Provided by: libcrypt-dev_4.4.27-1_amd64 
NAME
crypt — storage format for hashed passphrases and available hashing methods
DESCRIPTION
The hashing methods implemented by crypt(3) are designed only to process user passphrases for storage and
authentication; they are not suitable for use as general-purpose cryptographic hashes.
Passphrase hashing is not a replacement for strong passphrases. It is always possible for an attacker
with access to the hashed passphrases to guess and check possible cleartext passphrases. However, with a
strong hashing method, guessing will be too slow for the attacker to discover a strong passphrase.
All of the hashing methods use a “salt” to perturb the hash function, so that the same passphrase may
produce many possible hashes. Newer methods accept longer salt strings. The salt should be chosen at
random for each user. Salt defeats a number of attacks:
1. It is not possible to hash a passphrase once and then test it against each account's stored hash;
the hash calculation must be repeated for each account.
2. It is not possible to tell whether two accounts use the same passphrase without successfully
guessing one of the phrases.
3. Tables of precalculated hashes of commonly used passphrases must have an entry for each possible
salt, which makes them impractically large.
All of the hashing methods are also deliberately engineered to be slow; they use many iterations of an
underlying cryptographic primitive to increase the cost of each guess. The newer hashing methods allow
the number of iterations to be adjusted, using the “CPU time cost” parameter to crypt_gensalt(3). This
makes it possible to keep the hash slow as hardware improves.
FORMAT OF HASHED PASSPHRASES
All of the hashing methods supported by crypt(3) produce a hashed passphrase which consists of four
components: prefix, options, salt, and hash. The prefix controls which hashing method is to be used, and
is the appropriate string to pass to crypt_gensalt(3) to select that method. The contents of options,
salt, and hash are up to the method. Depending on the method, the prefix and options components may be
empty.
The setting argument to crypt(3) must begin with the first three components of a valid hashed passphrase,
but anything after that is ignored. This makes authentication simple: hash the input passphrase using
the stored passphrase as the setting, and then compare the result to the stored passphrase.
Hashed passphrases are always entirely printable ASCII, and do not contain any whitespace or the
characters ‘:’, ‘;’, ‘*’, ‘!’, or ‘\’. (These characters are used as delimiters and special markers in
the passwd(5) and shadow(5) files.)
The syntax of each component of a hashed passphrase is up to the hashing method. ‘$’ characters usually
delimit components, and the salt and hash are usually encoded as numerals in base 64. The details of
this base-64 encoding vary among hashing methods. The common “base64” encoding specified by RFC 4648 is
usually not used.
AVAILABLE HASHING METHODS
This is a list of all the hashing methods supported by crypt(3), in decreasing order of strength. Many
of the older methods are now considered too weak to use for new passphrases. The hashed passphrase
format is expressed with extended regular expressions (see regex(7)) and does not show the division into
prefix, options, salt, and hash.
yescrypt
yescrypt is a scalable passphrase hashing scheme designed by Solar Designer, which is based on Colin
Percival's scrypt. Recommended for new hashes.
Prefix
"$y$"
Hashed passphrase format
\$y\$[./A-Za-z0-9]+\$[./A-Za-z0-9]{,86}\$[./A-Za-z0-9]{43}
Maximum passphrase length
unlimited
Hash size
256 bits
Salt size
up to 512 (128+ recommended) bits
CPU time cost parameter
1 to 11 (logarithmic)
gost-yescrypt
gost-yescrypt uses the output from the yescrypt hashing method in place of a hmac message. Thus, the
yescrypt crypto properties are superseded by the GOST R 34.11-2012 (Streebog) hash function with a 256
bit digest. This hashing method is useful in applications that need modern passphrase hashing methods,
but require to rely on the cryptographic properties of GOST algorithms. The GOST R 34.11-2012 (Streebog)
hash function has been published by the IETF as RFC 6986. Recommended for new hashes.
