README for libxcrypt
libxcrypt is a modern library for one-way hashing of passwords. It
supports a wide variety of both modern and historical hashing methods:
yescrypt, gost-yescrypt, scrypt, bcrypt, sha512crypt, sha256crypt,
md5crypt, SunMD5, sha1crypt, NT, bsdicrypt, bigcrypt, and descrypt.
It provides the traditional Unix
crypt_r interfaces, as
well as a set of extended interfaces pioneered by Openwall Linux,
libxcrypt is intended to be used by
passwd(1), and other
similar programs; that is, to hash a small number of passwords during
an interactive authentication dialogue with a human. It is not
suitable for use in bulk password-cracking applications, or in any
other situation where speed is more important than careful handling of
sensitive data. However, it is intended to be fast and lightweight
enough for use in servers that must field thousands of login attempts
Authorship and Licensing
libxcrypt is currently maintained by Björn Esser and Zack Weinberg. Many people have contributed to the code making up libxcrypt, often under the aegis of a different project. Please see the AUTHORS and THANKS files for a full set of credits.
libxcrypt as a whole is licensed under the GNU Lesser General Public License (version 2.1, or at your option, any later version). However, many individual files may be reused under more permissive licenses if separated from the library. Please see the LICENSING file for a comprehensive inventory of licenses, and COPYING.LIB for the terms of the LGPL.
Bug Reports, Feature Requests, Contributions, Etc.
libxcrypt is currently maintained at Github: the canonical repository URL is https://github.com/besser82/libxcrypt. Please file bug reports at https://github.com/besser82/libxcrypt/issues. This is also the appropriate place to suggest new features, offer patches, etc. All your feedback is welcome and will eventually receive a response, but this is a spare-time project for all of the present maintainers, so please be patient.
Build Requirements and Instructions
To build from a tarball release, the only tools required are the
standard Unix shell environment (including an implementation of AWK)
and a C compiler. Follow the generic build and installation
instructions in the file
INSTALL. There are two package-specific
./configure --help for more detail on these options.
man -l crypt.5 for more detail on the hashing algorithms that
can be enabled or disabled by
--enable-hashes. You can do both of
these things before building the software.
Building from a Git checkout additionally requires the Autotools
./bootstrap at the top level of the source tree, and then
follow the instructions in
INSTALL (which is created by that command).
The oldest versions of Autotools components that are known to work are: autoconf 2.69, automake 1.14, libtool 2.4.6, pkg-config 0.29. If you test with an older version of one of these and find that it works, please let us know. We are not deliberately requiring newer versions; we just can’t conveniently test older versions ourselves.
libxcrypt should be buildable with any ISO C1999-compliant C compiler,
with one critical exception: the symbol versioning macros in
crypt-port.h only work with compilers that implement certain GCC and
GNU Binutils extensions (
A few C2011 features are used; the intention is not to use any of them without a fallback, but we do not currently test this. A few POSIX and nonstandard-but-widespread Unix APIs are also used; again, the intention is not to use any of them without a fallback, but we do not currently test this. In particular, the crypt_gensalt functions may not always be able to retrieve cryptographically-sound random numbers from the operating system; if you call these functions with a null pointer for the “rbytes” argument, be prepared for them to fail.
As of mid-2018, GCC and LLVM don’t support link-time optimization of
libraries that use symbol versioning. If you build libxcrypt with
either of these compilers, do not use
-flto. See GCC bug 48200
for specifics; the problem is very similar for LLVM. Because this is,
at its root, a set of missing compiler features, we expect link-time
optimization won’t work in other C compilers either, but we haven’t
tested it ourselves.
On Linux-based systems, by default libxcrypt will be binary backward compatible with the libcrypt.so.1 shipped as part of the GNU C Library. This means that all existing binary executables linked against glibc’s libcrypt should work unmodified with this library’s libcrypt.so.1. We have taken pains to provide exactly the same symbol versions as were used by glibc on various CPU architectures, and to account for the variety of ways in which the Openwall extensions were patched into glibc’s libcrypt by some Linux distributions. (For instance, compatibility symlinks for SUSE’s “libowcrypt” are provided.)
However, the converse is not true: programs linked against libxcrypt
will not work with glibc’s libcrypt. Also, programs that use certain
legacy APIs supplied by glibc’s libcrypt (
fcrypt) cannot be compiled against
Binary backward compatibility can be disabled by supplying the
--disable-obsolete-api switch to
configure, in which case libxcrypt
will install libcrypt.so.2 instead of libcrypt.so.1. This
configuration is always used on all operating systems other than
Linux. We are willing to consider adding binary backward
compatibility for other operating systems’ existing libcrypts, but we
don’t currently plan to do that work ourselves.
Individual hash functions may be enabled or disabled by use of the
--enable-hashes switch to
configure. The default is to enable all
supported hashes. Disabling the traditional ‘des’ hash algorithm
--disable-obsolete-api. Security-conscious environments
without backward compatibility constraints are encouraged to use
--enable-hashes=strong, which enables only the hash functions that
are strong enough to be safe for newly hashed passwords.
The original implementation of the SunMD5 hashing algorithm has a bug,
which is mimicked by libxcrypt to be fully compatible with hashes
generated on (Open)Solaris. According to the only existing
documentation of this algorithm, its hashes were supposed to have
$md5[,rounds=%u]$<salt>$<checksum>, and include only the
$md5[,rounds=%u]$<salt> in the salt digest
step. However, almost all hashes encountered in production
environments have the format
(note the double $$). Unfortunately, it is not merely a cosmetic
difference: hashes of this format incorporate the first $ after the
salt within the salt digest step, so the resulting checksum is
different. The documentation hints that this stems from a bug within
the production implementation’s parser. This bug causes the
implementation to return
$$-format hashes when passed a
configuration string that ends with
$. It returns the intended
original format and checksum only if there is at least one letter
The NT algorithm, in its original implementation, never came with any
gensalt function, because the algorithm does not use any. libxcrypt
ships a bogus
gensalt function for the NT algorithm, which simply
$3$__not_used__XXXXXXXXXXXXXX, where the
Xs stand for some
more or less random salt. There is no difference in the resulting hash
returned by the
crypt function, whether using one of the hashes
gensalt or simply using
$3$ as a setting for hashing a
password with NT.
glibc’s libcrypt could optionally be configured to use Mozilla’s NSS library’s implementations of the cryptographic primitives md5crypt, sha256crypt, and sha512crypt. This option is not available in libxcrypt, because we do not currently believe it is a desirable option. The stated rationale for the option was to source all cryptographic primitives from a library that has undergone FIPS certification, but we believe FIPS certification would need to cover all of libxcrypt itself to have any meaningful value. Moreover, the strongest hashing methods, yescrypt and bcrypt, use cryptographic primitives that are not available from NSS, so the certification would not cover any part of what will hopefully be the most used code paths.