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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 re-
sult 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. Ap-
plications 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 ob-
ject 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 argu-
ment. 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 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 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 stan-
dard.
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 sys-
tems support the hashing method that was used. However, the set of supported hashing meth-
ods 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 re-
turned 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 ap-
plications 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 inter-
face 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 allo-
cate. 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
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