glibc/manual/crypt.texi

@c This node must have no pointers.
@node Cryptographic Functions
@c @node Cryptographic Functions, Debugging Support, System Configuration, Top
@chapter DES Encryption and Password Handling
@c %MENU% DES encryption and password handling

On many systems, it is unnecessary to have any kind of user
authentication; for instance, a workstation which is not connected to a
network probably does not need any user authentication, because to use
the machine an intruder must have physical access.

Sometimes, however, it is necessary to be sure that a user is authorized
to use some service a machine provides---for instance, to log in as a
particular user id (@pxref{Users and Groups}).  One traditional way of
doing this is for each user to choose a secret @dfn{password}; then, the
system can ask someone claiming to be a user what the user's password
is, and if the person gives the correct password then the system can
grant the appropriate privileges.

If all the passwords are just stored in a file somewhere, then this file
has to be very carefully protected.  To avoid this, passwords are run
through a @dfn{one-way function}, a function which makes it difficult to
work out what its input was by looking at its output, before storing in
the file.

@Theglibc{} provides a one-way function that is compatible with
the behavior of the @code{crypt} function introduced in FreeBSD 2.0.
It supports two one-way algorithms: one based on the MD5
message-digest algorithm that is compatible with modern BSD systems,
and the other based on the Data Encryption Standard (DES) that is
compatible with Unix systems.

@menu
* Legal Problems::              This software can get you locked up, or worse.
* getpass::                     Prompting the user for a password.
* crypt::                       A one-way function for passwords.
* Unpredictable Bytes::         Randomness for cryptography purposes.
@end menu

@node Legal Problems
@section Legal Problems

Because of the continuously changing state of the law, it's not possible
to provide a definitive survey of the laws affecting cryptography.
Instead, this section warns you of some of the known trouble spots; this
may help you when you try to find out what the laws of your country are.

Some countries require that you have a license to use, possess, or import
cryptography.  These countries are believed to include Byelorussia,
Burma, India, Indonesia, Israel, Kazakhstan, Pakistan, Russia, and Saudi
Arabia.

Some countries restrict the transmission of encrypted messages by radio;
some telecommunications carriers restrict the transmission of encrypted
messages over their network.

Many countries have some form of export control for encryption software.
The Wassenaar Arrangement is a multilateral agreement between 33
countries (Argentina, Australia, Austria, Belgium, Bulgaria, Canada, the
Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary,
Ireland, Italy, Japan, Luxembourg, the Netherlands, New Zealand, Norway,
Poland, Portugal, the Republic of Korea, Romania, the Russian
Federation, the Slovak Republic, Spain, Sweden, Switzerland, Turkey,
Ukraine, the United Kingdom and the United States) which restricts some
kinds of encryption exports.  Different countries apply the arrangement
in different ways; some do not allow the exception for certain kinds of
``public domain'' software (which would include this library), some
only restrict the export of software in tangible form, and others impose
significant additional restrictions.

The United States has additional rules.  This software would generally
be exportable under 15 CFR 740.13(e), which permits exports of
``encryption source code'' which is ``publicly available'' and which is
``not subject to an express agreement for the payment of a licensing fee or
royalty for commercial production or sale of any product developed with
the source code'' to most countries.

The rules in this area are continuously changing.  If you know of any
information in this manual that is out-of-date, please report it to
the bug database.  @xref{Reporting Bugs}.

@node getpass
@section Reading Passwords

When reading in a password, it is desirable to avoid displaying it on
the screen, to help keep it secret.  The following function handles this
in a convenient way.

@deftypefun {char *} getpass (const char *@var{prompt})
@standards{BSD, unistd.h}
@safety{@prelim{}@mtunsafe{@mtasuterm{}}@asunsafe{@ascuheap{} @asulock{} @asucorrupt{}}@acunsafe{@acuterm{} @aculock{} @acucorrupt{}}}
@c This function will attempt to create a stream for terminal I/O, but
@c will fallback to stdio/stderr.  It attempts to change the terminal
@c mode in a thread-unsafe way, write out the prompt, read the password,
@c then restore the terminal mode.  It has a cleanup to close the stream
@c in case of (synchronous) cancellation, but not to restore the
@c terminal mode.

