function core_sha1(x, len)
{
  /* append padding */
  x[len >> 5] |= 0x80 << (24 - len % 32);
  x[((len + 64 >> 9) << 4) + 15] = len;

  var w = new Array(80);
  var a =  1732584193;
  var b = -271733879;
  var c = -1732584194;
  var d =  271733878;
  var e = -1009589776;

  var i, j, t, olda, oldb, oldc, oldd, olde;
  for (i = 0; i < x.length; i += 16)
  {
    olda = a;
    oldb = b;
    oldc = c;
    oldd = d;
    olde = e;

    for (j = 0; j < 80; j++)
    {
      if (j < 16) { w[j] = x[i + j]; }
      else { w[j] = rol(w[j-3] ^ w[j-8] ^ w[j-14] ^ w[j-16], 1); }
      t = safe_add(safe_add(rol(a, 5), sha1_ft(j, b, c, d)),
                       safe_add(safe_add(e, w[j]), sha1_kt(j)));
      e = d;
      d = c;
      c = rol(b, 30);
      b = a;
      a = t;
    }

    a = safe_add(a, olda);
    b = safe_add(b, oldb);
    c = safe_add(c, oldc);
    d = safe_add(d, oldd);
    e = safe_add(e, olde);
  }
  return [a, b, c, d, e];
}
Decodes a base64 string.
@param {String} input The string to decode.