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CRYPT(3) Library Functions Manual CRYPT(3)

crypt, crypt_checkpass, setkey, encrypt, des_setkey, des_cipher, bcrypt_gensalt, bcrypt — password hashing

#include <stdlib.h>

int
setkey(const char *key);


#include <unistd.h>

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

int
crypt_checkpass(const char *password, const char *hash);

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);

#include <pwd.h>

char *
bcrypt_gensalt(u_int8_t log_rounds);

char *
bcrypt(const char *key, const char *salt);

The () 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 () 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 ‘$2’ chooses Blowfish hashing; see below for more information.

The () function is provided to simplify checking a user's password. If both the hash and the password are the empty string, authentication is a success. Otherwise, the password is hashed and compared to the provided hash. If the hash is NULL, authentication will always fail, but a default amount of work is performed to simulate the hashing operation. A successful match will return 0. A failure will return -1 and set errno.

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.

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.

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.

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 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 (), (), 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 () 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 () to process it.

The argument to () 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 () 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 (), (), and des_setkey() functions all manipulate the same key space.

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.

encrypt(1), login(1), passwd(1), blowfish(3), getpass(3), md5(3), passwd(5)

A rotor-based crypt() function appeared in Version 3 AT&T UNIX. The current style crypt() first appeared in Version 7 AT&T UNIX.

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

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.

May 16, 2014 OpenBSD-5.6