doc/crypt.3 - develop - niceSOFT/libxcrypt

doc/crypt.3
512 строк11 KB
Björn Esser
lib/crypt.c: Do not overwrite the output buffer too early.
04 ноя 2025, 14:34
04 ноя 2025, 14:3425d5a2e
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.\" Revised by Zack Weinberg <zackw at panix.com> in 2017.
.\" Converted to mdoc format by Zack Weinberg in 2018.
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.Dd October 11, 2017
.Dt CRYPT 3
.Os "Openwall Project"
.Sh NAME
.Nm crypt , crypt_r , crypt_rn , crypt_ra
.Nd passphrase hashing
.Sh LIBRARY
.Lb libcrypt
.Sh SYNOPSIS
.In crypt.h
.Ft "char *"
.Fo crypt
.Fa "const char *phrase"
.Fa "const char *setting"
.Fc
.Ft "char *"
.Fo crypt_r
.Fa "const char *phrase"
.Fa "const char *setting"
.Fa "struct crypt_data *data"
.Fc
.Ft "char *"
.Fo crypt_rn
.Fa "const char *phrase"
.Fa "const char *setting"
.Fa "struct crypt_data *data"
.Fa "int size"
.Fc
.Ft "char *"
.Fo crypt_ra
.Fa "const char *phrase"
.Fa "const char *setting"
.Fa "void **data"
.Fa "int *size"
.Fc
.Sh DESCRIPTION
The
.Nm crypt ,
.Nm crypt_r ,
.Nm crypt_rn ,
and
.Nm crypt_ra
functions irreversibly
.Dq hash
.Fa phrase
for storage in the system password database
.Pq Xr shadow 5
using a cryptographic
.Dq hashing method.
The result of this operation is called a
.Dq hashed passphrase
or just a
.Dq hash.
Hashing methods are described in
.Xr crypt 5 .
.Pp
.Fa setting
controls which hashing method to use,
and also supplies various parameters to the chosen method,
most importantly a random
.Dq salt
which ensures that no two stored hashes are the same,
even if the
.Fa phrase
strings are the same.
.Pp
The
.Fa data
argument to
.Nm crypt_r
is a structure of type
.Vt "struct crypt_data" .
It has at least these fields:
.Bd -literal -offset indent
struct crypt_data {
    char output[CRYPT_OUTPUT_SIZE];
    char setting[CRYPT_OUTPUT_SIZE];
    char input[CRYPT_MAX_PASSPHRASE_SIZE];
    char initialized;
};
.Ed
.Pp
Upon a successful return from
.Nm crypt_r ,
the hashed passphrase will be stored in
.Fa output .
Applications are encouraged, but not required, to use the
.Fa input
and
.Fa setting
fields to store the strings that they will pass as
.Fa input phrase
and
.Fa setting
to
.Nm crypt_r .
This will make it easier to erase all sensitive data
after it is no longer needed.
.Pp
The
.Fa initialized
field must be set to zero before the first time a
.Vt "struct crypt_data"
object is first used in a call to
.Fn crypt_r .
We recommend zeroing the entire object,
not just
.Fa initialized
and not just the documented fields,
before the first use.
(Of course, do this before storing anything in
.Fa setting
and
.Fa input . )
.Pp
The
.Fa data
argument to
.Nm crypt_rn
should also point to a
.Vt "struct crypt_data"
object, and
.Fa size
should be the size of that object, cast to
.Vt int .
When used with
.Nm crypt_rn ,
the entire
.Fa data
object (except for the
.Fa input
and
.Fa setting
fields) must be zeroed before its first use;
this is not just a recommendation, as it is for
.Nm crypt_r .
Otherwise, the fields of the object have the same uses that they do for
.Nm crypt_r .
.Pp
On the first call to
.Nm crypt_ra ,
.Fa data
should be the address of a
.Vt "void *"
variable set to NULL, and
.Fa size
should be the address of an
.Vt int
variable set to zero.
.Nm crypt_ra
will allocate and initialize a
.Vt "struct crypt_data"
object, using
.Xr malloc 3 ,
and write its address and size into the variables pointed to by
.Fa data
and
.Fa size .
These can be reused in subsequent calls.
After the application is done hashing passphrases,
it should deallocate the
.Vt "struct crypt_data"
object using
.Xr free 3 .
.Sh RETURN VALUES
Upon successful completion,
.Nm crypt ,
.Nm crypt_r ,
.Nm crypt_rn ,
and
.Nm 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
.Fa setting
in other calls to
.Nm crypt ,
.Nm crypt_r ,
.Nm crypt_rn ,
and
.Nm crypt_ra ,
and as
.Fa prefix
in calls to
.Nm crypt_gensalt ,
.Nm crypt_gensalt_rn ,
and
.Nm crypt_gensalt_ra .
It will be entirely printable ASCII,
and will not contain whitespace
or the characters
.Sq Li \&: ,
.Sq Li \&; ,
.Sq Li \&* ,
.Sq Li \&! ,
or
.Sq Li \&\e .
See
.Xr crypt 5
for more detail on the format of hashed passphrases.
.Pp
.Nm crypt
places its result in a static storage area,
which will be overwritten by subsequent calls to
.Nm crypt .
It is not safe to call
.Nm crypt
from multiple threads simultaneously.
It's also not recommended to use the pointer
returned as an argument for another call to
.Nm crypt ,
as some implementations, including earlier
releases of libxcrypt, may overwrite the underlying
static output buffer before computing the hash.
.Pp
.Nm crypt_r ,
.Nm crypt_rn ,
and
.Nm crypt_ra
place their result in the
.Fa output
field of their
.Fa data
argument.
It is safe to call them from multiple threads simultaneously,
as long as a separate
.Fa data
object is used for each thread.
