1CRYPT(3P) POSIX Programmer's Manual CRYPT(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 crypt — string encoding function (CRYPT)
13
15 #include <unistd.h>
16 char *crypt(const char *key, const char *salt);
18
20 The crypt() function is a string encoding function. The algorithm is
21 implementation-defined.
22 The key argument points to a string to be encoded. The salt argument
24 shall be a string of at least two bytes in length not including the
25 null character chosen from the set:
26 a b c d e f g h i j k l m n o p q r s t u v w x y z
29 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
30 0 1 2 3 4 5 6 7 8 9 . /
31 The first two bytes of this string may be used to perturb the encoding
33 algorithm.
34 The return value of crypt() points to static data that is overwritten
36 by each call.
37 The crypt() function need not be thread-safe.
39
41 Upon successful completion, crypt() shall return a pointer to the
42 encoded string. The first two bytes of the returned value shall be
43 those of the salt argument. Otherwise, it shall return a null pointer
44 and set errno to indicate the error.
45
47 The crypt() function shall fail if:
48 ENOSYS The functionality is not supported on this implementation.
50 The following sections are informative.
52
54 Encoding Passwords
55 The following example finds a user database entry matching a particular
56 user name and changes the current password to a new password. The
57 crypt() function generates an encoded version of each password. The
58 first call to crypt() produces an encoded version of the old password;
59 that encoded password is then compared to the password stored in the
60 user database. The second call to crypt() encodes the new password
61 before it is stored.
62 The putpwent() function, used in the following example, is not part of
64 POSIX.1‐2008.
65 #include <unistd.h>
68 #include <pwd.h>
69 #include <string.h>
70 #include <stdio.h>
71 ...
72 int valid_change;
73 int pfd; /* Integer for file descriptor returned by open(). */
74 FILE *fpfd; /* File pointer for use in putpwent(). */
75 struct passwd *p;
76 char user[100];
77 char oldpasswd[100];
78 char newpasswd[100];
79 char savepasswd[100];
80 ...
81 valid_change = 0;
82 while ((p = getpwent()) != NULL) {
83 /* Change entry if found. */
84 if (strcmp(p->pw_name, user) == 0) {
85 if (strcmp(p->pw_passwd, crypt(oldpasswd, p->pw_passwd)) == 0) {
86 strcpy(savepasswd, crypt(newpasswd, user));
87 p->pw_passwd = savepasswd;
88 valid_change = 1;
89 }
90 else {
91 fprintf(stderr, "Old password is not valid\n");
92 }
93 }
94 /* Put passwd entry into ptmp. */
95 putpwent(p, fpfd);
96 }
97
99 The values returned by this function need not be portable among XSI-
100 conformant systems.
101 Several implementations offer extensions via characters outside of the
103 set specified for the salt argument for specifying alternative algo‐
104 rithms; while not portable, these extensions may offer better security.
105 The use of crypt() for anything other than password hashing is not rec‐
106 ommended.
107
109 None.
110
112 None.
113
115 encrypt(), setkey()
116 The Base Definitions volume of POSIX.1‐2017, <unistd.h>
118
120 Portions of this text are reprinted and reproduced in electronic form
121 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
122 table Operating System Interface (POSIX), The Open Group Base Specifi‐
123 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
124 Electrical and Electronics Engineers, Inc and The Open Group. In the
125 event of any discrepancy between this version and the original IEEE and
126 The Open Group Standard, the original IEEE and The Open Group Standard
127 is the referee document. The original Standard can be obtained online
128 at http://www.opengroup.org/unix/online.html .
129 Any typographical or formatting errors that appear in this page are
131 most likely to have been introduced during the conversion of the source
132 files to man page format. To report such errors, see https://www.ker‐
133 nel.org/doc/man-pages/reporting_bugs.html .
134IEEE/The Open Group 2017 CRYPT(3P)