@code{getpass} outputs @var{prompt}, then reads a string in from the
terminal without echoing it.  It tries to connect to the real terminal,
@file{/dev/tty}, if possible, to encourage users not to put plaintext
passwords in files; otherwise, it uses @code{stdin} and @code{stderr}.
@code{getpass} also disables the INTR, QUIT, and SUSP characters on the
terminal using the @code{ISIG} terminal attribute (@pxref{Local Modes}).
The terminal is flushed before and after @code{getpass}, so that
characters of a mistyped password are not accidentally visible.

In other C libraries, @code{getpass} may only return the first
@code{PASS_MAX} bytes of a password.  @Theglibc{} has no limit, so
@code{PASS_MAX} is undefined.

The prototype for this function is in @file{unistd.h}.  @code{PASS_MAX}
would be defined in @file{limits.h}.
@end deftypefun

This precise set of operations may not suit all possible situations.  In
this case, it is recommended that users write their own @code{getpass}
substitute.  For instance, a very simple substitute is as follows:

@smallexample
@include mygetpass.c.texi
@end smallexample

The substitute takes the same parameters as @code{getline}
(@pxref{Line Input}); the user must print any prompt desired.

@node crypt
@section Encrypting Passwords

@deftypefun {char *} crypt (const char *@var{key}, const char *@var{salt})
@standards{BSD, crypt.h}
@standards{SVID, crypt.h}
@safety{@prelim{}@mtunsafe{@mtasurace{:crypt}}@asunsafe{@asucorrupt{} @asulock{} @ascuheap{} @ascudlopen{}}@acunsafe{@aculock{} @acsmem{}}}
@c Besides the obvious problem of returning a pointer into static
@c storage, the DES initializer takes an internal lock with the usual
@c set of problems for AS- and AC-Safety.  The FIPS mode checker and the
@c NSS implementations of may leak file descriptors if canceled.  The
@c The MD5, SHA256 and SHA512 implementations will malloc on long keys,
@c and NSS relies on dlopening, which brings about another can of worms.

The @code{crypt} function takes a password, @var{key}, as a string, and
a @var{salt} character array which is described below, and returns a
printable ASCII string which starts with another salt.  It is believed
that, given the output of the function, the best way to find a @var{key}
that will produce that output is to guess values of @var{key} until the
original value of @var{key} is found.

The @var{salt} parameter does two things.  Firstly, it selects which
algorithm is used, the MD5-based one or the DES-based one.  Secondly, it
makes life harder for someone trying to guess passwords against a file
containing many passwords; without a @var{salt}, an intruder can make a
guess, run @code{crypt} on it once, and compare the result with all the
passwords.  With a @var{salt}, the intruder must run @code{crypt} once
for each different salt.

For the MD5-based algorithm, the @var{salt} should consist of the string
@code{$1$}, followed by up to 8 characters, terminated by either
another @code{$} or the end of the string.  The result of @code{crypt}
will be the @var{salt}, followed by a @code{$} if the salt didn't end
with one, followed by 22 characters from the alphabet
@code{./0-9A-Za-z}, up to 34 characters total.  Every character in the
@var{key} is significant.

For the DES-based algorithm, the @var{salt} should consist of two
characters from the alphabet @code{./0-9A-Za-z}, and the result of
@code{crypt} will be those two characters followed by 11 more from the
same alphabet, 13 in total.  Only the first 8 characters in the
@var{key} are significant.

The MD5-based algorithm has no limit on the useful length of the
password used, and is slightly more secure.  It is therefore preferred
over the DES-based algorithm.

When the user enters their password for the first time, the @var{salt}
should be set to a new string which is reasonably random.  To verify a
password against the result of a previous call to @code{crypt}, pass
the result of the previous call as the @var{salt}.
@end deftypefun

The following short program is an example of how to use @code{crypt} the
first time a password is entered.  Note that the @var{salt} generation
is just barely acceptable; in particular, it is not unique between
machines, and in many applications it would not be acceptable to let an
attacker know what time the user's password was last set.

@smallexample
@include genpass.c.texi
@end smallexample

The next program shows how to verify a password.  It prompts the user
for a password and prints ``Access granted.'' if the user types
@code{GNU libc manual}.

