密碼強度

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密碼強度量度一個密碼有幾難畀未有受權嘅人或電腦估到。

一般我地據啲密碼幾難估,將渠地分做「弱」(weak)、「破得」(vulnerable)同埋「強」(strong)。所謂「強弱」呢個字係相對嘅,只係響某特定密碼系統之內至有意義。 一串密碼嘅質素(necessary quality) 有賴成套密碼系統點樣限制任何人估中個密碼,無論係個用户嘅朋友定係一架電腦亂撞曬百萬計嘅可能性。響密碼學之中,呢幾個字都有咁上下準確意思。例如,passwords generally are not suitable for use as encryption keys。但係,就算一串「強」密碼都可能被人偷、呃、逼供(extorted)出來、用鍵盤 logger收集、傳送途中截聽、或者用其他方法發現。

廢密碼[編輯]

廢密碼係話個密碼好短,好簡單,好易畀暴力破解到using a subset of all possible passwords, such as words in the 字典, , words based on the user name or common variations on these themes. Passwords that can be easily guessed by acquaintances of the user, such as a 生日 and 寵物名, are also considered weak.[1]

一啲好易估嘅密碼:

  • admin -- too easily guessed
  • 1234 -- too easily guessed
  • abc123 -- too easily guessed
  • susan -- common personal name
  • password -- trivially guessed, used very often
  • p@$$\/\/0rd -- simple letter substitutions are pre-programmed into cracking tools
  • rover -- common name for a pet, also a dictionary word
  • 12/3/75 -- date, possibly of personal importance
  • December12 -- Using the date of a forced password change is very common
  • nbusr123 -- probably a user name, and if so, very easily guessed
  • asdf -- a sequence of adjacent letters on many keyboards
  • qwerty -- a sequence of adjacent letters on many keyboards
  • aaaa -- repeated letters, can be guessed

This short list is a very small sampling; there are many other examples that are considered weak for the same or similar reasons.[2] Additionally, any password that has been published as an example may be considered weak, simply on the basis of its publication.

3.8 percent of passwords are a single word found in a dictionary, and another 12 percent are a word plus a final digit, two-thirds of the time that digit is 1[3]

Many users do not change the default password that comes with many computer security systems. Lists of default passwords are available on the Internet.[4]

A password might be guessable if a user chooses an easily-discovered piece of personal information (such as a student ID number, a friend's name, a birthday, a telephone number, or a license plate number). Personal data about individuals are now available from various sources, many on-line, and can often be obtained by someone using social engineering techniques, such as posing as an opinion surveyor.

A password is often vulnerable if it can be found in a list. Dictionaries in machine-readable form are available for many languages, and there exist lists of commonly-chosen passwords. In tests on live systems, dictionary attacks are so routinely successful that software implementing this kind of attack is available for many systems.

A too-short password, perhaps chosen for ease of typing, is vulnerable if an attacker can obtain the cryptographic hash of the password. Computers are now fast enough to try all alphabetic passwords shorter than 7 characters, for example.

Departing employees, programmers and systems administrators often know widely disclosed passwords that are rarely changed. Easily guessed passwords may enable expensive losses through mischief, fraud or revenge.

堅嘅密碼[編輯]

A strong password is sufficiently long, random, or otherwise producible only by the user who chose it, such that successfully guessing it will require more time than the password cracker is willing to use guessing it. The length of time deemed to be too long will vary with the attacker, the attacker's resources, the ease with which a password can be tried, and the value of the password to the attacker. A student's password might not be worth more than a few seconds of computer time, while a password controlling access to a large bank's electronic money transfer system might be worth many weeks of computer time.

Examples of stronger passwords include:

  • t3wahSetyeT4 -- not a dictionary word, has both alpha and numeric characters
  • 4pRte!ai@3 -- not a dictionary word, has both cases of alpha, plus numeric, and punctuation characters
  • MoOoOfIn245679 -- long, with both alpha cases and numeric characters
  • Convert_100£ to Euros! -- phrases can be long, memorable and contain an extended symbol to increase their strength
  • 1382465304H -- a mixture of numbers and a letter at the end, generally used on mass accounts set password eg, schools, business.
  • Tpftcits4Utg! -- A mixture of varying-case letters, numbers, and symbols. It is memorable as an initialism of "The password for this computer is too strong for you to guess!"

Note: these particular strings are no longer strong passwords, because they have been published.

