path: root/lib/libcrypt/crypt.3

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.\" Manual page, using -mandoc macros
.\"
.Dd January 19, 1997
.Dt CRYPT 3
.Os "FreeSec 1.0"
.Sh NAME
.Nm crypt
.Nd Trapdoor encryption
.Sh LIBRARY
.Lb libcrypt
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft char *
.Fn crypt "const char *key" "const char *salt"
.Sh DESCRIPTION
The
.Fn crypt
function performs password hashing with additional code added to
deter key search attempts.  Different algorithms can be used to
in the hash.
.\"
.\" NOTICE:
.\" If you add more algorithms, make sure to update this list
.\" and the default used for the Traditional format, below.
.\"
Currently these include the
.Tn NBS
.Tn Data Encryption Standard (DES) ,
and
.Tn MD5 .
The algorithm used will depend upon the format of the Salt--following
the Modular Crypt Format (MCF)--and if
.Tn DES
is installed or not.
.Pp
The first argument to
.Nm
is the data to hash (usually a password), in a
.Dv null Ns -terminated
string.
The second is the salt, in one of three forms:
.Pp
.Bl -tag -width Traditional -compact -offset indent
.It Extended
If it begins with an underscore
.Pq Dq _
then the
.Tn DES
Extended Format
is used in interpreting both the the key and the salt, as outlined below.
.It Modular
If it begins with the string
.Dq $digit$
then the Modular Crypt Format is used, as outlined below.
.It Traditional
If neither of the above is true, it assumes the Traditional Format,
using the entire string as the salt (or the first portion).
.El
.Pp
All routines are designed to be time-consuming.  A brief test on a
.Tn Pentium
166/MMX shows the
.Tn DES
crypt to do approximately 2640 crypts
a CPU second and MD5 to do about 62 crypts a CPU second.
.Ss DES Extended Format:
.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
.Tn DES
key as follows:
the first group of 56 bits becomes the initial
.Tn DES
key.
For each additional group, the XOR of the encryption of the current
.Tn DES
key with itself and the group bits becomes the next
.Tn DES
key.
.Pp
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
.Fa count
and
.Fa salt .
.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
.Tn 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 salt
followed by the encoded 64-bit encryption.
.Ss "Modular" crypt:
.Pp
If the salt begins with the string 
.Fa $digit$
then the Modular Crypt Format is used.  The
.Fa 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 16 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.
.Pp
Currently supported algorithms are:
.Pp
.Bl -tag -width 012345678 -compact -offset indent
.It 1
MD5
.El
.Pp
Other crypt formats may be easilly added.  An example salt would be:
.Bl -tag -offset indent
.It Cm "$3$thesalt$rest"
.El
.Pp
.Ss "Traditional" crypt:
.Pp
The algorithm used will depend upon whether
.Tn DES
is installed or not.  If it is,
.Tn DES
will be used.  Otherwise, the best algorithm is used, which is currently
.\"
.\" NOTICE: Also make sure to update this
.\"
MD5.
.Pp
How the salt is used will depend upon the algorithm for the hash.  For
best results, specify at least two characters of salt.
.Sh RETURN VALUES
.Pp
.Fn crypt
returns a pointer to the encrypted value on success, and NULL on failure.
Note: this is not a standard behaviour, AT&T
.Fn crypt
will always return a pointer to a string.
.Sh SEE ALSO
.Xr login 1 ,
.Xr passwd 1 ,
.Xr cipher 3 ,
.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 data.
.Sh HISTORY
A rotor-based
.Fn crypt
function appeared in
.At v6 .
The current style
.Fn crypt
first appeared in
.At v7 .
.Pp
The
.Tn DES
section of the code (FreeSec 1.0) was developed outside the United
States of America as an unencumbered replacement for the U.S.-only
.Nx
libcrypt encryption library.
Users should be aware that this code (and programs staticly linked with it)
may not be exported from the U.S., although it apparently can be imported.
.Sh AUTHORS
Originally written by
.An David Burren Aq davidb@werj.com.au ,
later additions and changes by
.An Poul-henning Kamp ,
.An Mark R V Murray
and
.An Kris Kennaway .