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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 Standard, with variations intended (among other things) to frustrate use of hardware implementations 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 password, 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.