crypt —
storage
format for hashed passphrases and available hashing methods
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:
- 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.
- It is not possible to tell whether two accounts use the
same passphrase without successfully guessing one of the phrases.
- 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.
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.
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 is a scalable passphrase hashing scheme designed by Solar Designer,
which is based on Colin Percival's scrypt. Recommended for new hashes.
- Prefix
-
- Hashed
passphrase format
-
- 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 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
-
- Hashed
passphrase format
-
- 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 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
-
- Hashed
passphrase format
-
- Maximum
passphrase length
- unlimited
- Hash
size
- 256 bits
- Salt
size
- up to 512 (128+ recommended) bits
- CPU
time cost parameter
- 6 to 11 (logarithmic)
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
-
- Hashed
passphrase format
-
- 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.
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
-
- Hashed
passphrase format
-
- Maximum
passphrase length
- unlimited
- Hash
size
- 512 bits
- Salt
size
- 6 to 96 bits
- CPU
time cost parameter
- 1000 to 999,999,999
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
-
- Hashed
passphrase format
-
- Maximum
passphrase length
- unlimited
- Hash
size
- 256 bits
- Salt
size
- 6 to 96 bits
- CPU
time cost parameter
- 1000 to 999,999,999
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
-
- Hashed
passphrase format
-
- Maximum
passphrase length
- unlimited
- Hash
size
- 160 bits
- Salt
size
- 6 to 384 bits
- CPU
time cost parameter
- 4 to 4,294,967,295
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
-
- Hashed
passphrase format
-
- Maximum
passphrase length
- unlimited
- Hash
size
- 128 bits
- Salt
size
- 48 bits
- CPU
time cost parameter
- 4096 to 4,294,963,199
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
-
- Hashed
passphrase format
-
- Maximum
passphrase length
- unlimited
- Hash
size
- 128 bits
- Salt
size
- 6 to 48 bits
- CPU
time cost parameter
- 1000
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
-
- 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)
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
-
- 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
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
-
- 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
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
-
- Hashed
passphrase format
-
- Maximum
passphrase length
- unlimited
- Hash
size
- 256 bits
- Salt
size
- 0 bits
- CPU
time cost parameter
- 1
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.