@smallexample
@include testpass.c.texi
@end smallexample

@deftypefun {char *} crypt_r (const char *@var{key}, const char *@var{salt}, {struct crypt_data *} @var{data})
@standards{GNU, crypt.h}
@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{} @asulock{} @ascuheap{} @ascudlopen{}}@acunsafe{@aculock{} @acsmem{}}}
@c Compared with crypt, this function fixes the @mtasurace:crypt
@c problem, but nothing else.

The @code{crypt_r} function does the same thing as @code{crypt}, but
takes an extra parameter which includes space for its result (among
other things), so it can be reentrant.  @code{data@w{->}initialized} must be
cleared to zero before the first time @code{crypt_r} is called.

The @code{crypt_r} function is a GNU extension.
@end deftypefun

The @code{crypt} and @code{crypt_r} functions are prototyped in the
header @file{crypt.h}.

@node Unpredictable Bytes
@section Generating Unpredictable Bytes

Some cryptographic applications (such as session key generation) need
unpredictable bytes.

In general, application code should use a deterministic random bit
generator, which could call the @code{getentropy} function described
below internally to obtain randomness to seed the generator.  The
@code{getrandom} function is intended for low-level applications which
need additional control over the blocking behavior.

@deftypefun int getentropy (void *@var{buffer}, size_t @var{length})
@standards{GNU, sys/random.h}
@safety{@mtsafe{}@assafe{}@acsafe{}}

This function writes @var{length} bytes of random data to the array
starting at @var{buffer}, which must be at most 256 bytes long.  The
function returns zero on success.  On failure, it returns @code{-1} and
@code{errno} is updated accordingly.

The @code{getentropy} function is declared in the header file
@file{sys/random.h}.  It is derived from OpenBSD.

The @code{getentropy} function is not a cancellation point.  A call to
@code{getentropy} can block if the system has just booted and the kernel
entropy pool has not yet been initialized.  In this case, the function
will keep blocking even if a signal arrives, and return only after the
entropy pool has been initialized.

The @code{getentropy} function can fail with several errors, some of
which are listed below.

@table @code
@item ENOSYS
The kernel does not implement the required system call.

@item EFAULT
The combination of @var{buffer} and @var{length} arguments specifies
an invalid memory range.

@item EIO
More than 256 bytes of randomness have been requested, or the buffer
could not be overwritten with random data for an unspecified reason.

@end table

@end deftypefun

@deftypefun ssize_t getrandom (void *@var{buffer}, size_t @var{length}, unsigned int @var{flags})
@standards{GNU, sys/random.h}
@safety{@mtsafe{}@assafe{}@acsafe{}}

This function writes @var{length} bytes of random data to the array
starting at @var{buffer}.  On success, this function returns the number
of bytes which have been written to the buffer (which can be less than
@var{length}).  On error, @code{-1} is returned, and @code{errno} is
updated accordingly.

The @code{getrandom} function is declared in the header file
@file{sys/random.h}.  It is a GNU extension.

The following flags are defined for the @var{flags} argument:

@table @code
@item GRND_RANDOM
Use the @file{/dev/random} (blocking) pool instead of the
@file{/dev/urandom} (non-blocking) pool to obtain randomness.  If the
@code{GRND_RANDOM} flag is specified, the @code{getrandom} function can
block even after the randomness source has been initialized.

@item GRND_NONBLOCK
Instead of blocking, return to the caller immediately if no data is
available.
@end table

The @code{getrandom} function is a cancellation point.

Obtaining randomness from the @file{/dev/urandom} pool (i.e., a call
without the @code{GRND_RANDOM} flag) can block if the system has just
booted and the pool has not yet been initialized.

The @code{getrandom} function can fail with several errors, some of
which are listed below.  In addition, the function may not fill the
buffer completely and return a value less than @var{length}.

@table @code
@item ENOSYS
The kernel does not implement the @code{getrandom} system call.

@item EAGAIN
No random data was available and @code{GRND_NONBLOCK} was specified in
@var{flags}.

@item EFAULT
The combination of @var{buffer} and @var{length} arguments specifies
an invalid memory range.

@item EINTR
The system call was interrupted.  During the system boot process, before
the kernel randomness pool is initialized, this can happen even if
@var{flags} is zero.

@item EINVAL
The @var{flags} argument contains an invalid combination of flags.
@end table

@end deftypefun