crypt
CRYPT(5) BSD File Formats Manual CRYPT(5)
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 re-
peated 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 in-
crease the cost of each guess. The newer hashing methods allow the num-
ber 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 hard-
ware 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 compo-
nents 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 con-
tain 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. '$' characters usually delimit components, and the salt and hash
are usually encoded as numerals in base 64. The details of this base-64
encoding vary among hashing methods. The common "base64" encoding speci-
fied by RFC 4648 is usually not used.
AVAILABLE HASHING METHODS
This is a list of all the hashing methods supported by crypt(3), in de-
creasing order of strength. Many of the older methods are now considered
too weak to use for new passphrases. The hashed passphrase format is ex-
pressed with extended regular expressions (see regex(7)) and does not
show the division into prefix, options, salt, and hash.
yescrypt
yescrypt is a scalable passphrase hashing scheme designed by Solar De-
signer, which is based on Colin Percival's scrypt. Recommended for new
hashes.
Prefix
"$y$"
Hashed passphrase format
\$y\$[./A-Za-z0-9]+\$[./A-Za-z0-9]{,86}\$[./A-Za-z0-9]{43}
Maximum passphrase length
unlimited
Hash size
256 bits
Salt size
up to 512 bits
CPU time cost parameter
1 to 11 (logarithmic)
gost-yescrypt
gost-yescrypt uses the output from the yescrypt hashing method in place
of a hmac message. Thus, the yescrypt crypto properties are superseeded
by the GOST R 34.11-2012 (Streebog) hash function with a 256 bit digest.
This hashing method is useful in applications that need modern passphrase
hashing methods, but require to rely on the cryptographic properties of
GOST algorithms. The GOST R 34.11-2012 (Streebog) hash function has been
published by the IETF as RFC 6986. Recommended for new hashes.
Prefix
"$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 bits
CPU time cost parameter
1 to 11 (logarithmic)
scrypt
scrypt is a password-based key derivation function created by Colin Per-
cival, 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 bits
CPU time cost parameter
6 to 11 (logarithmic)
bcrypt
A hash based on the Blowfish block cipher, modified to have an extra-ex-
pensive 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 distribu-
tions.
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 "$2x$" pro-
vide bug-compatibility with crypt_blowfish 1.0.4 and earlier, which in-
correctly 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. Ac-
ceptable 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]+\$)?[./0-9A-Za-z]{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. Ac-
ceptable 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]+\$)?[./0-9A-Za-z]{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\$[^$]{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 pos-
sible 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 sup-
ports 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.
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