關於BASE 24 ，BASE 64原理以及實現程序
來源 https://wangye.org/blog/archives/5/
可能很多人聽說過Base64編碼，很少有人聽說過Base24編碼，Base24編碼主要應用在序列號生成上，其實基本的算法思想都是一樣的，只是編碼的模式有點變化。
Base64所對應的編碼表是
ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=
共計64位。
而Base24所對應的編碼表是
BCDFGHJKMPQRTVWXY2346789
共計24位，這里主要去掉了一些對於序列號來說不容易識別和容易混淆的字符。
具體算法網上都有介紹我就不再贅述，下面直接談怎么編碼實現吧:-)
對於Base64算法的C語言實現，大家可以參考Bob Trower的經典開源項目b64.c，網址是http://base64.sourceforge.net/。
在這里我要談的是如何用ASP進行Base64編碼，網上搜索出的一大堆算法不是太復雜就是運行不准確，其實ASP的Base64編碼可以很簡單，當然要用到服務器的一個COM組件XmlDom，具體思路是這樣的，首先創建個臨時的節點tmp，然后給結點tmp，設置datatype = “bin.base64″，這樣就可以進行相應的編解碼了，對於文本字符串的編解碼還要用ADODB.Stream進行一次編碼轉換。
Base64的ASP實現：
Class CBase64 Private objXmlDom Private objXmlNode ' GetObjectParam() 這個函數實現參考了開源項目PJBlog Private Function GetObjectParam() On Error Resume Next Dim Temp GetObjectParam = "Microsoft.XMLDOM" Err = 0 Dim TmpObj Set TmpObj = Server.CreateObject(GetObjectParam) Temp = Err.Number If Temp = 1 Or Temp = -2147221005 Then GetObjectParam = "Msxml2.DOMDocument.5.0" End If Err.Clear Set TmpObj = Nothing Err = 0 End Function Private Sub Class_Initialize() Set objXmlDom = Server.CreateObject(GetObjectParam()) End Sub Private Sub Class_Terminate() Set objXmlDom = Nothing End Sub Public Function encode(AnsiCode) encode = "" Set objXmlNode = objXmlDom.createElement("tmp") objXmlNode.datatype = "bin.base64" objXmlNode.nodeTypedvalue = AnsiCode encode = objXmlNode.Text Set objXmlNode = Nothing End Function Public Function decode(base64Code) On Error Resume Next decode = "" Set objXmlNode = objXmlDom.createElement("tmp") objXmlNode.datatype = "bin.base64" objXmlNode.Text = base64Code decode = objXmlNode.nodeTypedvalue Set objXmlNode = Nothing If Err Then Err.Clear End If End Function ' 以下函數編碼字符串 Public Function encodeText(ByVal str) On Error Resume Next Dim ado, r: r = "" If str <> "" Then Set ado = Server.CreateObject("ADODB.Stream") With ado .Charset = "gb2312" .Type = 2 If .State = 0 Then .Open .WriteText str .Position = 0 .Type = 1 r = encode(.Read(-1)) .Close End With Set ado = Nothing End If If Err Then Err.Clear: r = "" encodeText = r End Function ' 以下函數解碼字符串 Public Function decodeText(ByVal str) On Error Resume Next Dim ado, r: r = "" If str <> "" Then Set ado = Server.CreateObject("ADODB.Stream") With ado .Charset = "gb2312" .Type = 1 If .State = 0 Then .Open .Write (decode(str)) .Position = 0 .Type = 2 r = .ReadText(-1) .Close End With Set ado = Nothing End If If Err Then Err.Clear: r = "" decodeText = r End Function End Class
寫到這里想起以前項目里改寫過一段Base64和16進制互轉的代碼，在這里與大家分享下。
' Base64轉16進制 Function B64ToHex(ByVal strContent) Dim i,strReturned, b64pad, _ b64map, chTemp, intLocate, k , slop strReturned = "" : k = 0 b64map="ABCDEFGHIJKLMNOPQRSTUVWXYZ" &_ "abcdefghijklmnopqrstuvwxyz0123456789+/" b64pad="=" For i=0 To Len(strContent)-1 chTemp = Mid(strContent, i+1, 1) If chTemp = b64pad Then Exit For intLocate = InStr(1, b64map, chTemp, 0)-1 If intLocate > -1 Then Select Case K Case 0 strReturned = strReturned &_ Int2Char(Int(intLocate / 4)) slop = intLocate And 