NAME

       crypt_gensalt, crypt_gensalt_rn, crypt_gensalt_ra — encode settings for passphrase hashing

LIBRARY

       Crypt Library (libcrypt, -lcrypt)

SYNOPSIS

       #include <crypt.h>

       char *
       crypt_gensalt(const char *prefix, unsigned long count, const char *rbytes, int nrbytes);

       char *
       crypt_gensalt_rn(const char * prefix,      unsigned long count,      const char *rbytes,     int nrbytes,
           char * output, int output_size);

       char *
       crypt_gensalt_ra(const char *prefix, unsigned long count, const char *rbytes, int nrbytes);

DESCRIPTION

       The crypt_gensalt, crypt_gensalt_rn, and crypt_gensalt_ra functions compile  a  string  for  use  as  the
       setting  argument  to  crypt, crypt_r, crypt_rn, and crypt_ra.  prefix selects the hashing method to use.
       count controls the CPU time cost of the hash; the valid range for count and the  exact  meaning  of  “CPU
       time  cost”  depends  on the hashing method, but larger numbers correspond to more costly hashes.  rbytes
       should point to nrbytes cryptographically random bytes for use as “salt.”

       If prefix is a null pointer, the best available hashing method will be selected.  (CAUTION: if prefix  is
       an empty string, the “traditional” DES-based hashing method will be selected; this method is unacceptably
       weak  by  modern  standards.)   If  count is 0, a low default cost will be selected.  If rbytes is a null
       pointer, an appropriate number of random bytes will be obtained from the operating system, and nrbytes is
       ignored.

       See crypt(5) for other strings that can be used as prefix, and valid values of count for each.

RETURN VALUES

       crypt_gensalt, crypt_gensalt_rn, and crypt_gensalt_ra return a pointer  to  an  encoded  setting  string.
       This string will be entirely printable ASCII, and will not contain whitespace or the characters ‘:’, ‘;’,
       ‘*’,  ‘!’, or ‘\’.  See crypt(5) for more detail on the format of this string.  Upon error, they return a
       null pointer and set errno to an appropriate error code.

       crypt_gensalt places its result in a static storage area, which will be overwritten by  subsequent  calls
       to  crypt_gensalt.   It is not safe to call crypt_gensalt from multiple threads simultaneously.  However,
       it is safe to pass the string returned by crypt_gensalt  directly  to  crypt  without  copying  it;  each
       function has its own static storage area.

       crypt_gensalt_rn  places its result in the supplied output buffer, which has output_size bytes of storage
       available.  output_size should be greater than or equal to CRYPT_GENSALT_OUTPUT_SIZE.

       crypt_gensalt_ra allocates memory for its result using malloc(3).  It should be freed with free(3)  after
       use.

       Upon  error,  in  addition  to returning a null pointer, crypt_gensalt and crypt_gensalt_rn will write an
       invalid setting string to their output buffer, if there is enough space; this string will  begin  with  a
       ‘*’ and will not be equal to prefix.

ERRORS

       EINVAL             prefix  is  invalid  or not supported by this implementation; count is invalid for the
                          requested prefix; the input nrbytes is insufficient for the smallest valid  salt  with
                          the requested prefix.

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

       ENOMEM             Failed to allocate internal scratch memory.
                          crypt_gensalt_ra only: failed to allocate memory for the compiled setting string.

       ENOSYS, EACCES, EIO, etc.
                          Obtaining  random  bytes  from the operating system failed.  This can only happen when
                          rbytes is a null pointer.

FEATURE TEST MACROS

       The following macros are defined by <crypt.h>:

       CRYPT_GENSALT_IMPLEMENTS_DEFAULT_PREFIX
               A null pointer can be specified as the prefix argument.

       CRYPT_GENSALT_IMPLEMENTS_AUTO_ENTROPY
               A null pointer can be specified as the rbytes argument.

PORTABILITY NOTES

       The functions crypt_gensalt, crypt_gensalt_rn, and crypt_gensalt_ra are not part of any  standard.   They
       originate  with  the  Openwall project.  A function with the name crypt_gensalt also exists on Solaris 10
       and newer, but its prototype and semantics differ.

       The default prefix and auto entropy features are  available  since  libxcrypt  version  4.0.0.   Portable
       software  can  use  feature  test macros to find out whether null pointers can be used for the prefix and
       rbytes arguments.

       The set of supported hashing methods varies considerably from system to system.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).
       ┌───────────────────┬───────────────┬──────────────────────────────┐
       │ Interface         │ Attribute     │ Value                        │
       ├───────────────────┼───────────────┼──────────────────────────────┤
       │ crypt_gensalt     │ Thread safety │ MT-Unsafe race:crypt_gensalt │
       ├───────────────────┼───────────────┼──────────────────────────────┤
       │ crypt_gensalt_rn, │ Thread safety │ MT-Safe                      │
       │ crypt_gensalt_ra  │               │                              │
       └───────────────────┴───────────────┴──────────────────────────────┘

SEE ALSO

       crypt(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_GENSALT(3)