#include <unistd.h> char *crypt(const char *key, const char *salt);
The key argument points to a string to be encoded. 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.
The following sections are informative.
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.1-2008.
#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); }
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.
The Base Definitions volume of POSIX.1-2017, <unistd.h>
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