From CryptoDox, The Online Encyclopedia on Cryptography and Information Security
A password is a form of secret authentication data that is used to control access to a resource. The password is kept secret from those not allowed access, and those wishing to gain access are tested on whether or not they know the password and are granted or denied access accordingly.
The use of passwords goes back to ancient times. Sentries guarding a location would challenge for a password. They would only allow a person in if they knew the password. In modern times, passwords are used to control access to protected computer operating systems, mobile phones, cable TV decoders, automated teller machines (ATMs), etc. A typical computer user may require passwords for many purposes: logging in to computer accounts, retrieving email from servers, accessing files, databases, networks, web sites, and even reading the morning newspaper online.
Despite the name, there is no need for passwords to be actual words; indeed passwords which are not actual words are harder to guess (a desirable property), but are generally harder for users to remember (an undesirable property). Note that password is often used to describe what would be more accurately called a passphrase. Passcode is sometimes taken to imply that the information used is purely numeric, such as the personal identification number (PIN) commonly used for ATM access. Passwords are generally short enough to be memorized.
Designing a personal, user-friendly password
Passwords vary in the degree of public awareness, security protection and frequency of change. The most public, and therefore least secure, password might be one that is given to members of a group, a committee or some other organization. For instance, "publiclibrary", "internet", "AAAfinancecommittee" or "password" are all examples of easily remembered passwords, more or less publicly known passwords.
Less easily attacked passwords might be built from such a basic form, for instance, "smith12nov34street" or "AAAchairpersonSUE". These are slightly more secure, but being relatively easily predictable should not be relied upon to actually block unauthorized access. Effective access control requires passwords which are more difficult to guess or to find automatically, less publicly known (ideally not at all), and these are the subject of much of the rest of this article.
Security and convenience
In controlling access to anything, trade-offs are made between security and convenience. If a resource is protected by a password, then security is increased with a consequent loss of convenience for users. The amount of security and inconvenience inherent in a particular password system or policy are affected by several factors addressed below. However, there is generally no one universal 'best' way to set a balance between security and convenience for all cases.
Some password protected systems pose little or no risk to a user if compromised, for example a password allowing access to a free information web site with no confidential data. Others pose modest economic or privacy risk, as for instance a password used to access e-mail or a security lock code for a mobile telephone. Still others could have very serious consequences if compromised, such as passwords used to limit access to AIDS treatment records, control a power transmission grid, or access to personnel records (consider the risk of identity theft in this instance).
Factors in the security of a password system
The security of a password-protected system depends on several factors. The system must, of course, be designed for sound overall security, without which no password protection can have any significance. Early passwords on many systems were limited to a few numbers, or upper-case-letters, only often in prescribed patterns limiting the number of possible passwords. Most passwords today usually have fewer such limits. User input is determined by several limiting factors: allowable inputs (numbers / letters, non-visual codes and/or other keys / device inputs), minimum & maximum of time required for input, availability of cut / delete / paste / copy for input, and error/noise tolerance errors in the password or communications input. Some system administrators also enforce other limitations on passwords, such as compulsory change schedules, safe-password analysis feedback, and compulsory length / composition limits. See computer security and computer insecurity.
Here are some password management issues that must be considered:
Rate at which an attacker can try out guessed passwords
The rate at which an attacker can submit guessed passwords to the system is a key factor in determining system security. Some systems impose a long time out (several seconds) after a small number (e.g., a maximum of three) of failed password entry attempts. Absent other vulnerabilities, such systems can be secure with relatively simple passwords, if they are not easily guessed. Examples of passwords that are easily guessed include the name of a relative or pet, an automobile license plate number, and such default passwords as admin, 123456, or letmein. 
Other systems store or transmit a cryptographic hash of the password in a manner that makes the hash value accessible to an attacker. When this is done, and it is very common (to most observers' surprise or despair), an attacker can work off-line, rapidly testing candidate passwords against the true password's hash value.
Lists of common passwords are widely available and can further speed the process. (See Password cracking.) A sufficiently complex password used in a system with a good hash algorithm can defeat such attacks as the work factor imposed on such an attacker can be made impossible in practice. Passwords that are used to generate cryptographic keys, e.g. for disk encryption or Wi-Fi security, are also subject to high rate guessing. Stronger passwords are needed in such systems.
Form of stored passwords
Some computer systems store passwords, against which to compare user attempts, as cleartext. If an attacker gains access to such an internal password file, all passwords would be compromised. If some users employ the same password for multiple accounts, those will be compromised as well. More secure systems store each password in a cryptographically protected form, so access to the actual password will be difficult for a snooper who gains internal access to the system, while validation still remains possible.
Email is sometimes used to distribute passwords. Since most email is sent as cleartext, it is available without effort during transport to any eavesdropper. Further, it will be stored on at least two computers as cleartext -- the sender's and the receipients's. If it passes through intermediate systems during its travels, it will likely be stored on those as well. Emailed passwords are generally an insecure method of distribution.
