Passphrase

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A passphrase is a sequence of words or other text used to control access to a computer system, program or data. It is similar to a password in usage, but a passphrase is generally longer for added security. Passphrases are often used to control both access to, and the operation of, cryptographic programs and systems, especially those that derive an encryption key from a passphrase. 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. [1]

Contents

Security [2]

Considering that the entropy of written English is less than 1.1 bits per character, [3] 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]

4 bits (1st character) + 14 bits (characters 2–8) + 18 bits (characters 9–20) + 3 bits (characters 21–23) + 6 bits (bonus for upper case, lower case, and alphanumeric) = 45 bits

(This calculation does not take into account that this is a well-known quote from the operetta H.M.S. Pinafore. An MD5 hash of this passphrase can be cracked in 4 seconds using crackstation.net, indicating that the phrase is found in password cracking databases.)

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, 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. [5]

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 if they are randomly 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 the 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. [6]

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 if frequently changed, chosen using an appropriate policy, not found in dictionaries, sufficiently random, and/or if the system prevents online guessing, etc.[ citation needed ], such as:

But passwords are typically not safe to use as keys for standalone security systems such as encryption systems that expose data to enable offline password guessing by an attacker. [7] Passphrases are theoretically stronger, and so should make a 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. 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. [8]

When used in cryptography, commonly the passphrase protects a long machine generated key, and the key protects the data. The key is so long a brute force attack directly on the data is impossible. A key derivation function is used, involving many thousands of iterations (salted & hashed), to slow down password cracking attacks.

Passphrases selection

Typical advice about choosing a passphrase includes suggestions that it should be: [9]

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 7,7766 = 221,073,919,720,733,357,899,776 combinations, providing about 78 bits of entropy. (The number 7776 was chosen to allow words to be selected by throwing five dice. 7776 = 65) 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.

The PGP Passphrase FAQ [10] 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 (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 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.[ citation needed ]

See also

Related Research Articles

<span class="mw-page-title-main">Password</span> Used for user authentication to prove identity or access approval

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A key in cryptography is a piece of information, usually a string of numbers or letters that are stored in a file, which, when processed through a cryptographic algorithm, can encode or decode cryptographic data. Based on the used method, the key can be different sizes and varieties, but in all cases, the strength of the encryption relies on the security of the key being maintained. A key's security strength is dependent on its algorithm, the size of the key, the generation of the key, and the process of key exchange.

<span class="mw-page-title-main">Brute-force attack</span> Cryptanalytic method for unauthorized users to access data

In cryptography, a brute-force attack consists of an attacker submitting many passwords or passphrases with the hope of eventually guessing correctly. The attacker systematically checks all possible passwords and passphrases until the correct one is found. Alternatively, the attacker can attempt to guess the key which is typically created from the password using a key derivation function. This is known as an exhaustive key search.

The Yarrow algorithm is a family of cryptographic pseudorandom number generators (CSPRNG) devised by John Kelsey, Bruce Schneier, and Niels Ferguson and published in 1999. The Yarrow algorithm is explicitly unpatented, royalty-free, and open source; no license is required to use it. An improved design from Ferguson and Schneier, Fortuna, is described in their book, Practical Cryptography

A cryptographically secure pseudorandom number generator (CSPRNG) or cryptographic pseudorandom number generator (CPRNG) is a pseudorandom number generator (PRNG) with properties that make it suitable for use in cryptography. It is also loosely known as a cryptographic random number generator (CRNG).

<span class="mw-page-title-main">Cryptographic hash function</span> Hash function that is suitable for use in cryptography

A cryptographic hash function (CHF) is a hash algorithm that has special properties desirable for a cryptographic application:

<span class="mw-page-title-main">Key derivation function</span> Function that derives secret keys from a secret value

In cryptography, a key derivation function (KDF) is a cryptographic algorithm that derives one or more secret keys from a secret value such as a master key, a password, or a passphrase using a pseudorandom function. KDFs can be used to stretch keys into longer keys or to obtain keys of a required format, such as converting a group element that is the result of a Diffie–Hellman key exchange into a symmetric key for use with AES. Keyed cryptographic hash functions are popular examples of pseudorandom functions used for key derivation.

In cryptanalysis and computer security, password cracking is the process of recovering passwords from data that has been stored in or transmitted by a computer system in scrambled form. A common approach is to repeatedly try guesses for the password and to check them against an available cryptographic hash of the password. Another type of approach is password spraying, which is often automated and occurs slowly over time in order to remain undetected, using a list of common passwords.

S/KEY is a one-time password system developed for authentication to Unix-like operating systems, especially from dumb terminals or untrusted public computers on which one does not want to type a long-term password. A user's real password is combined in an offline device with a short set of characters and a decrementing counter to form a single-use password. Because each password is only used once, they are useless to password sniffers.

