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39-512: More formally, crypt provides cryptographic key derivation functions for password validation and storage on Unix systems. There is an unrelated crypt utility in Unix, which is often confused with the C library function. To distinguish between the two, writers often refer to the utility program as crypt(1) , because it is documented in section 1 of the Unix manual pages , and refer to the C library function as crypt(3) , because its documentation

78-535: A majority of MCF hashes. A number of extra application-defined methods exist. The original implementation of the crypt() library function in Third Edition Unix mimicked the M-209 cipher machine. Rather than encrypting the password with a key, which would have allowed the password to be recovered from the encrypted value and the key, it used the password itself as a key, and the password database contained

117-579: A method by James Reeds , and a new Enigma-based version appeared in Version 7 , which Reeds and Peter J. Weinberger also broke. Linux distributions generally do not include a Unix compatible version of the crypt command. This is largely due to a combination of three major factors: The source code to several old versions of the crypt command is available in The Unix Heritage Society's Unix Archive. The recent crypt source code

156-405: A multiword passphrase , using a password manager program or manually typing a password. It is possible to achieve a time–space tradeoff by pre-computing a list of hashes of dictionary words and storing these in a database using the hash as the key . This requires a considerable amount of preparation time, but this allows the actual attack to be executed faster. The storage requirements for

195-462: A number of Linux distributions (including, but not limited to, Fedora and Debian) have switched to libxcrypt , an ABI-compatible implementation that additionally supports new algorithms, including bcrypt and yescrypt. The musl C library supports schemes 1, 2, 5, and 6, plus the tradition DES scheme. The traditional DES code is based on the BSD FreeSec , with modification to be compatible with

234-552: A scheme based on the SHA-2 (SHA-256 and SHA-512) hash functions. The printable form of these hashes starts with $ 5$ (for SHA-256) or $ 6$ (for SHA-512) depending on which SHA variant is used. Its design is similar to the MD5-based crypt, with a few notable differences: The specification and sample code have been released into the public domain; it is often referred to as "SHAcrypt". Additional formats, if any, are described in

273-414: A simple statistical model similar to a dictionary-attack that takes a set of plain texts as input and processes it to guess plausible plaintexts, and does not require user interaction. There is also a Unix password hash function with the same name, crypt . Though both are used for securing data in some sense, they are otherwise essentially unrelated. To distinguish between the two, writers often refer to

312-480: A standard state, then uses this state to perform a block encryption using part of the key, and uses the result of that encryption (really, a hashing) to replace some of the subkeys. Then it uses this modified state to encrypt another part of the key, and uses the result to replace more of the subkeys. It proceeds in this fashion, using a progressively modified state to hash the key and replace bits of state, until all subkeys have been set. The number of rounds of keying

351-427: A tendency to choose short passwords that are ordinary words or common passwords; or variants obtained, for example, by appending a digit or punctuation character. Dictionary attacks are often successful, since many commonly used password creation techniques are covered by the available lists, combined with cracking software pattern generation. A safer approach is to randomly generate a long password (15 letters or more) or

390-476: Is a power of two, which is an input to the algorithm. The number is encoded in the textual hash, e.g. $ 2y$ 10... FreeBSD implemented support for the NT LAN Manager hash algorithm to provide easier compatibility with NT accounts via MS-CHAP . The NT-Hash algorithm is known to be weak, as it uses the deprecated md4 hash algorithm without any salting. FreeBSD used the $ 3$ prefix for this. Its use

429-599: Is also included in the Perl , PHP , Pike , Python (although it is now deprecated as of 3.11), and Ruby programming languages. Over time various algorithms have been introduced. To enable backward compatibility , each scheme started using some convention of serializing the password hashes that was later called the Modular Crypt Format (MCF). Old crypt(3) hashes generated before the de facto MCF standard may vary from scheme to scheme. A well-defined subset of

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468-558: Is available in the OpenSolaris project. A public domain version is available from the Crypt Breaker's Workbench. Enhanced symmetric encryption utilities are available for Linux (and should also be portable to any other Unix-like system) including mcrypt and ccrypt . While these provide support for much more sophisticated and modern algorithms, they can be used to encrypt and decrypt files which are compatible with

507-610: Is based on trying all the strings in a pre-arranged listing. Such attacks originally used words found in a dictionary (hence the phrase dictionary attack ); however, now there are much larger lists available on the open Internet containing hundreds of millions of passwords recovered from past data breaches. There is also cracking software that can use such lists and produce common variations, such as substituting numbers for similar-looking letters . A dictionary attack tries only those possibilities which are deemed most likely to succeed. Dictionary attacks often succeed because many people have

546-452: Is considered to be obsolete. The program is usually used as a filter , and it has traditionally been implemented using a "rotor machine" algorithm based on the Enigma machine . It is considered to be cryptographically far too weak to provide any security against brute-force attacks by modern, commodity personal computers . Some versions of Unix shipped with an even weaker version of