These passwords are relatively long and use combinations of lower and upper case letters, digits, and symbols. The longer and the wider the variety of symbol choices, the more intensive the password cracking effort or well matched the Rainbow table must be to defeat the password, assuming that suitable password hashing and protection methods are in place. Further, not using a single word makes password cracking word lists a less effective form of direct brute force attack. Note that some systems do not allow symbols like #, @ and ! in passwords and they may be hard to find on different keyboards. In such cases, adding another letter or number may offer equivalent security.

The above examples, having been published in this article as password examples, are no longer good choices; examples from publicly-accessible discussions about passwords are obviously good candidates for inclusion in a dictionary to be used for a dictionary attack. However, beware that even "strong" passwords (by this limited criterion), and especially human-chosen passwords, are not equivalent to a strong encryption key, and should not be used as such, if for no other reason than that they contain no unprintable characters. Passphrases and password-authenticated key agreement methods have been used to address this limitation.

Passwords can be found by using so-called brute force password generators. In the simplest case, these are small programs that simply try all possible combinations. A 3 GHz processor can generate approximately 3 million passwords a second. A ten letters password such as '4pRte!ai@3', because there are about 95 keys available, is one of 95^{10} possibilities, which would take approximately 632860 years to be found assuming purely random possible password generation. A password containing fifteen random upper-case letters would be just as safe and might be easier for some people to remember and type.

However, certain weak password hashing methods can betray a password much more quickly by reducing the number of combinations needed or increasing the speed at which guesses may be rejected to crack an otherwise "strong" password. Further, certain precomputed tables such as rainbow tables can increase the cracking speed considerably.

A formula which calculates the number of possible passwords is: maximumCombinations=nrAvailableChars^{PasswordLength}. Using only the 26 lowercase letters and a password length of 7 characters the number of combinations is relatively small: 26^{7}=8.03 billion combinations. This may appear large to some, but in a time when common computers can generate 3 million passwords a second, it will only take 45 minutes to find the password.

Passwords longer than 7 characters using non-dictionary words are therefore to be preferred in an attempt to use 'good' passwords. However, the majority of computer users don't observe such precautions, in part because they are hard to remember.

A simple example can be used to investigate the resistance of passwords to brute force search.[5] Experimenting with this program will make clear that a password such as apple is very easy to discover, since it can be found in less than 30 seconds by an exceptionally simple approach.

亂嚟嘅密碼[編輯]

[未記出處或冇根據]穏陣嘅密碼係一長串亂嚟嘅字元, but such passwords are generally the most difficult to remember. For the same number of characters, a password is stronger if it includes a mix of upper and lower case letters, numbers and other symbols (when allowed). The difficulty in remembering such a password increases the chance that the user will write down the password, which makes it more vulnerable to a different attack (in this case the paper being lost or stolen and the password discovered). Whether this represents a net reduction in security depends on whether the primary threat to security is internal or external.

A password can, at first sight, be random, but if you really examine it, it is just a pattern. One of these types of passwords is 26845. Although short, it is not easily guessed. But the person who created the password is able to remember it, because it's just the four direction keys on the square number board (found at the right of most keyboards), plus a five in the middle. If you practice it, it is just one swift motion of moving 2 fingers around the board (which is very easy to use).

Forcing users to use system-created random passwords ensures the password will have no connection with that user and shouldn't be found in any dictionary. Several operating systems have included such a feature. Many also include password aging, requiring users to choose new passwords regularly, commonly after 30 or 45 days. Many users resent such measures, particularly in the absence of effective security awareness training. The imposition of strong random passwords may encourage users to write down passwords, store them in PDAs or cellphones, share them with others against memory failure, increasing the risk of disclosure.

The following measures can increase acceptance of random password use, if carefully used:

  • Create a training program. The initial advantages of random passwords need to be laid out for people. Also, update training for those found to disclose passwords.
  • Reward users of random passwords by reducing the rate of forced password changes to periods longer than 3 months. Typically 4, 6 or 12 month forced password reset periods work. Spreading out the password change process may have an advantage to human memory processes.
  • Compensate for disclosed passwords by building a vigilant account closure process for departing users and/or a process to show each user a last login notification banner.
  • Automate a password reset system as a safety net. This will also reduce Helpdesk call volume concerning password resets. This must be done with care, however, because easily guessable password reset keys bypass the advantages of a strong password system!

Mnemonic passwords[編輯]

Some users develop mnemonic phrases that generate seemingly random passwords—for instance, the first letter of each word. Another way to make "random" passwords more memorable is to use random words (see diceware) or syllables instead of randomly chosen letters.

