MD5的全称是Message-Digest Algorithm 5（信息-摘要算法），在90年代初由MIT Laboratory for Computer Science和RSA Data Security Inc的Ronald L. Rivest开发出来，经MD2、MD3和MD4发展而来。它的作用是让大容量信息在用数字签名软件签署私人密匙前被"压缩"成一种保密的格式（就是把一个任意长度的字节串变换成一定长的大整数）。不管是MD2、MD4还是MD5，它们都需要获得一个随机长度的信息并产生一个128位的信息摘要。虽然这些算法的结构或多或少有些相似，但MD2的设计与MD4和MD5完全不同，那是因为MD2是为8位机器做过设计优化的，而MD4和MD5却是面向32位的电脑。这三个算法的描述和C语言源代码在Internet RFCs 1321中有详细的描述http://www.ietf.org/rfc/rfc1321.txt），这是一份最权威的文档，由Ronald L. Rivest在1992年8月向IEFT提交。

　　Rivest在1989年开发出MD2算法。在这个算法中，首先对信息进行数据补位，使信息的字节长度是16的倍数。然后，以一个16位的检验和追加到信息末尾。并且根据这个新产生的信息计算出散列值。后来，Rogier和Chauvaud发现如果忽略了检验和将产生MD2冲突。MD2算法的加密后结果是唯一的--既没有重复。

　　为了加强算法的安全性，Rivest在1990年又开发出MD4算法。MD4算法同样需要填补信息以确保信息的字节长度加上448后能被512整除（信息字节长度mod 512 = 448）。然后，一个以64位二进制表示的信息的最初长度被添加进来。信息被处理成512位Damg?rd/Merkle迭代结构的区块，而且每个区块要通过三个不同步骤的处理。Den Boer和Bosselaers以及其他人很快的发现了攻击MD4版本中第一步和第三步的漏洞。Dobbertin向大家演示了如何利用一部普通的个人电脑在几分钟内找到MD4完整版本中的冲突（这个冲突实际上是一种漏洞，它将导致对不同的内容进行加密却可能得到相同的加密后结果）。毫无疑问，MD4就此被淘汰掉了。

　　尽管MD4算法在安全上有个这么大的漏洞，但它对在其后才被开发出来的好几种信息安全加密算法的出现却有着不可忽视的引导作用。除了MD5以外，其中比较有名的还有SHA-1、RIPE-MD以及HAVAL等。

　　一年以后，即1991年，Rivest开发出技术上更为趋近成熟的MD5算法。它在MD4的基础上增加了"安全-带子"（Safety-Belts）的概念。虽然MD5比MD4稍微慢一些，但却更为安全。这个算法很明显的由四个和MD4设计有少许不同的步骤组成。在MD5算法中，信息-摘要的大小和填充的必要条件与MD4完全相同。Den Boer和Bosselaers曾发现MD5算法中的假冲突（Pseudo-Collisions），但除此之外就没有其他被发现的加密后结果了。

　　Van Oorschot和Wiener曾经考虑过一个在散列中暴力搜寻冲突的函数（Brute-Force Hash Function），而且他们猜测一个被设计专门用来搜索MD5冲突的机器（这台机器在1994年的制造成本大约是一百万美元）可以平均每24天就找到一个冲突。但单从1991年到2001年这10年间，竟没有出现替代MD5算法的MD6或被叫做其他什么名字的新算法这一点，我们就可以看出这个瑕疵并没有太多的影响MD5的安全性。上面所有这些都不足以成为MD5的在实际应用中的问题。并且，由于MD5算法的使用不需要支付任何版权费用的，所以在一般的情况下（非绝密应用领域。但即便是应用在绝密领域内，MD5也不失为一种非常优秀的中间技术），MD5怎么都应该算得上是非常安全的了。

				

						

								

										

												

														

																

																		

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																				代码分析
																		
																		
Coding by Sunrise_Chen. 
																
