Use the Windows Crypto API for Base64 encoding/decoding

function StringToBase64(instr: string; Flags: dword): string;
var
  sz: dword;
begin
  CryptBinaryToStringA(pointer(instr), Length(instr), Flags, nil, sz);
  SetLength(result, sz);
  CryptBinaryToStringA(pointer(instr), Length(instr), Flags, pointer(result), sz);
end;

cstr := StringToBase64(instr, CRYPT_STRING_BASE64);

function StringFromBase64(instr: string; Flags: dword): string;
var
  sz, skip: dword;
begin
  CryptStringToBinaryA(pointer(instr), Length(instr), Flags, nil, sz, skip,
             Flags);
  SetLength(result, sz);
  CryptStringToBinaryA(pointer(instr), Length(instr), Flags, pointer(result),
            sz, skip, Flags);
end;
cstr := StringFromBase64(cstr, CRYPT_STRING_BASE64);

procedure Base64EncFile(infilename, outfilename: string);
// copies file in infile to outfile.  Uses file mapping to read, but regular
// Delphi "file" to write.

var
  FileHandle: THandle;
  MapHandle: THandle;
  ViewPointer: Pointer;
  outptr: Pointer;
  outfile: file;
  outsize: longint;
begin
  FileHandle := CreateFile(Pchar(infilename), GENERIC_READ,
      FILE_SHARE_READ or FILE_SHARE_WRITE,nil, OPEN_EXISTING,
      FILE_ATTRIBUTE_NORMAL or FILE_FLAG_SEQUENTIAL_SCAN, 0);
  MapHandle := CreateFileMapping(FileHandle, nil, PAGE_READONLY, 0, 0, nil);
  ViewPointer := MapViewOfFile(MapHandle, FILE_MAP_READ, 0, 0, 0);
  if ViewPointer <> nil then
    try
      Assign(outfile, outfilename);
      rewrite(outfile, 1);
      CryptBinaryToStringA(ViewPointer, GetFileSize(FileHandle, nil),
                CRYPT_STRING_BASE64, nil, outsize);
      GetMem(outptr, outsize);
      CryptBinaryToStringA(ViewPointer, GetFileSize(FileHandle, nil),
                CRYPT_STRING_BASE64, outptr, outsize);
      BlockWrite(outfile, outptr^, outsize);
      Close(outfile);
      FreeMem(outptr);
    finally
      UnMapViewofFile(ViewPointer);
    end;
  CloseHandle(maphandle);
  CloseHandle(FileHandle);
end;