的密码是基于一般信息比如一个字典单词，那么一个攻击者就会使用所谓的“暴力破解”方法确定你的密码，继而危害你用密码保护的帐户和数据。 这个指南示范了使用应用程序产生一个可用于Ubuntu上的强有力密码。这个指南不推荐一个形式上的密码方针，比如类似于密码变换计划，重新使用阻碍策略。这些原则超出了这个指南的范围，并且要求读者仔细看本指南末尾列出的用来更深入了解密码和形式密码方针的资源。

Target Audience

This guide is for anyone with sufficient experience with the GNU/Linux command-line, including installing packages using apt-get, and executing command-line tools who also has an interest in passwords, and generating strong passwords with a command-line utility.

目标受众

这个指南是用于任何有使用GNU／Linux命令行丰富经验的人，包括了那些使用apt-get安装文件，以及对密码有兴趣用执行命令行工具的人，还有就是能用熟练命令行 产生密码的人。

Strong Passwords

Applications, and libraries exist for your Ubuntu system to assist in generating, or enforcing strong passwords. A strong password is defined as any password which meets the following criteria:

• At least eight (8) characters in length.
• Does not contain your user name, real name, organization name, family member's names or names of your pets.
• Does not contain your birth date.
• Does not contain a complete dictionary word.
• Is significantly different from your previous password.
• Should contain three (3) of the following character types.
• Lowercase Alphabetical (a, b, c, etc.)
• Uppercase Alphabetical (A, B, C, etc.)
• Numerics (0, 1, 2, etc.)
• Special Characters (@, %, !, etc.)

Generating Strong Passwords in Ubuntu

Now that we've establish what constitutes a strong password. How do we have our computers help generate strong passwords automatically? This section will cover the Automatic Password Generator application (APG) which is available in Ubuntu via System Administration (universe). APG does precisely what its name implies: it automatically generates passwords. Not only does APG generate passwords, it will generate strong passwords for you as well. If you wish to study APG further, you may find all information at the APG website. (see the Resources section of this guide) It should be noted here that Ubuntu's password functions for user accounts do provide some means of enforcing strong passwords for user accounts, but there are cases, and applications where the strength of the password cannot be enforced in this manner, and so this guide exists to help the user generate acceptable strong passwords.

Installing APG

To install APG, ensure you have enabled the Universe Packages, (see UniversePackages) and have a live connection to the Internet, then issue the following command at a command prompt: sudo apt-get install apg You will be prompted for a password. The password being requested is your user password, or the same password you use when issuing commands with the sudo command. After authentication, the APG package will be downloaded, and installed. You are now ready to begin using, and further configuring APG to your liking.

Using APG

The default behavior of APG installed on Ubuntu via apt-get when executed, is to prompt for random data from standard input (typically the keyboard) and upon pressing enter, display six (6) strong "pronounceable" passwords. They are referred to as "pronounceable" because the passwords may actually be pronounced like regular words. In fact, APG includes the pronunciation in parentheses () to the right of each password. Additionally, they are strong passwords because they include a mix of lowercase, uppercase, and numeric characters. An example of executing apg and the default output of APG is presented here: apg APG prompts for the input of random keyboard keys, and then, after pressing ENTER, produces output similar to this:

queafWodEis5 (queaf-Wod-Eis-FIVE)
WoudElIc6 (Woud-El-Ic-SIX)
GorIacNewt8 (Gor-Iac-Newt-EIGHT)
ShratUplEov7 (Shrat-Upl-E-ov-SEVEN)
hexLyafByff1 (hex-Lyaf-Byff-ONE)
Irkyorn9 (Irk-yorn-NINE)

If you would like to enforce different behavior from APG, there are many options you may pass to the apg command for the purpose of changing the default output, and behavior of APG. For example, to generate random character passwords instead of the pronounceable passwords, you may invoke APG with the -a 1 option, where -a is the algorithm APG uses, and 1 selects random mode. The following command would produce six (6) random character passwords with no prompting to input random data: apg -a 1 Upon executing the above command, APG outputs a list of six (6) random character passwords similar to these:

