1バイトずつ key [0] ～ key [7] と排他的論理和を取っていくループ。

var k0 = key [0];
var k1 = key [1];
var k2 = key [2];
var k3 = key [3];
var k4 = key [4];
var k5 = key [5];
var k6 = key [6];
var k7 = key [7];

var sb = new StringBuffer (http.data);
var len = sb.length;

sb.append ('*'.multiply (32));
for (i = 0; i < len;) {
  sb [i++] = sb.charCodeAt (i) ^ k0;
  sb [i++] = sb.charCodeAt (i) ^ k1;
  sb [i++] = sb.charCodeAt (i) ^ k2;
  sb [i++] = sb.charCodeAt (i) ^ k3;
  sb [i++] = sb.charCodeAt (i) ^ k4;
  sb [i++] = sb.charCodeAt (i) ^ k5;
  sb [i++] = sb.charCodeAt (i) ^ k6;
  sb [i++] = sb.charCodeAt (i) ^ k7;
  sb [i++] = sb.charCodeAt (i) ^ k0;
// 中略
  sb [i++] = sb.charCodeAt (i) ^ k7;
  sb [i++] = sb.charCodeAt (i) ^ k0;
  sb [i++] = sb.charCodeAt (i) ^ k1;
  sb [i++] = sb.charCodeAt (i) ^ k2;
  sb [i++] = sb.charCodeAt (i) ^ k3;
  sb [i++] = sb.charCodeAt (i) ^ k4;
  sb [i++] = sb.charCodeAt (i) ^ k5;
  sb [i++] = sb.charCodeAt (i) ^ k6;
  sb [i++] = sb.charCodeAt (i) ^ k7;
}
sb.delete (len);

ここは19世紀か。

まったくループを展開しない場合に比べて、ちょっとだけ速くなります。ちょっとだけ。
まじめに速くしたいなら外部プロセスに投げるべき。