3 : k = 1 Case 1 strReturned = strReturned &_ Int2Char( (slop * 4) Or (Int(intLocate / 16)) ) slop = intLocate And &h0f : k = 2 Case 2 strReturned = strReturned &_ Int2Char(slop) & Int2Char(Int(intLocate / 4)) slop = intLocate And 3 : k=3 Case Else strReturned = strReturned &_ Int2Char( (slop * 4) Or (Int(intLocate / 16)) ) &_ Int2Char(intLocate And &h0f) k = 0 End Select End If Next If k=1 Then strReturned = strReturned & Int2Char(slop * 4) B64ToHex = strReturned End Function ' 16進制轉Base64 Function HexToB64(ByVal strContent) Dim i, c, strReturned, b64map, b64pad, intLen b64map="ABCDEFGHIJKLMNOPQRSTUVWXYZ" &_ "abcdefghijklmnopqrstuvwxyz0123456789+/" b64pad="=" intLen = Len(strContent) For i=0 To intLen-3 Step 3 c=Clng("&h" & Mid(strContent,i+1,3)) strReturned = strReturned &_ Mid(b64map, Int(c / 64 +1), 1) &_ Mid(b64map, (c And 63)+1, 1) Next If i+1=intLen Then c =Clng("&h" & Mid(strContent,i+1,1)) strReturned = strReturned & Mid(b64map, c * 4 + 1, 1) ElseIf i+2=intLen Then c =Clng("&h" & Mid(strContent,i+1,2)) strReturned = strReturned & Mid(b64map, Int(c / 4) + 1, 1) &_ Mid(b64map, (c And 3)*16+1, 1) End If While (Len(strReturned) And 3) >0 strReturned = strReturned & b64pad Wend HexToB64 = strReturned End Function
有關Base24的代碼實現網上也有很多，這里給出兩種不同程序語言的實現供參考。
Base24的C語言實現
static const char sel[] = { 'B','C','D','F','G', 'H','J','K','M','P', 'Q','R','T','V','W', 'X','Y','2','3','4', '6','7','8','9', '\0'}; char *b24e(char *buf, unsigned char *byst, size_t sizeOfBytes) { int i = 0; unsigned char *p = byst; while ((size_t)(i = (p-byst)) < sizeOfBytes) { buf[2*i] = sel[((*p) >> 4)]; buf[(2*i)+1] = sel[23 - ((*p) & 0x0f)]; p++; } buf[(2*i)+1] = '\0'; return buf; } unsigned char *b24d(unsigned char *buf, char *str, size_t countOfChars) { size_t i; char *p = str; char *loc[2]; unsigned char n[2]; if (countOfChars % 2) return NULL; for (i = 0; i < (countOfChars>>1); i++) { loc[0] = strchr( sel, str[2*i] ); loc[1] = strchr( sel, str[ ( 2*i ) + 1 ] ); if (loc[0] == NULL || loc[1] == NULL) return NULL; n[0] = (unsigned char)( loc[0] - sel ); n[1] = 23 - (unsigned char)( loc[1] - sel ); buf[i] = (unsigned char)((n[0] << 4) | n[1]); } return buf; }
后來由於項目需要，我特地改寫了C#版。
public static class Base24 { private static string sel = "BCDFGHJKMPQRTVWXY2346789"; public static string Encode(string Text) { int i = 0; int Pos = 0; char []Buf = new char[Text.Length<<1]; while ((i = Pos) < Text.Length) { Buf[i<<1] = sel[(Text[Pos]) >> 4]; Buf[(i<<1) + 1] = sel[23 - (Text[Pos] & 0x0F)]; Pos++; } return new string(Buf); } public static string Decode(string Text) { if (Text.Length % 2 != 0) return null; int [] NPos = new int[2]; char[] N = new char[2]; char[] Buf = new char[Text.Length >> 1]; for (int i = 0; i < (Text.Length >> 1); i++) { NPos[0] = sel.IndexOf(Text[ i<<1 ]); NPos[1] = 23 - sel.IndexOf(Text[(i<<1) + 1]); if (NPos[0] < 0 || NPos[1] < 0) { return null; } Buf[i] = ((char)((NPos[0] << 4) | NPos[1])); } return new string(Buf); } }
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