| CRYPT(3) | Library Functions Manual | CRYPT(3) |
crypt, crypt_r,
crypt_rn, crypt_ra —
passphrase hashing
Crypt Library (libcrypt, -lcrypt)
#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);
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).
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.
EINVALERANGECRYPT_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.ENOMEMcrypt_ra only: failed to allocate memory for
data.ENOSYS
or
EOPNOTSUPPcrypt 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.
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).
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 |
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.
crypt_gensalt(3), getpass(3), getpwent(3), shadow(3), login(1), passwd(1), crypt(5), passwd(5), shadow(5), pam(8)
| October 11, 2017 | Openwall Project |