ENCRYPT(3) Linux Programmer's Manual ENCRYPT(3)

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 int 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, it is unchanged.
ENOSYS
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.

See crypt(3).

In glibc 2.2, these functions use the DES algorithm. 

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


int
main(void)
{
    char key[64];
    char orig[9] = "eggplant";
    char buf[64];
    char txt[9];
    int i, j;


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


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


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

cbc_crypt(3), crypt(3), ecb_crypt(3),
2018-04-30