MD5簡介
MD5的全稱是Message-Digest Algorithm 5，在90年代初由MIT的計算機科學實驗室和RSA Data Security Inc發明，經MD2、MD3和MD4發展而來。
Message-Digest泛指字節串(Message)的Hash變換，就是把一個任意長度的字節串變換成一定長的大整數。請注意我使用了“字節串”而不是“字符串”這個詞，是因為這種變換只與字節的值有關，與字符集或編碼方式無關。
MD5將任意長度的“字節串”變換成一個128bit的大整數，并且它是一個不可逆的字符串變換算法，換句話說就是，即使你看到源程序和算法描述，也無法將一個MD5的值變換回原始的字符串，從數學原理上說，是因為原始的字符串有無窮多個，這有點象不存在反函數的數學函數。
MD5的典型應用是對一段Message(字節串)產生fingerprint(指紋)，以防止被“篡改”。舉個例子，你將一段話寫在一個叫readme.txt文件中，并對這個readme.txt產生一個MD5的值并記錄在案，然后你可以傳播這個文件給別人，別人如果修改了文件中的任何內容，你對這個文件重新計算MD5時就會發現。如果再有一個第三方的認證機構，用MD5還可以防止文件作者的“抵賴”，這就是所謂的數字簽名應用。
MD5還廣泛用于加密和解密技術上，在很多操作系統中，用戶的密碼是以MD5值（或類似的其它算法）的方式保存的，用戶Login的時候，系統是把用戶輸入的密碼計算成MD5值，然后再去和系統中保存的MD5值進行比較，而系統并不“知道”用戶的密碼是什么。
一些黑客破獲這種密碼的方法是一種被稱為“跑字典”的方法。有兩種方法得到字典，一種是日常搜集的用做密碼的字符串表，另一種是用排列組合方法生成的，先用MD5程序計算出這些字典項的MD5值，然后再用目標的MD5值在這個字典中檢索。
即使假設密碼的最大長度為8，同時密碼只能是字母和數字，共26+26+10=62個字符，排列組合出的字典的項數則是P(62,1)+P(62,2)….+P(62,8)，那也已經是一個很天文的數字了，存儲這個字典就需要TB級的磁盤組，而且這種方法還有一個前提，就是能獲得目標賬戶的密碼MD5值的情況下才可以。
在很多電子商務和社區應用中，管理用戶的Account是一種最常用的基本功能，盡管很多Application Server提供了這些基本組件，但很多應用開發者為了管理的更大的靈活性還是喜歡采用關系數據庫來管理用戶，懶惰的做法是用戶的密碼往往使用明文或簡單的變換后直接保存在數據庫中，因此這些用戶的密碼對軟件開發者或系統管理員來說可以說毫無保密可言，本文的目的是介紹MD5的Java Bean的實現，同時給出用MD5來處理用戶的Account密碼的例子，這種方法使得管理員和程序設計者都無法看到用戶的密碼，盡管他們可以初始化它們。但重要的一點是對于用戶密碼設置習慣的保護。
有興趣的讀者可以從這里取得MD5也就是RFC 1321的文本。http://www.ietf.org/rfc/rfc1321.txt 
以下就是該文本的摘要:
--------------------------------------------------------------

Network Working Group                                          R. Rivest
Request for Comments: 1321           MIT Laboratory for Computer Science
                                             and RSA Data Security, Inc.
                                                              April 1992

                     The MD5 Message-Digest Algorithm

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard.  Distribution of this memo is
   unlimited.

Acknowlegements

   We would like to thank Don Coppersmith, Burt Kaliski, Ralph Merkle,
   David Chaum, and Noam Nisan for numerous helpful comments and
   suggestions.

Table of Contents

   1. Executive Summary                                                1
   2. Terminology and Notation                                         2
   3. MD5 Algorithm Description                                        3
   4. Summary                                                          6
   5. Differences Between MD4 and MD5                                  6
   References                                                          7
   APPENDIX A - Reference Implementation                               7
   Security Considerations                                            21
   Author's Address                                                   21

1. Executive Summary

   This document describes the MD5 message-digest algorithm. The
   algorithm takes as input a message of arbitrary length and produces
   as output a 128-bit "fingerprint" or "message digest" of the input.
   It is conjectured that it is computationally infeasible to produce
   two messages having the same message digest, or to produce any
   message having a given prespecified target message digest. The MD5
   algorithm is intended for digital signature applications, where a
   large file must be "compressed" in a secure manner before being
   encrypted with a private (secret) key under a public-key cryptosystem
   such as RSA.




Rivest                                                          [Page 1]

RFC 1321              MD5 Message-Digest Algorithm            April 1992


   The MD5 algorithm is designed to be quite fast on 32-bit machines. In
   addition, the MD5 algorithm does not require any large substitution
   tables; the algorithm can be coded quite compactly.

   The MD5 algorithm is an extension of the MD4 message-digest algorithm
   1,2]. MD5 is slightly slower than MD4, but is more "conservative" in
   design. MD5 was designed because it was felt that MD4 was perhaps
   being adopted for use more quickly than justified by the existing
   critical review; because MD4 was designed to be exceptionally fast,
   it is "at the edge" in terms of risking successful cryptanalytic
   attack. MD5 backs off a bit, giving up a little in speed for a much
   greater likelihood of ultimate security. It incorporates some
   suggestions made by various reviewers, and contains additional
   optimizations. The MD5 algorithm is being placed in the public domain
   for review and possible adoption as a standard.

[1] [2]  下一頁
   For OSI-based applications, MD5's object identifier is

   md5 OBJECT IDENTIFIER ::=
     iso(1) member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 5}

   In the X.509 type AlgorithmIdentifier [3], the parameters for MD5
   should have type NULL.

2. Terminology and Notation

   In this document a "word" is a 32-bit quantity and a "byte" is an
   eight-bit quantity. A sequence of bits can be interpreted in a
   natural manner as a sequence of bytes, where each consecutive group
   of eight bits is interpreted as a byte with the high-order (most
   significant) bit of each byte listed first. Similarly, a sequence of
   bytes can be interpreted as a sequence of 32-bit words, where each
   consecutive group of four bytes is interpreted as a word with the
   low-order (least significant) byte given first.

   Let x_i denote "x sub i". If the subscript is an expression, we
   surround it in braces, as in x_{i+1}. Similarly, we use ^ for
   superscripts (exponentiation), so that x^i denotes x to the i-th
   power.

   Let the symbol "+" denote addition of words (i.e., modulo-2^32
   addition). Let X <<< s denote the 32-bit value obtained by circularly
   shifting (rotating) X left by s bit positions. Let not(X) denote the
   bit-wise complement of X, and let X v Y denote the bit-wise OR of X
   and Y. Let X xor Y denote the bit-wise XOR of X and Y, and let XY
   denote the bit-wise AND of X and Y.



Rivest                                                          [Page 2]

RFC 1321              MD5 Message-Digest Algorithm            April 1992


3. MD5 Algorithm Description

   We begin by supposing that we have a b-bit message as input, and that
   we wish to find its message digest. Here b is an arbitrary
   nonnegative integer; b may be zero, it need not be a multiple of
   eight, and it may be arbitrarily large. We imagine the bits of the
   message written down as follows:

          m_0 m_1 ... m_{b-1}

.........................................

      The MD5 test suite (driver option "-x") should print the following
   results:

/* 然后進行如下操作 */ 
A = A + AA 
B = B + BB 
C = C + CC 
D = D + DD 

end /* 結束對I的循環*/ 

6、輸出結果。