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CRYPT(3)		       Library functions		      CRYPT(3)

NAME
       crypt, crypt_r, crypt_rn, crypt_ra, crypt_gensalt, crypt_gensalt_rn,
       crypt_gensalt_ra - password hashing

SYNOPSIS
       #define _XOPEN_SOURCE
       #include <unistd.h>

       char *crypt(const char *key, const char *setting);

       #define _GNU_SOURCE
       #include <crypt.h>

       char *crypt_r(const char *key, const char *setting, struct crypt_data
	       *data);

       #define _OW_SOURCE
       #include <crypt.h>

       char *crypt_rn(const char *key, const char *setting, void *data, int
	       size);
       char *crypt_ra(const char *key, const char *setting, void **data, int
	       *size);
       char *crypt_gensalt(const char *prefix, unsigned long count, const char
	       *input, int size);
       char *crypt_gensalt_rn(const char *prefix, unsigned long count, const
	       char *input, int size, char *output, int output_size);
       char *crypt_gensalt_ra(const char *prefix, unsigned long count, const
	       char *input, int size);

DESCRIPTION
       The crypt, crypt_r, crypt_rn, and crypt_ra functions calculate a	 cryp‐
       tographic  hash function of key with one of a number of supported meth‐
       ods as requested with setting, which is also used to pass  a  salt  and
       possibly	 other	parameters  to the chosen method.  The hashing methods
       are explained below.

       Unlike crypt, the functions crypt_r, crypt_rn and  crypt_ra  are	 reen‐
       trant.	They  place  their result and possibly their private data in a
       data area of size bytes as passed to them by an application  and/or  in
       memory  they allocate dynamically.  Some hashing algorithms may use the
       data area to cache precomputed intermediate values across calls.	 Thus,
       applications  must  properly  initialize the data area before its first
       use.  crypt_r requires that only data->initialized be  reset  to	 zero;
       crypt_rn	 and  crypt_ra	require	 that  either  the entire data area is
       zeroed or, in the case of crypt_ra, *data is NULL.  When called with  a
       NULL  *data  or insufficient *size for the requested hashing algorithm,
       crypt_ra uses realloc(3) to allocate  the  required  amount  of	memory
       dynamically.  Thus, crypt_ra has the additional requirement that *data,
       when non-NULL, must point to an area allocated either with  a  previous
       call to crypt_ra or with a malloc(3) family call.  The memory allocated
       by crypt_ra should be freed with free(3).

       The crypt_gensalt,  crypt_gensalt_rn,  and  crypt_gensalt_ra  functions
       compile	a  string  for use as setting - with the given prefix (used to
       choose a hashing method), the iteration count (if supported by the cho‐
       sen method) and up to size cryptographically random input bytes for use
       as the actual salt.  If count is 0, a low default will be picked.   The
       random  bytes may be obtained from /dev/urandom.	 Unlike crypt_gensalt,
       the functions  crypt_gensalt_rn	and  crypt_gensalt_ra  are  reentrant.
       crypt_gensalt_rn	 places its result in the output buffer of output_size
       bytes.  crypt_gensalt_ra allocates memory for its  result  dynamically.
       The memory should be freed with free(3).

RETURN VALUE
       Upon successful completion, the functions crypt, crypt_r, crypt_rn, and
       crypt_ra return a pointer to a string containing the setting  that  was
       actually used and a printable encoding of the hash function value.  The
       entire 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_gen‐
       salt, crypt_gensalt_rn, and crypt_gensalt_ra.

       The behavior of crypt on errors isn't well standardized.	  Some	imple‐
       mentations  simply  can't  fail (unless the process dies, in which case
       they obviously can't return), others return NULL	 or  a	fixed  string.
       Most  implementations  don't  set  errno, but some do.  SUSv2 specifies
       only returning NULL and setting errno as a valid behavior, and  defines
       only one possible error (ENOSYS, "The functionality is not supported on
       this  implementation.")	 Unfortunately,	 most  existing	  applications
       aren't  prepared	 to  handle  NULL returns from crypt.  The description
       below corresponds to this implementation of crypt and crypt_r only, and
       to  crypt_rn and crypt_ra.  The behavior may change to match standards,
       other implementations or existing applications.

       crypt and crypt_r may only fail (and return) when passed an invalid  or
       unsupported  setting,  in  which	 case they return a pointer to a magic
       string that is shorter than 13 characters and is guaranteed  to	differ
       from  setting.	This  behavior	is  safe  for older applications which
       assume that crypt can't fail,  when  both  setting  new	passwords  and
       authenticating against existing password hashes.	 crypt_rn and crypt_ra
       return NULL to indicate failure.	 All four  functions  set  errno  when
       they fail.

