CRYPT(3)

NAME
crypt, encrypt, setkey - DES encryption

SYNOPSIS
#include <unistd.h>
char *
crypt(const char *key, const char *salt);
void
encrypt(char *block, int edflag);
#include <stdlib.h>
void
setkey(const char *key);

DESCRIPTION
The crypt() function performs password encryption, based on the NBS Data Encryption Standard (DES). Additional code has been added to deter key search attempts. The first argument to crypt() is a null-terminated string, typically a user's typed password. The second is in one of two forms: if it begins with an underscore (``_''), an extended format is used in interpreting both the key and the salt value, as outlined below.

  Extended crypt:

The key is divided into groups of 8 characters (the last group is null-padded) and the low-order 7 bits of each each character (56 bits per group) are used to form the DES key as follows: the first group of 56 bits becomes the initial DES key. For each additional group, the XOR of the encryption of the current DES key with itself and the group bits becomes the next DES key.

The salt is a 9-character array consisting of an underscore, followed by 4 bytes of iteration count and 4 bytes of salt. These are encoded as printable characters, 6 bits per character, least significant character first. The values 0 to 63 are encoded as ``./0-9A-Za-z''. This allows 24 bits for both count and salt.

  Traditional crypt:

The first 8 bytes of the key are null-padded, and the low-order 7 bits of each character is used to form the 56-bit DES key.

The salt is a 2-character array of the ASCII-encoded salt. Thus, only 12 bits of salt are used. count is set to 25.

  Algorithm:

The salt introduces disorder in the DES algorithm in one of 16777216 or 4096 possible ways (ie. with 24 or 12 bits: if bit i of the salt is set, then bits i and i+24 are swapped in the DES E-box output).

The DES key is used to encrypt a 64-bit constant, using count iterations of DES. The value returned is a null-terminated string, 20 or 13 bytes (plus null) in length, consisting of the salt, followed by the encoded 64-bit encryption.

The functions, encrypt() and setkey() provide access to the DES algorithm itself. setkey() is passed a 64-byte array of binary values (numeric 0 or 1). A 56-bit key is extracted from this array by dividing the array into groups of 8 and ignoring the last bit in each group. That bit is reserved for a byte parity check by DES, but is ignored by these functions.

The block argument to encrypt() is also a 64-byte array of binary values. If the value of edflag is 0, block is encrypted; otherwise, it is decrypted. The result is returned in the original array block, after using the key specified by setkey() to process it.

The function crypt() returns a pointer to the encrypted value on success, and NULL on failure.

The crypt() and setkey() functions all manipulate the same key space.

SEE ALSO
login(1), passwd(1), getpass(3), compat(5), passwd(5)

LEGACY SYNOPSIS
#include <unistd.h>

int
encrypt(char *block, int edflag);

The function encrypt() returns 0 on success and 1 on failure.

void
setkey(const char *key);

The include file <unistd.h> is necessary and sufficient for the setkey() function.

BUGS
The crypt() function returns a pointer to static data, and subsequent calls to crypt() will modify the same object.

HISTORY
A rotor-based crypt() function appeared in AT&T UNIX. The current style crypt() first appeared in AT&T UNIX.

This library (FreeSec 1.0) was developed outside the United States of America as an unencumbered replacement for the U.S.-only libcrypt encryption library. Programs linked against the crypt() interface may be exported from the U.S.A. only if they use crypt() solely for authentication purposes and avoid use of the other programmer interfaces listed above. Special care has been taken in the library so that programs which only use the crypt() interface do not pull in the other components.

AUTHOR
David Burren <davidb@werj.com.au>