CRYPT_RN(3) Library functions CRYPT_RN(3)
NAME
crypt, crypt_r, crypt_rn, crypt_ra - passphrase hashing
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, void *data, int
size);
char *crypt_ra(const char *phrase, const char *setting, void **data,
int *size);
Link with -lcrypt.
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 hashing methods are explained below.
The crypt_data structure passed to crypt_r 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 setting and phrase 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
crypt_data object is first used in a call to crypt_r. We recommend
zeroing the entire crypt_data 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 crypt_data object,
and size should be the size of that object, cast to int. When used
with crypt_rn, the entire crypt_data object must be zeroed before its
first use; this is not just a recommendation, as it is for crypt_r.
(setting and phrase are still allowed to be used.) 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 crypt_data
object, using malloc(3), and write its address and size into *data and
*size. These can be reused in subsequent calls. After the application
is done hashing passphrases, it should deallocate *data using free(3).
RETURN VALUE
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 with other calls to crypt, crypt_r, crypt_rn, and
crypt_ra, and as prefix with 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 over-
written 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 the crypt_data object that they are supplied with; it is safe to
call them from multiple threads simultaneously, as long as a separate
crypt_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 crypt_data object, 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 appli-
cations 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 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 installa-
tion, or the hashing method requested by setting is not sup-
ported. 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 imple-
mentations 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. Many existing applications
are not prepared to handle null pointers returned by crypt. The behav-
ior described above for this implementation, setting errno and return-
ing 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
crypt_data without breaking binary compatibility. The crypt_rn inter-
face 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_RN(3)
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