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