path: root/manual/crypt.texi

@node Cryptographic Functions
@c @node Cryptographic Functions, , Top, 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 authorised
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.

The GNU C library already provides a one-way function based on MD5.  The
@code{crypt} add-on provides additional compatibility with the standard
UNIX one-way function based on the Data Encryption Standard.

It also provides support for Secure RPC, and some library functions that
can be used to perform normal DES encryption.

The add-on is not included in the main distribution of the GNU C library
because some governments, most notably those of France, Russia, 
and the US, have very restrictive rules governing the distribution and
use of encryption software.  The first section below tries to describe some
of those rules.

@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 UNIX passwords.
* DES Encryption::              Routines for DES encryption.
@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 licence to use, posess, or import
cryptography.  These countries are believed to include Byelorussia,
Burma, France, 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 using
the @code{glibcbug} script. @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.

@comment unistd.h
@comment BSD
@deftypefun {char *} getpass (const char * @var{prompt})

@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.  The GNU C library 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 ../crypt/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

@comment crypt.h
@comment BSD, SVID
@deftypefun {char *} crypt (const char * @var{key}, const char * @var{salt})

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.  If the @code{crypt} add-on is not installed,
trying to use the DES-based algorithm will return an empty string and
set @code{errno} to @code{EOPNOTSUPP}.

The MD5-based algorithm is available in the GNU C library even if the
@code{crypt} add-on is not installed.  It also 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 ../crypt/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 ../crypt/testpass.c.texi
@end smallexample

@comment crypt.h
@comment GNU
@deftypefun {char *} crypt_r (const char * @var{key}, const char * @var{salt}, {struct crypt_data *} @var{data})

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 DES Encryption
@section DES Encryption

The Data Encryption Standard is described in the US Government Federal
Information Processing Standards (FIPS) 46-3 published by the National
Institute of Standards and Technology.  The DES has been very thoroughly
analysed since it was developed in the late 1970s, and no new
significant flaws have been found.  

However, the DES uses only a 56-bit key (plus 8 parity bits), and a
machine has been built in 1998 which can search through all possible
keys in about 6 days, which cost about US$200000; faster searches would
be possible with more money.  This makes simple DES unsecure for most
purposes, and NIST no longer permits new US government systems
to use simple DES.

For serious encryption functionality, it is recommended that one of the
many free encryption libraries be used instead of these routines.

The DES is a reversible operation which takes a 64-bit block and a
64-bit key, and produces another 64-bit block.  Usually the bits are
numbered so that the most-significant bit, the first bit, of each block
is numbered 1.

Under that numbering, every 8th bit of the key (the 8th, 16th, and so
on) is not used by the encryption algorithm itself.  But the key must
have odd parity; that is, out of bits 1 through 8, and 9 through 16, and
so on, there must be an odd number of `1' bits, and this completely
specifies the unused bits.

@comment crypt.h
@comment BSD, SVID
@deftypefun void setkey (const char * @var{key})

The @code{setkey} function sets an internal data structure to be an
expanded form of @var{key}.  @var{key} is specified as an array of 64
bits each stored in a @code{char}, the first bit is @code{key[0]} and
the 64th bit is @code{key[63]}.  The @var{key} should have the correct
parity.
@end deftypefun

@comment crypt.h
@comment BSD, SVID
@deftypefun void encrypt (char * @var{block}, int @var{edflag})

The @code{encrypt} function encrypts @var{block} if
@var{edflag} is 0, otherwise it decrypts @var{block}, using a key
previously set by @code{setkey}.  The result is
placed in @var{block}.

Like @code{setkey}, @var{block} is specified as an array of 64 bits each
stored in a @code{char}, but there are no parity bits in @var{block}.
@end deftypefun

@comment crypt.h
@comment GNU
@deftypefun void setkey_r (const char * @var{key}, {struct crypt_data *} @var{data})
@comment crypt.h
@comment GNU
@deftypefunx void encrypt_r (char * @var{block}, int @var{edflag}, {struct crypt_data *} @var{data})

These are reentrant versions of @code{setkey} and @code{encrypt}.  The
only difference is the extra parameter, which stores the expanded
version of @var{key}.  Before calling @code{setkey_r} the first time, 
@code{data->initialised} must be cleared to zero.
@end deftypefun

The @code{setkey_r} and @code{encrypt_r} functions are GNU extensions.
@code{setkey}, @code{encrypt}, @code{setkey_r}, and @code{encrypt_r} are
defined in @file{crypt.h}.  

If the @code{crypt} add-on is not used to build the library, programs
that use these four functions will crash when the functions are called.
If this is a problem, the @code{ecb_crypt} function described below is
recommended instead.

