path: root/manual/crypt.texi

@node Cryptographic Functions, Debugging Support, System Configuration, Top
@chapter Cryptographic Functions
@c %MENU% Passphrase storage and strongly unpredictable bytes.

@Theglibc{} includes only a few special-purpose cryptographic
functions: one-way hash functions for passphrase storage, and access
to a cryptographic randomness source, if one is provided by the
operating system.  Programs that need general-purpose cryptography
should use a dedicated cryptography library, such as
@uref{https://www.gnu.org/software/libgcrypt/,,libgcrypt}.

Many countries place legal restrictions on the import, export,
possession, or use of cryptographic software.  We deplore these
restrictions, but we must still warn you that @theglibc{} may be
subject to them, even if you do not use the functions in this chapter
yourself.  The restrictions vary from place to place and are changed
often, so we cannot give any more specific advice than this warning.

@menu
* Passphrase Storage::          One-way hashing for passphrases.
* Unpredictable Bytes::         Randomness for cryptographic purposes.
@end menu

@node Passphrase Storage
@section Passphrase Storage
@cindex passphrase hashing
@cindex one-way hashing
@cindex hashing, passphrase

Sometimes 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{passphrase}; then, the
system can ask someone claiming to be a user what the user's passphrase
is, and if the person gives the correct passphrase then the system can
grant the appropriate privileges.  (Traditionally, these were called
``passwords,'' but nowadays a single word is too easy to guess.)

Programs that handle passphrases must take special care not to reveal
them to anyone, no matter what.  It is not enough to keep them in a
file that is only accessible with special privileges.  The file might
be ``leaked'' via a bug or misconfiguration, and system administrators
shouldn't learn everyone's passphrase even if they have to edit that
file for some reason.  To avoid this, passphrases should also be
converted into @dfn{one-way hashes}, using a @dfn{one-way function},
before they are stored.

A one-way function is easy to compute, but there is no known way to
compute its inverse.  This means the system can easily check
passphrases, by hashing them and comparing the result with the stored
hash.  But an attacker who discovers someone's passphrase hash can
only discover the passphrase it corresponds to by guessing and
checking.  The one-way functions are designed to make this process
impractically slow, for all but the most obvious guesses.  (Do not use
a word from the dictionary as your passphrase.)

@Theglibc{} provides an interface to four one-way functions, based on
the SHA-2-512, SHA-2-256, MD5, and DES cryptographic primitives.  New
passphrases should be hashed with either of the SHA-based functions.
The others are too weak for newly set passphrases, but we continue to
support them for verifying old passphrases.  The DES-based hash is
especially weak, because it ignores all but the first eight characters
of its input.

@deftypefun {char *} crypt (const char *@var{phrase}, const char *@var{salt})
@standards{X/Open, unistd.h}
@standards{GNU, 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.
@c The NSS implementations may leak file descriptors if cancelled.
@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 function @code{crypt} converts a passphrase string, @var{phrase},
into a one-way hash suitable for storage in the user database.  The
string that it returns will consist entirely of printable ASCII
characters.  It will not contain whitespace, nor any of the characters
@samp{:}, @samp{;}, @samp{*}, @samp{!}, or @samp{\}.

The @var{salt} parameter controls which one-way function is used, and
it also ensures that the output of the one-way function is different
for every user, even if they have the same passphrase.  This makes it
harder to guess passphrases from a large user database.  Without salt,
the attacker could make a guess, run @code{crypt} on it once, and
compare the result with all the hashes.  Salt forces the attacker to
make separate calls to @code{crypt} for each user.

To verify a passphrase, pass the previously hashed passphrase as the
@var{salt}.  To hash a new passphrase for storage, set @var{salt} to a
string consisting of a prefix plus a sequence of randomly chosen
characters, according to this table:

@multitable @columnfractions .2 .1 .3
@headitem One-way function @tab Prefix @tab Random sequence
@item SHA-2-512
@tab @samp{$6$}
@tab 16 characters
@item SHA-2-256
@tab @samp{$5$}
@tab 16 characters
@item MD5
@tab @samp{$1$}
@tab 8 characters
@item DES
@tab @samp{}
@tab 2 characters
@end multitable

In all cases, the random characters should be chosen from the alphabet
@code{./0-9A-Za-z}.

With all of the hash functions @emph{except} DES, @var{phrase} can be
arbitrarily long, and all eight bits of each byte are significant.
With DES, only the first eight characters of @var{phrase} affect the
output, and the eighth bit of each byte is also ignored.

@code{crypt} can fail.  Some implementations return @code{NULL} on
failure, and others return an @emph{invalid} hashed passphrase, which
will begin with a @samp{*} and will not be the same as @var{salt}.  In
either case, @code{errno} will be set to indicate the problem.  Some
of the possible error codes are:

@table @code
@item EINVAL
@var{salt} is invalid; neither a previously hashed passphrase, nor a
well-formed new salt for any of the supported hash functions.

@item EPERM
The system configuration forbids use of the hash function selected by
@var{salt}.

@item ENOMEM
Failed to allocate internal scratch storage.

@item ENOSYS
@itemx EOPNOTSUPP
Hashing passphrases is not supported at all, or the hash function
selected by @var{salt} is not supported.  @Theglibc{} does not use
these error codes, but they may be encountered on other operating
systems.
@end table

@code{crypt} uses static storage for both internal scratchwork and the
string it returns.  It is not safe to call @code{crypt} from multiple
threads simultaneously, and the string it returns will be overwritten
by any subsequent call to @code{crypt}.

