CRYPT(3)

NAME
crypt, bcrypt_gensalt, bcrypt - password hashing

SYNOPSIS
#include <stdlib.h>

#include <unistd.h>
char *
crypt(const char *key, const char *setting);
#include <pwd.h>
char *
bcrypt_gensalt(u_int8_t log_rounds);
char *
bcrypt(const char *key, const char *salt);

DESCRIPTION
These functions are deprecated in favor of crypt_checkpass(3) and crypt_newhash(3).

The crypt() function performs password hashing based on the NBS Data Encryption Standard (DES). Additional code has been added to deter key search attempts and to use stronger hashing algorithms.

The first argument to crypt() is a NUL-terminated string key, typically a user's typed password. The second, setting, is in one of three forms: if it begins with an underscore (‘_’) then an extended format is used in interpreting both the key and the setting, as outlined below. If it begins with a string character (‘$’) and a number then a different algorithm is used depending on the number. At the moment ‘$2’ chooses Blowfish hashing; see below for more information.

  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 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.

  Blowfish crypt
The Blowfish version of crypt has 128 bits of salt in order to make building dictionaries of common passwords space consuming. The initial state of the Blowfish cipher is expanded using the salt and the password repeating the process a variable number of rounds, which is encoded in the password string. The maximum password length is 72. The final Blowfish password entry is created by encrypting the string

“OrpheanBeholderScryDoubt”

with the Blowfish state 64 times.

The version number, the logarithm of the number of rounds and the concatenation of salt and hashed password are separated by the ‘$’ character. An encoded ‘8’ would specify 256 rounds. A valid Blowfish password looks like this:

“$2b$12$FPWWO2RJ3CK4FINTw0Hi8OiPKJcX653gzSS.jqltHFMxyDmmQ0Hqq”.

The whole Blowfish password string is passed as setting for interpretation.

  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.

  DES Algorithm
The salt introduces disorder in the DES algorithm in one of 16777216 or 4096 possible ways (i.e., 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 NUL-terminated string, 20 or 13 bytes (plus NUL) in length, consisting of the setting followed by the encoded 64-bit encryption.

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

SEE ALSO
encrypt(1), login(1), passwd(1), blowfish(3), crypt_checkpass(3), getpass(3), passwd(5)

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

AUTHORS
David Burren <davidb@werj.com.au> wrote the original DES functions.

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

With DES hashing, passwords containing the byte 0x80 use less key entropy than other passwords. This is an implementation bug, not a bug in the DES cipher.