CRYPT_RN(3) Library functions CRYPT_RN(3)
NAME
crypt, crypt_r, crypt_rn, crypt_ra - passphrase hashing
SYNOPSIS
#include <crypt.h>
char *crypt(const char *phrase, const char *setting);
char *crypt_r(const char *phrase, const char *setting, struct
crypt_data *data);
char *crypt_rn(const char *phrase, const char *setting, void *data, int
size);
char *crypt_ra(const char *phrase, const char *setting, void **data,
int *size);
Link with -lcrypt.
DESCRIPTION
The crypt, crypt_r, crypt_rn, and crypt_ra functions irreversibly
"hash" phrase for storage in the system password database (shadow(5))
using a cryptographic "hashing method." The result of this operation
is called a "hashed passphrase" or just a "hash." Hashing methods are
described in crypt(5).
setting controls which hashing method to use, and also supplies various
parameters to the chosen method, most importantly a random "salt" which
ensures that no two stored hashes are the same, even if the phrase
strings are the same. The hashing methods are explained below.
The crypt_data structure passed to crypt_r has at least these fields:
struct crypt_data {
char output[CRYPT_OUTPUT_SIZE];
char setting[CRYPT_OUTPUT_SIZE];
char phrase[CRYPT_MAX_PASSPHRASE_SIZE];
char initialized;
};
Upon a successful return from crypt_r, the hashed passphrase will be
stored in output. Applications are encouraged, but not required, to
use the setting and phrase fields to store the strings that they will
pass as phrase and setting to crypt_r. This will make it easier to
erase all sensitive data after it is no longer needed.
The initialized field must be set to zero before the first time a
crypt_data object is first used in a call to crypt_r. We recommend
zeroing the entire crypt_data object, not just initialized and not just
the documented fields, before the first use. (Of course, do this
before storing anything in setting and phrase.)
The data argument to crypt_rn should also point to a crypt_data object,
and size should be the size of that object, cast to int. When used
with crypt_rn, the entire crypt_data object must be zeroed before its
first use; this is not just a recommendation, as it is for crypt_r.
(setting and phrase are still allowed to be used.) Otherwise, the
fields of the object have the same uses that they do for crypt_r.
On the first call to crypt_ra, data should be the address of a void *
variable set to NULL, and size should be the address of an int variable
set to zero. crypt_ra will allocate and initialize a crypt_data
object, using malloc(3), and write its address and size into *data and
*size. These can be reused in subsequent calls. After the application
is done hashing passphrases, it should deallocate *data using free(3).
RETURN VALUE
Upon successful completion, crypt, crypt_r, crypt_rn, and crypt_ra
return a pointer to a string which encodes both the hashed passphrase,
and the settings that were used to encode it. This string is directly
usable as setting with other calls to crypt, crypt_r, crypt_rn, and
crypt_ra, and as prefix with calls to crypt_gensalt, crypt_gensalt_rn,
and crypt_gensalt_ra. It will be entirely printable ASCII, and will
not contain whitespace or the characters `:', `;', `*', `!', or `\'.
See crypt(5) for more detail on the format of hashed passphrases.
crypt places its result in a static storage area, which will be over-
written by subsequent calls to crypt. It is not safe to call crypt
from multiple threads simultaneously.
crypt_r, crypt_rn, and crypt_ra place their result in the output field
of the crypt_data object that they are supplied with; it is safe to
call them from multiple threads simultaneously, as long as a separate
crypt_data object is used for each thread.
Upon error, crypt_r, crypt_rn, and crypt_ra write an invalid hashed
passphrase to the output field of their crypt_data object, and crypt
writes an invalid hash to its static storage area. This string will be
shorter than 13 characters, will begin with a `*', and will not compare
equal to setting.
Upon error, crypt_rn and crypt_ra return a null pointer. crypt_r and
crypt may also return a null pointer, or they may return a pointer to
the invalid hash, depending on how libcrypt was configured. (The
option to return the invalid hash is for compatibility with old appli-
cations that assume that crypt cannot return a null pointer. See
PORTABILITY NOTES below.)
All four functions set errno when they fail.
ERRORS
EINVAL setting is invalid, or requests a hashing method that is not
supported.
ERANGE crypt_rn only: size is too small for the hashing method
requested by setting.
ENOMEM Failed to allocate internal scratch memory.
crypt_ra only: failed to allocate memory for *data.
ENOSYS or EOPNOTSUPP
Hashing passphrases is not supported at all on this installa-
tion, or the hashing method requested by setting is not sup-
ported. These error codes are not used by this version of
libcrypt, but may be encountered on other systems.
PORTABILITY NOTES
crypt is included in POSIX, but crypt_r, crypt_rn, and crypt_ra are not
part of any standard.
POSIX does not specify any hashing methods, and does not require hashed
passphrases to be portable between systems. In practice, hashed
passphrases are portable as long as both systems support the hashing
method that was used. However, the set of supported hashing methods
varies considerably from system to system.
The behavior of crypt on errors isn't well standardized. Some imple-
mentations simply can't fail (except by crashing the program), others
return a null pointer or a fixed string. Most implementations don't
set errno, but some do. POSIX specifies returning a null pointer and
setting errno, but it defines only one possible error, ENOSYS, in the
case where crypt is not supported at all. Many existing applications
are not prepared to handle null pointers returned by crypt. The behav-
ior described above for this implementation, setting errno and return-
ing an invalid hashed passphrase different from setting, is chosen to
make these applications fail closed when an error occurs.
