lib/libcrypt/crypt.3

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.Dd August 10, 2016
.Dt CRYPT 3
.Os
.Sh NAME
.Nm crypt
.Nd Trapdoor encryption
.Sh LIBRARY
.Lb libcrypt
.Sh SYNOPSIS
.In unistd.h
.Ft char *
.Fn crypt "const char *key" "const char *salt"
.Ft char *
.Fn crypt_r "const char *key" "const char *salt" "struct crypt_data *data"
.Ft const char *
.Fn crypt_get_format "void"
.Ft int
.Fn crypt_set_format "const char *string"
.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) ,
.Tn MD5
hash,
.Tn NT-Hash
.Pq compatible with Microsoft's NT scheme
and
.Tn Blowfish .
The algorithm used will depend upon the format of the Salt
.Po
following
the Modular Crypt Format
.Pq MCF
.Pc ,
if
.Tn DES
and/or
.Tn Blowfish
is installed or not, and whether
.Fn crypt_set_format
has been called to change the default.
.Pp
The first argument to
.Nm
is the data to hash
.Pq usually a password ,
in a
.Dv NUL 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 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
.Pq or the first portion .
.El
.Pp
All routines are designed to be time-consuming.
.Ss DES Extended Format:
The
.Ar key
is divided into groups of 8 characters
.Pq the last group is NUL-padded
and the low-order 7 bits of each character
.Pq 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
.Dq ./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
.Po
i.e., 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
.Pc .
.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 NUL Ns -terminated
string, 20 or 13 bytes
.Pq plus NUL
in length, consisting of the
.Ar salt
followed by the encoded 64-bit encryption.
.Ss Modular crypt:
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 maximum length of the salt used depends upon the module.
The salt must be terminated with the end of the string character
.Pq NUL
or a dollar sign.
Any characters after the dollar sign are ignored.
.Pp
Currently supported algorithms are:
.Pp
.Bl -enum -compact -offset indent
.It
MD5
.It
Blowfish
.It
NT-Hash
.It
(unused)
.It
SHA-256
.It
SHA-512
.El
.Pp
Other crypt formats may be easily added.
An example salt would be:
.Bl -tag -width 6n -offset indent
.It Cm "$4$thesalt$rest"
.El
.Ss Traditional crypt:
The algorithm used will depend upon whether
.Fn crypt_set_format
has been called and whether a global default format has been specified.
Unless a global default has been specified or
.Fn crypt_set_format
has set the format to something else, the built-in default format is
used.
This is currently
.\"
.\" NOTICE: Also make sure to update this
.\"
DES
if it is available, or MD5 if not.
.Pp
How the salt is used will depend upon the algorithm for the hash.
For
best results, specify at least eight characters of salt.
.Pp
The
.Fn crypt_get_format
function returns a constant string that represents the name of the
algorithm currently used.
Valid values are
.\"
.\" NOTICE: Also make sure to update this, too, as well
.\"
.Ql des ,
.Ql blf ,
.Ql md5 ,
.Ql sha256 ,
.Ql sha512
and
.Ql nth .
.Pp
The
.Fn crypt_set_format
function sets the default encoding format according to the supplied
.Fa string .
.Pp
The
.Fn crypt_r
function behaves identically to
.Fn crypt ,
except that the resulting string is stored in
.Fa data ,
making it thread-safe.
.Sh RETURN VALUES
The
.Fn crypt
and
.Fn crypt_r
functions return 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.
.Pp
The
.Fn crypt_set_format
function will return 1 if the supplied encoding format was valid.
Otherwise, a value of 0 is returned.
.Sh SEE ALSO
.Xr login 1 ,
.Xr passwd 1 ,
.Xr getpass 3 ,
.Xr passwd 5
.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.
.Pp
The
.Fn crypt_r
function was added in
.Fx 12.0 .
.Sh AUTHORS
.An -nosplit
Originally written by
.An David Burren Aq Mt davidb@werj.com.au ,
later additions and changes by
.An Poul-Henning Kamp ,
.An Mark R V Murray ,
.An Michael Bretterklieber ,
.An Kris Kennaway ,
.An Brian Feldman ,
.An Paul Herman
and
.An Niels Provos .
.Sh BUGS
The
.Fn crypt
function returns a pointer to static data, and subsequent calls to
.Fn crypt
will modify the same data.
Likewise,
.Fn crypt_set_format
modifies static data.
.Pp
The NT-hash scheme does not use a salt,
and is not hard
for a competent attacker
to break.
Its use is not recommended.