585-503: Is followed by 4 characters representing the number of rounds then 4 characters for the salt. The BSDi algorithm also supports longer passwords, using DES to fold the initial long password down to the eight 7-bit bytes supported by the original algorithm. Poul-Henning Kamp designed a baroque and (at the time) computationally expensive algorithm based on the MD5 message digest algorithm. MD5 itself would provide good cryptographic strength for

624-471: Is in manual section 3. This same crypt function is used both to generate a new hash for storage and also to hash a proffered password with a recorded salt for comparison. Modern Unix implementations of the crypt library routine support a variety of hash schemes. The particular hash algorithm used can be identified by a unique code prefix in the resulting hashtext, following a de facto standard called Modular Crypt Format. The crypt() library function

663-468: Is not recommended, as it is easily broken. The commonly used MD5 based scheme has become easier to attack as computer power has increased. Although the Blowfish-based system has the option of adding rounds and thus remain a challenging password algorithm, it does not use a NIST -approved algorithm. In light of these facts, Ulrich Drepper  [ de ] of Red Hat led an effort to create

702-419: Is passed through a thousand iterations of a function which rehashes it together with the passphrase and salt in a manner that varies between rounds. The output of the last of these rounds is the resulting passphrase hash. The fixed iteration count has caused this scheme to lose the computational expense that it once enjoyed and variable numbers of rounds are now favoured. In June 2012, Poul-Henning Kamp declared

741-569: The crypt function which supports the DES, MD5, and (since version 2.7) SHA-2 based hashing algorithms mentioned above. Ulrich Drepper, the glibc maintainer, rejected bcrypt (scheme 2) support since it isn't approved by NIST . A public domain crypt_blowfish library is available for systems without bcrypt. It has been integrated into glibc in SUSE Linux . In August 2017, glibc announced plans to remove its crypt implementation completely. In response,

780-595: The crypt(1) command in order to comply with contemporaneous laws and regulations that limited the exportation of cryptographic software . Some of these were simply implementations of the Caesar cipher (effectively no more secure than ROT13 , which is implemented as a Caesar cipher with a well-known key). Cryptographer Robert Morris wrote a M-209 -based crypt , which first appeared in Version 3 Unix , to encourage codebreaking experiments; Morris managed to break crypt by hand. Dennis Ritchie automated decryption with

819-492: The man pages of implementations. BigCrypt is the modified version of DES-Crypt used on HP-UX, Digital Unix, and OSF/1. The main difference between it and DES is that BigCrypt uses all the characters of a password, not just the first 8, and has a variable length hash. Crypt16 is the minor modification of DES, which allows passwords of up to 16 characters. Used on Ultrix and Tru64. The GNU C Library (glibc) used by almost all Linux distributions provides an implementation of

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858-496: The DES-based algorithm vulnerable to dictionary attacks, and Unix and Unix-like systems such as Linux have used "shadow" files for a long time, migrating just the password hash values out of the account file ( /etc/passwd ) and into a file (conventionally named /etc/shadow ) which can only be read by privileged processes. To increase the computational cost of password breaking, some Unix sites privately started increasing

897-641: The Modular Crypt Format was created during the Password Hashing Competition . The format is defined as: $ <id>[$ <param>=<value>(,<param>=<value>)*][$ <salt>[$ <hash>]] where The radix-64 encoding in crypt is called B64 and uses the alphabet ./0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz which is different than the more common RFC 4648 base64 $ 8$ aes256-gcm$ hmac-sha2-256$ 100$ y/4YMC4YDLU$ fzYDI4jjN6YCyQsYLsaf8A$ Ilu4jLcZarD9YnyD /Hejww$ okhBlc0cGakSqYxKww The PHC subset covers

936-484: The algorithm insecure and encouraged users to migrate to stronger password scramblers. Niels Provos and David Mazières designed a crypt() scheme called bcrypt based on Blowfish , and presented it at USENIX in 1999. The printable form of these hashes starts with $ 2$ , $ 2a$ , $ 2b$ , $ 2x$ or $ 2y$ depending on which variant of the algorithm is used: Blowfish is notable among block ciphers for its expensive key setup phase. It starts off with subkeys in

975-497: The entire password database simultaneously. In detail, the user's password is truncated to eight characters, and those are coerced down to only 7-bits each; this forms the 56-bit DES key. That key is then used to encrypt an all-bits-zero block, and then the ciphertext is encrypted again with the same key, and so on for a total of 25 DES encryptions. A 12-bit salt is used to perturb the encryption algorithm, so standard DES implementations can't be used to implement crypt(). The salt and

1014-426: The final ciphertext are encoded into a printable string in a form of base64 . This is technically not encryption since the data (all bits zero) is not being kept secret; it's widely known to all in advance. However, one of the properties of DES is that it's very resistant to key recovery even in the face of known plaintext situations. It is theoretically possible that two different passwords could result in exactly

