.\"
.\" Copyright (c) 2000 Poul-Henning Kamp <phk@FreeBSD.org>
.\"
.\" Permission to use, copy, modify, and distribute this software for any
.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
.\"
.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
.\"
.\" If we meet some day, and you think this stuff is worth it, you
.\" can buy me a beer in return. Poul-Henning Kamp
.\"
.\" 	$OpenBSD: md5.3,v 1.2 2014/04/03 15:55:29 beck Exp $
.\"
.Dd $Mdocdate: April 3 2014 $
.Dt MD5 3
.Os
.Sh NAME
.Nm MD5Init ,
.Nm MD5Update ,
.Nm MD5Pad ,
.Nm MD5Final ,
.Nm MD5Transform ,
.Nm MD5End ,
.Nm MD5File ,
.Nm MD5FileChunk ,
.Nm MD5Data
.Nd calculate the RSA Data Security, Inc., ``MD5'' message digest
.Sh SYNOPSIS
.In sys/types.h
.In md5.h
.Ft void
.Fn MD5Init "MD5_CTX *context"
.Ft void
.Fn MD5Update "MD5_CTX *context" "const u_int8_t *data" "size_t len"
.Ft void
.Fn MD5Pad "MD5_CTX *context"
.Ft void
.Fn MD5Final "u_int8_t digest[MD5_DIGEST_LENGTH]" "MD5_CTX *context"
.Ft void
.Fn MD5Transform "u_int32_t state[4]" "u_int8_t block[MD5_BLOCK_LENGTH]"
.Ft "char *"
.Fn MD5End "MD5_CTX *context" "char *buf"
.Ft "char *"
.Fn MD5File "const char *filename" "char *buf"
.Ft "char *"
.Fn MD5FileChunk "const char *filename" "char *buf" "off_t offset" "off_t length"
.Ft "char *"
.Fn MD5Data "const u_int8_t *data" "size_t len" "char *buf"
.Sh DESCRIPTION
The MD5 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 doesn't disclose the actual input.
.Pp
MD5 has been broken; it should only be used where necessary for
backward compatibility.
The attack on MD5 is in the nature of finding
.Dq collisions
\- 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 MD5Init ,
.Fn MD5Update ,
and
.Fn MD5Final
functions are the core functions.
Allocate an
.Vt MD5_CTX ,
initialize it with
.Fn MD5Init ,
run over the data with
.Fn MD5Update ,
and finally extract the result using
.Fn MD5Final .
.Pp
The
.Fn MD5Pad
function can be used to apply padding to the message digest as in
.Fn MD5Final ,
but the current context can still be used with
.Fn MD5Update .
.Pp
The
.Fn MD5Transform
function is used by
.Fn MD5Update
to hash 512-bit blocks and forms the core of the algorithm.
Most programs should use the interface provided by
.Fn MD5Init ,
.Fn MD5Update
and
.Fn MD5Final
instead of calling
.Fn MD5Transform
directly.
.Pp
.Fn MD5End
is a wrapper for
.Fn MD5Final
which converts the return value to an MD5_DIGEST_STRING_LENGTH-character
(including the terminating '\e0')
.Tn ASCII
string which represents the 128 bits in hexadecimal.
.Pp
.Fn MD5File
calculates the digest of a file, and uses
.Fn MD5End
to return the result.
If the file cannot be opened, a null pointer is returned.
.Pp
.Fn MD5FileChunk
behaves like
.Fn MD5File
but calculates the digest only for that portion of the file starting at
.Fa offset
and continuing for
.Fa length
bytes or until end of file is reached, whichever comes first.
A zero
.Fa length
can be specified to read until end of file.
A negative
.Fa length
or
.Fa offset
will be ignored.
.Fn MD5Data
calculates the digest of a chunk of data in memory, and uses
.Fn MD5End
to return the result.
.Pp
When using
.Fn MD5End ,
.Fn MD5File ,
.Fn MD5FileChunk ,
or
.Fn MD5Data ,
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 MD5_DIGEST_STRING_LENGTH
characters of buffer space.
.Sh SEE ALSO
.Xr cksum 1 ,
.Xr md5 1 ,
.Xr rmd160 3 ,
.Xr sha1 3 ,
.Xr sha2 3
.Rs
.%A H. Dobbertin
.%D 1995
.%J CryptoBytes
.%N 1(3):5
.%T Alf Swindles Ann
.Re
.Rs
.%A MJ. B. Robshaw
.%D November 12, 1996
.%J RSA Laboratories Bulletin
.%N 4
.%T On Recent Results for MD4 and MD5
.Re
.Rs
.%A Hans Dobbertin
.%T Cryptanalysis of MD5 Compress
.Re
.Sh STANDARDS
.Rs
.%A R. Rivest
.%D April 1992
.%R RFC 1321
.%T The MD5 Message Digest Algorithm
.Re
.Sh HISTORY
These functions appeared in
.Ox 2.0 .
.Sh AUTHORS
The original MD5 routines were developed by
.Tn RSA
Data Security, Inc., and published in the above references.
This code is derived from a public domain implementation written by Colin Plumb.
.Pp
The
.Fn MD5End ,
.Fn MD5File ,
.Fn MD5FileChunk ,
and
.Fn MD5Data
helper functions are derived from code written by Poul-Henning Kamp.
.Sh BUGS
Collisions have been found for the full version of MD5.
The use of
.Xr sha2 3
is recommended instead.