CRYPT(3)

NAME
crypt, setkey, encrypt, des_setkey, des_cipher - password hashing

LIBRARY
libcrypt

SYNOPSIS
#include <unistd.h>
char *
crypt(const char *key, const char *setting);
int
encrypt(char *block, int flag);
int
des_setkey(const char *key);
int
des_cipher(const char *in, char *out, long salt, int count);
#include <stdlib.h>
int
setkey(const char *key);

DESCRIPTION
The crypt() function performs password hashing. The password hashing scheme used by crypt() is dependent upon the contents of the NUL-terminated string setting. 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. If setting begins with the ‘_’ character, DES password hashing with a user specified number of perturbations is selected. If setting begins with any other character, DES password hashing with a fixed number of perturbations is selected.

  DES password hashing
The DES password hashing scheme is derived from the NBS Data Encryption Standard. Additional code has been added to deter key search attempts and to use stronger hashing algorithms. In the DES case, the second argument to crypt() is a character array, 9 bytes in length, consisting of an underscore (‘_’) followed by 4 bytes of iteration count and 4 bytes of salt. Both the iteration count and the salt are encoded with 6 bits per character, least significant bits first. The values 0 to 63 are encoded by the characters ‘./0-9A-Za-z’, respectively.

The salt is used to induce disorder in to the DES algorithm in one of 16777216 possible ways (specifically, if bit i of the salt is set then bits i and i+24 are swapped in the DES “E” box output). The key is divided into groups of 8 characters (a short final 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 group bits and the encryption of the DES key with itself becomes the next DES key. Then the final DES key is used to perform count cumulative encryptions of a 64-bit constant yielding a ‘ciphertext’. The value returned is a NUL-terminated string, 20 bytes in length, consisting of the setting followed by the encoded 64-bit ‘ciphertext’.

For compatibility with historical versions of crypt(), the setting may consist of 2 bytes of salt, encoded as above, in which case an iteration count of 25 is used, fewer perturbations of DES are available, at most 8 characters of key are used, and the returned value is a NUL-terminated string 13 bytes in length.

The functions encrypt(), setkey(), des_setkey() and des_cipher() allow limited access to the DES algorithm itself. The key argument to setkey() is a 64 character array of binary values (numeric 0 or 1). A 56-bit key is derived from this array by dividing the array into groups of 8 and ignoring the last bit in each group.

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

The des_setkey() and des_cipher() functions are faster but less portable than setkey() and encrypt(). The argument to des_setkey() is a character array of length 8. The least significant bit in each character is ignored and the next 7 bits of each character are concatenated to yield 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. The salt specifies perturbations to DES as described above.

  MD5 password hashing
For the MD5 password hashing scheme, the version number (in this case ‘1’), salt and the hashed password are separated by the ‘$’ character. An encoded password hash looks like:

    $1$2qGr5PPQ$eT08WBFev3RPLNChixg0H

The entire encoded MD5 password hash is passed as setting for interpretation.

  Argon2 password hashing
Argon2 is a memory-hard password hashing algorithm. crypt() provides all three variants: argon2i, argon2d, and argon2id. It is recommended to use argon2id, which provides a hybrid combination using argon2i on the first pass, and argon2d on the remaining passes. We parameterize on three variables. First, m_cost (m), specifies the memory usage in KB. Second, t_cost (t), specifies the number of iterations. Third, parallelism (p) specifies the number of threads. This is currently ignored and one thread will always be used. An encoded Argon2 password hash looks like:

    $argon2id$v=19$m=4096,t=6,p=1$qCatF9a1s/6TgcYB$ \
       yeYYrU/rh7E+LI2CAeHTSHVB3iO+OXiNIUHu6NPeTfo

containing five fields delimited by ‘$’. The fields, in order, are variant name, version, parameter set, 128-bit salt, and Argon2 hash encoded in base64. The entire encoded Argon2 password hash is required to be processed correctly.

  Blowfish bcrypt
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 hash. The maximum password length is 72. The final Blowfish password output 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. An encoded Blowfish password hash looks like:

    $2a$12$eIAq8PR8sIUnJ1HaohxX2O9x9Qlm2vK97LJ5dsXdmB.eXF42qjchC

The entire encoded Blowfish password hash is passed as setting for interpretation.

RETURN VALUES
The function crypt() returns a pointer to the encoded hash on success.

The behavior of crypt() on errors isn't well standardized. Some implementations simply can't fail (unless the process dies, in which case they obviously can't return), others return NULL or a fixed string. Most implementations don't set errno, but some do. susv2 specifies only returning NULL and setting errno as a valid behavior, and defines only one possible error (ENOSYS, “The functionality is not supported on this implementation.”) Unfortunately, most existing applications aren't prepared to handle NULL returns from crypt(). The description below corresponds to this implementation of crypt() only. The behavior may change to match standards, other implementations or existing applications.

crypt() may only fail (and return) when passed an invalid or unsupported setting, in which case it returns a pointer to a magic string that is shorter than 13 characters and is guaranteed to differ from setting. This behavior is safe for older applications which assume that crypt() can't fail, when both setting new passwords and authenticating against existing password hashes.

The functions setkey(), encrypt(), des_setkey(), and des_cipher() return 0 on success and 1 on failure. Historically, the functions setkey() and encrypt() did not return any value. They have been provided return values primarily to distinguish implementations where hardware support is provided but not available or where the DES encryption is not available due to the usual political silliness.

SEE ALSO
login(1), passwd(1), pwhash(1), getpass(3), md5(3), passwd(5), passwd.conf(5) Argon2: the memory-hard function for password hashing and other applications, Alex Biryukov, Daniel Dinu, Dmitry Khovratovich, 2017, University of Luxembourg, https://www.password-hashing.net/. Mathematical Cryptology for Computer Scientists and Mathematicians, Wayne Patterson, 1987, ISBN 0-8476-7438-X. Password Security: A Case History, R. Morris, Ken Thompson, Communications of the ACM, vol. 22, pp. 594-597, Nov. 1979. DES will be Totally Insecure within Ten Years, M.E. Hellman, IEEE Spectrum, vol. 16, pp. 32-39, July 1979.

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

BUGS
Dropping the least significant bit in each character of the argument to des_setkey() is ridiculous.

The crypt() function leaves its result in an internal static object and returns a pointer to that object. Subsequent calls to crypt() will modify the same object.

Before NetBSD 6.0 crypt() returned either NULL or: on error.

The term ‘encryption’ for password hashing does not match the terminology of modern cryptography, but the name of the library is entrenched.

A library for password hashing has no business directly exposing the DES cipher itself, which is obsolete and broken as a cipher.