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 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 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 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 phrase and setting 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 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
     phrase.)

     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 object (except for the phrase 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 object 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 argument.  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 applications 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 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 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 interface makes it
     impossible to change the size of struct crypt_data without breaking binary compatibility.  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, crypt_ra │ Thread safety │ MT-Safe              │
     └────────────────────────────┴───────────────┴──────────────────────┘

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)