diff options
| -rw-r--r-- | crypt/crypt.texi | 412 |
1 files changed, 0 insertions, 412 deletions
diff --git a/crypt/crypt.texi b/crypt/crypt.texi deleted file mode 100644 index 1a4177a..0000000 --- a/crypt/crypt.texi +++ /dev/null @@ -1,412 +0,0 @@ -@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}. |