CRYPT
Section: File Formats (5)
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BSD mandoc
Openwall Project
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 genera-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:
-
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.
Most 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
``processing cost''
parameter to
crypt_gensalt3.
For memor-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 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_gensalt3
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
Fa 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 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 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 bas-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),
roughly 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.
While yescrypt's strength against password guessing attacks comes from its
algorithm design, its cryptographic security is guaranteed by its use of
SH-256 on the outer layer.
The SH-256 hash function has been published by NIST in FIPS PUB 18-2
(and its subsequent revisions such as FIPS PUB 18-4)
and by the IETF as RFC 4634 (and subsequently RFC 6234).
Recommended for new hashes.
- Prefix
-
Bf "sh"
Ef
- Hashed passphrase format
-
Bf -literal
$y$
Ef
- Maximum passphrase length
-
$y$[./-Z-z-9]+$[./-Z-z-9]{,86}$[./-Z-z-9]{43} characters
- Hash size
-
256 bits
- Effective key size
-
8 bits
- Salt size
-
256 bits
- Processing cost parameter
-
up to 512 (128+ recommended)
gos-yescrypt
gos-yescrypt uses the output from yescrypt as an input message to HMAC with
the GOST R 34.1-2012 (Streebog) hash function with a 25-bit digest.
Thus, yescrypt's cryptographic properties are superseded by those of the GOST
hash function.
This hashing method is useful in applications that need modern passphrase
hashing, but have to rely on GOST algorithms.
The GOST R 34.1-2012 (Streebog) hash function has been published by the IETF
as RFC 6986.
Acceptable for new hashes where required.
- Prefix
-
Bf "sh"
Ef
- Hashed passphrase format
-
Bf -literal
$gy$
Ef
- Maximum passphrase length
-
$gy$[./-Z-z-9]+$[./-Z-z-9]{,86}$[./-Z-z-9]{43} characters
- Hash size
-
256 bits
- Effective key size
-
8 bits
- Salt size
-
256 bits
- Processing cost parameter
-
up to 512 (128+ recommended)
scrypt
scrypt is a passwor-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
larg-scale custom hardware attacks by requiring large amounts of memory.
In 2016, the scrypt algorithm was published by IETF as RFC 7914.
- Prefix
-
Bf "sh"
Ef
- Hashed passphrase format
-
Bf -literal
$7$
Ef
- Maximum passphrase length
-
$7$[./-Z-z-9]{11,97}$[./-Z-z-9]{43} characters
- Hash size
-
256 bits
- Effective key size
-
8 bits
- Salt size
-
256 bits
- Processing cost parameter
-
up to 512 (128+ recommended)
bcrypt
A hash based on the Blowfish block cipher,
modified to have an extr-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
-
Bf "sh"
Ef
- Hashed passphrase format
-
Bf -literal
$2b$
Ef
- Maximum passphrase length
-
$2[abxy]$[-9]{2}$[./-Z-z-9]{53} characters
- Hash size
-
184 bits
- Effective key size
-
8 bits
- Salt size
-
184 bits
- Processing cost parameter
-
128
The alternative prefix "$2y$" is equivalent to "$2b$".
It exists for historical reasons only.
The alternative prefixes "$2a$" and "$2x$"
provide bu-compatibility with crypt_blowfish 1.0.4 and earlier,
which incorrectly processed characters with the 8th bit set.
sha512crypt
A hash based on SH-2 with 51-bit output,
originally developed by Ulrich 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
-
Bf "sh"
Ef
- Hashed passphrase format
-
Bf -literal
$6$
Ef
- Maximum passphrase length
-
$6$(rounds=[-9][-9]+$)?[^$:rsn]{1,16}$[./-9-Z-z]{86} characters
- Hash size
-
512 bits
- Effective key size
-
8 bits
- Salt size
-
512 bits
- Processing cost parameter
-
6 to 96
sha256crypt
A hash based on SH-2 with 25-bit output,
originally developed by Ulrich 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
-
Bf "sh"
Ef
- Hashed passphrase format
-
Bf -literal
$5$
Ef
- Maximum passphrase length
-
$5$(rounds=[-9][-9]+$)?[^$:rsn]{1,16}$[./-9-Z-z]{43} characters
- Hash size
-
256 bits
- Effective key size
-
8 bits
- Salt size
-
256 bits
- Processing cost parameter
-
6 to 96
sha1crypt
A hash based on HMA-SHA1.
