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.\" "THE BEER-WARE LICENSE" (Revision 42):
.\" <phk@FreeBSD.org> wrote this file.  As long as you retain this notice you
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.\" $FreeBSD: release/9.1.0/lib/libmd/mdX.3 186941 2009-01-09 11:45:13Z phk $
.\"
.Dd February 11, 1999
.Dt MDX 3
.Os
.Sh NAME
.Nm MDXInit ,
.Nm MDXUpdate ,
.Nm MDXPad ,
.Nm MDXFinal ,
.Nm MDXEnd ,
.Nm MDXFile ,
.Nm MDXFileChunk ,
.Nm MDXData
.Nd calculate the RSA Data Security, Inc., ``MDX'' message digest
.Sh LIBRARY
.Lb libmd
.Sh SYNOPSIS
.In sys/types.h
.In mdX.h
.Ft void
.Fn MDXInit "MDX_CTX *context"
.Ft void
.Fn MDXUpdate "MDX_CTX *context" "const void *data" "unsigned int len"
.Ft void
.Fn MDXPad "MDX_CTX *context"
.Ft void
.Fn MDXFinal "unsigned char digest[16]" "MDX_CTX *context"
.Ft "char *"
.Fn MDXEnd "MDX_CTX *context" "char *buf"
.Ft "char *"
.Fn MDXFile "const char *filename" "char *buf"
.Ft "char *"
.Fn MDXFileChunk "const char *filename" "char *buf" "off_t offset" "off_t length"
.Ft "char *"
.Fn MDXData "const void *data" "unsigned int len" "char *buf"
.Sh DESCRIPTION
The MDX functions calculate a 128-bit cryptographic checksum (digest)
for any number of input bytes.
A cryptographic checksum is a one-way
hash-function, that is, you cannot find (except by exhaustive search)
the input corresponding to a particular output.
This net result is a
.Dq fingerprint
of the input-data, which does not disclose the actual input.
.Pp
MD2 is the slowest, MD4 is the fastest and MD5 is somewhere in the middle.
MD2 can only be used for Privacy-Enhanced Mail.
MD4 has now been broken; it should only be used where necessary for
backward compatibility.
MD5 has not yet (1999-02-11) been broken, but sufficient attacks have been
made that its security is in some doubt.
The attacks on both MD4 and MD5
are both in the nature of finding
.Dq collisions
\[en]
that is, multiple
inputs which hash to the same value; it is still unlikely for an attacker
to be able to determine the exact original input given a hash value.
.Pp
The
.Fn MDXInit ,
.Fn MDXUpdate ,
and
.Fn MDXFinal
functions are the core functions.
Allocate an
.Vt MDX_CTX ,
initialize it with
.Fn MDXInit ,
run over the data with
.Fn MDXUpdate ,
and finally extract the result using
.Fn MDXFinal .
.Pp
The
.Fn MDXPad
function can be used to pad message data in same way
as done by
.Fn MDXFinal
without terminating calculation.
.Pp
The
.Fn MDXEnd
function is a wrapper for
.Fn MDXFinal
which converts the return value to a 33-character
(including the terminating '\e0')
.Tn ASCII
string which represents the 128 bits in hexadecimal.
.Pp
The
.Fn MDXFile
function calculates the digest of a file, and uses
.Fn MDXEnd
to return the result.
If the file cannot be opened, a null pointer is returned.
The
.Fn MDXFileChunk
function is similar to
.Fn MDXFile ,
but it only calculates the digest over a byte-range of the file specified,
starting at
.Fa offset
and spanning
.Fa length
bytes.
If the
.Fa length
parameter is specified as 0, or more than the length of the remaining part
of the file,
.Fn MDXFileChunk
calculates the digest from
.Fa offset
to the end of file.
The
.Fn MDXData
function calculates the digest of a chunk of data in memory, and uses
.Fn MDXEnd
to return the result.
.Pp
When using
.Fn MDXEnd ,
.Fn MDXFile ,
or
.Fn MDXData ,
the
.Fa buf
argument can be a null pointer, in which case the returned string
is allocated with
.Xr malloc 3
and subsequently must be explicitly deallocated using
.Xr free 3
after use.
If the
.Fa buf
argument is non-null it must point to at least 33 characters of buffer space.
.Sh SEE ALSO
.Xr md2 3 ,
.Xr md4 3 ,
.Xr md5 3 ,
.Xr sha 3
.Rs
.%A B. Kaliski
.%T The MD2 Message-Digest Algorithm
.%O RFC 1319
.Re
.Rs
.%A R. Rivest
.%T The MD4 Message-Digest Algorithm
.%O RFC 1186
.Re
.Rs
.%A R. Rivest
.%T The MD5 Message-Digest Algorithm
.%O RFC 1321
.Re
.Rs
.%A H. Dobbertin
.%T Alf Swindles Ann
.%J CryptoBytes
.%N 1(3):5
.%D 1995
.Re
.Rs
.%A MJ. B. Robshaw
.%T On Recent Results for MD2, MD4 and MD5
.%J RSA Laboratories Bulletin
.%N 4
.%D November 12, 1996
.Re
.Sh HISTORY
These functions appeared in
.Fx 2.0 .
.Sh AUTHORS
The original MDX routines were developed by
.Tn RSA
Data Security, Inc., and published in the above references.
This code is derived directly from these implementations by
.An Poul-Henning Kamp Aq phk@FreeBSD.org
.Pp
Phk ristede runen.
.Sh BUGS
No method is known to exist which finds two files having the same hash value,
nor to find a file with a specific hash value.
There is on the other hand no guarantee that such a method does not exist.
.Pp
MD2 has only been licensed for use in Privacy Enhanced Mail.
Use MD4 or MD5 if that is not what you are doing.