CRYPT(3)

NAME
crypt - Trapdoor encryption

LIBRARY
libcrypt

SYNOPSIS
#include <unistd.h>
char *
crypt(const char *key, const char *salt);

DESCRIPTION
The crypt() function performs password hashing with additional code added to deter key search attempts. Different algorithms can be used to in the hash. Currently these include the NBS Data Encryption Standard (DES), and MD5. The algorithm used will depend upon the format of the Salt--following the Modular Crypt Format (MCF)--and if DES is installed or not.

The first argument to crypt is the data to hash (usually a password), in a null-terminated string. The second is the salt, in one of three forms:

Extended
    If it begins with an underscore (“_”) then the DES Extended Format is used in interpreting both the the key and the salt, as outlined below.
Modular
    If it begins with the string “$digit$” then the Modular Crypt Format is used, as outlined below.
Traditional
    If neither of the above is true, it assumes the Traditional Format, using the entire string as the salt (or the first portion).

All routines are designed to be time-consuming. A brief test on a Pentium 166/MMX shows the DES crypt to do approximately 2640 crypts a CPU second and MD5 to do about 62 crypts a CPU second.

  DES Extended Format:

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.

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.

  Modular crypt:

If the salt begins with the string $digit$ then the Modular Crypt Format is used. The digit represents which algorithm is used in encryption. Following the token is the actual salt to use in the encryption. The length of the salt is limited to 16 characters--because the length of the returned output is also limited (_PASSWORD_LEN). The salt must be terminated with the end of the string (NULL) or a dollar sign. Any characters after the dollar sign are ignored.

Currently supported algorithms are:

1   MD5

Other crypt formats may be easilly added. An example salt would be:

$3$thesalt$rest

  Traditional crypt:

The algorithm used will depend upon whether DES is installed or not. If it is, DES will be used. Otherwise, the best algorithm is used, which is currently MD5.

How the salt is used will depend upon the algorithm for the hash. For best results, specify at least two characters of salt.

RETURN VALUES

crypt() returns a pointer to the encrypted value on success, and NULL on failure. Note: this is not a standard behaviour, AT&T crypt() will always return a pointer to a string.

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

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

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

The DES section of the code (FreeSec 1.0) was developed outside the United States of America as an unencumbered replacement for the U.S.-only NetBSD libcrypt encryption library. Users should be aware that this code (and programs staticly linked with it) may not be exported from the U.S., although it apparently can be imported.

AUTHORS
Originally written by David Burren <davidb@werj.com.au>, later additions and changes by Poul-henning Kamp, Mark R V Murray and Kris Kennaway.