313 lines
7.6 KiB
Groff
313 lines
7.6 KiB
Groff
.\" FreeSec: libcrypt for NetBSD
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.\"
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.\" Copyright (c) 1994 David Burren
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 4. Neither the name of the author nor the names of other contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd April 9, 2011
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.Dt CRYPT 3
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.Os
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.Sh NAME
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.Nm crypt
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.Nd Trapdoor encryption
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.Sh LIBRARY
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.Lb libcrypt
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.Sh SYNOPSIS
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.In unistd.h
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.Ft char *
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.Fn crypt "const char *key" "const char *salt"
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.Ft const char *
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.Fn crypt_get_format "void"
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.Ft int
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.Fn crypt_set_format "const char *string"
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.Sh DESCRIPTION
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The
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.Fn crypt
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function performs password hashing with additional code added to
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deter key search attempts.
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Different algorithms can be used to
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in the hash.
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.\"
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.\" NOTICE:
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.\" If you add more algorithms, make sure to update this list
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.\" and the default used for the Traditional format, below.
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.\"
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Currently these include the
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.Tn NBS
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.Tn Data Encryption Standard (DES) ,
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.Tn MD5
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hash,
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.Tn NT-Hash
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(compatible with Microsoft's NT scheme)
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and
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.Tn Blowfish .
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The algorithm used will depend upon the format of the Salt (following
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the Modular Crypt Format (MCF)), if
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.Tn DES
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and/or
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.Tn Blowfish
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is installed or not, and whether
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.Fn crypt_set_format
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has been called to change the default.
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.Pp
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The first argument to
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.Nm
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is the data to hash (usually a password), in a
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.Dv null Ns -terminated
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string.
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The second is the salt, in one of three forms:
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.Pp
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.Bl -tag -width Traditional -compact -offset indent
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.It Extended
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If it begins with an underscore
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.Pq Dq _
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then the
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.Tn DES
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Extended Format
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is used in interpreting both the key and the salt, as outlined below.
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.It Modular
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If it begins with the string
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.Dq $digit$
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then the Modular Crypt Format is used, as outlined below.
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.It Traditional
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If neither of the above is true, it assumes the Traditional Format,
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using the entire string as the salt (or the first portion).
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.El
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.Pp
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All routines are designed to be time-consuming.
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A brief test on a
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.Tn Pentium
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166/MMX shows the
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.Tn DES
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crypt to do approximately 2640 crypts
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a CPU second and MD5 to do about 62 crypts a CPU second.
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.Ss DES Extended Format:
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.Pp
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The
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.Ar key
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is divided into groups of 8 characters (the last group is null-padded)
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and the low-order 7 bits of each character (56 bits per group) are
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used to form the
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.Tn DES
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key as follows:
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the first group of 56 bits becomes the initial
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.Tn DES
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key.
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For each additional group, the XOR of the encryption of the current
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.Tn DES
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key with itself and the group bits becomes the next
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.Tn DES
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key.
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.Pp
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The salt is a 9-character array consisting of an underscore followed
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by 4 bytes of iteration count and 4 bytes of salt.
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These are encoded as printable characters, 6 bits per character,
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least significant character first.
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The values 0 to 63 are encoded as ``./0-9A-Za-z''.
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This allows 24 bits for both
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.Fa count
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and
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.Fa salt .
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.Pp
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The
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.Fa salt
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introduces disorder in the
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.Tn DES
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algorithm in one of 16777216 or 4096 possible ways
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(i.e., with 24 or 12 bits: if bit
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.Em i
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of the
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.Ar salt
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is set, then bits
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.Em i
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and
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.Em i+24
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are swapped in the
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.Tn DES
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E-box output).
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.Pp
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The
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.Tn DES
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key is used to encrypt a 64-bit constant using
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.Ar count
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iterations of
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.Tn DES .
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The value returned is a
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.Dv null Ns -terminated
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string, 20 or 13 bytes (plus null) in length, consisting of the
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.Ar salt
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followed by the encoded 64-bit encryption.