1053-399: The glibc UFC-Crypt . Darwin's native crypt() provides limited functionality, supporting only DES and BSDi. OS X uses a few systems for its own password hashes, ranging from the old NeXTStep netinfo to the newer directory services (ds) system. Crypt (Unix) In Unix computing, crypt or enigma is a utility program used for encryption. Due to the ease of breaking it, it

1092-466: The number of encryption rounds on an ad hoc basis. This had the side effect of making their crypt() incompatible with the standard crypt() : the hashes had the same textual form, but were now calculated using a different algorithm. Some sites also took advantage of this incompatibility effect, by modifying the initial block from the standard all-bits-zero. This did not increase the cost of hashing, but meant that precomputed hash dictionaries based on

1131-446: The password hash, but it is designed to be quite quick to calculate relative to the strength it provides. The crypt() scheme is designed to be expensive to calculate, to slow down dictionary attacks. The printable form of MD5 password hashes starts with $ 1$ . This scheme allows users to have any length password, and they can use any characters supported by their platform (not just 7-bit ASCII). (In practice many implementations limit

1170-429: The password length, but they generally support passwords far longer than any person would be willing to type.) The salt is also an arbitrary string, limited only by character set considerations. First the passphrase and salt are hashed together, yielding an MD5 message digest. Then a new digest is constructed, hashing together the passphrase, the salt, and the first digest, all in a rather complex form. Then this digest

1209-450: The pre-computed tables were once a major cost, but now they are less of an issue because of the low cost of disk storage . Pre-computed dictionary attacks are particularly effective when a large number of passwords are to be cracked. The pre-computed dictionary needs be generated only once, and when it is completed, password hashes can be looked up almost instantly at any time to find the corresponding password. A more refined approach involves

crypt (C) - Misplaced Pages Continue

1248-569: The result of encrypting the password with this key. The original password encryption scheme was found to be too fast and thus subject to brute force enumeration of the most likely passwords. In Seventh Edition Unix , the scheme was changed to a modified form of the DES algorithm. A goal of this change was to make encryption slower. In addition, the algorithm incorporated a 12-bit salt in order to ensure that an attacker would be forced to crack each password independently as opposed to being able to target

1287-578: The same hash. Thus the password is never "decrypted": it is merely used to compute a result, and the matching results are presumed to be proof that the passwords were "the same." The advantages of this method have been that the hashtext can be stored and copied among Unix systems without exposing the corresponding plaintext password to the system administrators or other users. This portability has worked for over 30 years across many generations of computing architecture, and across many versions of Unix from many vendors. The traditional DES-based crypt algorithm

1326-459: The standard crypt() could not be applied. BSDi used a slight modification of the classic DES-based scheme. BSDi extended the salt to 24 bits and made the number of rounds variable (up to 2-1). The chosen number of rounds is encoded in the stored password hash, avoiding the incompatibility that occurred when sites modified the number of rounds used by the original scheme. These hashes are identified by starting with an underscore ( _ ), which

1365-528: The traditional crypt(1) command by providing the correct command line options. Programs for breaking crypt(1) encryption are widely available. Bob Baldwin's public domain Crypt Breaker's Workbench, which was written in 1984-1985, is an interactive tool that provides successive plaintext guesses that must be corrected by the user. It also provides a working crypt(1) implementation used by modern BSD distributions. Peter Selinger's unixcrypt-breaker uses

1404-437: The use of rainbow tables , which reduce storage requirements at the cost of slightly longer lookup-times. See LM hash for an example of an authentication system compromised by such an attack. Pre-computed dictionary attacks, or "rainbow table attacks", can be thwarted by the use of salt , a technique that forces the hash dictionary to be recomputed for each password sought, making precomputation infeasible, provided that

1443-611: The utility program as crypt(1) , because it is documented in section 1 of the Unix manual pages , and refer to the password hash function as crypt(3) , because its documentation is in section 3 of the manual. Dictionary attack In cryptanalysis and computer security , a dictionary attack is an attack using a restricted subset of a keyspace to defeat a cipher or authentication mechanism by trying to determine its decryption key or passphrase , sometimes trying thousands or millions of likely possibilities often obtained from lists of past security breaches. A dictionary attack

1482-415: Was originally chosen because DES was resistant to key recovery even in the face of "known plaintext" attacks, and because it was computationally expensive. On the earliest Unix machines it took over a full second to compute a password hash. This also made it reasonably resistant to dictionary attacks in that era. At that time password hashes were commonly stored in an account file ( /etc/passwd ) which

1521-429: Was readable to anyone on the system. (This account file was also used to map user ID numbers into names, and user names into full names, etc.). In the three decades since that time, computers have become vastly more powerful. Moore's Law has generally held true, so the computer speed and capacity available for a given financial investment has doubled over 20 times since Unix was first written. This has long since left

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