       The  functions  crypt_gensalt,  crypt_gensalt_rn,  and crypt_gensalt_ra
       return a pointer to the compiled string for setting, or NULL  on	 error
       in which case errno is set.

ERRORS
       EINVAL crypt,  crypt_r,	crypt_rn,  crypt_ra: setting is invalid or not
	      supported by this implementation;

	      crypt_gensalt,  crypt_gensalt_rn,	 crypt_gensalt_ra:  prefix  is
	      invalid  or  not	supported  by  this  implementation;  count is
	      invalid for the requested prefix; the input size is insufficient
	      for  the smallest valid salt with the requested prefix; input is
	      NULL.

       ERANGE crypt_rn: the provided data area size is	insufficient  for  the
	      requested hashing algorithm;

	      crypt_gensalt_rn:	 output_size is too small to hold the compiled
	      setting string.

       ENOMEM crypt (original glibc only): failed to allocate memory  for  the
	      output buffer (which subsequent calls would re-use);

	      crypt_ra:	 *data	is  NULL  or  *size  is	 insufficient  for the
	      requested hashing algorithm and realloc(3) failed;

	      crypt_gensalt_ra: failed to allocate  memory  for	 the  compiled
	      setting string.

       ENOSYS crypt (SUSv2): the functionality is not supported on this imple‐
	      mentation;

	      crypt, crypt_r (glibc 2.0 to 2.0.1 only): the  crypt  add-on  is
	      not  compiled  in	 and setting requests something other than the
	      MD5-based algorithm.

       EOPNOTSUPP
	      crypt, crypt_r (glibc 2.0.2 to 2.1.3 only): the crypt add-on  is
	      not  compiled  in	 and setting requests something other than the
	      MD5-based algorithm.

HASHING METHODS
       The implemented hashing methods are intended specifically for  process‐
       ing  user  passwords  for  storage and authentication; they are at best
       inefficient for most other purposes.

       It is important to understand that password hashing is not  a  replace‐
       ment  for strong passwords.  It is always possible for an attacker with
       access to password hashes to try guessing candidate  passwords  against
       the  hashes.  There are, however, certain properties a password hashing
       method may have which make these key search attacks somewhat harder.

       All of the hashing methods use salts such that the same key may produce
       many  possible  hashes.	 Proper	 use  of  salts may defeat a number of
       attacks, including:

       1.     The ability to try candidate passwords against  multiple	hashes
	      at the price of one.

       2.     The use of pre-hashed lists of candidate passwords.

       3.     The  ability  to determine whether two users (or two accounts of
	      one user) have the same or different passwords without  actually
	      having to guess one of the passwords.

       The  key	 search attacks depend on computing hashes of large numbers of
       candidate passwords.  Thus, the computational cost of a	good  password
       hashing	method	must be high - but of course not too high to render it
       impractical.

       All hashing methods implemented within the  crypt,  crypt_r,  crypt_rn,
       and  crypt_ra interfaces use multiple iterations of an underlying cryp‐
       tographic primitive specifically in order to increase the cost of  try‐
       ing a candidate password.  Unfortunately, due to hardware improvements,
       the hashing methods which have a fixed cost  become  increasingly  less
       secure over time.

       In addition to salts, modern password hashing methods accept a variable
       iteration count.	 This makes it possible to adapt  their	 cost  to  the
       hardware improvements while still maintaining compatibility.

