A passphrase is a sequence of words or other text used to control access to a
computer system, program or data. A passphrase is similar to a password in
usage, but is generally longer for added security. Passphrases are often used to
control both access to, and operation of, cryptographic programs and systems.
Passphrases are particularly applicable to systems that use the passphrase as an
encryption key. The origin of the term is by analogy with password. The modern
concept of passphrases is believed to have been invented by Sigmund N. Porter
in 1982. Security
Considering that the entropy of written English is less than 1.1 bits per
character, passphrases can be relatively weak. NIST has estimated that the
23-character passphrase "IamtheCapitanofthePina4" contains a
45-bit strength. The equation employed here is:
4 bits + 14 bits + 18 bits + 3 bits + 6 bits = 45 bits
(It is not clear whether or not this calculation takes into account that this
is a well-known quote from the operetta H.M.S. Pinafore.)
Using this guideline, to achieve the 80-bit strength recommended for high
security by NIST, a passphrase would need to be 58 characters long, assuming
a composition that includes uppercase and alphanumeric.
There is room for debate regarding the applicability of this equation,
depending on the number of bits of entropy assigned. For example, the
characters in five-letter words each contain 2.3 bits of entropy, which would
mean only a 35-character passphrase is necessary to achieve 80 bit strength.
If the words or components of a passphrase may be found in a language
dictionary—especially one available as electronic input to a software
program—the passphrase is rendered more vulnerable to dictionary attack. This is
a particular issue if the entire phrase can be found in a book of quotations or
phrase compilations. However, the required effort can be made
impracticably high if there are enough words in the passphrase and how randomly
they are chosen and ordered in the passphrase. The number of combinations
which would have to be tested under sufficient conditions make a dictionary
attack so difficult as to be infeasible. These are difficult conditions to meet,
and selecting at least one word that cannot be found in any dictionary
significantly increases passphrase strength.
If passphrases are chosen by humans they are usually biased by frequency of
particular words in natural language. In the case of four word phrases, actual
entropy rarely exceeds 30 bits. On the other hand, user-selected passwords tend
to be much weaker than that and encouraging users to use even 2-word
passphrases may be able to raise entropy from below 10 bits to over 20 bits.
For example, the widely used cryptography standard OpenPGP requires
that a user make up a passphrase that must be entered whenever decrypting or
signing messages. Internet services like Hushmail provide free encrypted e-mail
or file sharing services, but the security present depends almost entirely
on the quality of the chosen passphrase. Compared to passwords
Passphrases differ from passwords. A password is usually short—six to ten
characters. Such passwords may be adequate for various applications such
as: Logging onto computer systems
Negotiating keys in an interactive setting
Enabling a smart-card or PIN for an ATM card cannot be extracted)
But passwords are typically not safe to use as keys for standalone security
systems that expose data to enable offline password guessing by an
attacker. Passphrases are theoretically stronger, and so should make a better
choice in these cases. First, they usually are much longer—20 to 30
characters or more is typical—making some kinds of brute force attacks
entirely impractical. Second, if well chosen, they will not be found in any
phrase or quote dictionary, so such dictionary attacks will be almost
impossible. Third, they can be structured to be more easily memorable
than passwords without being written down, reducing the risk of hardcopy
theft. However, if a passphrase is not protected appropriately by the
authenticator and the clear-text passphrase is revealed its use is no
better than other passwords. For this reason it is recommended that
passphrases not be reused across different or unique sites and services.
In 2012, two Cambridge University researchers analyzed passphrases from
the Amazon PayPhrase system and found that a significant percentage are easy
to guess due to common cultural references such as movie names and
sports teams, losing much of the potential of using long passwords.
When used in cryptography, commonly the password protects a long key, and the
key protects the data. The key is so long a brute force attack is impossible.
A key derivation function is used, involving many thousands of iterations,
to slow down password cracking attacks. Passphrase selection
Typical advice about choosing a passphrase includes suggestions that it
should be: Long enough to be hard to guess
Not a famous quotation from literature, holy books, et cetera
Hard to guess by intuition—even by someone who knows the user well
Easy to remember and type accurately For better security, any easily
memorable encoding at the user's own level can be applied.
Not reused between sites, applications and other different sources.
Example methods One method to create a strong passphrase
is to use dice to select words at random from a long list, a technique often
referred to as diceware. While such a collection of words might appear to
violate the "not from any dictionary" rule, the security is based entirely on
the large number of possible ways to choose from the list of words and not
from any secrecy about the words themselves. For example, if there are
7776 words in the list and six words are chosen randomly, then there are 77766 =
221073919720733357899776 combinations, providing about 78 bits of entropy.
Random word sequences may then be memorized using techniques such as the
memory palace. Another is to choose two phrases, turn
one into an acronym, and include it in the second, making the final passphrase.
For instance, using two English language typing exercises, we have the following.
The quick brown fox jumps over the lazy dog, becomes tqbfjotld. Including it in,
Now is the time for all good men to come to the aid of their country, might
produce, Now is the time for all good tqbfjotld to come to the aid of their
country as the passphrase. There are several points to note here,
all relating to why this example passphrase is not a good one.
It has appeared in public and so should be avoided by everyone.
It is long and requires a good typist as typing errors are much more likely for
extended phrases. Individuals and organizations serious
about cracking computer security have compiled lists of passwords derived in
this manner from the most common quotations, song lyrics, and so on.
The PGP Passphrase FAQ suggests a procedure that attempts a better balance
between theoretical security and practicality than this example. All
procedures for picking a passphrase involve a tradeoff between security and
ease of use; security should be at least "adequate" while not "too seriously"
annoying users. Both criteria should be evaluated to match particular
situations. Another supplementary approach to
frustrating brute-force attacks is to derive the key from the passphrase using
a deliberately slow hash function, such as PBKDF2 as described in RFC 2898.
Windows support If backward compatibility with Microsoft
LAN Manager is not needed, in versions of Windows NT, a passphrase can be used
as a substitute for a Windows password. If the passphrase is longer than 14
characters, this will also avoid the generation of a very weak LM hash.
Unix support In recent versions of Unix-like
operating systems such as Linux, OpenBSD, NetBSD, Solaris and FreeBSD, up
to 255-character passphrases can be used.
See also Keyfile
Password-based cryptography References
^ Sigmund N. Porter. "A password extension for improved human factors".
Computers and Security, 1(1):54-56, January 1982.
^ Matt Mahoney. "Refining the Estimated Entropy of English by Shannon Game
Simulation". Florida Institute of Technology. Retrieved March 27, 2008.
^ "Electronic Authentication Guideline". NIST. Retrieved April 7, 2008.
^ Jesper M. Johansson. "The Great Debates: Pass Phrases vs. Passwords.
Part 2 of 3". Microsoft Corporation. Retrieved March 27, 2008.
^ Joseph Bonneau, Ekaterina Shutova, Linguistic properties of multi-word
passphrases, University of Cambridge ^ Godwin, Dan. "Passphrases only
marginally more secure than passwords because of poor choices". Retrieved 9
December 2014. ^ Lundin, Leigh. "PINs and Passwords,
Part 2". Passwords. Orlando: SleuthSayers.
^ Randall T. Williams. "The Passphrase FAQ". Retrieved 2006-12-11.
External links Diceware page
Passkool - A deterministic "pronounceable" password generator
Passphrase FAQs Readable Passphrase Generator at
codeplex.com in C# with API. Passphrase - a web-based encrypted
password generator and management tool. xkcd Password Strength common-viewed
explanation of concept