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diff --git a/manual/crypt.texi b/manual/crypt.texi new file mode 100644 index 0000000000..1a4177a024 --- /dev/null +++ b/manual/crypt.texi @@ -0,0 +1,412 @@ +@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}. |