CRYPT (3) | C library functions | Unix Manual Pages

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CRYPT (3) | C library functions | Unix Manual Pages | :man▋

The first argument to crypt is the data to hash (usually a password), in a null -terminated string. The second is the salt, in one of three forms:

Extended	If it begins with an underscore ("_") then the DES Extended Format is used in interpreting both the key and the salt, as outlined below.
Modular	If it begins with the string "$digit$" then the Modular Crypt Format is used, as outlined below.
Traditional
	If neither of the above is true, it assumes the Traditional Format, using the entire string as the salt (or the first portion).

All routines are designed to be time-consuming. A brief test on a Pentium 166/MMX shows the DES crypt to do approximately 2640 crypts a CPU second and MD5 to do about 62 crypts a CPU second.

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 salt 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 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 null -terminated string, 20 or 13 bytes (plus null) in length, consisting of the salt followed by the encoded 64-bit encryption.

"Modular" crypt:

If the salt begins with the string $digit$ then the Modular Crypt Format is used. The digit represents which algorithm is used in encryption. Following the token is the actual salt to use in the encryption. The length of the salt is limited to 8 characters--because the length of the returned output is also limited (_PASSWORD_LEN). The salt must be terminated with the end of the string (NULL) or a dollar sign. Any characters after the dollar sign are ignored.

Currently supported algorithms are:

MD5
Blowfish
NT-Hash

Other crypt formats may be easily added. An example salt would be:

"$4$thesalt$rest"

"Traditional" crypt:

The algorithm used will depend upon whether crypt_set_format has been called and whether a global default format has been specified. Unless a global default has been specified or crypt_set_format has set the format to something else, the built-in default format is used. This is currently DES if it is available, or MD5 if not.

How the salt is used will depend upon the algorithm for the hash. For best results, specify at least two characters of salt.

The crypt_get_format function returns a constant string that represents the name of the algorithm currently used. Valid values are ‘des’, ‘blf’, ‘md5’ and ‘nth’.

The crypt_set_format function sets the default encoding format according to the supplied string.

The global default format can be set using the /etc/auth.conf file using the crypt_default property.