A common cryptographically based scheme stores only a "hashed" form of the plaintext password. When a user types in a password on such a system, it is run through the hashing algorithm, and if the hash value generated from the user's entry matches the hash stored in the password database, the user is permitted access. The hash value is created by applying a cryptographic hash function to a string consisting of the submitted password and, usually, another value known as a salt. The salt prevents attackers from building a list of hash values for common passwords. MD5 and SHA1 are frequently used cryptographic hash functions. A modified version of DES was used in early Unix systems.
The UNIX DES function was iterated to make the hash function slow, to further frustrate automated guessing attacks, and used the password candidate as a key to encrypt a fixed value, thus blocking yet another attack on the password hashing system. A more flexible function for iterated hashed passwords is described in PKCS-5.
If the hash function is well designed, it will be computationally infeasible to reverse it to find the plaintext directly. However, many systems do not protect their hashed passwords adequately, and if an attacker can gain access to hashed values he can use widely available tools which compare the encrypted outcome of every word from some collection, such as a dictionary. Long lists of possible passwords in many languages are widely available (eg, on the Internet) and the tools try common variations as well. The existence of these dictionary attack tools demonstrates the relative strengths of different password choices against such attacks. Use of a key derivation function can reduce this risk.
Methods of verifying a password over a network
A variety of methods have been used to verify passwords in a network setting:
Simple transmission of the password
Passwords can be vulnerable to interception (ie, "snooping") while being transmitted to the authenticating machine or person. If the password is carried as electrical signals on unsecured physical wiring between the user access point and the central system controlling the password database, it is subject to snooping by wiretapping methods. If it is carried as packetitzed data over the Internet, anyone able to watch the packets containing the logon information can snoop with a very low probability of detection.
An example of cleartext transmission of passwords is this website. When you log into your Wikipedia account (if you are not an administrator) your username and password are sent from your computer through the Internet via cleartext. Anyone can read them in transit and potentially log into your account. But because anyone can gain access to the site—without logging in—there is little need to encrypt transmissions.
Another example of transmission vulnerability is email. Emailed passwords may be read by anyone with access to the transmission medium. Using client-side encryption will only protect transmission from the POP server to the client. Previous or subsequent relays of the email will not be protected and the email will be stored on multiple computers in cleartext.
Transmission through encrypted channels
The risk of interception of passwords sent over the Internet can be reduced by, among other approaches, using the Transport Layer Security (TLS, previously called SSL) feature built into many Internet browsers. Most browsers display a closed lock icon when TLS is in use. See cryptography for other ways in which the passing of information can be made more secure.
Hash-based challenge-response methods
Unfortunately, there is a conflict between stored hashed-passwords and hash-based challenge-response authentication; the latter requires a client to prove to a server that he knows what the shared secret (ie, password) is, and to do this, the server must be able to obtain the shared secret from its stored form. On Unix-type systems doing remote authentication, the shared secret usually becomes the hashed form and has the serious limitation of exposing passwords to offline guessing attacks.
Zero-knowledge password proofs
Moving a step further, augmented systems for password-authenticated key agreement (e.g. AMP, B-SPEKE, PAK-Z, SRP-6) avoid both the conflict and limitation of hash-based methods; An augmented system allows a client to prove knowledge of the password to a server, where the server knows only a (not exactly) hashed password, and where the unhashed password is required to gain access.
Procedures for changing passwords
Usually, a system must provide a way to change a password, either because a user believes the current password has been (or might have been) compromised, or as a precautionary measure. If a new password is passed to the system in an unencrypted form, security can be lost (e.g., via wiretapping) even before the new password can even be installed in the password database. If the new password is given to a compromised employee, little is gained. Some web sites include the user-selected password in an unencrypted confirmation e-mail message, with the obvious increased vulnerability.
Identity management systems are increasingly used to automate issuance of replacements for lost passwords, a feature called self service password reset. The user's identity is verified by asking questions and comparing the answers to ones previously stored (ie, at account initialization). Typical questions include "Where were you born?," "What is your favorite movie?" or "What is the name of your pet?" In many cases the answers to these questions can be relatively easily guessed, determined by research, or obtained through social engineering, and so this is less than reliable as a verification technique. While many users have been trained never to reveal a password, few consider the name of their favorite movie to require similar care.
"Password aging" is a feature of some operating systems which forces users to change passwords frequently (eg, quarterly, monthly or even more often), thus ensuring that a stolen password will become unusable more or less quickly. Most users are not so familiar with passwords and computers as to be comfortable with this, so such policies usually earn some protest and foot-dragging at best and hostility at worst. These features are therefore not always used. In any case, the security benefits are limited because attackers often exploit a password as soon as it is compromised. In many cases, particularly with administrative or "root" accounts, once an attacker has gained access, he can make alterations to the operating system that will allow him future access even after the initial password he used expires (one example of this is a rootkit).
