Crypt

Crypt

crypt, setkey, encrypt, des_setkey, des_cipher, — DES encryption

SYNOPSIS

#include

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

void
setkey(char *key);

void
encrypt(char *block, int flag);

int
des_setkey(const char *key);

int
des_cipher(const char *in, char *out, long salt, int count);

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. The first argument to crypt() is a null-terminated
string, typically a user’s typed password. The second is in one of two
forms: if it begins with an underscore (“_”) then an extended format is
used in interpreting both the key and the setting, as outlined below.

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 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.

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.

Algorithm:

The salt introduces disorder in the DES algorithm in one of 16777216 or
4096 possible ways (ie. 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 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.

EOF

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