path: root/lib/libc/crypt/crypt.3

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.Dd March 9, 1994
.Dt CRYPT 3
.Os "FreeSec 1.0"
.Sh NAME
.Nm crypt ,
.Nm setkey ,
.Nm encrypt ,
.Nm des_setkey ,
.Nm des_cipher ,
.Nd DES encryption
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft char
.Fn *crypt "const char *key" "const char *setting"
.Ft int
.Fn setkey "char *key"
.Ft int
.Fn encrypt "char *block" "int flag"
.Ft int
.Fn des_setkey "const char *key"
.Ft int
.Fn des_cipher "const char *in" "char *out" "int32_t salt" "int count"
.Sh DESCRIPTION
The
.Fn crypt
function performs password encryption, based on the
.Tn NBS
Data Encryption Standard (DES).
Additional code has been added to deter key search attempts.
The first argument to
.Fn crypt
is a
.Dv null Ns -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.
.Ss Extended crypt:
.Pp
The
.Ar 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.
.Pp
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
.Fa count
and
.Fa salt .
.Ss "Traditional" crypt:
.Pp
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
.Tn DES
key.
.Pp
The setting is a 2-character array of the ASCII-encoded salt.
Thus only 12 bits of
.Fa salt
are used.
.Fa count
is set to 25.
.Ss Algorithm:
.Pp
The
.Fa salt
introduces disorder in the
.Tn DES
algorithm in one of 16777216 or 4096 possible ways
(ie. with 24 or 12 bits: if bit
.Em i
of the
.Ar salt
is set, then bits
.Em i
and
.Em i+24
are swapped in the
.Tn DES
E-box output).
.Pp
The DES key is used to encrypt a 64-bit constant using
.Ar count
iterations of
.Tn DES .
The value returned is a
.Dv null Ns -terminated
string, 20 or 13 bytes (plus null) in length, consisting of the
.Ar setting
followed by the encoded 64-bit encryption.
.Pp
The functions,
.Fn encrypt ,
.Fn setkey ,
.Fn des_setkey
and
.Fn des_cipher
provide access to the
.Tn DES
algorithm itself.
.Fn 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.
.Pp
The
.Fa block
argument to
.Fn encrypt
is also a 64-byte array of binary values.
If the value of
.Fa flag
is 0,
.Fa block
is encrypted otherwise it is decrypted.
The result is returned in the original array
.Fa block
after using the key specified by
.Fn setkey
to process it.
.Pp
The argument to
.Fn 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
.Fn des_cipher
encrypts (or decrypts if
.Fa count
is negative) the 64-bits stored in the 8 characters at
.Fa in
using
.Xr abs 3
of
.Fa count
iterations of
.Tn DES
and stores the 64-bit result in the 8 characters at
.Fa out
(which may be the same as
.Fa in
).
The
.Fa salt
specifies perturbations to the
.Tn DES
E-box output as described above.
.Pp
The function
.Fn crypt
returns a pointer to the encrypted value on success, and NULL on failure.
The functions
.Fn setkey ,
.Fn encrypt ,
.Fn des_setkey ,
and
.Fn des_cipher
return 0 on success and 1 on failure.
.Pp
The
.Fn crypt ,
.Fn setkey
and
.Fn des_setkey
functions all manipulate the same key space.
.Sh SEE ALSO
.Xr login 1 ,
.Xr passwd 1 ,
.Xr getpass 3 ,
.Xr passwd 5
.Sh BUGS
The
.Fn crypt
function returns a pointer to static data, and subsequent calls to
.Fn crypt
will modify the same object.
.Sh HISTORY
A rotor-based
.Fn crypt
function appeared in
.At v6 .
The current style
.Fn crypt
first appeared in
.At v7 .
.Pp
This library (FreeSec 1.0) was developed outside the United States of America
as an unencumbered replacement for the U.S.-only libcrypt encryption
library.
Programs linked against the 
.Fn crypt
interface may be exported from the U.S.A. only if they use 
.Fn crypt
solely for authentication purposes and avoid use of
the other programmer interfaces listed above. Special care has been taken
in the library so that programs which only use the 
.Fn crypt
interface do not pull in the other components.
.Sh AUTHOR
David Burren <davidb@werj.com.au>