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 Openwall Project