Due to historical restrictions on the export of cryptographic software
from the USA, crypt is an optional POSIX component. Applications
should therefore be prepared for crypt not to be available, or to
always fail (setting errno to ENOSYS) at runtime.
POSIX specifies that crypt is declared in unistd.h, but only if the
macro _XOPEN_CRYPT is defined and has a value greater than or equal to
zero. Since libcrypt does not provide unistd.h, it declares crypt,
crypt_r, crypt_rn, and crypt_ra in crypt.h instead.
On a minority of systems (notably recent versions of Solaris), crypt
uses a thread-specific static storage buffer, which makes it safe to
call from multiple threads simultaneously, but does not prevent each
call within a thread from overwriting the results of the previous one.
BUGS
Some implementations of crypt, upon error, return an invalid hash that
is stored in a read-only location or only initialized once, which means
that it is only safe to erase the buffer pointed to by the crypt return
value if an error did not occur.
struct crypt_data may be quite large (32kB in this implementation of
libcrypt; over 128kB in some other implementations). This is large
enough that it may be unwise to allocate it on the stack.
Some recently designed hashing methods need even more scratch memory,
but the crypt_r interface makes it impossible to change the size of
crypt_data without breaking binary compatibility. The crypt_rn inter-
face could accommodate larger allocations for specific hashing methods,
but the caller of crypt_rn has no way of knowing how much memory to
allocate. crypt_ra does the allocation itself, but can only make a
single call to malloc(3).
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
+-------------------+---------------+----------------------+
|Interface | Attribute | Value |
+-------------------+---------------+----------------------+
|crypt | Thread safety | MT-Unsafe race:crypt |
+-------------------+---------------+----------------------+
|crypt_r, crypt_rn, | Thread safety | MT-Safe |
|crypt_ra | | |
+-------------------+---------------+----------------------+
HISTORY
A rotor-based crypt function appeared in Version 6 AT&T UNIX. The
"traditional" DES-based crypt first appeared in Version 7 AT&T UNIX.
crypt_r originates with the GNU C Library. There's also a crypt_r
function on HP-UX and MKS Toolkit, but the prototypes and semantics
differ.
crypt_rn and crypt_ra originate with the Openwall project.
SEE ALSO
crypt_gensalt(3), getpass(3), getpwent(3), shadow(3), login(1),
passwd(1), crypt(5), passwd(5), shadow(5), pam(8)
Openwall Project October 11, 2017 CRYPT_RN(3)
CRYPT(3P) POSIX Programmer's Manual CRYPT(3P)
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
NAME
crypt -- string encoding function (CRYPT)
SYNOPSIS
#include <unistd.h>
char *crypt(const char *key, const char *salt);
DESCRIPTION
The crypt() function is a string encoding function. The algorithm is
implementation-defined.
The key argument points to a string to be encoded. The salt argument
shall be a string of at least two bytes in length not including the
null character chosen from the set:
a b c d e f g h i j k l m n o p q r s t u v w x y z
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
0 1 2 3 4 5 6 7 8 9 . /
The first two bytes of this string may be used to perturb the encoding
algorithm.
The return value of crypt() points to static data that is overwritten
by each call.
The crypt() function need not be thread-safe.
RETURN VALUE
Upon successful completion, crypt() shall return a pointer to the
encoded string. The first two bytes of the returned value shall be
those of the salt argument. Otherwise, it shall return a null pointer
and set errno to indicate the error.
ERRORS
The crypt() function shall fail if:
ENOSYS The functionality is not supported on this implementation.
The following sections are informative.
EXAMPLES
Encoding Passwords
The following example finds a user database entry matching a particular
user name and changes the current password to a new password. The
crypt() function generates an encoded version of each password. The
first call to crypt() produces an encoded version of the old password;
that encoded password is then compared to the password stored in the
user database. The second call to crypt() encodes the new password
before it is stored.
The putpwent() function, used in the following example, is not part of
POSIX.1-2008.
#include <unistd.h>
#include <pwd.h>
#include <string.h>
#include <stdio.h>
...
int valid_change;
int pfd; /* Integer for file descriptor returned by open(). */
FILE *fpfd; /* File pointer for use in putpwent(). */
struct passwd *p;
char user[100];
char oldpasswd[100];
char newpasswd[100];
char savepasswd[100];
...
valid_change = 0;
while ((p = getpwent()) != NULL) {
/* Change entry if found. */
if (strcmp(p->pw_name, user) == 0) {
if (strcmp(p->pw_passwd, crypt(oldpasswd, p->pw_passwd)) == 0) {
strcpy(savepasswd, crypt(newpasswd, user));
p->pw_passwd = savepasswd;
valid_change = 1;
}
else {
fprintf(stderr, "Old password is not valid\n");
}
}
/* Put passwd entry into ptmp. */
putpwent(p, fpfd);
}
APPLICATION USAGE
The values returned by this function need not be portable among XSI-
conformant systems.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
encrypt(), setkey()
The Base Definitions volume of POSIX.1-2008, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 CRYPT(3P)
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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