man > crypt_ra(3)

CRYPT(3)                               Library Functions Manual                             CRYPT(3)

NAME
       crypt, crypt_r, crypt_rn, crypt_ra — passphrase hashing

LIBRARY
       Crypt Library (libcrypt, -lcrypt)

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, struct crypt_data *data, int size);

       char *
       crypt_ra(const char *phrase, const char *setting, void **data, int *size);

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  re‐
       sult  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 data argument to crypt_r is a structure of type struct crypt_data.  It has at least these
       fields:

             struct crypt_data {
                 char output[CRYPT_OUTPUT_SIZE];
                 char setting[CRYPT_OUTPUT_SIZE];
                 char input[CRYPT_MAX_PASSPHRASE_SIZE];
                 char initialized;
             };

       Upon a successful return from crypt_r, the hashed passphrase will be stored in  output.   Ap‐
       plications are encouraged, but not required, to use the input and setting fields to store the
       strings that they will pass as input 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 struct crypt_data object is
       first  used  in  a  call  to  crypt_r().   We  recommend  zeroing the entire object, not just
       initialized and not just the documented fields, before the first use.  (Of  course,  do  this
       before storing anything in setting and input.)

       The  data  argument  to  crypt_rn  should  also point to a struct crypt_data object, and size
       should be the size of that object, cast to int.  When used with crypt_rn, the entire data ob‐
       ject (except for the input and setting fields) must be zeroed before its first use;  this  is
       not  just  a  recommendation, as it is for crypt_r.  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 struct crypt_data object, using malloc(3), and write its address and  size  into
       the  variables  pointed to by data and size.  These can be reused in subsequent calls.  After
       the application is done hashing passphrases, it should deallocate the struct  crypt_data  ob‐
       ject using free(3).

RETURN VALUES
       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 in other calls to crypt, crypt_r, crypt_rn,
       and  crypt_ra,  and  as   prefix   in   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 overwritten 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  their  data  argu‐
       ment.   It  is  safe to call them from multiple threads simultaneously, as long as a separate
       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  data argument, 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 applica‐
       tions 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             phrase is too long (more than CRYPT_MAX_PASSPHRASE_SIZE  characters;  some
                          hashing methods may have lower limits).
                          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 installation,  or  the
                          hashing  method  requested by setting is not supported.  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  sys‐
       tems 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 implementations 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.  Some older applications are not prepared to handle null pointers  returned
       by  crypt.  The behavior described above for this implementation, setting errno and returning
       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  inter‐
       face makes it impossible to change the size of struct crypt_data without breaking binary com‐
       patibility.  The crypt_rn interface 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(3)