Patterned passwords[編輯]

Any pattern in a password makes guessing (automated or not) easier. As of October 2005, employees of the UK Government are advised to use passwords of the following form: consonant, vowel, consonant, consonant, vowel, consonant, number, number (for example pinray45). This pattern is called an Environ password[需要參考資料] and is case-insensitive.

This pattern of alternating vowel and consonant characters causes the password to be pronounceable and thus more memorable. Unfortunately, such patterns severely reduce the password's information entropy, making bruteforce password cracking considerably more efficient.

Training[編輯]

In large password files the awareness of password strength may be roughly measured by the time it takes to crack 50% of the password hashes present. With a large password file, using a standard Pentium 4 system, Microsoft password hashing techniques and an under-trained user group making password choices, it may be safely estimated that 50% of the password hashes will crack in less than two minutes. However, with careful attention to password strength advice, password choices can outlast the patience of a password cracking effort.

User training versus a cracking time metric for large user base password files:[需要參考資料]

User training actions Metric: 50% password hashes cracked
No Training Time ≤ 2 minutes
Sporadic User Training Time ≤ 4 minutes
General User Training Time ≥ 45 minutes
Focused User Training Time ≥ 24 hours
Motivated User Population Time ≥ 1 week
Random Password Group Time ≥ 2 weeks
Contest Winners Time ≥ 1 month

Note: times are based on reports from authorized password cracking efforts that used John the Ripper, without a dictionary, on Microsoft LM Hashes, run on a Pentium 4, 2 GHz system. Only results from password files containing greater than 3,000 password hashes are included.[需要參考資料]

Pre-computed rainbow tables represent a determined computing investment. The greater the range of keys included and the length in password places covered, the greater the number of combinations to compute, and the greater the expense of time or money to obtain them.

Factors to consider

  • Key space
  • Investment in computer time
  • Parallel computing team coordination
  • Raw patience of a determined computing effort
  • Predictive accuracy
  • Encryption type
  • Storage space
  • Distribution or purchase costs

Rainbow Table Defenses

  • Exceeding the password length of commonly available tables.
  • Exceeding the keyspace of commonly available tables.
  • Avoiding the use or exceeding the useful length of old hashing methods.
  • Preventing the capture of password hashes.
  • Using salts.

Passwords weakened by hashing vulnerabilities[編輯]

Knowing which password hashing methods are used to protect your password can help you choose a more effective one. In this training example the effect of position in password selection is illustrated. Microsoft has updated its password hashing methods over time, but the LM hashing method often remains because of backward compatible features of newer Microsoft systems.

Training Samples to avoid.

Password Quality Notes
Crunchy Bad It is a word. 100,000 words can be tested in less than 0.03 seconds.
Crunchy! Bad The symbol "!" amounts to a second, single digit, password and will not last long.
CrunchyPretzel Bad "Pretzel" amounts to a second password, that is a word.

Why are these sample passwords at risk?

  • Each 7 places in a Microsoft LM password hash may be cracked separately.
  • These passwords show unfortunate placement problems in LM hashing protection.
Password Full Length Cracking Lengths First Half 1 2 3 4 5 6 7 Second Half 8 9 10 11 12 13 14
Crunchy 7 7 + 0 Crunchy C r u n c h y
Crunchy! 8 7 + 1 Crunchy C r u n c h y !  !
CrunchyPretzel 14 7 + 7 Crunchy C r u n c h y Pretzel P r e t z e l
  • LM hashing is only used by Microsoft for passwords of 14 places or less.
  • Longer passwords will not be subject to backward compatible hashing weakness.
  • By evading LM hashing, substantial improvements in cracking delay are added.
  • An extra password place beyond 14 can frustrate a cracking effort by millions of years by avoiding LM password hashing.
  • Such a plan can work even inside a network where shorter passwords are backward compatible to LM hashing.
  • Support and use of LM hashes can be turned off if no pre-Windows 2000 systems are in use.[6]

Guarding user passwords[編輯]

Computer users are generally advised "never write a password down anywhere, no matter what" and "never use a password for more than one account." These maxims, while sound in theory, ignore the reality that an ordinary computer user may have dozens of password-protected accounts. The multitude of accounts often ends up with users having the same password everywhere. A user's attempt to comply will often result in many forgotten passwords, even for important accounts.

If passwords are written down, they should never be kept in obvious places such as address books, Rolodex files, under drawers or keyboards or behind pictures. Perhaps the worst, but all too common, method is a note near the computer. Better locations are a safe deposit box or a locked file approved for information of comparable sensitivity to that protected by the password. Software is available for popular hand-held computers that can store passwords for numerous accounts in encrypted form. Another approach is to use a single password for low security accounts and select separate, strong passwords for a smaller number of high-value applications such as online banking. Manual or offline forms of this approach are also in use. For example, a phone directory may be used to keep a map between computer accounts and password hints.