Originally developed by Simon Gerraty for NetBSD.
Not as weak as the DE-based hashes below,
but SH-1 is so cheap on modern hardware
that it should not be used for new hashes.
- Prefix
-
Bf "sh"
Ef
- Hashed passphrase format
-
Bf -literal
$sha1
Ef
- Maximum passphrase length
-
$sha1$[-9][-9]+$[./-9-Z-z]{1,64}$[./-9-Z-z]{8,64}[./-9-Z-z]{32} characters
- Hash size
-
160 bits
- Effective key size
-
8 bits
- Salt size
-
160 bits
- Processing cost parameter
-
6 to 384
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 DE-based hashes below,
but MD5 is so cheap on modern hardware
that it should not be used for new hashes.
- Prefix
-
Bf "sh"
Ef
- Hashed passphrase format
-
Bf -literal
$md5
Ef
- Maximum passphrase length
-
$md5(,rounds=[-9][-9]+)?$[./-9-Z-z]{8}${1,2}[./-9-Z-z]{22} characters
- Hash size
-
128 bits
- Effective key size
-
8 bits
- Salt size
-
128 bits
- Processing cost parameter
-
48
md5crypt
A hash based on the MD5 algorithm, originally developed by
Pou-Henning Kamp for FreeBSD.
Supported on most free Unixes and newer versions of Solaris.
Not as weak as the DE-based hashes below,
but MD5 is so cheap on modern hardware
that it should not be used for new hashes.
Processing cost is not adjustable.
- Prefix
-
Bf "sh"
Ef
- Hashed passphrase format
-
Bf -literal
$1$
Ef
- Maximum passphrase length
-
$1$[^$:rsn]{1,8}$[./-9-Z-z]{22} characters
- Hash size
-
128 bits
- Effective key size
-
8 bits
- Salt size
-
128 bits
- Processing cost parameter
-
6 to 48
bsdicrypt (BSDI extended DES)
An extension of traditional DES,
which eliminates the length limit,
increases 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
-
Bf "sh"
Ef
- Hashed passphrase format
-
Bf -literal
_
Ef
- Maximum passphrase length
-
_[./-9-Z-z]{19} characters
- Hash size
-
up to 56 bits
- Effective key size
-
7 bits
- Salt size
-
64 bits
- Processing cost parameter
-
24
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
-
Bf "sh"
Ef
- Hashed passphrase format
-
Bf -literal
Ef
- Maximum passphrase length
-
[./-9-Z-z]{13} characters
- Hash size
-
up to 56 bits
- Effective key size
-
7 bits
- Salt size
-
64 bits
- Processing cost parameter
-
12
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
-
Bf "sh"
Ef
- Hashed passphrase format
-
Bf -literal
Ef
- Maximum passphrase length
-
[./-9-Z-z]{13,178} characters
- Hash size
-
up to 56 bits
- Effective key size
-
7 bits
- Salt size
-
up to 1024 bits
- Processing cost parameter
-
12
NT
The hashing method used for network authentication
in some versions of the SMB/CIFS protocol.
Available, for cros-compatibility's sake, on FreeBSD.
Based on MD4.
Has no salt or tunable cost parameter.
It is so weak that almost
any
huma-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
-
Bf "sh"
Ef
- Hashed passphrase format
-
Bf -literal
$3$
Ef
- Maximum passphrase length
-
$3$$[-9-f]{32} characters
- Hash size
-
256 bits
- Effective key size
-
8 bits
- Salt size
-
256 bits
- Processing cost parameter
-
0
SEE ALSO
crypt(3),
crypt_gensalt3,
getpwent(3),
passwd(5),
shadow(5),
pam(8)
-
Niels Provos
David Mazieres
A Futur-Adaptable Password Scheme
Proceedings of the 1999 USENIX Annual Technical Conference
June 1999
-
Robert Morris
Ken Thompson
Password Security: A Case History
Communications of the ACM
22
11
1979
Index
- NAME
-
- DESCRIPTION
-
- FORMAT OF HASHED PASSPHRASES
-
- AVAILABLE HASHING METHODS
-
- yescrypt
-
- gost-yescrypt
-
- scrypt
-
- bcrypt
-
- sha512crypt
-
- sha256crypt
-
- sha1crypt
-
- SunMD5
-
- md5crypt
-
- bsdicrypt (BSDI extended DES)
-
- descrypt (Traditional DES)
-
- bigcrypt
-
- NT
-
- SEE ALSO
-
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