@ -1,8 +1,14 @@
@c This node must have no pointers.
@node Cryptographic Functions
@c @node Cryptographic Functions, Debugging Support, System Configuration, Top
@chapter DES Encryption and Password Handling
@c %MENU% DES encryption and password handling
@node Cryptographic Functions, Debugging Support, System Configuration, Top
@chapter Cryptographic Functions
@c %MENU% Password storage and strongly unpredictable bytes
@menu
* crypt:: A one-way function for passwords.
* Unpredictable Bytes:: Randomness for cryptography purposes.
@end menu
@node crypt
@section Encrypting Passwords
On many systems, it is unnecessary to have any kind of user
authentication; for instance, a workstation which is not connected to a
@ -30,103 +36,6 @@ message-digest algorithm that is compatible with modern BSD systems,
and the other based on the Data Encryption Standard (DES) that is
compatible with Unix systems.
@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 passwords.
* Unpredictable Bytes:: Randomness for cryptography purposes.
@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 license to use, possess, or import
cryptography. These countries are believed to include Byelorussia,
Burma, 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 to
the bug database. @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.
@deftypefun {char *} getpass (const char *@var{prompt})
@standards{BSD, unistd.h}
@safety{@prelim{}@mtunsafe{@mtasuterm{}}@asunsafe{@ascuheap{} @asulock{} @asucorrupt{}}@acunsafe{@acuterm{} @aculock{} @acucorrupt{}}}
@c This function will attempt to create a stream for terminal I/O, but
@c will fallback to stdio/stderr. It attempts to change the terminal
@c mode in a thread-unsafe way, write out the prompt, read the password,
@c then restore the terminal mode. It has a cleanup to close the stream
@c in case of (synchronous) cancellation, but not to restore the
@c terminal mode.
@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. @Theglibc{} 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 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
@deftypefun {char *} crypt (const char *@var{key}, const char *@var{salt})
@standards{BSD, crypt.h}
@standards{SVID, crypt.h}
@ -177,6 +86,23 @@ password against the result of a previous call to @code{crypt}, pass
the result of the previous call as the @var{salt}.
@end deftypefun
@deftypefun {char *} crypt_r (const char *@var{key}, const char *@var{salt}, {struct crypt_data *} @var{data})
@standards{GNU, crypt.h}
@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{} @asulock{} @ascuheap{} @ascudlopen{}}@acunsafe{@aculock{} @acsmem{}}}
@c Compared with crypt, this function fixes the @mtasurace:crypt
@c problem, but nothing else.
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}.
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
@ -195,23 +121,6 @@ for a password and prints ``Access granted.'' if the user types
@include testpass.c.texi
@end smallexample
@deftypefun {char *} crypt_r (const char *@var{key}, const char *@var{salt}, {struct crypt_data *} @var{data})
@standards{GNU, crypt.h}
@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{} @asulock{} @ascuheap{} @ascudlopen{}}@acunsafe{@aculock{} @acsmem{}}}
@c Compared with crypt, this function fixes the @mtasurace:crypt
@c problem, but nothing else.
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 Unpredictable Bytes
@section Generating Unpredictable Bytes
Zack Weinberg
8 years ago
Florian Weimer