function Decode(const S: AnsiString): AnsiString;
const
 Map: array[Char] of Byte = (0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 62, 0, 0, 0, 63, 52, 53,
  54, 55, 56, 57, 58, 59, 60, 61, 0, 0, 0, 0, 0, 0, 0, 0, 1, 2,
  3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
  20, 21, 22, 23, 24, 25, 0, 0, 0, 0, 0, 0, 26, 27, 28, 29, 30,
  31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45,
  46, 47, 48, 49, 50, 51, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0);
var
 I: LongInt;
begin
 case Length(S) of
  2:
  begin
  I := Map[S[1]] + (Map[S[2]] shl 6);
  SetLength(Result, 1);
  Move(I, Result[1], Length(Result))
  end;
  3:
  begin
  I := Map[S[1]] + (Map[S[2]] shl 6) + (Map[S[3]] shl 12);
  SetLength(Result, 2);
  Move(I, Result[1], Length(Result))
  end;
  4:
  begin
  I := Map[S[1]] + (Map[S[2]] shl 6) + (Map[S[3]] shl 12) +
  (Map[S[4]] shl 18);
  SetLength(Result, 3);
  Move(I, Result[1], Length(Result))
  end
 end
end;

function Encode(const S: AnsiString): AnsiString;
const
 Map: array[0..63] of Char = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ' +
  'abcdefghijklmnopqrstuvwxyz0123456789+/';
var
 I: LongInt;
begin
 I := 0;
 Move(S[1], I, Length(S));
 case Length(S) of
  1:
  Result := Map[I mod 64] + Map[(I shr 6) mod 64];
  2:
  Result := Map[I mod 64] + Map[(I shr 6) mod 64] +
  Map[(I shr 12) mod 64];
  3:
  Result := Map[I mod 64] + Map[(I shr 6) mod 64] +
  Map[(I shr 12) mod 64] + Map[(I shr 18) mod 64]
 end
end;

type TAByte = array [0..maxInt-1] of byte;

type TPAByte = ^TAByte;

function Encode(data:string) : string; overload;

const b64 : array [0..63] of char = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';

var ic,len : integer;

pi, po : TPAByte;

c1 : dword;

begin

len:=length(data);

if len > 0 then begin

SetLength(result, ((len + 2) div 3) * 4);

pi := pointer(data);

po := pointer(result);

for ic := 1 to len div 3 do begin

c1 := pi^[0] shl 16 + pi^[1] shl 8 + pi^[2];

po^[0] := byte(b64[(c1 shr 18) and $3f]);

po^[1] := byte(b64[(c1 shr 12) and $3f]);

po^[2] := byte(b64[(c1 shr 6) and $3f]);

po^[3] := byte(b64[(c1 ) and $3f]);

inc(dword(po), 4);

inc(dword(pi), 3);

end;

case len mod 3 of

1 : begin

c1 := pi^[0] shl 16;

po^[0] := byte(b64[(c1 shr 18) and $3f]);

po^[1] := byte(b64[(c1 shr 12) and $3f]);

po^[2] := byte('=');

po^[3] := byte('=');

end;

2 : begin

c1 := pi^[0] shl 16 + pi^[1] shl 8;

po^[0] := byte(b64[(c1 shr 18) and $3f]);

po^[1] := byte(b64[(c1 shr 12) and $3f]);

po^[2] := byte(b64[(c1 shr 6) and $3f]);

po^[3] := byte('=');

end;

end;

end else

result := '';

end;

function Decode(data:string) : string; overload;

var i1,i2,len : integer;

pi, po : TPAByte;

ch1 : char;

c1 : dword;

begin

len:=length(data);

if (len > 0) and (len mod 4 = 0) then begin

len := len shr 2;

SetLength(result, len * 3);

pi := pointer(data);

po := pointer(result);

for i1 := 1 to len do begin

c1 := 0;

i2 := 0;

while true do begin

ch1 := char(pi^[i2]);

case ch1 of

'A'..'Z' : c1 := c1 or (dword(ch1) - byte('A') );

'a'..'z' : c1 := c1 or (dword(ch1) - byte('a') + 26);

'0'..'9' : c1 := c1 or (dword(ch1) - byte('0') + 52);

'+' : c1 := c1 or 62;

'/' : c1 := c1 or 63;

else begin

if i2 = 3 then begin

po^[0] := c1 shr 16;

po^[1] := byte(c1 shr 8);

SetLength(result, Length(result) - 1);

end else begin

po^[0] := c1 shr 10;

