NAME
crypt — password
hashing function (CRYPT)
SYNOPSIS
char *crypt(const char *key, const char *salt);
DESCRIPTION
The crypt () function hashes a password
for storage in the user database. The algorithm is
implementation-defined.
The key argument points to a password to be hashed. The salt argument shall be a string of at least two bytes in length not including the null character chosen from the set:
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 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 0 1 2 3 4 5 6 7 8 9 . /
The first two bytes of this string may be used to perturb the encoding algorithm.
The return value of crypt () points to
static data that is overwritten by each call.
The crypt () function need not be
thread-safe.
RETURN VALUE
Upon successful completion, crypt () shall
return a pointer to the hashed password; the first two bytes of the returned
value shall be those of the salt argument. Otherwise, it
shall return a null pointer and set
errno to
indicate the error.
ERRORS
The crypt () function shall fail if:
- [ENOSYS]
- The functionality is not supported on this implementation.
EXAMPLES
Encoding Passwords
The following example finds a user database entry matching a
particular user name and changes the current password to a new password. The
crypt () function generates an encoded version of
each password. The first call to crypt () produces
an encoded version of the old password; that encoded password is then
compared to the password stored in the user database. The second call to
crypt () encodes the new password before it is
stored.
The putpwent () function, used in the following example, is not part of POSIX.1-2024.
#include <unistd.h>
#include <pwd.h>
#include <string.h>
#include <stdio.h>
...
int valid_change;
int pfd; /* Integer for file descriptor returned by open(). */
FILE *fpfd; /* File pointer for use in putpwent(). */
struct passwd *p;
char user[100];
char oldpasswd[100];
char newpasswd[100];
char savepasswd[100];
...
valid_change = 0;
while ((p = getpwent()) != NULL) {
/* Change entry if found. */
if (strcmp(p->pw_name, user) == 0) {
if (strcmp(p->pw_passwd, crypt(oldpasswd, p->pw_passwd)) == 0) {
strcpy(savepasswd, crypt(newpasswd, user));
p->pw_passwd = savepasswd;
valid_change = 1;
}
else {
fprintf(stderr, "Old password is not valid\n");
}
}
/* Put passwd entry into ptmp. */
putpwent(p, fpfd);
}
APPLICATION USAGE
The values returned by this function need not be portable among XSI-conformant systems.
Several implementations offer extensions via characters outside of
the set specified for the salt argument for specifying
alternative algorithms; while not portable, these extensions may offer
better security. The use of crypt () for anything
other than password hashing is not recommended.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
XBD <unistd.h>
CHANGE HISTORY
First released in Issue 1. Derived from Issue 1 of the SVID.
Issue 5
Normative text previously in the APPLICATION USAGE section is moved to the DESCRIPTION.
Issue 7
Austin Group Interpretation 1003.1-2001 #156 is applied.
SD5-XSH-ERN-178 is applied.
POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0073 [899] is applied.
Issue 8
Austin Group Defect 1192 is applied, clarifying that
crypt () is intended for password hashing, not for
general string encoding.