														
												
										
								
								

										

												

														

																

																		
 Source Code
																
																

																		

																				

																						
<%
	'--------------------------------------
	' COCOON Disk Manager v3 -- MD5 Function
	'
	'	经典 MD5 算法的ASP实现
	'                      -- 摘自某国外网站
	'--------------------------------------
	Private Const BITS_TO_A_BYTE = 8
	Private Const BYTES_TO_A_WORD = 4
	Private Const BITS_TO_A_WORD = 32
	
	Private m_lOnBits(30)
	Private m_l2Power(30)
	
	Private Function LShift(lValue, iShiftBits)
		If iShiftBits = 0 Then
			LShift = lValue
			Exit Function
		ElseIf iShiftBits = 31 Then
			If lValue And 1 Then
				LShift = &H80000000
			Else
				LShift = 0
			End If
			Exit Function
		ElseIf iShiftBits < 0 Or iShiftBits > 31 Then
			Err.Raise 6
		End If
	
		If (lValue And m_l2Power(31 - iShiftBits)) Then
			LShift = ((lValue And m_lOnBits(31 - (iShiftBits + 1))) * m_l2Power(iShiftBits)) Or &H80000000
		Else
			LShift = ((lValue And m_lOnBits(31 - iShiftBits)) * m_l2Power(iShiftBits))
		End If
	End Function
	
	Private Function RShift(lValue, iShiftBits)
		If iShiftBits = 0 Then
			RShift = lValue
			Exit Function
		ElseIf iShiftBits = 31 Then
			If lValue And &H80000000 Then
				RShift = 1
			Else
				RShift = 0
			End If
			Exit Function
		ElseIf iShiftBits < 0 Or iShiftBits > 31 Then
			Err.Raise 6
		End If
	
		RShift = (lValue And &H7FFFFFFE) \ m_l2Power(iShiftBits)
	
		If (lValue And &H80000000) Then
			RShift = (RShift Or (&H40000000 \ m_l2Power(iShiftBits - 1)))
		End If
	End Function
	
	Private Function RotateLeft(lValue, iShiftBits)
		RotateLeft = LShift(lValue, iShiftBits) Or RShift(lValue, (32 - iShiftBits))
	End Function
	
	Private Function AddUnsigned(lX, lY)
		Dim lX4
		Dim lY4
		Dim lX8
		Dim lY8
		Dim lResult
	
		lX8 = lX And &H80000000
		lY8 = lY And &H80000000
		lX4 = lX And &H40000000
		lY4 = lY And &H40000000
		
		lResult = (lX And &H3FFFFFFF) + (lY And &H3FFFFFFF)
	
		If lX4 And lY4 Then
			lResult = lResult Xor &H80000000 Xor lX8 Xor lY8
		ElseIf lX4 Or lY4 Then
			If lResult And &H40000000 Then
				lResult = lResult Xor &HC0000000 Xor lX8 Xor lY8
			Else
				lResult = lResult Xor &H40000000 Xor lX8 Xor lY8
			End If
		Else
			lResult = lResult Xor lX8 Xor lY8
		End If
	
		AddUnsigned = lResult
	End Function
	
	Private Function md5_F(x, y, z)
		md5_F = (x And y) Or ((Not x) And z)
	End Function
	
	Private Function md5_G(x, y, z)
		md5_G = (x And z) Or (y And (Not z))
	End Function
	
	Private Function md5_H(x, y, z)
		md5_H = (x Xor y Xor z)
	End Function
	
	Private Function md5_I(x, y, z)
		md5_I = (y Xor (x Or (Not z)))
	End Function
	
	Private Sub md5_FF(a, b, c, d, x, s, ac)
		a = AddUnsigned(a, AddUnsigned(AddUnsigned(md5_F(b, c, d), x), ac))
		a = RotateLeft(a, s)
		a = AddUnsigned(a, b)
	End Sub
	
	Private Sub md5_GG(a, b, c, d, x, s, ac)
		a = AddUnsigned(a, AddUnsigned(AddUnsigned(md5_G(b, c, d), x), ac))
		a = RotateLeft(a, s)
		a = AddUnsigned(a, b)
	End Sub
	
