在各种登陆系统中，用户密码的md5加密是不可少的。
这么说吧，即使你通过各种手段看到了用户的登陆密码，但那一串古古怪怪的东西你也不可能知道它是什么，这就是保护。
md5加密运算是不可逆的，就是说你不能通过那一串古古怪怪的东西算出它原始的样子。
下面是完整的md5代码，不要去理解算法了，用到时拿去直接调用。
Private Const BITS_TO_A_BYTE = 8Private Const BYTES_TO_A_WORD = 4Private Const BITS_TO_A_WORD = 32 Private m_lOnBits(30)Private m_l2Power(30) Private Function LShift(lValue, iShiftBits)If iShiftBits = 0 ThenLShift = lValueExit FunctionElseIf iShiftBits = 31 ThenIf lValue And 1 ThenLShift = &H80000000ElseLShift = 0End IfExit FunctionElseIf iShiftBits < 0 Or iShiftBits > 31 ThenErr.Raise 6End If If (lValue And m_l2Power(31 - iShiftBits)) ThenLShift = ((lValue And m_lOnBits(31 - (iShiftBits + 1))) * m_l2Power(iShiftBits)) Or &H80000000ElseLShift = ((lValue And m_lOnBits(31 - iShiftBits)) * m_l2Power(iShiftBits))End IfEnd Function Private Function RShift(lValue, iShiftBits)If iShiftBits = 0 ThenRShift = lValueExit FunctionElseIf iShiftBits = 31 ThenIf lValue And &H80000000 ThenRShift = 1ElseRShift = 0End IfExit FunctionElseIf iShiftBits < 0 Or iShiftBits > 31 ThenErr.Raise 6End If RShift = (lValue And &H7FFFFFFE) / m_l2Power(iShiftBits) If (lValue And &H80000000) ThenRShift = (RShift Or (&H40000000 / m_l2Power(iShiftBits - 1)))End IfEnd Function Private Function RotateLeft(lValue, iShiftBits)RotateLeft = LShift(lValue, iShiftBits) Or RShift(lValue, (32 - iShiftBits))End Function Private Function AddUnsigned(lX, lY)Dim lX4Dim lY4Dim lX8Dim lY8Dim lResult lX8 = lX And &H80000000lY8 = lY And &H80000000lX4 = lX And &H40000000lY4 = lY And &H40000000 lResult = (lX And &H3FFFFFFF) + (lY And &H3FFFFFFF) If lX4 And lY4 ThenlResult = lResult Xor &H80000000 Xor lX8 Xor lY8ElseIf lX4 Or lY4 ThenIf lResult And &H40000000 ThenlResult = lResult Xor &HC0000000 Xor lX8 Xor lY8ElselResult = lResult Xor &H40000000 Xor lX8 Xor lY8End IfElselResult = lResult Xor lX8 Xor lY8End If AddUnsigned = lResultEnd 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 lMessageLengthDim lNumberOfWordsDim lWordArray()Dim lBytePositionDim lByteCountDim lWordCount Const MODULUS_BITS = 512Const 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 = 0lByteCount = 0Do Until lByteCount >= lMessageLengthlWordCount = lByteCount / BYTES_TO_A_WORDlBytePosition = (lByteCount Mod BYTES_TO_A_WORD) * BITS_TO_A_BYTElWordArray(lWordCount) = lWordArray(lWordCount) Or LShift(Asc(Mid(sMessage, lByteCount + 1, 1)), lBytePosition)lByteCount = lByteCount + 1Loop lWordCount = lByteCount / BYTES_TO_A_WORDlBytePosition = (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 = lWordArrayEnd Function Private Function WordToHex(lValue)Dim lByteDim lCount For lCount = 0 To 3lByte = RShift(lValue, lCount * BITS_TO_A_BYTE) And m_lOnBits(BITS_TO_A_BYTE - 1)WordToHex = WordToHex & Right("0" & Hex(lByte), 2)NextEnd Function Public Function MD5(sMessage, stype)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 xDim kDim AADim BBDim CCDim DDDim aDim bDim cDim d Const S11 = 7Const S12 = 12Const S13 = 17Const S14 = 22Const S21 = 5Const S22 = 9Const S23 = 14Const S24 = 20Const S31 = 4Const S32 = 11Const S33 = 16Const S34 = 23Const S41 = 6Const S42 = 10Const S43 = 15Const S44 = 21 x = ConvertToWordArray(sMessage) a = &H67452301b = &HEFCDAB89c = &H98BADCFEd = &H10325476 For k = 0 To UBound(x) Step 16AA = aBB = bCC = cDD = d md5_FF a, b, c, d, x(k + 0), S11, &HD76AA478md5_FF d, a, b, c, x(k + 1), S12, &HE8C7B756md5_FF c, d, a, b, x(k + 2), S13, &H242070DBmd5_FF b, c, d, a, x(k + 3), S14, &HC1BDCEEEmd5_FF a, b, c, d, x(k + 4), S11, &HF57C0FAFmd5_FF d, a, b, c, x(k + 5), S12, &H4787C62Amd5_FF c, d, a, b, x(k + 6), S13, &HA8304613md5_FF b, c, d, a, x(k + 7), S14, &HFD469501md5_FF a, b, c, d, x(k + 8), S11, &H698098D8md5_FF d, a, b, c, x(k + 9), S12, &H8B44F7AFmd5_FF c, d, a, b, x(k + 10), S13, &HFFFF5BB1md5_FF b, c, d, a, x(k + 11), S14, &H895CD7BEmd5_FF a, b, c, d, x(k + 12), S11, &H6B901122md5_FF d, a, b, c, x(k + 13), S12, &HFD987193md5_FF c, d, a, b, x(k + 14), S13, &HA679438Emd5_FF b, c, d, a, x(k + 15), S14, &H49B40821 md5_GG a, b, c, d, x(k + 1), S21, &HF61E2562md5_GG d, a, b, c, x(k + 6), S22, &HC040B340md5_GG c, d, a, b, x(k + 11), S23, &H265E5A51md5_GG b, c, d, a, x(k + 0), S24, &HE9B6C7AAmd5_GG a, b, c, d, x(k + 5), S21, &HD62F105Dmd5_GG d, a, b, c, x(k + 10), S22, &H2441453md5_GG c, d, a, b, x(k + 15), S23, &HD8A1E681md5_GG b, c, d, a, x(k + 4), S24, &HE7D3FBC8md5_GG a, b, c, d, x(k + 9), S21, &H21E1CDE6md5_GG d, a, b, c, x(k + 14), S22, &HC33707D6md5_GG c, d, a, b, x(k + 3), S23, &HF4D50D87md5_GG b, c, d, a, x(k + 8), S24, &H455A14EDmd5_GG a, b, c, d, x(k + 13), S21, &HA9E3E905md5_GG d, a, b, c, x(k + 2), S22, &HFCEFA3F8md5_GG c, d, a, b, x(k + 7), S23, &H676F02D9md5_GG b, c, d, a, x(k + 12), S24, &H8D2A4C8A md5_HH a, b, c, d, x(k + 5), S31, &HFFFA3942md5_HH d, a, b, c, x(k + 8), S32, &H8771F681md5_HH c, d, a, b, x(k + 11), S33, &H6D9D6122md5_HH b, c, d, a, x(k + 14), S34, &HFDE5380Cmd5_HH a, b, c, d, x(k + 1), S31, &HA4BEEA44md5_HH d, a, b, c, x(k + 4), S32, &H4BDECFA9md5_HH c, d, a, b, x(k + 7), S33, &HF6BB4B60md5_HH b, c, d, a, x(k + 10), S34, &HBEBFBC70md5_HH a, b, c, d, x(k + 13), S31, &H289B7EC6md5_HH d, a, b, c, x(k + 0), S32, &HEAA127FAmd5_HH c, d, a, b, x(k + 3), S33, &HD4EF3085md5_HH b, c, d, a, x(k + 6), S34, &H4881D05md5_HH a, b, c, d, x(k + 9), S31, &HD9D4D039md5_HH d, a, b, c, x(k + 12), S32, &HE6DB99E5md5_HH c, d, a, b, x(k + 15), S33, &H1FA27CF8md5_HH b, c, d, a, x(k + 2), S34, &HC4AC5665 md5_II a, b, c, d, x(k + 0), S41, &HF4292244md5_II d, a, b, c, x(k + 7), S42, &H432AFF97md5_II c, d, a, b, x(k + 14), S43, &HAB9423A7md5_II b, c, d, a, x(k + 5), S44, &HFC93A039md5_II a, b, c, d, x(k + 12), S41, &H655B59C3md5_II d, a, b, c, x(k + 3), S42, &H8F0CCC92md5_II c, d, a, b, x(k + 10), S43, &HFFEFF47Dmd5_II b, c, d, a, x(k + 1), S44, &H85845DD1md5_II a, b, c, d, x(k + 8), S41, &H6FA87E4Fmd5_II d, a, b, c, x(k + 15), S42, &HFE2CE6E0md5_II c, d, a, b, x(k + 6), S43, &HA3014314md5_II b, c, d, a, x(k + 13), S44, &H4E0811A1md5_II a, b, c, d, x(k + 4), S41, &HF7537E82md5_II d, a, b, c, x(k + 11), S42, &HBD3AF235md5_II c, d, a, b, x(k + 2), S43, &H2AD7D2BBmd5_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 If stype = 32 ThenMD5 = LCase(WordToHex(a) & WordToHex(b) & WordToHex(c) & WordToHex(d))ElseMD5 = LCase(WordToHex(b) & WordToHex(c))End IfEnd FunctionSub test()MsgBox MD5("a", 16) '16位加密MsgBox MD5("a", 32) '32位加密End Sub