The problem of password overload is quite real. Certain IT professionals may manage from 25 to 250 privileged passwords at any one time. In such cases, some safe storage area for password hints, private key or encrypting password become unavoidable.

At a 2005 security conference, an expert from Microsoft was quoted as saying: "I claim that password policy should say you should write down your password. I have 68 different passwords. If I am not allowed to write any of them down, guess what I am going to do? I am going to use the same password on every one of them."[7]

Whether it is worse to use weak passwords that are memorized or strong passwords that are written down can provoke fierce debate among experts. Practical security often requires balancing conflicting imperatives such as security requirements and human factors.

The problem was addressed in an interesting way by Steve Gibson during his "Security Now" podcasts with Leo Laporte; he suggested creating not a password, but a password algorithm that could be universally applied to a company name or other unique indicator. A simple algorithm, such as using every other letter from a name, would generate the password wkpda for a Wikipedia password; although this example is a fairly low-security password, minor variables to include numbers, capitalization and symbols can generate a password which is seemingly random, difficult or impossible to remember, but easy to re-generate at any time.

Rather than writing the passwords themselves, some write lists of cryptic clues to their own passwords that have relevance through personal experience. For example, if your favorite movie was Terminator and your cat's birthday was 2007-04-10 you could describe the password "terminator0410" as "Favorite movie cat is born without years".

In case that it may happen that you need to type the password while being abroad on a different PC, then consider that the PCs abroad may have a different keyboard layout than yours, and you should avoid passwords containing chars that are not present in other keyboard layouts.

Password discovery[編輯]

Passwords can be discovered by shoulder surfing, burglary, extortion, blackmail, threats, or other methods. Information diving is surprisingly fruitful for situations in which sensitive printed data is discarded with insufficient precaution; it is said to be part of the techniques which have produced the recent rise in identity theft. Approximate password length can be discovered even without shoulder surfing by simply counting keyboard clicks or noting finger motions. Research published by IBM in 2004 shows that each key on a keyboard has a distinctive acoustic signature, allowing keyed in data, including passwords, to be recovered by analyzing recordings from a covert listening device or "bug." See: Acoustic cryptanalysis.

Obtaining passwords by psychological manipulation of users is an example of social engineering. An attacker might telephone a user and say "Hi. Systems Control here. We're doing a security test. Can we have your password so we can proceed?" Systems administrators and other support staff will very rarely, if ever, need to know a user's password in order to perform their jobs. System administrators with "root" or superuser privileges can change the users' passwords without their permission, so they have no need whatsoever to ask for it. In addition, they will go out of their way not to ask for a password, precisely because they do not want to encourage the habit of giving passwords to anyone. Users do not generally appreciate that any of this is so, and are thus too often vulnerable to social engineering.

Password Betrayal is a method that takes advantage of a mixed mode technique. It uses the combined effect of differing strengths of password hashing and the habit of people to re-use the same password many times. A classic example of password betrayal would be a person re-using the same password on two independent computer systems. On one computer system the strong password is strongly protected. But on the other computer system the password protection is easily defeated. The under protected password is then re-used with other accounts known to belong to the same person. This password capture method can work even if the password is a strong or complex password choice.

Single sign-on solutions can be an advantage in reducing the risk of password betrayal. However, a person may access separate systems with differing password encryption strengths, password complexity rules, single sign-on product vendors and computer security vulnerabilities.

To help, a person can keep an index of which password is in use with multiple accounts. This strategy can mitigate this risk to some extent. The password use index allows the person to change groups of accounts with identical passwords as needed. But, a password use index should only contain password hints rather than direct password examples. Care should be taken to protect a password use index from easy duplication, sharing, exposure or capture.

References[編輯]

  1. Bidwell, Teri. Hack Proofing Your Identity in the Information Age. Syngress Publishing. 2002. 1931836515. 
  2. Bidwell, p. 87
  3. ZDNet Report: Net users picking safer passwords
  4. Default Password List Pnenoelit.de Retrieved on 2007-05-07
  5. http://www.phonax.com/brute_force.c Phonax.com Retrieved on 2007-05-07
  6. How to prevent Windows from storing a LAN manager hash of your password in Active Directory and local SAM databases
  7. Microsoft security guru: Jot down your passwords, News.com.com Retrieved on 2007-05-07

External links[編輯]