Forcing password change too frequently may make users more likely to forget which password is current, and there is a consequent temptation for users to either write their password down or to reuse an earlier password, which may negate any added security benefit. Implementing such a policy requires careful consideration of the relevant human factors.
Number of users per password
Sometimes a single password controls access to a device, for example, for a network router, or password-protected mobile phone. However, in the case of a computer system, a password is usually stored for each user name, thus making all access traceable (save, of course, in the case of users sharing passwords). A would-be user must give a name as well as a password. If the user supplies a password matching the one stored for the supplied user name, he or she is permitted further access into the computer system. This is also the case for a cash machine, except that the user name is the account number stored on the bank customer's card, and the PIN is usually quite short (4 to 6 digits).
Allotting separate passwords to each user of a system is preferable to having a single password shared by legitimate users of the system, certainly from a security viewpoint. This is partly because users are more willing to tell another person (who may not be authorized) a shared password than one exclusively for their use. Single passwords are also much less convenient to change because many people need to be told at the same time, and they make removal of a particular user's access more difficult. Per-user passwords are also essential if users are to be held accountable for their activities, such as making financial transactions or viewing medical records.
Design of the protected software
Common techniques used to improve the security of software systems protected by a password include:
Some of the more stringent policy enforcement measures can pose a risk of alienating users, possibly decreasing security. Be safe
Factors in the security of an individual password
Main article: Password strength
Alternatives to passwords for access control
The numerous ways in which reusable passwords can be compromised has prompted the development of other techniques. Unfortunately, few of them have become universally available for users seeking a more secure alternative.
With some "graphical" password techniques, such as web-based keyboards, the user memorizes a normal alphanumeric password, but enters that password by clicking on a sequence of pictures of numbers (or pictures of letters). The images are randomly slightly distorted every time, and the order of numbers in the pictures is also shuffled every time, so entering each guess of a dictionary attack is equivalent to solving a CAPTCHA -- infeasible for an automated attack.
Graphical passwords are an alternative means of authentication for log-in intended to be used in place of conventional password; they utilize images instead of text. In many implementations, such as Vidoop, the user is required to pick from a series of images in the correct sequence in order to gain access.
While some believe that graphical passwords would be harder to crack, others suggest that people will be just as likely to pick common images or sequences as they are to pick common passwords.Template:Fact
Website password systems
Passwords are used on websites to authenticate users and are usually server-side, meaning the browser sends the password to the server (by HTTP POST), the server checks the password and sends back the relevant content (or an access denied message). This process eliminates the possibility of local reverse engineering as the code used to authenticate the password does not reside on the local machine.
The transmission of the password through the browser in plaintext means it can be intercepted along its journey to the server. Most web authentication systems use SSL to establish an encrypted session between the browser and the server. This is done automatically by the browser and ensures integrity of the session.
It is customary to design password-verification systems such that the user cannot see what he/she types: instead of echoing the characters typed, a series of question marks or asterisks is displayed.This may have been a good idea once—in the days of UNIX time-sharing systems, where users talked to a computer via terminals, or in terminals or computers shared by many users, as in libraries, where it is actually possible for someone to look over the user's shoulder—but it has significant disadvantages. Most importantly, if a person makes a typing mistake once, he/she is likely to make it twice, unless he/she can actually see what characters were typed: muscles tend to repeat themselves. In the worst case, this can happen when the user is initially creating a password and is required to type it twice. A person who twice typed a password that is different from the intended one will never be able to use it: this is a common reason for a user to get an "invalid password" error every time he/she tries to log in. Unfortunately, this a common occurrence, and it is an occurrence that is an inevitable result of misguided design principles.
Attempting to crack passwords by trying as many possibilities as time and money permit is a brute force attack. A related method, rather more efficient in most cases, is a dictionary attack. In a dictionary attack, all words in one or more dictionaries are tested.
There are several programs available for password auditing and recovery such as L0phtCrack, John the Ripper, and Cain; some of which use password design vulnerabilities (as in the Microsoft LANManager system) to increase efficiency. Some are useful to system administrators as any password which can be found using one of these programs is most definitely a weak password and should be rejected as an unacceptable password choice.
According to Bruce Schneier, the most commonly used password is password1. <ref>http://www.wired.com/politics/security/commentary/securitymatters/2006/12/72300</ref>
History of passwords
Passwords have been used with computers since the earliest days of computing. MIT's CTSS, one of the first time sharing systems, was introduced in 1961. It had a LOGIN command that requested a user password. "After typing PASSWORD, the system turns off the printing mechanism, if possible, so that the user may type in his password with privacy." <ref> CTSS Programmers Guide, 2nd Ed., 1965</ref> Robert Morris invented the idea of storing login passwords in a hashed form as part of the Unix operating system. His algorithm, know as crypt(3), used a 12-bit salt and invoked a modified form of the DES algorithm 25 times to reduce the risk of dictionary attacks.