In cryptography, a salt is random data fed as an additional input to a one-way function that hashes data, a password or passphrase. Salting helps defend against attacks that use precomputed tables, by vastly growing the size of table needed for a successful attack. It also helps protect passwords that occur multiple times in a database, as a new salt is used for each password instance. Additionally, salting does not place any burden on users.

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A random password generator is a software program or hardware device that takes input from a random or pseudo-random number generator and automatically generates a password. Random passwords can be generated manually, using simple sources of randomness such as dice or coins, or they can be generated using a computer.

<span class="mw-page-title-main">Diceware</span> Method for generating passphrases using dice

Diceware is a method for creating passphrases, passwords, and other cryptographic variables using ordinary dice as a hardware random number generator. For each word in the passphrase, five rolls of a six-sided die are required. The numbers from 1 to 6 that come up in the rolls are assembled as a five-digit number, e.g. 43146. That number is then used to look up a word in a cryptographic word list. In the original Diceware list 43146 corresponds to munch. By generating several words in sequence, a lengthy passphrase can thus be constructed randomly.

A rainbow table is a precomputed table for caching the outputs of a cryptographic hash function, usually for cracking password hashes. Passwords are typically stored not in plain text form, but as hash values. If such a database of hashed passwords falls into the hands of an attacker, they can use a precomputed rainbow table to recover the plaintext passwords. A common defense against this attack is to compute the hashes using a key derivation function that adds a "salt" to each password before hashing it, with different passwords receiving different salts, which are stored in plain text along with the hash.

In cryptography, key stretching techniques are used to make a possibly weak key, typically a password or passphrase, more secure against a brute-force attack by increasing the resources it takes to test each possible key. Passwords or passphrases created by humans are often short or predictable enough to allow password cracking, and key stretching is intended to make such attacks more difficult by complicating a basic step of trying a single password candidate. Key stretching also improves security in some real-world applications where the key length has been constrained, by mimicking a longer key length from the perspective of a brute-force attacker.

<span class="mw-page-title-main">Password strength</span> Resistance of a password to being guessed

Password strength is a measure of the effectiveness of a password against guessing or brute-force attacks. In its usual form, it estimates how many trials an attacker who does not have direct access to the password would need, on average, to guess it correctly. The strength of a password is a function of length, complexity, and unpredictability.

OTPW is a one-time password system developed for authentication in Unix-like operating systems by Markus Kuhn. A user's real password is not directly transmitted across the network. Rather, a series of one-time passwords is created from a short set of characters and a set of one-time tokens. As each single-use password can only be used once, passwords intercepted by a password sniffer or key logger are not useful to an attacker.

The following outline is provided as an overview of and topical guide to cryptography:

crypt is a POSIX C library function. It is typically used to compute the hash of user account passwords. The function outputs a text string which also encodes the salt, and identifies the hash algorithm used. This output string forms a password record, which is usually stored in a text file.

In cryptography, a pepper is a secret added to an input such as a password during hashing with a cryptographic hash function. This value differs from a salt in that it is not stored alongside a password hash, but rather the pepper is kept separate in some other medium, such as a Hardware Security Module. Note that the National Institute of Standards and Technology refers to this value as a secret key rather than a pepper. A pepper is similar in concept to a salt or an encryption key. It is like a salt in that it is a randomized value that is added to a password hash, and it is similar to an encryption key in that it should be kept secret.

References

  1. ↑ Sigmund N. Porter. "A password extension for improved human factors". Computers and Security, 1(1):54-56, January 1982.
  2. ↑ Nosenko, Alex; Cheng, Yuan; Chen, Haiquan (August 27, 2022). "Password and Passphrase Guessing with Recurrent Neural Networks". Information Systems Frontiers. doi:10.1007/s10796-022-10325-x. ISSN   1387-3326.
  3. ↑ Matt Mahoney. "Refining the Estimated Entropy of English by Shannon Game Simulation". Florida Institute of Technology. Retrieved March 27, 2008.
  4. ↑ "Electronic Authentication Guideline" (PDF). NIST. Retrieved September 26, 2016.
  5. ↑ Jesper M. Johansson. "The Great Debates: Pass Phrases vs. Passwords. Part 2 of 3". Microsoft Corporation. Retrieved March 27, 2008.
  6. ↑ Joseph Bonneau, Ekaterina Shutova, Linguistic properties of multi-word passphrases, University of Cambridge
  7. ↑ Urbina, Ian (November 19, 2014). "The Secret Life of Passwords". The New York Times Magazine.
  8. ↑ Godwin, Dan (March 14, 2012). "Passphrases only marginally more secure than passwords because of poor choices" . Retrieved December 9, 2014.
  9. ↑ Lundin, Leigh (August 11, 2013). "PINs and Passwords, Part 2". Passwords. Orlando: SleuthSayers.
  10. ↑ Randall T. Williams (January 13, 1997). "The Passphrase FAQ" . Retrieved December 11, 2006.
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