md5(3)

NAME

MD5Init, MD5Update, MD5Pad, MD5Final, MD5End, MD5File,

MD5FileChunk

MD5Data - calculate the RSA Data Security, Inc., ``MD5''

message digest

LIBRARY

library ``libmd''

SYNOPSIS

#include <sys/types.h>
#include <md5.h>
void
MD5Init(MD5_CTX *context);
void
MD5Update(MD5_CTX  *context,  const  unsigned  char   *data,
unsigned int len);
void
MD5Pad(MD5_CTX *context);
void
MD5Final(unsigned char digest[16], MD5_CTX *context);
char *
MD5End(MD5_CTX *context, char *buf);
char *
MD5File(const char *filename, char *buf);
char *
MD5FileChunk(const char *filename, char *buf, off_t offset,
        off_t length);
char *
MD5Data(const  unsigned  char  *data, unsigned int len, char
*buf);

DESCRIPTION

The MD5 functions calculate a 128-bit cryptographic checksum

(digest) for
any number of input bytes. A cryptographic checksum is a

one-way hashfunction, 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 does not 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 now been
broken; it should only be used where necessary for backward

compatibility. MD5 has not yet (1999-02-11) been broken, but suffi

cient attacks
have been made that its security is in some doubt. The at

tacks on both
MD4 and MD5 are both in the nature of finding ``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.

The MD5Init(), MD5Update(), and MD5Final() functions are the

core functions. Allocate an MD5_CTX, initialize it with MD5Init(),

run over the
data with MD5Update(), and finally extract the result using

MD5Final

The MD5Pad() function can be used to pad message data in

same way as done
by MD5Final() without terminating calculation.

The MD5End() function is a wrapper for MD5Final() which con

verts the
return value to a 33-character (including the terminating
string which represents the 128 bits in hexadecimal.

The MD5File() function calculates the digest of a file, and

uses MD5End()
to return the result. If the file cannot be opened, a null

pointer is
returned. The MD5FileChunk() function is similar to

MD5File

only calculates the digest over a byte-range of the file

specified,
starting at offset and spanning length bytes. If the length

parameter is
specified as 0, or more than the length of the remaining

part of the
file, MD5FileChunk() calculates the digest from offset to

the end of
file. The MD5Data() function calculates the digest of a

chunk of data in
memory, and uses MD5End() to return the result.

When using MD5End(), MD5File(), or MD5Data(), the buf argu

ment can be a
null pointer, in which case the returned string is allocated

with
malloc(3) and subsequently must be explicitly deallocated

using free(3)
after use. If the buf argument is non-null it must point to

at least 33
characters of buffer space.

SEE ALSO

md2(3), md4(3), md5(3), sha(3)

B. Kaliski, The MD2 Message-Digest Algorithm, RFC 1319.

R. Rivest, The MD4 Message-Digest Algorithm, RFC 1186.

R. Rivest, The MD5 Message-Digest Algorithm, RFC 1321.

RSA Laboratories, Frequently Asked Questions About today's

Cryptography,
<http://www.rsa.com/rsalabs/faq/>.

H. Dobbertin, "Alf Swindles Ann", CryptoBytes, 1(3):5, 1995.

MJ. B. Robshaw, "On Recent Results for MD2, MD4 and MD5",

RSA
Laboratories Bulletin, 4, November 12, 1996.

HISTORY

These functions appeared in FreeBSD 2.0.

AUTHORS

The original MD5 routines were developed by RSA Data Securi

ty, Inc., and
published in the above references. This code is derived di

rectly from
these implementations by Poul-Henning Kamp <phk@FreeBSD.org>

Phk ristede runen.

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.

MD2 has only been licensed for use in Privacy Enhanced Mail.

Use MD4 or
MD5 if that is not what you are doing. Copyright (C)

1991-2, RSA Data
Security, Inc. Created 1991. All rights reserved.

License to copy and use this software is granted provided

that it is
identified as the "RSA Data Security, Inc. MD5 Message-Di

gest Algorithm"
in all material mentioning or referencing this software or

this function.

License is also granted to make and use derivative works

provided that
such works are identified as "derived from the RSA Data Se

curity, Inc.
MD5 Message-Digest Algorithm" in all material mentioning or

referencing
the derived work.

RSA Data Security, Inc. makes no representations concerning

either the
merchantability of this software or the suitability of this

software for
any particular purpose. It is provided "as is" without ex

press or implied
warranty of any kind.

These notices must be retained in any copies of any part of

this documentation and/or software.

BSD February 11, 1999

BSD