@comment rpc/des_crypt.h
@comment SUNRPC
@deftypefun int ecb_crypt (char * @var{key}, char * @var{blocks}, unsigned @var{len}, unsigned @var{mode})

The function @code{ecb_crypt} encrypts or decrypts one or more blocks
using DES.  Each block is encrypted independently.

The @var{blocks} and the @var{key} are stored packed in 8-bit bytes, so
that the first bit of the key is the most-significant bit of
@code{key[0]} and the 63rd bit of the key is stored as the
least-significant bit of @code{key[7]}.  The @var{key} should have the
correct parity.

@var{len} is the number of bytes in @var{blocks}.  It should be a
multiple of 8 (so that there is a whole number of blocks to encrypt).
@var{len} is limited to a maximum of @code{DES_MAXDATA} bytes.

The result of the encryption replaces the input in @var{blocks}.

The @var{mode} parameter is the bitwise OR of two of the following:

@table @code
@comment rpc/des_crypt.h
@comment SUNRPC
@item DES_ENCRYPT
@findex DES_ENCRYPT
This constant, used in the @var{mode} parameter, specifies that
@var{blocks} is to be encrypted.

@comment rpc/des_crypt.h
@comment SUNRPC
@item DES_DECRYPT
@findex DES_DECRYPT
This constant, used in the @var{mode} parameter, specifies that
@var{blocks} is to be decrypted.

@comment rpc/des_crypt.h
@comment SUNRPC
@item DES_HW
@findex DES_HW
This constant, used in the @var{mode} parameter, asks to use a hardware
device.  If no hardware device is available, encryption happens anyway,
but in software.

@comment rpc/des_crypt.h
@comment SUNRPC
@item DES_SW
@findex DES_SW
This constant, used in the @var{mode} parameter, specifies that no
hardware device is to be used.
@end table

The result of the function will be one of these values:

@table @code
@comment rpc/des_crypt.h
@comment SUNRPC
@item DESERR_NONE
@findex DESERR_NONE
The encryption succeeded.

@comment rpc/des_crypt.h
@comment SUNRPC
@item DESERR_NOHWDEVICE
@findex DESERR_NOHWDEVICE
The encryption succeeded, but there was no hardware device available.

@comment rpc/des_crypt.h
@comment SUNRPC
@item DESERR_HWERROR
@findex DESERR_HWERROR
The encryption failed because of a hardware problem.  In the GNU
library, this error code is also returned if the @code{crypt} add-on was
not used to build the library.

@comment rpc/des_crypt.h
@comment SUNRPC
@item DESERR_BADPARAM
@findex DESERR_BADPARAM
The encryption failed because of a bad parameter, for instance @var{len}
is not a multiple of 8 or @var{len} is larger than @code{DES_MAXDATA}.
@end table
@end deftypefun

@comment rpc/des_crypt.h
@comment SUNRPC
@deftypefun int DES_FAILED (int @var{err})
This macro returns 1 if @var{err} is a `success' result code from
@code{ecb_crypt} or @code{cbc_crypt}, and 0 otherwise.
@end deftypefun

@comment rpc/des_crypt.h
@comment SUNRPC
@deftypefun int cbc_crypt (char * @var{key}, char * @var{blocks}, unsigned @var{len}, unsigned @var{mode}, char * @var{ivec})

The function @code{cbc_crypt} encrypts or decrypts one or more blocks
using DES in Cipher Block Chaining mode.

For encryption in CBC mode, each block is exclusive-ored with @var{ivec}
before being encrypted, then @var{ivec} is replaced with the result of
the encryption, then the next block is processed.  Decryption is the
reverse of this process.

This has the advantage that blocks which are the same before being
encrypted are very unlikely to be the same after being encrypted, making
it much harder to detect patterns in the data.

Usually, @var{ivec} is set to 8 random bytes before encryption starts.
Then the 8 random bytes are transmitted along with the encrypted data
(without themselves being encrypted), and passed back in as @var{ivec}
for decryption.  Another possibility is to set @var{ivec} to 8 zeroes
initially, and have the first the block encrypted consist of 8 random
bytes.

Otherwise, all the parameters are similar to those for @code{ecb_crypt}.
@end deftypefun

@comment rpc/des_crypt.h
@comment SUNRPC
@deftypefun void des_setparity (char * @var{key})

The function @code{des_setparity} changes the 64-bit @var{key}, stored
packed in 8-bit bytes, to have odd parity by altering the low bits of
each byte.
@end deftypefun

The @code{ecb_crypt}, @code{cbc_crypt}, and @code{des_setparity}
functions and their accompanying macros are all defined in the header
@file{rpc/des_crypt.h}.