Ubuntu Manpage: crypt, crypt_r, crypt_rn, crypt_ra

Provided by: libcrypt-dev_4.4.28-2_amd64

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.