S:w[AOe<
!_IlD#_G/~
)lL`avASe6
SgzlI:lU?h
~Wc(0b"{
'4fU25w%}

If you wish to always enforce prompting for random data from standard input (keyboard), to ensure the most secure passwords possible, you should explicitly tell APG to do so with the -s option. Another example would be the generation of four (4) WiFi Protected Access (WPA) Pre Shared Keys (WPA PSKs) having the maximum supported WPA PSK length of 63 random characters for use with modern WiFi equipment, such as a Wireless Access Point (WAP). To generate such a sequence, and use random data prompting, issue the following command at a command prompt: apg -s -a 1 -m 63 -n 4 After supplying the random keyboard data, and pressing ENTER a list of strong WPA PSKs will be presented, similar to these:

+DIaz*<DmO6VAFR/cQ>B^'[mtB;J1ABi*n"B<=|_My"5bZv#*iRJH!0oCcf!,@Z
os#?3;sC2}/A>lG^e*9%9}pd^&A\'eLkdC,lk#jB,bcg70I|q@U:^VZP"}>3z?G
rj??<_Ej%-:2LW;4q_?53$ab$U_~1I(({`^LN1Hd&&gKj+Dw;EqR<\qH_VQ{`s!
"x3up};b~/jU6Vo,t">Dr~f_U`NSNr30JmOD@c'Y:p2wvc"0s?$MtT<S5Qr\AZi

Many other options are available. You should consult the APG manual page, and the APG website for further information. (see the Resources section of this guide)

Generating Strong Passwords Manually

Some people find that they remember their password better when they come up with one on their own. This section provides an example of how to manually generate a strong password. Keep in mind that because the examples here are viewable by anyone, these exact results should be avoided. The first step is to choose a phrase which you can easily remember, but is at least 8 words long. This can be a favourite quote, a line from your favourite song, something you heard randomly while walking around, or even an obscure reference to a movie you like. For this example, we'll choose "`To be or not to be, that is the question`". Now, convert your phrase to a single word. Exactly how you do so is not important, as long as you remember how you did it! We'll take the first letter of each word, which gives us "`tbontbtitq`". Now, we have one character class, the lower case letter. To get upper case letters, simply randomly capitalize letters. We'll capitalize each third letter, and end up with "`tbOntBtiTq`". Now, we'll get some numbers. That '`O`' looks a lot like the number zero, so let's replace '`O`' with '`0`': "`tb0ntBtiTq`". There's a lot of '`T`' characters in there, so let's also replace one of them with the number '`7`': "`tb0n7BtiTq`". Finally, we'll go after a symbol or two. With a little imagination, you can see how the symbol '`<sup>`' and the letter '`l`' beside each other somewhat resemble the general shape of the letter '`N`', so replace the '`n`' with those: "`tb0^l7BtiTq`". Just to make it a little better, we'll add one last symbol. You'll notice that the exclaimation point (`!`) resembles a lower case '`i`' upside down. Replace the '`i`' with '`!`' to get the final password: "`tb0</sup>l7Bt!Tq`". Just keep in mind that certain characters (like: ^,`,',~,ß,...) might be hard to locate or enter on a foreign keyboard (e.g. when travelling or using a different keyboard layout - think US 104-key layout vs. Canadian French layout).

Resources

Additional password information, command information, and formal password policy guidelines are available via the following resources: pwgen is an alternative to apg that will create reasonable secure passwords that are easier to remember.

Local System Resources

WWW Resources

APG website Bad Password Policies SANS Password Policy Guideline (PDF Document) Simple Formula For Strong Passwords (PDF Document) Strong Password Policies The Diceware Passphrase Home Page Steve Gibson's Ultra High Security Password Generator