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.\"	$NetBSD: mdX.3,v 1.12 2018/05/23 06:08:01 wiz Exp $
.\"
.\" ----------------------------------------------------------------------------
.\" "THE BEER-WARE LICENSE" (Revision 42):
.\" <phk@login.dkuug.dk> wrote this file.  As long as you retain this notice you
.\" can do whatever you want with this stuff. If we meet some day, and you think
.\" this stuff is worth it, you can buy me a beer in return.   Poul-Henning Kamp
.\" ----------------------------------------------------------------------------
.\"
.\"	from FreeBSD Id: mdX.3,v 1.7 1996/10/22 16:28:56 phk Exp
.\"
.Dd May 22, 2018
.Dt MDX 3
.Os
.Sh NAME
.Nm MDXInit ,
.Nm MDXUpdate ,
.Nm MDXFinal ,
.Nm MDXEnd ,
.Nm MDXFile ,
.Nm MDXData
.Nd calculate the RSA Data Security, Inc.,
.Dq MDX
message digest
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In mdX.h
.Ft void
.Fn MDXInit "MDX_CTX *context"
.Ft void
.Fn MDXUpdate "MDX_CTX *context" "const unsigned char *data" "unsigned int len"
.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 MDXData "const unsigned char *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 ``fingerprint'' of the input-data, which doesn't 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 been criticized for being too weak, so MD5 was developed in
response as ``MD4 with safety-belts''.
When in doubt, use MD5.
.Pp
The
.Fn MDXInit ,
.Fn MDXUpdate ,
and
.Fn MDXFinal
functions are the core functions.
Allocate an MDX_CTX, initialize it with
.Fn MDXInit ,
run over the data with
.Fn MDXUpdate ,
and finally extract the result using
.Fn MDXFinal .
.Pp
.Fn MDXEnd
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
.Fn MDXFile
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.
.Fn MDXData
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
.Ar 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
.Ar 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 openssl_MD2 3 ,
.Xr openssl_MD4 3 ,
.Xr openssl_MD5 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 RSA Laboratories
.%T Frequently Asked Questions About today's Cryptography
.Re
.Sh HISTORY
These functions appeared in
.Nx 1.3 .
.Sh AUTHORS
.An -nosplit
The original MDX routines were developed by
.An RSA Data Security, Inc. ,
and published in the above references.
This code is derived directly from these implementations by
.An Poul-Henning Kamp Aq Mt phk@login.dkuug.dk .
.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 doesn't exist.