	Private Sub md5_HH(a, b, c, d, x, s, ac)
		a = AddUnsigned(a, AddUnsigned(AddUnsigned(md5_H(b, c, d), x), ac))
		a = RotateLeft(a, s)
		a = AddUnsigned(a, b)
	End Sub
	
	Private Sub md5_II(a, b, c, d, x, s, ac)
		a = AddUnsigned(a, AddUnsigned(AddUnsigned(md5_I(b, c, d), x), ac))
		a = RotateLeft(a, s)
		a = AddUnsigned(a, b)
	End Sub
	
	Private Function ConvertToWordArray(sMessage)
		Dim lMessageLength
		Dim lNumberOfWords
		Dim lWordArray()
		Dim lBytePosition
		Dim lByteCount
		Dim lWordCount
		
		Const MODULUS_BITS = 512
		Const CONGRUENT_BITS = 448
		
		lMessageLength = Len(sMessage)
		
		lNumberOfWords = (((lMessageLength + ((MODULUS_BITS - CONGRUENT_BITS) \ BITS_TO_A_BYTE)) \ (MODULUS_BITS \ BITS_TO_A_BYTE)) + 1) * (MODULUS_BITS \ BITS_TO_A_WORD)
		ReDim lWordArray(lNumberOfWords - 1)
		
		lBytePosition = 0
		lByteCount = 0
		Do Until lByteCount >= lMessageLength
			lWordCount = lByteCount \ BYTES_TO_A_WORD
			lBytePosition = (lByteCount Mod BYTES_TO_A_WORD) * BITS_TO_A_BYTE
			lWordArray(lWordCount) = lWordArray(lWordCount) Or LShift(Asc(Mid(sMessage, lByteCount + 1, 1)), lBytePosition)
			lByteCount = lByteCount + 1
		Loop
	
		lWordCount = lByteCount \ BYTES_TO_A_WORD
		lBytePosition = (lByteCount Mod BYTES_TO_A_WORD) * BITS_TO_A_BYTE
		
		lWordArray(lWordCount) = lWordArray(lWordCount) Or LShift(&H80, lBytePosition)
		
		lWordArray(lNumberOfWords - 2) = LShift(lMessageLength, 3)
		lWordArray(lNumberOfWords - 1) = RShift(lMessageLength, 29)
	
		ConvertToWordArray = lWordArray
	End Function
	
	Private Function WordToHex(lValue)
		Dim lByte
		Dim lCount
		
		For lCount = 0 To 3
			lByte = RShift(lValue, lCount * BITS_TO_A_BYTE) And m_lOnBits(BITS_TO_A_BYTE - 1)
			WordToHex = WordToHex & Right("0" & Hex(lByte), 2)
		Next
	End Function
	
	Public Function MD5(sMessage)
		m_lOnBits(0) = CLng(1)
		m_lOnBits(1) = CLng(3)
		m_lOnBits(2) = CLng(7)
		m_lOnBits(3) = CLng(15)
		m_lOnBits(4) = CLng(31)
		m_lOnBits(5) = CLng(63)
		m_lOnBits(6) = CLng(127)
		m_lOnBits(7) = CLng(255)
		m_lOnBits(8) = CLng(511)
		m_lOnBits(9) = CLng(1023)
		m_lOnBits(10) = CLng(2047)
		m_lOnBits(11) = CLng(4095)
		m_lOnBits(12) = CLng(8191)
		m_lOnBits(13) = CLng(16383)
		m_lOnBits(14) = CLng(32767)
		m_lOnBits(15) = CLng(65535)
		m_lOnBits(16) = CLng(131071)
		m_lOnBits(17) = CLng(262143)
		m_lOnBits(18) = CLng(524287)
		m_lOnBits(19) = CLng(1048575)
		m_lOnBits(20) = CLng(2097151)
		m_lOnBits(21) = CLng(4194303)
		m_lOnBits(22) = CLng(8388607)
		m_lOnBits(23) = CLng(16777215)
		m_lOnBits(24) = CLng(33554431)
		m_lOnBits(25) = CLng(67108863)
		m_lOnBits(26) = CLng(134217727)
		m_lOnBits(27) = CLng(268435455)
		m_lOnBits(28) = CLng(536870911)
		m_lOnBits(29) = CLng(1073741823)
		m_lOnBits(30) = CLng(2147483647)
		
