Base64算法是把3个8位字符(24)转换成4个6位字符(32),因此编码后的长度会扩大1/3,
进行编码转换时需要用到一张Base64的编码表:
Table 1: The Base64 Alphabet
Value Encoding Value Encoding Value Encoding Value Encoding
0 A 17 R 34 i 51 z
1 B 18 S 35 j 52 0
2 C 19 T 36 k 53 1
3 D 20 U 37 l 54 2
4 E 21 V 38 m 55 3
5 F 22 W 39 n 56 4
6 G 23 X 40 o 57 5
7 H 24 Y 41 p 58 6
8 I 25 Z 42 q 59 7
9 J 26 a 43 r 60 8
10 K 27 b 44 s 61 9
11 L 28 c 45 t 62 +
12 M 29 d 46 u 63 /
13 N 30 e 47 v
14 O 31 f 48 w (pad) =
15 P 32 g 49 x
16 Q 33 h 50 y
在delphi中可以简单的将其保存为一个常量:
BaseTable:string='ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz
0123456789+/=';
编码过程是这样的,第一个字符通过右移2位获得第一个目标字符的Base64表位置,根据
这个数值取到表上相应的字符,就是第一个目标字符
,然后将第一个字符左移6位加上第二个字符右移4位,即获得第二个目标字符,再将第二
个字符左移4位加上第三个字符右移6位,获得第三个
目标字符,最后取第三个字符的右6位即获得第四个目标字符.
const BaseTable:string='ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';
var
function DecodeBase64(Source:string):string; //解码函数
function FindInTable(CSource:char):integer; //
function EncodeBase64(Source:string):string; //编码函数
implementation
{$R *.DFM}
function FindInTable(CSource:char):integer;
begin
result:=Pos(string(CSource),BaseTable)-1;
end;
////
function DecodeBase64(Source:string):string;
var
SrcLen,Times,i:integer;
x1,x2,x3,x4,xt:byte;
begin
result:='';
SrcLen:=Length(Source);
Times:=SrcLen div 4;
for i:=0 to Times-1 do
begin
x1:=FindInTable(Source[1+i*4]);
x2:=FindInTable(Source[2+i*4]);
x3:=FindInTable(Source[3+i*4]);
x4:=FindInTable(Source[4+i*4]);
x1:=x1 shl 2;
xt:=x2 shr 4;
x1:=x1 or xt;
x2:=x2 shl 4;
result:=result+chr(x1);
if x3= 64 then break;
xt:=x3 shr 2;
x2:=x2 or xt;
x3:=x3 shl 6;
result:=result+chr(x2);
if x4=64 then break;
x3:=x3 or x4;
result:=result+chr(x3);
end;
end;
/////
function EncodeBase64(Source:string):string;
var
Times,LenSrc,i:integer;
x1,x2,x3,x4:char;
xt:byte;
begin
result:='';
LenSrc:=length(Source);
if LenSrc mod 3 =0 then Times:=LenSrc div 3
else Times:=LenSrc div 3 + 1;
for i:=0 to times-1 do
begin
if LenSrc >= (3+i*3) then
begin
x1:=BaseTable[(ord(Source[1+i*3]) shr 2)+1];
xt:=(ord(Source[1+i*3]) shl 4) and 48;
xt:=xt or (ord(Source[2+i*3]) shr 4);
x2:=BaseTable[xt+1];
xt:=(Ord(Source[2+i*3]) shl 2) and 60;
xt:=xt or (ord(Source[3+i*3]) shr 6);
x3:=BaseTable[xt+1];
xt:=(ord(Source[3+i*3]) and 63);
x4:=BaseTable[xt+1];
end
else if LenSrc>=(2+i*3) then
begin
x1:=BaseTable[(ord(Source[1+i*3]) shr 2)+1];
xt:=(ord(Source[1+i*3]) shl 4) and 48;
xt:=xt or (ord(Source[2+i*3]) shr 4);
x2:=BaseTable[xt+1];
xt:=(ord(Source[2+i*3]) shl 2) and 60;
x3:=BaseTable[xt+1];
x4:='=';
end else
begin
x1:=BaseTable[(ord(Source[1+i*3]) shr 2)+1];
xt:=(ord(Source[1+i*3]) shl 4) and 48;
x2:=BaseTable[xt+1];
x3:='=';
x4:='=';
end;
result:=result+x1+x2+x3+x4;
end;
end;