CRYPT(5) File Formats and Conversions 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 ran-
dom 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. Tables of precalculated hashes of commonly used passphrases must
have an entry for each possible salt, which makes them impracti-
cally large.
3. It is not possible to tell whether two accounts use the same
passphrase without successfully guessing one of the phrases.
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" parame-
ter 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 libcrypt 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 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 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 hash-
ing method. `$' characters usually delimit components, and the salt
and hash are usually encoded as numerals in base 64. However, the
details of the base-64 encoding vary among hashing methods and are usu-
ally not compatible with the common "base64" encoding.
AVAILABLE HASHING METHODS
This is a list of all the hashing methods supported by libcrypt, in
decreasing order of strength. Many of the older methods are now con-
sidered too weak to use for new passphrases. The encoded passphrase
format is expressed with extended regular expressions (see regex(7))
and does not show the division into prefix, options, salt, and hash.
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 distri-
butions. Recommended for new password hashes.
prefix "$2b$"
Encoded passphrase format
\$2[abxy]\$[0-9]{2}\$[./A-Za-z0-9]{53}
Maximum password 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.
SHA-2-512
A hash based on SHA-2 with 512-bit output, originally developed by
Ulrich Drepper for GNU libc. Supported on Linux but not common else-
where. Acceptable for new password hashes. The default CPU time cost
parameter is 5000, which is too low for modern hardware.
prefix "$6$"
Encoded passphrase format
\$6\$(rounds=[1-9][0-9]+\$)?[./0-9A-Za-z]{1,16}\$[./0-9A-Za-
z]{86}
Maximum password length
unlimited
Hash size
512 bits
Salt size
6 to 96 bits
CPU time cost parameter
1000 to 999,999,999
SHA-2-256
A hash based on SHA-2 with 256-bit output, originally developed by
Ulrich Drepper for GNU libc. Supported on Linux but not common else-
where. Acceptable for new password hashes. The default CPU time cost
parameter is 5000, which is too low for modern hardware.
prefix "$5$"
Encoded passphrase format
\$5\$(rounds=[1-9][0-9]+\$)?[./0-9A-Za-z]{1,16}\$[./0-9A-Za-
z]{43}
Maximum password length
unlimited
Hash size
256 bits
Salt size
6 to 96 bits
CPU time cost parameter
1000 to 999,999,999
SHA-1
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 "$sha1"
Encoded passphrase format
\$sha1\$[1-9][0-9]+\$[./0-9A-Za-z]{1,64}\$[./0-9A-Za-
z]{8,64}[./0-9A-Za-z]{32}
Maximum password length
unlimited
Hash size
160 bits
Salt size
6 to 384 bits
CPU time cost parameter
1 to 4,294,967,295
MD5 (Sun)
A hash based on the MD5 algorithm, with additional cleverness to make
precomputation difficult, originally developed by Alec David Muffet for
Solaris. Not adopted elsewhere, to our knowledge. Not as weak as the
DES-based hashes below, but MD5 is so cheap on modern hardware that it
should not be used for new hashes.
prefix "$md5"
Encoded passphrase format
\$md5(,rounds=[1-9][0-9]+)?\$[./0-9A-Za-z]{8}\${1,2}[./0-9A-Za-
z]{22}
Maximum password length
unlimited
Hash size
128 bits
Salt size
48 bits
CPU time cost parameter
4096 to 4,294,963,199
MD5 (FreeBSD)
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 "$1$"
Encoded passphrase format
\$1\$[^$]{1,8}\$[./0-9A-Za-z]{22}
Maximum password length
unlimited
Hash size
128 bits
Salt size
6 to 48 bits
CPU time cost parameter
1000
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 origi-
nates 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 "_"
Encoded passphrase format
_[./0-9A-Za-z]{19}
Maximum password length
unlimited (ignores 8th bit)
Hash size
64 bits (effectively 56)
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 charac-
ters, 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)
Encoded passphrase format
[./0-9A-Za-z]{13,178}
Maximum password length
128 characters (ignores 8th bit)
Hash size
up to 1024 bits (effectively up to 896)
Salt size
12 bits
CPU time cost parameter
25
Traditional DES-based
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)
Encoded passphrase format
[./0-9A-Za-z]{13}
Maximum password length
8 characters (ignores 8th bit)
Hash size
64 bits (effectively 56)
Salt size
12 bits
CPU time cost parameter
25
NTHASH
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 "$3$"
Encoded passphrase format
\$3\$\$[0-9a-f]{32}
Maximum password length
unlimited
Hash size
256 bits
Salt size
0 bits
CPU time cost parameter
1
SEE ALSO
crypt(3), crypt_r(3), crypt_ra(3), crypt_rn(3), crypt_gensalt(3), getp-
went(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, June 1999.
https://www.usenix.org/events/usenix99/provos.html
Robert Morris and Ken Thompson. Password Security: A Case History.
Communications of the ACM, Volume 22, Issue 11, 1979.
http://wolfram.schneider.org/bsd/7thEdManVol2/password/password.pdf
Openwall Project October 11, 2017 CRYPT(5)
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