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CRYPT(3)                 BSD Library Functions Manual                 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 cur-rent current
     rent 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 Version 6 AT&T UNIX.  The current style crypt() first
     appeared in Version 7 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() inter-face interface
     face 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>

FreeSec 1.0                      March 9, 1994                     FreeSec 1.0