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简单的实现MD5加密算法

C++/object oriented translation and modification of MD5.

Version: 1.00 (28 Aug 95)
Version: 1.02 (22 Sep 97)

 Translation and modification (c) 1995 by Mordechai T. Abzug 
 Thanks to Martin Cleaver for for making it happy on Windows NT and Solaris.

 This translation/ modification is provided "as is," without express or 
 implied warranty of any kind.

 The translator/ modifier does not claim (1) that MD5 will do what you think 
 it does; (2) that this translation/ modification is accurate; or (3) that 
 this software is "merchantible."  (Language for this disclaimer partially 
 copied from the disclaimer below).

Based on:

   MD5.H - header file for MD5C.C
   MD5C.C - RSA Data Security, Inc., MD5 message-digest algorithm
   MDDRIVER.C - test driver for MD2, MD4 and MD5

   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-Digest
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
Security, 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 express or implied warranty of any kind.

These notices must be retained in any copies of any part of this
documentation and/or software.

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A note on compilation:

	MD5 assumes that an unsigned char is at least a byte long, that an
	unsigned short int is at least two bytes long, and that an unsigned
	int is at least four bytes long.  If your system is different, you 
	should modify md5.hh accordingly.


HOW TO USE:

(1) standalone:  

	Driver.cc provides a nice interface for simple uses of md5.  The 
	included makefile will do fine.  However, if this is the limit of your
	md5 use, you may as well use the original C source, which is somewhat 
	smaller.

(2) as an object-oriented module:

	The MD5 objects in this module are MD5 contexts that are initialized, 
	then updated zero or more times, then finalized, and then have their 
	digest accessed zero or more times.  Note that they *must* be 
	"finalized" between initialization and the availability of the digest;
	this is part of the definition of MD5.

	The updating is done with a message, either with a stdio file opened 
	for reading (it is the user's responsibility to make sure that the file
	is open), with an ifstream (again, the user's responsibility), with an 
	istream, or with an unsigned char string (typecast if you're using
	normal char strings).  If you initialized with the default contructor
	(see later), you can update a single MD5 object multiple times, even
	with different types of objects (say, a file and then a string).  Note
	that a stdio file is closed after an update, and the streams must no
	longer be "good".  Example:  "context.update(file);".

	Initialization of the object depends on the type of updating you
	intend to do (which is why it's here).  If you want a general-purpose
	MD5 object capable of being updated multiple times, with any object
	type, use the default constructor (ie. "MD5 context;").  If you only
	want to use the MD5 object for one update, you can initialize with
	that object; this does an implicit finalization Example: "MD5 context
	(cin);".

	Finalization must be done explicitly on object initialized with the
	default constructor (see above); finalization is implicit with all
	other contrsuctors.  Once finalization has occurred, update is an
	error.  Example:  "context.finalize();"

	After finalizing, you can look at the digest.  MD5::raw_digest()
	returns an 16-member array of unsigned chars (bytes).
	MD5::hex_digest() returns a 33-byte long character string that is a
	hex representation of the raw digest.  There is also a << operator for
	ostreams.  Example:  "unsigned char *digest=context.hex_digest();".
	Example:  'cout << "Digest is:  " << context << endl;'.

	See driver.cc for more examples.