There are two general approaches to password management:
Password generators which use a hash function, like the SS64 password generator, are easy to use and will repeatedly regenerate the same password when given the same inputs but they do have limitations - the only way to change a password is to enter a different master password or a different salt value. All the generated passwords are the same length.
Password managers which store passwords have the flexibility to apply different complexity rules to each password or to store a pre-existing password - often required when a password needs to be shared between a team of people. The downside of the storage approach is that the password storage (file/database) needs to be managed carefully - secured, backed up and synchronised to all the devices where you will need to use the passwords. If the password store is lost or corrupted you will lose all the passwords! Destructive viruses such as CryptoLocker can also make a password store unreadable.
The capacity of a password can be calculated from length + complexity, to give a measure of password entropy. A password containing only upper and lower case characters has 52 possible characters in each digit, adding numerals and other characters will increase this further.
Entropy denotes the uncertainty in the value of a password and is conventionally expressed in bits. If a password of k bits is chosen at random there are 2k possible values
|a-z||a-z, A-Z, 0-9|
|8||26 bits||37 bits||47 bits|
|10||33 bits||47 bits||59 bits|
|11||36 bits||51 bits||65 bits|
|12||39 bits||56 bits||71 bits|
|13||43 bits||61 bits||77 bits|
|14||46 bits||65 bits||80 bits|
|15||49 bits||70 bits||89 bits|
|20||66 bits||94 bits||119 bits|
Maximum Time to crack a password based on its entropy
|Entropy||Maximum Time to crack
at 350 billion guesses/Sec
|47 bits||0.223 Hours|
|59 bits||457.50 Hours|
|65 bits||3.342 Years|
|71 bits||213.92 Years|
|77 bits||13,690 Years|
|80 bits||109,527.95 Years|
|89 bits||56078315.93 Years.|
|119 bits||6.0213633 e+16 Years|
High password entropy will help to protect against 'old school' brute force attacks, but passwords like aaaaaaaaaaaaaaa or schoolofhardknocks or any common phrase or saying will still be less secure than a random string of the same length. This is because a rainbow table can be used to lookup almost any known phrase or pattern very quickly.
For non-random passwords the calculation of entropy can be modified by applying a set of rules to account for typical language patterns (Shannon Entropy).
There is no definitive answer to the question of the password strength required to resist a brute force attack but NIST recommend 80 bits for the most secure passwords.
Sensitive systems or situations will demand longer passwords, but high entropy long passwords containing random characters and numbers are more difficult for anyone to memorize. Truly secure passwords quickly become impractical unless you use software to automatically retrieve (or generate) the passwords.
Pass phrases (also known as Diceware) a long password consisting of several random words such as correct horse battery staple have been popularised by XKCD. Pass phrases don't offer any significant advantage or disadvantage compared to a traditional password of the same entropy: Research paper (PDF). Novice typists may find the longer length difficult to type and they may exceed the password length limitations of some websites.
Some badly coded websites only store the first 10 or 15 characters of a password, if a long password is silently truncated to correcthorse thats suddenly a lot less secure than you would expect. Up until 2012 Microsoft Hotmail did this.
It is possible that an intruder could attack a popular password generator by passing a rainbow table of common passwords through the same hash algorithm to generate potential passwords in bulk. To prevent this, it is still important when using any password manager to choose the master password carefully, a long, random, difficult-to-guess string.
Although rarely enforced for website logins, there are some advantages to changing passwords on a regular basis. If the length of your password means that it could be brute force cracked in 50 days, then changing the password every 49 days makes it impossible for such an attack to succeed.
Conversely if the password is truly strong, there is little to be gained in changing it. Mathematically, moving from never changing one’s password to changing it at every single login attempt (pass or fail) will on average only double the number of attempts it would take to brute force the password. In comparison, adding a single extra character to the password length will make it an order of magnitude more secure.
If a sensitive password has been discovered by a third party, then changing it after 30 days will revoke that access, but in many cases this will be locking the stable door after the horse has bolted.
If you do need to change passwords regularly then a password manager with an encrypted database will allow individual passwords to be changed as required.