It's also not recommended to use the pointer
returned as an argument for another call to
.Nm crypt_r ,
.Nm crypt_rn ,
and
.Nm crypt_ra
using the same
.Fa data
object for subsequent calls, as some implementations,
including earlier releases of libxcrypt, may overwrite the
.Fa output
field of the passed
.Fa data
argument before computing the hash.
Calling
.Nm crypt_ra
with the
.Fa phrase
and/or
.Fa setting
parameters located within a passed
.Fa data
object which is smaller than the size of
.Vt "struct crypt_data"
is not recommended, as some implementations, including
earlier releases of libxcrypt, may not preserve the
passed literals, if the
.Fa data
object needs to be altered.
.Pp
Upon error,
.Nm crypt_r ,
.Nm crypt_rn ,
and
.Nm crypt_ra
write an
.Em invalid
hash to the
.Fa output
field of their
.Fa data
argument, and
.Nm crypt
writes an invalid hash to its static storage area.
This string will be shorter than 13 characters,
will begin with a
.Sq Li \&* ,
and will not compare equal to
.Fa setting .
.Pp
Upon error,
.Nm crypt_rn
and
.Nm crypt_ra
return a null pointer.
.Nm crypt_r
and
.Nm 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
.Nm crypt
cannot return a null pointer.
See
.Sx PORTABILITY NOTES
below.)
.Pp
All four functions set
.Va errno
when they fail.
When the functions succeed, the value of
.Va errno
is unspecified and must not be relied upon.
.Sh ERRORS
.Bl -tag -width Er
.It Er EINVAL
.Fa setting
is invalid, or requests a hashing method that is not supported.
.It Er ERANGE
.Fa phrase
is too long
(more than
.Dv CRYPT_MAX_PASSPHRASE_SIZE
characters; some hashing methods may have lower limits).
.br
.Nm crypt_rn
only:
.Fa size
is too small for the hashing method requested by
.Fa setting .
.It Er ENOMEM
Failed to allocate internal scratch memory.
.br
.Nm crypt_ra
only: failed to allocate memory for
.Fa data .
.It Er ENOSYS No or Er EOPNOTSUPP
Hashing passphrases is not supported at all on this installation,
or the hashing method requested by
.Fa setting
is not supported.
These error codes are not used by this version of libcrypt,
but may be encountered on other systems.
.El
.Sh PORTABILITY NOTES
.Nm crypt
is included in POSIX, but
.Nm crypt_r ,
.Nm crypt_rn ,
and
.Nm crypt_ra
are not part of any standard.
.Pp
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.
.Pp
The behavior of
.Nm 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
.Va errno ,
but some do.
POSIX specifies returning a null pointer and setting
.Va errno ,
but it defines only one possible error,
.Er ENOSYS ,
in the case where
.Nm crypt
is not supported at all.
Some older applications are not prepared to handle null pointers
returned by
.Nm crypt .
The behavior described above for this implementation,
setting
.Va errno
and returning an invalid hash different from
.Fa setting ,
is chosen to make these applications fail closed when an error occurs.
.Pp
Due to historical restrictions
on the export of cryptographic software from the USA,
.Nm crypt
is an optional POSIX component.
Applications should therefore be prepared for
.Nm crypt
not to be available,
or to always fail (setting
.Va errno
to
.Er ENOSYS )
at runtime.
.Pp
POSIX specifies that
.Nm crypt
is declared in
.In unistd.h ,
but only if the macro
.Dv _XOPEN_CRYPT
is defined and has a value greater than or equal to zero.
Since libcrypt does not provide
.In unistd.h ,
it declares
.Nm crypt ,
.Nm crypt_r ,
.Nm crypt_rn ,
and
.Nm crypt_ra
in
.In crypt.h
instead.
.Pp
On a minority of systems (notably recent versions of Solaris),
.Nm 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.
.Sh BUGS
Some implementations of
.Nm 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
.Nm crypt
return value if an error did not occur.
.Pp
.Vt "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.
.Pp
Some recently designed hashing methods need even more scratch memory,
but the
.Nm crypt_r
interface makes it impossible to change the size of
.Vt "struct crypt_data"
without breaking binary compatibility.
The
.Nm crypt_rn
interface could accommodate larger allocations for specific hashing methods,
but the caller of
.Nm crypt_rn
has no way of knowing how much memory to allocate.
.Nm crypt_ra
does the allocation itself,
but can only make a single call to
.Xr malloc 3 .
.Sh ATTRIBUTES
For an explanation of the terms used in this section, see
.Xr attributes 7 .
.TS
allbox;
lb lb lb
l l l.
Interface	Attribute	Value
T{
.Nm crypt
T}	Thread safety	MT-Unsafe race:crypt
T{
.Nm crypt_r ,
.Nm crypt_rn ,
.Nm crypt_ra
T}	Thread safety	MT-Safe
.TE
.sp
.Sh HISTORY
A rotor-based
.Nm crypt
function appeared in
.At v6 .
The
.Dq traditional
DES-based
.Nm crypt
first appeared in
.At v7 .
.Pp
.Nm crypt_r
originates with the GNU C Library.
There's also a
.Nm crypt_r
function on HP-UX and MKS Toolkit, but the prototypes and semantics
differ.
.Pp
.Nm crypt_rn
and
.Nm crypt_ra
originate with the Openwall project.
.Sh SEE ALSO
.Xr crypt_gensalt 3 ,
.Xr getpass 3 ,
.Xr getpwent 3 ,
.Xr shadow 3 ,
.Xr login 1 ,
.Xr passwd 1 ,
.Xr crypt 5 ,
.Xr passwd 5 ,
.Xr shadow 5 ,
.Xr pam 8