       The  following  hashing	methods	 are  or may be implemented within the
       described interfaces:

     Traditional DES-based
       This method is  supported  by  almost  all  implementations  of	crypt.
       Unfortunately,  it  no  longer  offers adequate security because of its
       many limitations.  Thus, it should not be used for new passwords unless
       you  absolutely have to be able to migrate the password hashes to other
       systems.

       prefix "" (empty string);
	      a string matching ^[./0-9A-Za-z]{2} (see regex(7))

       Encoding syntax
	      [./0-9A-Za-z]{13}

       Maximum password length
	      8 (uses 7-bit characters)

       Effective key size
	      up to 56 bits

       Hash size
	      64 bits

       Salt size
	      12 bits

       Iteration count
	      25

     Extended BSDI-style DES-based
       This method is used on BSDI and is also available on at	least  NetBSD,
       OpenBSD, and FreeBSD due to the use of David Burren's FreeSec library.

       prefix "_"

       Encoding syntax
	      _[./0-9A-Za-z]{19}

       Maximum password length
	      unlimited (uses 7-bit characters)

       Effective key size
	      up to 56 bits

       Hash size
	      64 bits

       Salt size
	      24 bits

       Iteration count
	      1 to 2**24-1 (must be odd)

     FreeBSD-style MD5-based
       This  is	 Poul-Henning  Kamp's MD5-based password hashing method origi‐
       nally developed for FreeBSD.  It is currently supported	on  many  free
       Unix-like  systems,  on	Solaris	 10,  and it is a part of the official
       glibc.  Its main disadvantage is the fixed iteration  count,  which  is
       already too low for the currently available hardware.

       prefix "$1$"

       Encoding syntax
	      \$1\$[^$]{1,8}\$[./0-9A-Za-z]{22}

       Maximum password length
	      unlimited (uses 8-bit characters)

       Effective key size
	      limited by the hash size only

       Hash size
	      128 bits

       Salt size
	      6 to 48 bits

       Iteration count
	      1000

     SHA256 based
       This  is	 Ulrich	 Drepper's SHA256-based password hashing method origi‐
       nally developed for Linux.

       prefix "$5$"

       Encoding syntax
	      \$5\$(rounds=[0-9]{1,9}\$)?([./0-9A-Za-z]{1,16})?\$[./0-9A-Za-
	      z]{43}

       Maximum password length
	      unlimited (uses 8-bit characters)

       Effective key size
	      limited by the hash size only

       Hash size
	      256 bits

       Salt size
	      0 to 96 bits

       Iteration count
	      1000 to 999999999 (default 5000)

     SHA512 based
       This  is	 Ulrich	 Drepper's SHA512-based password hashing method origi‐
       nally developed for Linux.

       prefix "$6$"

       Encoding syntax
	      \$6\$(rounds=[0-9]{1,9}\$)?([./0-9A-Za-z]{1,16})?\$[./0-9A-Za-
	      z]{86}

       Maximum password length
	      unlimited (uses 8-bit characters)

       Effective key size
	      limited by the hash size only

       Hash size
	      512 bits

       Salt size
	      0 to 96 bits

       Iteration count
	      1000 to 999999999 (default 5000)

     OpenBSD-style Blowfish-based (bcrypt)
       bcrypt  was originally developed by Niels Provos and David Mazieres for
       OpenBSD and is also supported on recent versions of FreeBSD and NetBSD,
       on  Solaris  10, and on several GNU/*/Linux distributions.  It is, how‐
       ever, not a part of the official glibc.

       While both bcrypt and the BSDI-style DES-based hashing offer a variable
       iteration  count,  bcrypt  may  scale  to even faster hardware, doesn't
       allow for certain optimizations specific	 to  password  cracking	 only,
       doesn't	have the effective key size limitation, and uses 8-bit charac‐
       ters in passwords.

       prefix "$2y$"

       Encoding syntax
	      \$2[axy]\$[0-9]{2}\$[./A-Za-z0-9]{53}

       Maximum password length
	      72 (uses 8-bit characters)

       Effective key size
	      limited by the hash size only

       Hash size
	      184 bits

       Salt size
	      128 bits

       Iteration count
	      2**4 to 2**99 (current implementations are limited to 2**31
	      iterations)

       With  bcrypt,  the count passed to crypt_gensalt, crypt_gensalt_rn, and
       crypt_gensalt_ra is the base-2 logarithm of the actual iteration count.

       bcrypt hashes used the "$2a$" prefix since 1997.	 However, in  2011  an
       implementation  bug  was	 discovered  in crypt_blowfish (versions up to
       1.0.4 inclusive) affecting handling of password characters with the 8th
       bit set.	 Besides fixing the bug, to provide for upgrade strategies for
       existing systems, two new prefixes were introduced: "$2x$", which fully
       re-introduces the bug, and "$2y$", which guarantees correct handling of
       both 7- and 8-bit characters  (same  as	OpenBSD's  "$2a$").   Unfortu‐
       nately,	the behavior of "$2a$" on password characters with the 8th bit
       set has to be considered system-specific.  When generating new password
       hashes, the "$2y$" prefix should be used.  (If such hashes ever need to
       be migrated to a system that does not yet support this new prefix,  the
       prefix  in  migrated  copies  of	 the  already-generated	 hashes may be
       changed to "$2a$".)

