CRYPT(3) 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 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 docu-
mented 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 ob-
ject 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 vari-
ables 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 re-
turn a pointer to a string which encodes both the hashed passphrase, and
the settings that were used to encode it. This string is directly us-
able 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 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 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 hash 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 charac-
ters, 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. When the functions suc-
ceed, the value of errno is unspecified and must not be relied upon.
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 encoun-
tered 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 implemen-
tations simply can't fail (except by crashing the program), others re-
turn a null pointer or a fixed string. Most implementations don't set
errno, but some do. POSIX specifies returning a null pointer and set-
ting 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 de-
scribed above for this implementation, setting errno and returning an
invalid hash different from setting, is chosen to make these applica-
tions 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 mem-
ory 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 func-
tion 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(3)
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