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authorUlrich Drepper <drepper@redhat.com>2000-02-29 05:59:52 +0000
committerUlrich Drepper <drepper@redhat.com>2000-02-29 05:59:52 +0000
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-@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}.