freebsd-dev/lib/libmd/mdX.3

.\"
.\" ----------------------------------------------------------------------------
.\" "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
.\" ----------------------------------------------------------------------------
.\"
.\" $FreeBSD$
.\"
.Dd July 14, 1994
.Dt MDX 3
.Os FreeBSD 2
.Sh NAME
.Nm MDXInit ,
.Nm MDXUpdate ,
.Nm MDXFinal ,
.Nm MDXEnd ,
.Nm MDXFile ,
.Nm MDXData
.Nd calculate ``MDX'' cryptographic checksum
.Sh SYNOPSIS
.Fd #include <mdX.h>
.Ft void
.Fn MDXInit "MDX_CTX *context"
.Ft void
.Fn MDXUpdate "MDX_CTX *context" "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 "char *filename" "char *buf"
.Ft "char *"
.Fn MDXData "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.

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 critizised for being to weak, and MD5 was developed as a 
response to this as ``MD4 with safety-belts''.  If in doubt, use MD5.

The
.Fn MDXInit ,
.Fn MDXUpdate
and
.Fn MDXFinal
functions are the core functions.  Allocate a MDX_CTX, initialize it with
.Fn MDXInit
run over the data with
.Fn MDXUpdate
and finally extract the result using
.Fn MDXFinal .

.Fn MDXEnd
is a wrapper for
.Fn MDXFinal ,
which converts the return value to a 33 character (incl terminating NULL)
ascii string which represents the 128 bits in hexadecimal.

.Fn MDXFile
calculates the digest of a file, and uses 
.Fn MDXEnd
to return the result.
In case the file cannot be opened, NULL is returned.

.Fn MDXData
calculates the digest of a chunk of data in memory, and uses
.Fn MDXEnd
to return the result.

When using
.Fn MDXEnd ,
.Fn MDXFile
or
.Fn MDXData ,
the 
.Ar buf
argument can be NULL, 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 isn't NULL it must point to at least 33 characters of buffer space.
.Sh SEE ALSO
.Xr md2 3 ,
.Xr md4 3 ,
.Xr md5 3
.Pp
ISOC
Internet Request For Comments
.%T RFC1319 ,
.%T RFC1320
and
.%T RFC1321 .
.Pp
RSA Laboratories 
.%T Frequently Asked Questions About today's Cryptography . 
.Sh AUTHOR
The MD2, MD4 and MD5 is designed and written by Ron Rivest and published in
the above RFC's, including a reference implementation of each algorithm.

This code is derived directly from these implementations by Poul-Henning Kamp
<phk@login.dkuug.dk>

Phk ristede runen.
.Sh HISTORY
These functions appeared in
.Fx 2.0 .
.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.

MD2 has only been released for use in Privacy Enhanced eMail.
Use MD4 or MD5 if that isn't what you're doing.
.Sh COPYRIGHT