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Unix Version 7 - man page for crypt (v7 section 3)

CRYPT(3)			     Library Functions Manual				 CRYPT(3)

       crypt, setkey, encrypt - DES encryption

       char *crypt(key, salt)
       char *key, *salt;

       char *key;

       encrypt(block, edflag)
       char *block;

       Crypt  is  the  password encryption routine.  It is based on the NBS Data Encryption Stan-
       dard, with variations intended (among other things) to frustrate use of hardware implemen-
       tations of the DES for key search.

       The  first  argument  to  crypt	is  a user's typed password.  The second is a 2-character
       string chosen from the set [a-zA-Z0-9./].  The salt string is  used  to	perturb  the  DES
       algorithm  in  one  of 4096 different ways, after which the password is used as the key to
       encrypt repeatedly a constant string.  The returned value points to  the  encrypted  pass-
       word, in the same alphabet as the salt.	The first two characters are the salt itself.

       The  other  entries  provide  (rather  primitive) access to the actual DES algorithm.  The
       argument of setkey is a character array of length 64 containing only the  characters  with
       numerical value 0 and 1.  If this string is divided into groups of 8, the low-order bit in
       each group is ignored, leading to a 56-bit key which is set into the machine.

       The argument to the encrypt entry is likewise a character array of  length  64  containing
       0's  and 1's.  The argument array is modified in place to a similar array representing the
       bits of the argument after having been subjected to the DES algorithm using the key set by
       setkey.	If edflag is 0, the argument is encrypted; if non-zero, it is decrypted.

       passwd(1), passwd(5), login(1), getpass(3)

       The return value points to static data whose content is overwritten by each call.


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