@code{crypt} is specified in the X/Open Portability Guide and is
present on nearly all historical Unix systems.  However, the XPG does
not specify any one-way functions.

@code{crypt} is declared in @file{unistd.h}.  @Theglibc{} also
declares this function in @file{crypt.h}.
@end deftypefun

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

The function @code{crypt_r} is a thread-safe version of @code{crypt}.
Instead of static storage, it uses the memory pointed to by its
@var{data} argument for both scratchwork and the string it returns.
It can safely be used from multiple threads, as long as different
@var{data} objects are used in each thread.  The string it returns
will still be overwritten by another call with the same @var{data}.

@var{data} must point to a @code{struct crypt_data} object allocated
by the caller.  All of the fields of @code{struct crypt_data} are
private, but before one of these objects is used for the first time,
it must be initialized to all zeroes, using @code{memset} or similar.
After that, it can be reused for many calls to @code{crypt_r} without
erasing it again.  @code{struct crypt_data} is very large, so it is
best to allocate it with @code{malloc} rather than as a local
variable.  @xref{Memory Allocation}.

@code{crypt_r} is a GNU extension.  It is declared in @file{crypt.h},
as is @code{struct crypt_data}.
@end deftypefun

The following program shows how to use @code{crypt} the first time a
passphrase is entered.  It uses @code{getentropy} to make the salt as
unpredictable as possible; @pxref{Unpredictable Bytes}.

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

The next program demonstrates how to verify a passphrase.  It checks a
hash hardcoded into the program, because looking up real users' hashed
passphrases may require special privileges (@pxref{User Database}).
It also shows that different one-way functions produce different
hashes for the same passphrase.

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

@node Unpredictable Bytes
@section Generating Unpredictable Bytes
@cindex randomness source
@cindex random numbers, cryptographic
@cindex pseudo-random numbers, cryptographic
@cindex cryptographic random number generator
@cindex deterministic random bit generator
@cindex CRNG
@cindex CSPRNG
@cindex DRBG

Cryptographic applications often need some random data that will be as
difficult as possible for a hostile eavesdropper to guess.  For
instance, encryption keys should be chosen at random, and the ``salt''
strings used by @code{crypt} (@pxref{Passphrase Storage}) should also
be chosen at random.

Some pseudo-random number generators do not provide unpredictable-enough
output for cryptographic applications; @pxref{Pseudo-Random Numbers}.
Such applications need to use a @dfn{cryptographic random number
generator} (CRNG), also sometimes called a @dfn{cryptographically strong
pseudo-random number generator} (CSPRNG) or @dfn{deterministic random
bit generator} (DRBG).

Currently, @theglibc{} does not provide a cryptographic random number
generator, but it does provide functions that read random data from a
@dfn{randomness source} supplied by the operating system.  The
randomness source is a CRNG at heart, but it also continually
``re-seeds'' itself from physical sources of randomness, such as
electronic noise and clock jitter.  This means applications do not need
to do anything to ensure that the random numbers it produces are
different on each run.

The catch, however, is that these functions will only produce
relatively short random strings in any one call.  Often this is not a
problem, but applications that need more than a few kilobytes of
cryptographically strong random data should call these functions once
and use their output to seed a CRNG.

Most applications should use @code{getentropy}.  The @code{getrandom}
function is intended for low-level applications which need additional
control over blocking behavior.

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

This function writes exactly @var{length} bytes of random data to the
array starting at @var{buffer}.  @var{length} can be no more than 256.
On success, it returns zero.  On failure, it returns @math{-1}, and
@code{errno} is set to indicate the problem.  Some of the possible
errors are listed below.

@table @code
@item ENOSYS
The operating system does not implement a randomness source, or does
not support this way of accessing it.  (For instance, the system call
used by this function was added to the Linux kernel in version 3.17.)

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

@item EIO
@var{length} is larger than 256, or the kernel entropy pool has
suffered a catastrophic failure.
@end table

A call to @code{getentropy} can only block when the system has just
booted and the randomness source has not yet been initialized.
However, if it does block, it cannot be interrupted by signals or
thread cancellation.  Programs intended to run in very early stages of
the boot process may need to use @code{getrandom} in non-blocking mode
instead, and be prepared to cope with random data not being available
at all.

The @code{getentropy} function is declared in the header file
@file{sys/random.h}.  It is derived from OpenBSD.
@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 up to @var{length} bytes of random data to the
array starting at @var{buffer}.  The @var{flags} argument should be
either zero, or the bitwise OR of some of the following flags:

@table @code
@item GRND_RANDOM
Use the @file{/dev/random} (blocking) source instead of the
@file{/dev/urandom} (non-blocking) source to obtain randomness.

If this flag is specified, the call may block, potentially for quite
some time, even after the randomness source has been initialized.  If it
is not specified, the call can only block when the system has just
booted and the randomness source has not yet been initialized.

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

@item GRND_INSECURE
Write random data that may not be cryptographically secure.
@end table

Unlike @code{getentropy}, the @code{getrandom} function is a
cancellation point, and if it blocks, it can be interrupted by
signals.

On success, @code{getrandom} returns the number of bytes which have
been written to the buffer, which may be less than @var{length}.  On
error, it returns @math{-1}, and @code{errno} is set to indicate the
problem.  Some of the possible errors are:

@table @code
@item ENOSYS
The operating system does not implement a randomness source, or does
not support this way of accessing it.  (For instance, the system call
used by this function was added to the Linux kernel in version 3.17.)

@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

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

@end deftypefun