Prefix
"$gy$"
Hashed passphrase format
\$gy\$[./A-Za-z0-9]+\$[./A-Za-z0-9]{,86}\$[./A-Za-z0-9]{43}
Maximum passphrase length
unlimited
Hash size
256 bits
Salt size
up to 512 (128+ recommended) bits
CPU time cost parameter
1 to 11 (logarithmic)
scrypt
scrypt is a password-based key derivation function created by Colin Percival, originally for the Tarsnap
online backup service. The algorithm was specifically designed to make it costly to perform large-scale
custom hardware attacks by requiring large amounts of memory. In 2016, the scrypt algorithm was
published by IETF as RFC 7914.
Prefix
"$7$"
Hashed passphrase format
\$7\$[./A-Za-z0-9]{11,97}\$[./A-Za-z0-9]{43}
Maximum passphrase length
unlimited
Hash size
256 bits
Salt size
up to 512 (128+ recommended) bits
CPU time cost parameter
6 to 11 (logarithmic)
bcrypt
A hash based on the Blowfish block cipher, modified to have an extra-expensive key schedule. Originally
developed by Niels Provos and David Mazieres for OpenBSD and also supported on recent versions of FreeBSD
and NetBSD, on Solaris 10 and newer, and on several GNU/*/Linux distributions.
Prefix
"$2b$"
Hashed passphrase format
\$2[abxy]\$[0-9]{2}\$[./A-Za-z0-9]{53}
Maximum passphrase length
72 characters
Hash size
184 bits
Salt size
128 bits
CPU time cost parameter
4 to 31 (logarithmic)
The alternative prefix "$2y$" is equivalent to "$2b$". It exists for historical reasons only. The
alternative prefixes "$2a$" and "$2x$" provide bug-compatibility with crypt_blowfish 1.0.4 and earlier,
which incorrectly processed characters with the 8th bit set.
sha512crypt
A hash based on SHA-2 with 512-bit output, originally developed by Ulrich Drepper for GNU libc.
Supported on Linux but not common elsewhere. Acceptable for new hashes. The default CPU time cost
parameter is 5000, which is too low for modern hardware.
Prefix
"$6$"
Hashed passphrase format
\$6\$(rounds=[1-9][0-9]+\$)?[^$:\n]{1,16}\$[./0-9A-Za-z]{86}
Maximum passphrase length
unlimited
Hash size
512 bits
Salt size
6 to 96 bits
CPU time cost parameter
1000 to 999,999,999
sha256crypt
A hash based on SHA-2 with 256-bit output, originally developed by Ulrich Drepper for GNU libc.
Supported on Linux but not common elsewhere. Acceptable for new hashes. The default CPU time cost
parameter is 5000, which is too low for modern hardware.
Prefix
"$5$"
Hashed passphrase format
\$5\$(rounds=[1-9][0-9]+\$)?[^$:\n]{1,16}\$[./0-9A-Za-z]{43}
Maximum passphrase length
unlimited
Hash size
256 bits
Salt size
6 to 96 bits
CPU time cost parameter
1000 to 999,999,999
sha1crypt
A hash based on HMAC-SHA1. Originally developed by Simon Gerraty for NetBSD. Not as weak as the DES-
based hashes below, but SHA1 is so cheap on modern hardware that it should not be used for new hashes.
Prefix
"$sha1"
Hashed passphrase format
\$sha1\$[1-9][0-9]+\$[./0-9A-Za-z]{1,64}\$[./0-9A-Za-z]{8,64}[./0-9A-Za-z]{32}
Maximum passphrase length
unlimited
Hash size
160 bits
Salt size
6 to 384 bits
CPU time cost parameter
4 to 4,294,967,295
SunMD5
A hash based on the MD5 algorithm, with additional cleverness to make precomputation difficult,
originally developed by Alec David Muffet for Solaris. Not adopted elsewhere, to our knowledge. Not as
weak as the DES-based hashes below, but MD5 is so cheap on modern hardware that it should not be used for
new hashes.
