crypt

CRYPT(3)                  BSD 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 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, │ 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                Openwall Project