		m_l2Power(0) = CLng(1)
		m_l2Power(1) = CLng(2)
		m_l2Power(2) = CLng(4)
		m_l2Power(3) = CLng(8)
		m_l2Power(4) = CLng(16)
		m_l2Power(5) = CLng(32)
		m_l2Power(6) = CLng(64)
		m_l2Power(7) = CLng(128)
		m_l2Power(8) = CLng(256)
		m_l2Power(9) = CLng(512)
		m_l2Power(10) = CLng(1024)
		m_l2Power(11) = CLng(2048)
		m_l2Power(12) = CLng(4096)
		m_l2Power(13) = CLng(8192)
		m_l2Power(14) = CLng(16384)
		m_l2Power(15) = CLng(32768)
		m_l2Power(16) = CLng(65536)
		m_l2Power(17) = CLng(131072)
		m_l2Power(18) = CLng(262144)
		m_l2Power(19) = CLng(524288)
		m_l2Power(20) = CLng(1048576)
		m_l2Power(21) = CLng(2097152)
		m_l2Power(22) = CLng(4194304)
		m_l2Power(23) = CLng(8388608)
		m_l2Power(24) = CLng(16777216)
		m_l2Power(25) = CLng(33554432)
		m_l2Power(26) = CLng(67108864)
		m_l2Power(27) = CLng(134217728)
		m_l2Power(28) = CLng(268435456)
		m_l2Power(29) = CLng(536870912)
		m_l2Power(30) = CLng(1073741824)
		
		
		Dim x
		Dim k
		Dim AA
		Dim BB
		Dim CC
		Dim DD
		Dim a
		Dim b
		Dim c
		Dim d
		
		Const S11 = 7
		Const S12 = 12
		Const S13 = 17
		Const S14 = 22
		Const S21 = 5
		Const S22 = 9
		Const S23 = 14
		Const S24 = 20
		Const S31 = 4
		Const S32 = 11
		Const S33 = 16
		Const S34 = 23
		Const S41 = 6
		Const S42 = 10
		Const S43 = 15
		Const S44 = 21
		
		x = ConvertToWordArray(sMessage)
		
		a = &H67452301
		b = &HEFCDAB89
		c = &H98BADCFE
		d = &H10325476
		
		For k = 0 To UBound(x) Step 16
			AA = a
			BB = b
			CC = c
			DD = d
			
			md5_FF a, b, c, d, x(k + 0), S11, &HD76AA478
			md5_FF d, a, b, c, x(k + 1), S12, &HE8C7B756
			md5_FF c, d, a, b, x(k + 2), S13, &H242070DB
			md5_FF b, c, d, a, x(k + 3), S14, &HC1BDCEEE
			md5_FF a, b, c, d, x(k + 4), S11, &HF57C0FAF
			md5_FF d, a, b, c, x(k + 5), S12, &H4787C62A
			md5_FF c, d, a, b, x(k + 6), S13, &HA8304613
			md5_FF b, c, d, a, x(k + 7), S14, &HFD469501
			md5_FF a, b, c, d, x(k + 8), S11, &H698098D8
			md5_FF d, a, b, c, x(k + 9), S12, &H8B44F7AF
			md5_FF c, d, a, b, x(k + 10), S13, &HFFFF5BB1
			md5_FF b, c, d, a, x(k + 11), S14, &H895CD7BE
			md5_FF a, b, c, d, x(k + 12), S11, &H6B901122
			md5_FF d, a, b, c, x(k + 13), S12, &HFD987193
			md5_FF c, d, a, b, x(k + 14), S13, &HA679438E
			md5_FF b, c, d, a, x(k + 15), S14, &H49B40821
			
