CRYPT(3)

NAME
crypt, setkey, encrypt, des_setkey, des_cipher, bcrypt_gensalt, bcrypt, md5crypt - DES encryption

SYNOPSIS
#include <pwd.h>
#include <unistd.h>
char *
crypt(const char *key, const char *setting);
int
setkey(const char *key);
int
encrypt(char *block, int flag);
int
des_setkey(const char *key);
int
des_cipher(const char *in, char *out, int32_t salt, int count);
char *
bcrypt_gensalt(u_int8_t log_rounds);
char *
bcrypt(const char *key, const char *salt);
char *
md5crypt(const char *key, const char *salt);

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 and to use stronger hashing algorithms.

The first argument to crypt() is a NUL-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 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 a ‘$1’ chooses MD5 hashing and a ‘$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.

  MD5 crypt
For MD5 crypt the version number, salt and the hashed password are separated by the ‘$’ character. The maximum length of a password is limited by the length counter of the MD5 context, which is about 2**64. A valid MD5 password entry looks like this:

“$1$caeiHQwX$hsKqOjrFRRN6K32OWkCBf1”.

The whole MD5 password string is passed as setting for interpretation.

  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:

“$2a$12$eIAq8PR8sIUnJ1HaohxX2O9x9Qlm2vK97LJ5dsXdmB.eXF42qjchC”.

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.

The functions encrypt(), setkey(), des_setkey(), and des_cipher() provide access to the DES algorithm itself. setkey() is passed a 64-byte array of binary values (numeric 0 or 1). A 56-bit key is extracted from this array by dividing the array into groups of 8, and ignoring the last bit in each group. That bit is reserved for a byte parity check by DES, but is ignored by these functions.

The block argument to encrypt() is also a 64-byte array of binary values. If the value of flag is 0, block is encrypted otherwise it is decrypted. The result is returned in the original array block after using the key specified by setkey() to process it.

The argument to des_setkey() is a character array of length 8. The least significant bit (the parity bit) in each character is ignored, and the remaining bits are concatenated to form a 56-bit key. The function des_cipher() encrypts (or decrypts if count is negative) the 64-bits stored in the 8 characters at in using abs(3) of count iterations of DES and stores the 64-bit result in the 8 characters at out (which may be the same as in). The salt specifies perturbations to the DES E-box output as described above.

The function crypt() returns a pointer to the encrypted value on success, and NULL on failure. The functions setkey(), encrypt(), des_setkey(), and des_cipher() return 0 on success and 1 on failure.

The crypt(), setkey(), and des_setkey() functions all manipulate the same key space.

SEE ALSO
login(1), passwd(1), blowfish(3), getpass(3), md5(3), passwd(5)

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 libcrypt encryption library. Programs linked against the crypt() interface may be exported from the U.S.A. only if they use crypt() solely for authentication purposes and avoid use of the other programmer interfaces listed above. Special care has been taken in the library so that programs which only use the crypt() interface do not pull in the other components.

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

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