SetLength(result, Length(result) - 2);

end;

exit;

end;

end;

if i2 = 3 then

break;

inc(i2);

c1 := c1 shl 6;

end;

po^[0] := c1 shr 16;

po^[1] := byte(c1 shr 8);

po^[2] := byte(c1);

inc(dword(pi), 4);

inc(dword(po), 3);

end;

end else

result := '';

end;

....

var a,b:string;

begin

a:='aaa';

b:=Encode( a );

showmessage( b );

a:=Decode( b );

showmessage( a );

const

 Codes64 = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz+/';

function Encode64(S: string): string;

var

 i: Integer;

 a: Integer;

 x: Integer;

 b: Integer;

begin

 Result := '';

 a := 0;

 b := 0;

 for i := 1 to Length(s) do

 begin

  x := Ord(s[i]);

  b := b * 256 + x;

  a := a + 8;

  while a >= 6 do

  begin

  a := a - 6;

  x := b div (1 shl a);

  b := b mod (1 shl a);

  Result := Result + Codes64[x + 1];

  end;

 end;

 if a > 0 then

 begin

  x := b shl (6 - a);

  Result := Result + Codes64[x + 1];

 end;

end;

const
 Codes64 = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz+/';

function Decode64(S: string): string;
var
 i: Integer;
 a: Integer;
 x: Integer;
 b: Integer;
begin
 Result := '';
 a := 0;
 b := 0;
 for i := 1 to Length(s) do
 begin
  x := Pos(s[i], codes64) - 1;
  if x >= 0 then
  begin
  b := b * 64 + x;
  a := a + 6;
  if a >= 8 then
  begin
  a := a - 8;
  x := b shr a;
  b := b mod (1 shl a);
  x := x mod 256;
  Result := Result + chr(x);
  end;
  end
  else
  Exit;
 end;
end;

{ 64-битное декодирование файлов }
{ Arne de Bruijn }
uses dos;
var
 Base64: array[43..122] of byte;
var
 T: text;
 Chars: set of char;
 S: string;
 K, I, J: word;
 Buf: pointer;
 DShift: integer;
 F: file;
 B, B1: byte;
 Decode: array[0..63] of byte;
 Shift2: byte;
 Size, W: word;
begin
 FillChar(Base64, SizeOf(Base64), 255);
 J := 0;
 for I := 65 to 90 do
 begin
  Base64[I] := J;
  Inc(J);
 end;
 for I := 97 to 122 do
 begin
  Base64[I] := J;
  Inc(J);
 end;
 for I := 48 to 57 do
 begin
  Base64[I] := J;
  Inc(J);
 end;
 Base64[43] := J;
 Inc(J);
 Base64[47] := J;
 Inc(J);
 if ParamCount = 0 then
 begin
  WriteLn('UNBASE64 <mime-файл> [<выходной файл>]');
  Halt(1);
 end;
 S := ParamStr(1);
 assign(T, S);
 GetMem(Buf, 32768);
 SetTextBuf(T, Buf^, 32768);
{$I-}reset(T);
{$I+}
 if IOResult <> 0 then
 begin
  WriteLn('Ошибка считывания ', S);
  Halt(1);
 end;
 if ParamCount >= 2 then
  S := ParamStr(2)
 else
 begin
  write('Расположение:');
  ReadLn(S);
 end;
 assign(F, S);
{$I-}rewrite(F, 1);
{$I+}
 if IOResult <> 0 then
 begin
  WriteLn('Ошибка создания ', S);
  Halt(1);
 end;
 while not eof(T) do
 begin
  ReadLn(T, S);
  if (S <> '') and (pos(' ', S) = 0) and (S[1] >= #43) and (S[1] <= #122) and
  (Base64[byte(S[1])] <> 255) then
  begin
  FillChar(Decode, SizeOf(Decode), 0);
  DShift := 0;
  J := 0;
  Shift2 := 1;
  Size := 255;
  B := 0;
  for I := 1 to Length(S) do
  begin
  case S[I] of
  #43..#122: B1 := Base64[Ord(S[I])];
  else
  B1 := 255;
  end;
  if B1 = 255 then
  if S[I] = '=' then
  begin
  B1 := 0;
  if Size = 255 then
  Size := J;
  end
  else
  WriteLn('Ошибка символа:', S[I], ' (', Ord(S[I]), ')');
  if DShift and 7 = 0 then
  begin
  Decode[J] := byte(B1 shl 2);
  DShift := 2;
  end
  else
  begin
  Decode[J] := Decode[J] or Hi(word(B1) shl (DShift + 2));
  Decode[J + 1] := Lo(word(B1) shl (DShift + 2));
  Inc(J);
  Inc(DShift, 2);
  end;
  end;
  if Size = 255 then
  Size := J;
  BlockWrite(F, Decode, Size);
  end;
 end;
 Close(F);
 close(T);
end.