Factors in the security
of an individual password
Likelihood that a password can be guessed
Studies of production computer systems have for decades consistently shown that about 40% of all user-chosen passwords are readily guessed.
- Many users do not change the default password that comes with many computer security systems. Lists of default passwords are available on the Internet.
- A password might be guessable if a user chooses an easily-discovered piece of personal information as a password (such as a student ID number, a boy- or girlfriend'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 vulnerable if it can be found in a list. Dictionaries (often in computer-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.
A weak password would be one that was short or was a default, or which could be rapidly guessed by searching a subset of all possible passwords such as words in the dictionary, proper names, words based on the user name or common variations on these themes. A strong password would be sufficiently long, random, or otherwise producible only by the user who chose it, so that 'guessing' for it will require too long a time. The length of time deemed to be 'too long' will vary with the attacker, with the attacker's resources, with the ease with which a password can be tried, and with the importance of the password to the attacker. So a student's password might not be worth more than a few seconds of computer time, whilst a password controlling access to a large bank's electronic money transfer system might be worth many weeks of computer time.
'Weak' and 'strong' have meaning only with regard to specific password systems. The necessary quality of the password depends on how well the password system limits attempts to guess a user's password, whether by a person who knows the user well, or a computer trying millions of possibilities. In a cryptgraphic context, the terms can have considerable precision. For example, passwords generally are not suitable for use as encryption keys. But note that even a 'strong' password may still be stolen, tricked, or extorted from a user, collected from a keyboard logger, overheard by tapping some communications medium, or copied from a Post-It note or Rolodex.
Examples of weak passwords include admin, 1234, susan, password, rover and 12/3/75; which might be easily guessed (the last being likely to be a personally significant date readily discoverable with a little research), and would certainly be found with an automated dictionary search attack in a system that permitted such action. Examples of stronger passwords would be tastywheeT34, partei@34!, and #23kLLflux. These passwords are longer and use combinations of lower and upper case letters, digits and symbols. They are unlikely to be in any password cracking word list and are sufficiently long to make direct brute force search impractical in some systems. Note that some systems do not allow symbols like #, @ and ! in passwords and they may be hard to find on some country-specific keyboards. In such cases, adding another letter or number or two may offer equivalent security. Also note that, having been published in this article as password examples, these 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-looking passwords, and especially human-chosen passwords, are not equivalent to a strong encryption key, and should generally not be used as such. Passphrases and password-authenticated key agreement methods have been used to address this limitation.
The strongest method for generating passwords is to select sufficient characters at random, but such passwords are generally the most difficult to remember - those passwords are generated by the online password generator. Some users develop mnemonic phrases that have the random letters as the initial of each word. Another way to make random passwords more memorable is to use random words or syllables instead of random letters.
Personal mnemonics are sometimes recommended, that is, things that are memorable to you, but not to others, for example, the password Iw21wIfvP, a difficult to remember string, derives from "I was 21 when I first visited Paris", possibly easily remembered. However, if your first experience of Paris is important to you, it may be possible to guess this password from knowledge of you, and then this would not be a sensible password choice.
As of October 2005, employees of the UK Government are advised to use passwords of the following form: consonant, vowel, consonant, consonant, vowel, consanant, number, number (for example pinray45). Apparantly upper and lower case do not matter, and this form is called an Environ password.
Likelihood that a password can be remembered
The most secure passwords are long, random strings of characters. 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). Unfortunately from a security perspective, such passwords are quite hard for most people to remember.
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. While helpful from a security viewpoint, many users resent such measures and user cooperation is generally essential for security.
Computer users are generally advised "never write a password down anywhere, no matter what" and "never use the 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. They have the unintended consequence that many users select weak passwords, even for important accounts, and end up using the same password everywhere.
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. The worst, but all too common, location is a Post-it note near the computer. Better locations are a safety 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 high value applications such as on-line banking.
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."
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 requirements and human factors.
Likelihood that a password can be discovered
Passwords can be discovered by shoulder surfing, burglary, extortion, blackmail, threats, or other methods. Dumpster diving is surprisingly fruitful for situations in which sensitive printed data is discarded with insufficient precaution (as evidenced by the recent rise in identity theft). That is why having a strong password and a protection system like identityhawk can increase your chances of protection. 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.