Ubuntu Manpage: crypt, crypt_r, crypt_rn, crypt_ra

Provided by: libcrypt-dev_4.4.36-4build1_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 input[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 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 object (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 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.