| <html lang="en"> |
| <head> |
| <title>crypt - The GNU C Library</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="The GNU C Library"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="Cryptographic-Functions.html#Cryptographic-Functions" title="Cryptographic Functions"> |
| <link rel="prev" href="getpass.html#getpass" title="getpass"> |
| <link rel="next" href="DES-Encryption.html#DES-Encryption" title="DES Encryption"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| This file documents the GNU C library. |
| |
| This is Edition 0.12, last updated 2007-10-27, |
| of `The GNU C Library Reference Manual', for version |
| 2.8 (Sourcery G++ Lite 2011.03-41). |
| |
| Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002, |
| 2003, 2007, 2008, 2010 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being ``Free Software Needs Free Documentation'' |
| and ``GNU Lesser General Public License'', the Front-Cover texts being |
| ``A GNU Manual'', and with the Back-Cover Texts as in (a) below. A |
| copy of the license is included in the section entitled "GNU Free |
| Documentation License". |
| |
| (a) The FSF's Back-Cover Text is: ``You have the freedom to |
| copy and modify this GNU manual. Buying copies from the FSF |
| supports it in developing GNU and promoting software freedom.''--> |
| <meta http-equiv="Content-Style-Type" content="text/css"> |
| <style type="text/css"><!-- |
| pre.display { font-family:inherit } |
| pre.format { font-family:inherit } |
| pre.smalldisplay { font-family:inherit; font-size:smaller } |
| pre.smallformat { font-family:inherit; font-size:smaller } |
| pre.smallexample { font-size:smaller } |
| pre.smalllisp { font-size:smaller } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| <link rel="stylesheet" type="text/css" href="../cs.css"> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="crypt"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="DES-Encryption.html#DES-Encryption">DES Encryption</a>, |
| Previous: <a rel="previous" accesskey="p" href="getpass.html#getpass">getpass</a>, |
| Up: <a rel="up" accesskey="u" href="Cryptographic-Functions.html#Cryptographic-Functions">Cryptographic Functions</a> |
| <hr> |
| </div> |
| |
| <h3 class="section">32.3 Encrypting Passwords</h3> |
| |
| <!-- crypt.h --> |
| <!-- BSD, SVID --> |
| <div class="defun"> |
| — Function: char * <b>crypt</b> (<var>const char *key, const char *salt</var>)<var><a name="index-crypt-3677"></a></var><br> |
| <blockquote> |
| <p>The <code>crypt</code> function takes a password, <var>key</var>, as a string, and |
| a <var>salt</var> 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</var> |
| that will produce that output is to guess values of <var>key</var> until the |
| original value of <var>key</var> is found. |
| |
| <p>The <var>salt</var> 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</var>, an intruder can make a |
| guess, run <code>crypt</code> on it once, and compare the result with all the |
| passwords. With a <var>salt</var>, the intruder must run <code>crypt</code> once |
| for each different salt. |
| |
| <p>For the MD5-based algorithm, the <var>salt</var> should consist of the string |
| <code>$1$</code>, followed by up to 8 characters, terminated by either |
| another <code>$</code> or the end of the string. The result of <code>crypt</code> |
| will be the <var>salt</var>, followed by a <code>$</code> if the salt didn't end |
| with one, followed by 22 characters from the alphabet |
| <code>./0-9A-Za-z</code>, up to 34 characters total. Every character in the |
| <var>key</var> is significant. |
| |
| <p>For the DES-based algorithm, the <var>salt</var> should consist of two |
| characters from the alphabet <code>./0-9A-Za-z</code>, and the result of |
| <code>crypt</code> 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</var> are significant. |
| |
| <p>The MD5-based algorithm 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. |
| |
| <p>When the user enters their password for the first time, the <var>salt</var> |
| 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</code>, pass |
| the result of the previous call as the <var>salt</var>. |
| </p></blockquote></div> |
| |
| <p>The following short program is an example of how to use <code>crypt</code> the |
| first time a password is entered. Note that the <var>salt</var> 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. |
| |
| <pre class="smallexample"> #include <stdio.h> |
| #include <time.h> |
| #include <unistd.h> |
| #include <crypt.h> |
| |
| int |
| main(void) |
| { |
| unsigned long seed[2]; |
| char salt[] = "$1$........"; |
| const char *const seedchars = |
| "./0123456789ABCDEFGHIJKLMNOPQRST" |
| "UVWXYZabcdefghijklmnopqrstuvwxyz"; |
| char *password; |
| int i; |
| |
| /* <span class="roman">Generate a (not very) random seed. |
| You should do it better than this...</span> */ |
| seed[0] = time(NULL); |
| seed[1] = getpid() ^ (seed[0] >> 14 & 0x30000); |
| |
| /* <span class="roman">Turn it into printable characters from `seedchars'.</span> */ |
| for (i = 0; i < 8; i++) |
| salt[3+i] = seedchars[(seed[i/5] >> (i%5)*6) & 0x3f]; |
| |
| /* <span class="roman">Read in the user's password and encrypt it.</span> */ |
| password = crypt(getpass("Password:"), salt); |
| |
| /* <span class="roman">Print the results.</span> */ |
| puts(password); |
| return 0; |
| } |
| </pre> |
| <p>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</code>. |
| |
| <pre class="smallexample"> #include <stdio.h> |
| #include <string.h> |
| #include <unistd.h> |
| #include <crypt.h> |
| |
| int |
| main(void) |
| { |
| /* <span class="roman">Hashed form of "GNU libc manual".</span> */ |
| const char *const pass = "$1$/iSaq7rB$EoUw5jJPPvAPECNaaWzMK/"; |
| |
| char *result; |
| int ok; |
| |
| /* <span class="roman">Read in the user's password and encrypt it, |
| passing the expected password in as the salt.</span> */ |
| result = crypt(getpass("Password:"), pass); |
| |
| /* <span class="roman">Test the result.</span> */ |
| ok = strcmp (result, pass) == 0; |
| |
| puts(ok ? "Access granted." : "Access denied."); |
| return ok ? 0 : 1; |
| } |
| </pre> |
| <!-- crypt.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: char * <b>crypt_r</b> (<var>const char *key, const char *salt, struct crypt_data * data</var>)<var><a name="index-crypt_005fr-3678"></a></var><br> |
| <blockquote> |
| <p>The <code>crypt_r</code> function does the same thing as <code>crypt</code>, but |
| takes an extra parameter which includes space for its result (among |
| other things), so it can be reentrant. <code>data-><!-- /@w -->initialized</code> must be |
| cleared to zero before the first time <code>crypt_r</code> is called. |
| |
| <p>The <code>crypt_r</code> function is a GNU extension. |
| </p></blockquote></div> |
| |
| <p>The <code>crypt</code> and <code>crypt_r</code> functions are prototyped in the |
| header <samp><span class="file">crypt.h</span></samp>. |
| |
| </body></html> |
| |
