Password Strength

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Password strength is a measure of the effectiveness of a password in resisting guessing and brute-force attacks. In its usual form, it estimates how many trials an attacker who does not have direct access to the password would need, on average, to guess it correctly. The strength of a password is a function of length, complexity, and unpredictability.

Using strong passwords lowers overall risk of a security breach, but strong passwords do not replace the need for other effective security controls. The effectiveness of a password of a given strength is strongly determined by the design and implementation of the authentication system software, particularly how frequently password guesses can be tested by an attacker and how securely information on user passwords is stored and transmitted. Risks are also posed by several means of breaching computer security which are unrelated to password strength. Such means include wiretapping, phishing, keystroke logging, social engineering, dumpster diving, shoulder surfing, side-channel attacks, and software vulnerabilities.

Human-generated passwords

People are notoriously remiss at achieving sufficient entropy to produce satisfactory passwords.

Thus, in one analysis of over 3 million eight-character passwords, the letter "e" was used over 1.5 million times, while the letter "f" was used only 250,000 times. A uniform distribution would have had each character being used about 900,000 times. The most common number used is "1", whereas the most common letters are a, e, o, and r.

Users rarely make full use of larger character sets in forming passwords. For example, hacking results obtained from a MySpace phishing scheme in 2006 revealed 34,000 passwords, of which only 8.3% used mixed case, numbers, and symbols.

Note that the full strength associated with using the entire ASCII character set (numerals, mixed case letters and special characters) is only achieved if each character in the password is chosen randomly from that set. Capitalizing a letter and adding one or two numbers and a special character to a password will not achieve the same strength. If the numbers and special character are added in predictable ways, say at the beginning and end of the password, they could even lower password strength compared to an all letter random password of the same length.

Common Guidelines

Guidelines for choosing good passwords are typically designed to make passwords less easily discovered by intelligent guessing. Common guidelines advocated by proponents of software system security include:

  • Use a minimum password length of 12 to 14 characters if permitted.
  • Include lowercase and uppercase alphabetic characters, numbers and symbols if permitted.
  • Generate passwords randomly where feasible.
  • Avoid using the same password twice (e.g., across multiple user accounts and/or software systems).
  • Avoid character repetition, keyboard patterns, dictionary words, letter or number sequences, usernames, relative or pet names, romantic links (current or past) and biographical information (e.g., ID numbers, ancestors' names or dates).
  • Avoid using information that is or might become publicly associated with the user or the account.
  • Avoid using information that the user's colleagues and/or acquaintances might know to be associated with the user.
  • Do not use passwords which consist wholly of any simple combination of the aforementioned weak components.

Some guidelines advise against writing passwords down, while others, noting the large numbers of password protected systems users must access, encourage writing down passwords as long as the written password lists are kept in a safe place, not attached to a monitor or in an unlocked desk drawer.

The possible character set for a password can be constrained by different web sites or by the range of keyboards on which the password must be entered.

Examples of weak passwords

As with any security measure, passwords vary in effectiveness (i.e., strength); some are weaker than others. For example, the difference in weakness between a dictionary word and a word with obfuscation (i.e., letters in the password are substituted by, say, numbers — a common approach) may cost a password cracking device a few more seconds; this adds little strength. The examples below illustrate various ways weak passwords might be constructed, all of which are based on simple patterns which result in extremely low entropy, allowing them to be tested automatically at high speeds:
  • Default passwords (as supplied by the system vendor and meant to be changed at installation time): passworddefaultadminguest, etc. Lists of default passwords are widely available on the internet.
  • Dictionary words: chameleonRedSoxsandbagsbunnyhop!IntenseCrabtree, etc., including words in non-English dictionaries.
  • Words with numbers appended: password1deer2000john1234, etc., can be easily tested automatically with little lost time.
  • Words with simple obfuscation: p@ssw0rdl33th4x0rg0ldf1sh, etc., can be tested automatically with little additional effort. For example a domain administrator password compromised in the DigiNotar attack was reportedly Pr0d@dm1n.
  • Doubled words: crabcrabstopstoptreetreepasspass, etc.
  • Common sequences from a keyboard row: qwerty12345asdfghfred, etc.
  • Numeric sequences based on well known numbers such as 911 (9-1-1, 9/11), 314159... (pi), or 27182... (e), etc.
  • Identifiers: jsmith1231/1/1970555–1234, one's username, etc.
  • Anything personally related to an individual: license plate number, Social Security number, current or past telephone numbers, student ID, current address, previous addresses, birthday, sports team, relative's or pet's names/nicknames/birthdays/initials, etc., can easily be tested automatically after a simple investigation of person's details.

There are many other ways a password can be weak, corresponding to the strengths of various attack schemes; the core principle is that a password should have high entropy (usually taken to be equivalent to randomness) and not be readily derivable by any "clever" pattern, nor should passwords be mixed with information identifying the user. On-line services often provide a restore password function that a hacker can figure out and by doing so bypass a password. Choosing hard-to-guess restore password questions can further secure the password.

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