encrypt, setkey, encrypt_r, setkey_r — encrypt 64-bit messages


#define _XOPEN_SOURCE        /* See feature_test_macros(7) */
#include <unistd.h>
void encrypt( char block[64],
  int edflag);

#define _XOPEN_SOURCE        /* See feature_test_macros(7) */
#include <stdlib.h>
void setkey( const char *key);

#define _GNU_SOURCE          /* See feature_test_macros(7) */
#include <crypt.h>
void setkey_r( const char *key,
  struct crypt_data *data);
void encrypt_r( char *block,
  int edflag,
  struct crypt_data *data);
Each of these requires linking with −lcrypt.


These functions encrypt and decrypt 64-bit messages. The setkey() function sets the key used by encrypt(). The key argument used here is an array of 64 bytes, each of which has numerical value 1 or 0. The bytes key[n] where n=8*i-1 are ignored, so that the effective key length is 56 bits.

The encrypt() function modifies the passed buffer, encoding if edflag is 0, and decoding if 1 is being passed. Like the key argument, also block is a bit vector representation of the actual value that is encoded. The result is returned in that same vector.

These two functions are not reentrant, that is, the key data is kept in static storage. The functions setkey_r() and encrypt_r() are the reentrant versions. They use the following structure to hold the key data:

struct crypt_data {
  char   keysched[16 * 8];  
  char   sb0[32768];  
  char   sb1[32768];  
  char   sb2[32768];  
  char   sb3[32768];  
  char   crypt_3_buf[14];  
  char   current_salt[2];  
  long   current_saltbits;  
  int   direction;  
  int   initialized;  

Before calling setkey_r() set data−>initialized to zero.


These functions do not return any value.


Set errno to zero before calling the above functions. On success, errno is unchanged.


The function is not provided. (For example because of former USA export restrictions.)


Because they employ the DES block cipher, which is no longer considered secure, crypt(), crypt_r(), setkey(), and setkey_r() were removed in glibc 2.28. Applications should switch to a modern cryptography library, such as libgcrypt.


For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
encrypt(), setkey() Thread safety MT-Unsafe race:crypt
encrypt_r(), setkey_r() Thread safety MT-Safe


encrypt(), setkey(): POSIX.1-2001, POSIX.1-2008, SUS, SVr4.

The functions encrypt_r() and setkey_r() are GNU extensions.


Availability in glibc

See crypt(3).

Features in glibc

In glibc 2.2, these functions use the DES algorithm.


#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <crypt.h>

    char key[64];
    char orig[9] = "eggplant";
    char buf[64];
    char txt[9];

    for (int i = 0; i < 64; i++) {
        key[i] = rand() & 1;

    for (int i = 0; i < 8; i++) {
        for (int j = 0; j < 8; j++) {
            buf[i * 8 + j] = orig[i] >> j & 1;
    printf("Before encrypting: %s\n", orig);

    encrypt(buf, 0);
    for (int i = 0; i < 8; i++) {
        for (int j = 0, txt[i] = '\0'; j < 8; j++) {
            txt[i] |= buf[i * 8 + j] << j;
        txt[8] = '\0';
    printf("After encrypting:  %s\n", txt);

    encrypt(buf, 1);
    for (int i = 0; i < 8; i++) {
        for (int j = 0, txt[i] = '\0'; j < 8; j++) {
            txt[i] |= buf[i * 8 + j] << j;
        txt[8] = '\0';
    printf("After decrypting:  %s\n", txt);


cbc_crypt(3), crypt(3), ecb_crypt(3),


This page is part of release 5.11 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at−pages/.

  Copyright 2000 Nicolás Lichtmaier <>
Created 2000-07-22 00:52-0300

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Modified 2002-07-23 19:21:35 CEST 2002 Walter Harms

Modified 2003-04-04, aeb