@ -1,121 +1,200 @@
@node Cryptographic Functions, Debugging Support, System Configuration, Top
@chapter Cryptographic Functions
@c %MENU% Password storage and strongly unpredictable bytes
@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
* crypt:: A one-way function for passwords.
* Unpredictable Bytes:: Randomness for cryptography purposes.
* Passphrase Storage:: One-way hashing for passphrase s.
* Unpredictable Bytes:: Randomness for cryptographic purposes.
@end menu
@node crypt
@section Encrypting Passwords
@node Passphrase Storage
@section Passphrase Storage
@cindex passphrase hashing
@cindex one-way hashing
@cindex hashing, passphrase
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
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{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.
@deftypefun {char *} crypt (const char *@var{key}, const char *@var{salt})
@standards{BSD, crypt.h}
@standards{SVID, crypt.h}
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. The FIPS mode checker and the
@c NSS implementations of may leak file descriptors if canceled. The
@c set of problems for AS- and AC-Safety.
@c The NSS implementations may leak file descriptors if cancell ed.
@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}.
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{key}, const char *@var{salt}, {struct crypt_data *} @var{data})
@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 @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.
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 @code{crypt} and @code{crypt_r} functions are prototyped in the
header @file{crypt.h}.
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.
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 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}.
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
@ -123,93 +202,121 @@ for a password and prints ``Access granted.'' if the user types
@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.
@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 @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.
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 kernel does not implement the required system call.
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
More than 256 bytes of randomness have been requested, or the buffer
could not be overwritten with random data for an unspecified reason.
@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 @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:
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) 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.
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.
@end table
The @code{getrandom} function is a cancellation point.
Unlike @code{getentropy}, the @code{getrandom} function is a
cancellation point, and if it blocks, it can be interrupted by
signals.
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}.
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 kernel does not implement the @code{getrandom} system call.
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
@ -228,4 +335,7 @@ the kernel randomness pool is initialized, this can happen even if
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
Zack Weinberg
8 years ago
Florian Weimer