CRYPT(5) 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.
Most 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 impracti-
cally large.
Most 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 “processing cost” parameter
to crypt_gensalt(3). For memory-hard hashing methods such as yescrypt,
this parameter also adjusts the amount of memory needed to compute a
hash. Having this configurable makes it possible to keep password
guessing attacks against the hashes slow and costly as hardware im-
proves.
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 hashed passphrase as the setting, and then compare the result to
the stored hashed 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. ‘$’ characters usually delimit components, and the salt and
hash are usually encoded as numerals in base 64. The details of this
base-64 encoding vary among hashing methods. The common “base64” encod-
ing specified by RFC 4648 is usually not used.
AVAILABLE HASHING METHODS
This is a list of all the hashing methods supported by crypt(3), roughly
in decreasing order of strength. Many of the older methods are now con-
sidered too weak to use for new passphrases. The hashed passphrase for-
mat is expressed with extended regular expressions (see regex(7)) and
does not show the division into prefix, options, salt, and hash.
yescrypt
yescrypt is a scalable passphrase hashing scheme designed by Solar De-
signer, which is based on Colin Percival's scrypt. While yescrypt's
strength against password guessing attacks comes from its algorithm de-
sign, its cryptographic security is guaranteed by its use of SHA-256 on
the outer layer. The SHA-256 hash function has been published by NIST
in FIPS PUB 180-2 (and its subsequent revisions such as FIPS PUB 180-4)
and by the IETF as RFC 4634 (and subsequently RFC 6234). Recommended
for new hashes.
Prefix
"$y$"
Hashed passphrase format
\$y\$[./A-Za-z0-9]+\$[./A-Za-z0-9]{,86}\$[./A-Za-z0-9]{43}
Maximum passphrase length
unlimited
Hash size
256 bits
Salt size
up to 512 (128+ recommended) bits
Processing cost parameter
1 to 11 (logarithmic, also affects memory usage)
gost-yescrypt
gost-yescrypt uses the output from yescrypt as an input message to HMAC
with the GOST R 34.11-2012 (Streebog) hash function with a 256-bit di-
gest. Thus, yescrypt's cryptographic properties are superseded by those
of the GOST hash function. This hashing method is useful in applica-
tions that need modern passphrase hashing, but have to rely on GOST al-
gorithms. The GOST R 34.11-2012 (Streebog) hash function has been pub-
lished by the IETF as RFC 6986. Acceptable for new hashes where re-
quired.
Prefix
"$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
Processing cost parameter
1 to 11 (logarithmic, also affects memory usage)
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 cus-
tom 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
Processing cost parameter
6 to 11 (logarithmic, also affects memory usage)
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 distrib-
utions.
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
Processing 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 Ul-
rich Drepper for GNU libc. Supported on Linux but not common elsewhere.
Acceptable for new hashes. The default processing 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
Processing cost parameter
1000 to 999,999,999
sha256crypt
A hash based on SHA-2 with 256-bit output, originally developed by Ul-
rich Drepper for GNU libc. Supported on Linux but not common elsewhere.
Acceptable for new hashes. The default processing cost parameter is
5000, which is too low for modern hardware.
Prefix
"$5$"
Hashed passphrase format
\$5\$(rounds=[1-9][0-9]+\$)?[^$:\n]{1,16}\$[./0-9A-Za-z]{43}
Maximum passphrase length
unlimited
Hash size
256 bits
Salt size
6 to 96 bits
Processing cost parameter
1000 to 999,999,999
sha1crypt
A hash based on HMAC-SHA1. Originally developed by Simon Gerraty for
NetBSD. Not as weak as the DES-based hashes below, but SHA-1 is so
cheap on modern hardware that it should not be used for new hashes.
Prefix
"$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
Processing cost parameter
4 to 4,294,967,295
SunMD5
A hash based on the MD5 algorithm, originally developed by Alec David
Muffett for Solaris. Not adopted elsewhere, to our knowledge. Not as
weak as the DES-based hashes below, but MD5 is so cheap on modern hard-
ware 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
Processing 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. Process-
ing 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
Processing cost parameter
1000
bsdicrypt (BSDI extended DES)
An extension of traditional DES, which eliminates the length limit, in-
creases the salt size, and makes the time cost tunable. It originates
with BSDI BSD/OS and is also available on at least NetBSD, OpenBSD, and
FreeBSD due to the use of David Burren's FreeSec library. It is much
better than traditional DES and bigcrypt, 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
up to 56 bits
Salt size
24 bits
Processing cost parameter
1 to 16,777,215 (must be odd)
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 distinct passphrases, which it truncates 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
up to 56 bits
Salt size
12 bits
Processing cost parameter
25
bigcrypt
A weak extension of traditional DES, available on some commercial
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 separately and in parallel, which may make guessing
even easier than for traditional DES above. 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 56 bits
Salt size
12 bits
Processing 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. It is
so weak that almost any human-chosen 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
Processing 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 March 27, 2024 CRYPT(5)
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