Prefix
"$md5"
Hashed passphrase format
\$md5(,rounds=[1-9][0-9]+)?\$[./0-9A-Za-z]{8}\${1,2}[./0-9A-Za-z]{22}
Maximum passphrase length
unlimited
Hash size
128 bits
Salt size
48 bits
CPU time cost parameter
4096 to 4,294,963,199
md5crypt
A hash based on the MD5 algorithm, originally developed by Poul-Henning Kamp for FreeBSD. Supported on
most free Unixes and newer versions of Solaris. Not as weak as the DES-based hashes below, but MD5 is so
cheap on modern hardware that it should not be used for new hashes. CPU time cost is not adjustable.
Prefix
"$1$"
Hashed passphrase format
\$1\$[^$:\n]{1,8}\$[./0-9A-Za-z]{22}
Maximum passphrase length
unlimited
Hash size
128 bits
Salt size
6 to 48 bits
CPU time cost parameter
1000
bsdicrypt (BSDI extended DES)
A weak extension of traditional DES, which eliminates the length limit, increases the salt size, and
makes the time cost tunable. It originates with BSDI and is also available on at least NetBSD, OpenBSD,
and FreeBSD due to the use of David Burren's FreeSec library. It is better than bigcrypt and traditional
DES, but still should not be used for new hashes.
Prefix
"_"
Hashed passphrase format
_[./0-9A-Za-z]{19}
Maximum passphrase length
unlimited (ignores 8th bit)
Hash size
64 bits
Effective key size
56 bits
Salt size
24 bits
CPU time cost parameter
1 to 16,777,215 (must be odd)
bigcrypt
A weak extension of traditional DES, available on some System V-derived Unixes. All it does is raise the
length limit from 8 to 128 characters, and it does this in a crude way that allows attackers to guess
chunks of a long passphrase in parallel. It should not be used for new hashes.
Prefix
"" (empty string)
Hashed passphrase format
[./0-9A-Za-z]{13,178}
Maximum passphrase length
128 characters (ignores 8th bit)
Hash size
up to 1024 bits
Effective key size
up to 896 bits
Salt size
12 bits
CPU time cost parameter
25
descrypt (Traditional DES)
The original hashing method from Unix V7, based on the DES block cipher. Because DES is cheap on modern
hardware, because there are only 4096 possible salts and 2**56 possible hashes, and because it truncates
passphrases to 8 characters, it is feasible to discover any passphrase hashed with this method. It
should only be used if you absolutely have to generate hashes that will work on an old operating system
that supports nothing else.
Prefix
"" (empty string)
Hashed passphrase format
[./0-9A-Za-z]{13}
Maximum passphrase length
8 characters (ignores 8th bit)
Hash size
64 bits
Effective key size
56 bits
Salt size
12 bits
CPU time cost parameter
25
NT
The hashing method used for network authentication in some versions of the SMB/CIFS protocol. Available,
for cross-compatibility's sake, on FreeBSD. Based on MD4. Has no salt or tunable cost parameter. Like
traditional DES, it is so weak that any passphrase hashed with this method is guessable. It should only
be used if you absolutely have to generate hashes that will work on an old operating system that supports
nothing else.
Prefix
"$3$"
Hashed passphrase format
\$3\$\$[0-9a-f]{32}
Maximum passphrase length
unlimited
Hash size
256 bits
Salt size
0 bits
CPU time cost parameter
1
SEE ALSO
crypt(3), crypt_gensalt(3), getpwent(3), passwd(5), shadow(5), pam(8)
Niels Provos and David Mazieres, “A Future-Adaptable Password Scheme”, Proceedings of the 1999 USENIX
Annual Technical Conference, https://www.usenix.org/events/usenix99/provos.html, June 1999.
Robert Morris and Ken Thompson, “Password Security: A Case History”, Communications of the ACM, 11, 22,
http://wolfram.schneider.org/bsd/7thEdManVol2/password/password.pdf, 1979.
Openwall Project October 11, 2017 CRYPT(5)