       crypt_gensalt,  crypt_gensalt_rn,  and  crypt_gensalt_ra	 support   the
       "$2y$"  and "$2a$" prefixes (the latter for legacy programs or configu‐
       rations), but not "$2x$" (which must  not  be  used  for	 new  hashes).
       crypt,  crypt_r, crypt_rn, and crypt_ra support all three of these pre‐
       fixes.

PORTABILITY NOTES
       Programs using any of these functions on a glibc	 2.x  system  must  be
       linked against libcrypt.	 However, many Unix-like operating systems and
       older versions of the GNU C Library include the crypt function in libc.

       The crypt_r, crypt_rn, crypt_ra, crypt_gensalt,	crypt_gensalt_rn,  and
       crypt_gensalt_ra functions are very non-portable.

       The set of supported hashing methods is implementation-dependent.

CONFORMING TO
       The  crypt  function  conforms to SVID, X/OPEN, and is available on BSD
       4.3.  The strings returned by crypt are not  required  to  be  portable
       among conformant systems.

       crypt_r	is  a GNU extension.  There's also a crypt_r function on HP-UX
       and MKS Toolkit, but the prototypes and semantics differ.

       crypt_gensalt is an Openwall extension.	There's also  a	 crypt_gensalt
       function on Solaris 10, but the prototypes and semantics differ.

       crypt_rn, crypt_ra, crypt_gensalt_rn, and crypt_gensalt_ra are Openwall
       extensions.

HISTORY
       A rotor-based crypt function appeared in	 Version  6  AT&T  UNIX.   The
       "traditional" crypt first appeared in Version 7 AT&T UNIX.

       The crypt_r function was introduced during glibc 2.0 development.

BUGS
       The  return  values  of crypt and crypt_gensalt point to static buffers
       that are overwritten by subsequent  calls.   These  functions  are  not
       thread-safe.  (crypt on recent versions of Solaris uses thread-specific
       data and actually is thread-safe.)

       The strings returned by certain other implementations of crypt on error
       may  be	stored	in read-only locations or only initialized once, which
       makes it unsafe to always attempt  to  zero  out	 the  buffer  normally
       pointed	to  by the crypt return value as it would otherwise be prefer‐
       able for security reasons.  The problem could be avoided with  the  use
       of  crypt_r,  crypt_rn, or crypt_ra where the application has full con‐
       trol over output buffers of these functions (and	 often	over  some  of
       their  private  data  as	 well).	  Unfortunately,  the functions aren't
       (yet?) available on platforms where crypt has this undesired property.

       Applications using the thread-safe crypt_r  need	 to  allocate  address
       space  for  the	large (over 128 KB) struct crypt_data structure.  Each
       thread needs a separate instance of the structure.  The crypt_r	inter‐
       face  makes  it impossible to implement a hashing algorithm which would
       need to keep an even larger amount of private  data,  without  breaking
       binary  compatibility.	crypt_ra allows for dynamically increasing the
       allocation size as required by the hashing algorithm that  is  actually
       used.  Unfortunately, crypt_ra is even more non-portable than crypt_r.

       Multi-threaded  applications or library functions which are meant to be
       thread-safe should use crypt_gensalt_rn or crypt_gensalt_ra rather than
       crypt_gensalt.

SEE ALSO
       login(1),  passwd(1), crypto(3), encrypt(3), free(3), getpass(3), getp‐
       went(3),	 malloc(3),  realloc(3),  shadow(3),   passwd(5),   shadow(5),
       regex(7), pam(8)

       Niels  Provos  and David Mazieres.  A Future-Adaptable Password Scheme.
       Proceedings of the 1999 USENIX Annual Technical Conference, June 1999.
       http://www.usenix.org/events/usenix99/provos.html

       Robert Morris and Ken Thompson.	Password  Security:  A	Case  History.
       Unix Seventh Edition Manual, Volume 2, April 1978.
       http://plan9.bell-labs.com/7thEdMan/vol2/password

Openwall Project		 July 16, 2011			      CRYPT(3)
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