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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). Note that password is often used to describe what would be more accurately called a pass phrase. 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" or "AAAfinancecommittee" are all examples of easily remembered 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 relied upon to actually block unauthorized access. Effective access control requires passwords which are more difficult to guess or to find automatically, 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 proper 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. Others pose modest economic or privacy risk, 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 or control a power transmission grid.
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. Early passwords on many systems were limited to a few numbers or upper-case-letters only, often in prescribed patterns limiting possible passwords. Most passwords today usually have few 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 length of input, available of cut / delete / paste / copy for input, and error/noise tolerance of password or input errors. Some system administrators also enforce other limitations on passwords, such as compulsory change schedules, safe-password analysis feedback, and compulsory safe-no-crack 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 after a small number (e.g. 3) of failed password entry attempts. Absent other vulnerabilities, such systems can be secure with relatively simple passwords, as long as 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, 1234, or letmein (let me in). 
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, 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 as plain text. If an attacker gains access to the password file, all passwords are 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, whilst validation still remains possible.
A common cryptographic 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 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. A more flexible function for iterated hashed passwords is described in PKCS-5.
If the hash function is well designed, it is 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 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.
A poorly designed hash function can make attacks feasible even if a strong password is chosen. See LM hash for a widely deployed example.
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 (known as "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 over the Internet, anyone able to watch the packets containing the logon information can snoop with very little possibility of detection. Cable modems may be more vulnerable to snooping than DSL and dialup connections, and ethernet may or may not be snoopable, depending particularly on the choice of networking hardware and wiring. Some organizations have noted a significant increase in stolen passwords after users began using cable internet connections.
Transmission through encrypted channels
The risk of interception of passwords sent over the Internet can be reduced with 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 (the password) is, and to do this, the server end needs to be able to obtain the shared secret from its stored form. On Unix-type systems doing remote authentication, the shared secret becomes the hashed form and has the serious limitation that they expose passwords to offline guessing attack.
Zero-knowledge password proofs
Rather than transmitting the password, password-authenticated key agreement systems can perform a zero-knowledge password proof, which proves knowledge of the password without revealing it.
Taking it 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) 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 confirming e-mail message.
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. 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 guessed, determined by research, or obtained through social engineering, and so this is less than certain 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.
Longevity of a password
Forcing users to change passwords frequently (quarterly, monthly or even more often) ensures that a valid password in the wrong hands will eventually become unusable. Most users are not so familiar with passwords and computers, so you take the great risk of losing or gaining the hostility from users. Many operating systems provide such features, though they are not universally used. Their 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.
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 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 usually preferable to having a single password shared by legitimate users of the system. This is partly because people 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:
- not echoing the password on the display screen as it is being entered or obscuring it as it is typed by using asterisks or circular blobs
- allowing passwords of adequate length (some Unix systems limited passwords to 8 characters).
- requiring users to re-enter their password after a period of inactivity
- enforcing a password policy to ensure strong passwords
- requiring periodic password changes
- assigning passwords at random
- providing an alternative to keyboard entry
- using encrypted tunnels or password-authenticated key agreement to prevent network attacks on transmitted passwords
Some of the more stringent policy enforcement measures can pose a risk of alienating users, possibly decreasing security.
Factors in the security of an individual password
Studies of production computer systems have for decades consistently shown that about 40% of all user-chosen passwords are readily guessed. Password strength is the likelihood that a password can be guessed or discovered by an unauthorized person or computer. Passwords easily guessed are known as weak or vulnerable; passwords very difficult or impossible to guess are considered strong.
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.
- Single-use passwords. Having passwords which are only valid once makes many potential attacks ineffective. Most users find single use passwords extremely inconvenient. They have, however, been widely implemented in personal online banking, where they are known as TANs. As most home users only perform a small number of transactions each week, the single use issue has not lead to significant customer dissatisfaction in this case.
- Security tokens are similar to single-use passwords, but the value to be entered is displayed on a small fob and changes every minute or so.
- Access controls based on public key cryptography e.g. SSH. The necessary keys are too large to memorize (but see proposal Passmaze) and must be stored on a local computer, security token or portable memory device, such as a flash disk or floppy disk.
- Biometric methods promise authentication based on unalterable personal characteristics, but currently (2005) have high error rates and require additional hardware to scan, for example, fingerprints, irises, etc. They have proven easy to spoof in some famous incidents testing commercially available systems And, because these characteristics are unalterable, they cannot be changed if compromised, a highly important consideration in access control as a compromised access token is almost the very definition of insecure.
- Single sign-on technology is supposed to eliminate the need for having multiple passwords. Such schemes do not relieve user and administrators from choosing reasonable single passwords, nor system designers or administrators from ensuring that private access control information passed among systems enabling single sign-on is secure against attack. As yet, no satisfactory standard has been developed.
- Non-text-based passwords. Passwords are not restricted to be letters or numbers. This article demonstrates the use of mouse gestures to authenticate users. Since these are hard to describe, system administrators will have some difficulty resetting passwords on the user's behalf.
Website Password Systems
The head on attempt to crack passwords by trying as many possibilities as time and money allows is known as brute force attack. Another method is a dictionary attack. In a dictionary attack, all the words in a dictionary are tested to see if they are the password.
There are number of computer programs for password auditing and recovery such as L0phtCrack and Cain.
Passwords in fiction
Password use is often depicted in fiction, Illya Kuryakin 'proving' his identity to the U.N.C.L.E. security door with a code word, or Harry Potter giving a password to a magic painting to enter his dormitory. Famous fictional passwords include "open sesame" from the Arabian Nights' tale of The Forty Thieves, Rumpelstiltskin, and Swordfish from the speakeasy in the Marx Brothers film Horse Feathers.
- Password manager
- Password policy
- Password strength
- Password length parameter
- Password cracking
- Password-authenticated key agreement
- Password notification e-mail
- Password synchronization
- Pre-shared key
- Random password generator
- Self-service password reset
- Password myths and tips
- Random secure password generator with automatic mnemonic generation
- GRC's "Ultra High Security" Password Generator
- Pick a Safe Password
- Enforcing use of cryptographically strong password
- Password management best practices
- Statistics on password choices, practices, and risks
- List of default passwords listed by vendor
- Links for password-based cryptography
- Website password and membership management systems
- Wordlists and articles about Password Security
- A list of English words that are typed using alternating left and right-hand keystrokes
- A large list of default passwords for computing, networking, and voice applications
- Pronouncable Password Generator
- pwgen - open-source program to generate random pronouncable passwords
- APG (Automated Password Generator) - similar to pwgen
- Online Help On Changing Password For Favourite Web Services
- Microsoft security guru: Jot down your passwords
- More Secure Mnemonic-Passwords: User-Friendly Passwords for Real Humans by Stephan Vladimir Bugaj
- The Memorability and Security of Passwords ó Some Empirical Results (PDF)
- Security policies can weaken passwords
- Directory of Anonymous Usernames and Passwords
- Password Security: Itís Not That Hard (But You Still Canít Get It Right) by Ron Rothman
- Encrypting Passwords with Hashing