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.Ss "Modular" crypt:
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.Pp
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If the salt begins with the string
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.Fa $digit$
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then the Modular Crypt Format is used.
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The
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.Fa digit
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represents which algorithm is used in encryption.
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Following the token is
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the actual salt to use in the encryption.
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The length of the salt is limited
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to 8 characters--because the length of the returned output is also limited
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(_PASSWORD_LEN).
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The salt must be terminated with the end of the string
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(NULL) or a dollar sign.
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Any characters after the dollar sign are ignored.
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.Pp
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Currently supported algorithms are:
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.Pp
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.Bl -enum -compact -offset indent
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.It
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MD5
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.It
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Blowfish
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.It
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NT-Hash
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.It
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SHA-256
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.It
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SHA-512
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.El
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.Pp
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Other crypt formats may be easily added.
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An example salt would be:
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.Bl -tag -offset indent
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.It Cm "$4$thesalt$rest"
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.El
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.Pp
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.Ss "Traditional" crypt:
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.Pp
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The algorithm used will depend upon whether
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.Fn crypt_set_format
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has been called and whether a global default format has been specified.
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Unless a global default has been specified or
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.Fn crypt_set_format
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has set the format to something else, the built-in default format is
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used.
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This is currently
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.\"
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.\" NOTICE: Also make sure to update this
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.\"
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DES
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if it is available, or MD5 if not.
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.Pp
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How the salt is used will depend upon the algorithm for the hash.
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For
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best results, specify at least two characters of salt.
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.Pp
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The
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.Fn crypt_get_format
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function returns a constant string that represents the name of the
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algorithm currently used.
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Valid values are
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.\"
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.\" NOTICE: Also make sure to update this, too, as well
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.\"
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.Ql des ,
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.Ql blf ,
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.Ql md5 ,
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.Ql sha256 ,
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.Ql sha512
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and
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.Ql nth .
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.Pp
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The
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.Fn crypt_set_format
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function sets the default encoding format according to the supplied
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.Fa string .
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.Pp
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The global default format can be set using the
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.Pa /etc/auth.conf
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file using the
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.Va crypt_default
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property.
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.Sh RETURN VALUES
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The
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.Fn crypt
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function returns a pointer to the encrypted value on success, and NULL on
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failure.
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Note: this is not a standard behaviour, AT&T
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.Fn crypt
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will always return a pointer to a string.
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.Pp
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The
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.Fn crypt_set_format
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function will return 1 if the supplied encoding format was valid.
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Otherwise, a value of 0 is returned.
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.Sh SEE ALSO
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.Xr login 1 ,
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.Xr passwd 1 ,
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.Xr auth_getval 3 ,
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.Xr getpass 3 ,
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.Xr auth.conf 5 ,
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.Xr passwd 5
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.Sh HISTORY
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A rotor-based
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.Fn crypt
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function appeared in
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.At v6 .
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The current style
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.Fn crypt
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first appeared in
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.At v7 .
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.Pp
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The
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.Tn DES
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section of the code (FreeSec 1.0) was developed outside the United
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States of America as an unencumbered replacement for the U.S.-only
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.Nx
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libcrypt encryption library.
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.Sh AUTHORS
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.An -nosplit
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Originally written by
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.An David Burren Aq davidb@werj.com.au ,
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later additions and changes by
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.An Poul-Henning Kamp ,
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.An Mark R V Murray ,
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.An Michael Bretterklieber ,
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.An Kris Kennaway ,
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.An Brian Feldman ,
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.An Paul Herman
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and
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.An Niels Provos .
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.Sh BUGS
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The
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.Fn crypt
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function returns a pointer to static data, and subsequent calls to
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.Fn crypt
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will modify the same data.
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Likewise,
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.Fn crypt_set_format
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modifies static data.
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.Pp
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The NT-hash scheme does not use a salt,
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and is not hard
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for a competent attacker
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to break.
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Its use is not recommended.
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