CRYPT(3P) POSIX Programmer's Manual CRYPT(3P)

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

crypt — string encoding function (CRYPT)

#include <unistd.h>
char *crypt(const char *key, const char *salt);

The crypt() function is a string encoding function. The algorithm is implementation-defined.

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
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.

Upon successful completion, crypt() shall return a pointer to the encoded string. 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.

The crypt() function shall fail if:

The functionality is not supported on this implementation.

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);

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.



encrypt(), setkey()

The Base Definitions volume of POSIX.1‐2017, <unistd.h>

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see .

2017 IEEE/The Open Group