CRYPT(3)

NAME
crypt - DES trapdoor encryption

SYNOPSIS
char
*crypt(const char *key, const char *setting);

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 three forms: if it begins with an underscore (``_'') then an extended format is used in interpreting both the the key and the setting, as outlined below. if it begins with the string ``$1$'' then an exportable format is used.

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

  FreeBSD or Exportable crypt:

If the salt begins with ``$1$'' then the freely exportable MD5 algorithm is used to calculate a hash value, from which the password string is generated. The MD5 derived routine is designed to be time-consuming like the DES based version.

  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 setting followed by the encoded 64-bit encryption.

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

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

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 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.

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