			md5_GG a, b, c, d, x(k + 1), S21, &HF61E2562
			md5_GG d, a, b, c, x(k + 6), S22, &HC040B340
			md5_GG c, d, a, b, x(k + 11), S23, &H265E5A51
			md5_GG b, c, d, a, x(k + 0), S24, &HE9B6C7AA
			md5_GG a, b, c, d, x(k + 5), S21, &HD62F105D
			md5_GG d, a, b, c, x(k + 10), S22, &H2441453
			md5_GG c, d, a, b, x(k + 15), S23, &HD8A1E681
			md5_GG b, c, d, a, x(k + 4), S24, &HE7D3FBC8
			md5_GG a, b, c, d, x(k + 9), S21, &H21E1CDE6
			md5_GG d, a, b, c, x(k + 14), S22, &HC33707D6
			md5_GG c, d, a, b, x(k + 3), S23, &HF4D50D87
			md5_GG b, c, d, a, x(k + 8), S24, &H455A14ED
			md5_GG a, b, c, d, x(k + 13), S21, &HA9E3E905
			md5_GG d, a, b, c, x(k + 2), S22, &HFCEFA3F8
			md5_GG c, d, a, b, x(k + 7), S23, &H676F02D9
			md5_GG b, c, d, a, x(k + 12), S24, &H8D2A4C8A
			
			md5_HH a, b, c, d, x(k + 5), S31, &HFFFA3942
			md5_HH d, a, b, c, x(k + 8), S32, &H8771F681
			md5_HH c, d, a, b, x(k + 11), S33, &H6D9D6122
			md5_HH b, c, d, a, x(k + 14), S34, &HFDE5380C
			md5_HH a, b, c, d, x(k + 1), S31, &HA4BEEA44
			md5_HH d, a, b, c, x(k + 4), S32, &H4BDECFA9
			md5_HH c, d, a, b, x(k + 7), S33, &HF6BB4B60
			md5_HH b, c, d, a, x(k + 10), S34, &HBEBFBC70
			md5_HH a, b, c, d, x(k + 13), S31, &H289B7EC6
			md5_HH d, a, b, c, x(k + 0), S32, &HEAA127FA
			md5_HH c, d, a, b, x(k + 3), S33, &HD4EF3085
			md5_HH b, c, d, a, x(k + 6), S34, &H4881D05
			md5_HH a, b, c, d, x(k + 9), S31, &HD9D4D039
			md5_HH d, a, b, c, x(k + 12), S32, &HE6DB99E5
			md5_HH c, d, a, b, x(k + 15), S33, &H1FA27CF8
			md5_HH b, c, d, a, x(k + 2), S34, &HC4AC5665
			
			md5_II a, b, c, d, x(k + 0), S41, &HF4292244
			md5_II d, a, b, c, x(k + 7), S42, &H432AFF97
			md5_II c, d, a, b, x(k + 14), S43, &HAB9423A7
			md5_II b, c, d, a, x(k + 5), S44, &HFC93A039
			md5_II a, b, c, d, x(k + 12), S41, &H655B59C3
			md5_II d, a, b, c, x(k + 3), S42, &H8F0CCC92
			md5_II c, d, a, b, x(k + 10), S43, &HFFEFF47D
			md5_II b, c, d, a, x(k + 1), S44, &H85845DD1
			md5_II a, b, c, d, x(k + 8), S41, &H6FA87E4F
			md5_II d, a, b, c, x(k + 15), S42, &HFE2CE6E0
			md5_II c, d, a, b, x(k + 6), S43, &HA3014314
			md5_II b, c, d, a, x(k + 13), S44, &H4E0811A1
			md5_II a, b, c, d, x(k + 4), S41, &HF7537E82
			md5_II d, a, b, c, x(k + 11), S42, &HBD3AF235
			md5_II c, d, a, b, x(k + 2), S43, &H2AD7D2BB
			md5_II b, c, d, a, x(k + 9), S44, &HEB86D391
			
			a = AddUnsigned(a, AA)
			b = AddUnsigned(b, BB)
			c = AddUnsigned(c, CC)
			d = AddUnsigned(d, DD)
		Next
		
		MD5 = LCase(WordToHex(a) & WordToHex(b) & WordToHex(c) & WordToHex(d))
	End Function
%>