On-line services often provide a password restore function that can be used to reset the password. It is good practice to setup complex answers to such security questions particularly if the 'true' answer would be very simple for an attacker to discover. Password reset answers should be managed just like actual passwords.
|Password Generators (Cryptographic Hash)||Password Generators (store with reversible encryption)|
Nic Wolff’s password generator
Creates 10 character passwords. (SHA-1 hash)
Free open source software that encrypts passwords in a storage database. Created by Bruce Schneier and Counterpane Labs for MS Windows users.
By default PasswordSafe creates 8 character passwords.
Supported in: Windows, Linux (in beta)
A browser plug-in that encrypts passwords in a storage database and automates website logins. The basic plug-in is free but 'premium' features require a yearly subscription. Lastpass includes a strong password generator and can sync between devices. For syncing to take place, the encrypted passwords are stored online, which does require a certain amount of trust in the LastPass security team. LastPass offer a secure bookmarklet for the situation where you can't use a LastPass browser plugin.
By default Lastpass creates 8 character passwords.
Supported on: Firefox, Internet Explorer, Safari, Chrome, Opera, Windows Desktop, iPhone, iPad, Android, Blackberry.
|Pwdhash (MD 5 hash)
The default password length is proportional to the length of the master password.
Supported on: IE, Firefox, Chrome, Safari and anything else that can run a web browser.
A browser plug-in that encrypts passwords in a storage database and automates website logins. Founded in 2010 by Bernard Liautaud (Business Objects). The basic plug-in is free but 'premium' features require a yearly subscription. Dashlane includes a strong password generator and can sync between devices. For syncing to take place, the encrypted passwords are stored online, which does require a certain amount of trust in the Dashlane security team. Dashlane will display an impala against each form field that can be saved or filled in.
By default Dashlane creates 10 character passwords.
Supported on: Firefox, Internet Explorer, Safari, Chrome, iPhone, Android.
|SuperGenPass (SHA-1 hash)
SuperGenPass will generate the same password for different subdomains of the same website.
One downside is that the bookmarklet code contains a large number of hard-coded URL/domains, this may become difficult to keep up to date given the increasing expansion of top level domains.
By default SuperGenPass creates 10 character passwords.
Supported on: IE, Firefox, Chrome, Safari and anything else that can run a web browser.
|KeePass Password Safe
An open source application that encrypts passwords in a storage database. Includes a strong password generator. To sync between devices either copy the password database manually or use cloud storage (DropBox etc.)
By default KeePass creates 20 character passwords.
Supported on: Windows, Linux, Mac OS X, Andriod. Requires the Microsoft .NET Framework or Mono.
KeePassX is a compatible version for Mac OSX and Windows.
|PasswordMaker (MD 5 Hash or a choice of 12 others)
Has an unparalleled level of control over password generation, but this does mean that you have to remember all the options you have chosen in order to regenerate the same password.
By default PasswordMaker creates 8 character passwords.
Supported on: Windows, IE, Firefox, Chrome, Opera and anything else that can run a web browser.
A commercial application that encrypts passwords in a storage database. Strong support for Enterprise features. It has a browser plug-in to automate website logins and a strong password generator. The passwords are encrypted and stored on individual clients or optionally on a USB Flash Drive.
By default RoboForm creates 8 character passwords but this is fully customisable via policy.
Supported on: Windows, Firefox, Internet Explorer, Chrome, Safari, iPhone, iPad, Android, Blackberry.
A commercial application that encrypts passwords in a storage database. It has a browser plug-in to automate website logins, a strong password generator and an option to sync between devices. There is no plug-in for Mobile Safari but they do have a workaround.
By default 1Password creates 20 character passwords but this is fully customisable.
Supported on: Mac OS X, Windows, Linux, iOS, Andriod(read only)
Many password generators include web-browser integration with 'auto login' features, this can introduce a new security risk.
Many password generators utilise bookmarklets, particularly on mobile devices. There are are some XSS risks around bookmarklets.
Good password management solutions should be able to cope when a website changes it's url e.g. signup.example.com changes to login.example.com
“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” ~ Jesper Johansson (Microsoft)
A really strong password is one that nobody else has ever used - Joseph Bonneau (University of Cambridge)
NIST password recommendations (PDF)
Password entropy does not improve security Florida State University (PDF)
It is now possible to build GPU computer clusters that can cycle through as many as 350 billion guesses per second.