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[1] in 1982.
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Considering that the entropy of written English is less than 1.1 bits per character,[2] passphrases can be relatively weak. NIST has estimated that the 23 character pass phrase "IamtheCapitanofthePina4" contains a 45 bit-strength. The equation employed here is:[3]
Using this guideline, to achieve the 80-bit strength recommended for high security (non-military) 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, 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.[4]
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 (in time and cost) 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.
For example, the widely used cryptography standard OpenPGP requires that a user make up a passphrase that must be entered whenever encrypting, decrypting, or signing messages. Internet services like CryptoHeaven and Hushmail provide free encrypted e-mail or file sharing services, but the security present depends almost entirely on the quality of the chosen passphrase.
Passphrases differ from passwords. A password is usually short—six to ten characters. Such passwords may be adequate for various applications (if frequently changed, if chosen using an appropriate policy, if not found in dictionaries, if sufficiently random, and/or if the system prevents online guessing, etc.) such as:
But passwords are typically not safe to use as keys for standalone security systems (e.g., encryption systems) that expose data to enable offline password guessing by an attacker. Passphrases are generally stronger, and a clearly better choice in these cases. First, they usually are (and always should be) 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.. It should be noted however that 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.
Typical advice about choosing a passphrase includes suggestions that it should be:
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. (The number 7776 was chosen to allow words to be selected by throwing five dice. 7776 = 65)
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 pass phrase is not a good one.
The PGP Passphrase FAQ[5] 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.
If backward compatibility with Microsoft LAN Manager is not needed, in versions of Windows NT (including Windows 2000, Windows XP and later), a passphrase can be used as a substitute for a Windows password. If the passphrase is longer than 14 characters, this will also cause the very weak LM hash to not be generated.
In recent versions of Unix-like operating systems such as Linux, OpenBSD, NetBSD, Solaris and FreeBSD, up to 255 character passphrases can be used.