---
# System prepended metadata

title: PicoCTF Client-side-again

---

## 首頁(其他picoCTF writeup)
https://hackmd.io/@sunfrancis12/ry_LLAgp3

作者: [台中教育大學 白帽社](https://hackmd.io/@ntcuhack/index) -sunfrancis12
## Client-side-again

![](https://hackmd.io/_uploads/r19d7Mja3.png)

進來網頁，f12後看到一行超長的js
```
<script type="text/javascript">
  var _0x5a46=['f49bf}','_again_e','this','Password\x20Verified','Incorrect\x20password','getElementById','value','substring','picoCTF{','not_this'];(function(_0x4bd822,_0x2bd6f7){var _0xb4bdb3=function(_0x1d68f6){while(--_0x1d68f6){_0x4bd822['push'](_0x4bd822['shift']());}};_0xb4bdb3(++_0x2bd6f7);}(_0x5a46,0x1b3));var _0x4b5b=function(_0x2d8f05,_0x4b81bb){_0x2d8f05=_0x2d8f05-0x0;var _0x4d74cb=_0x5a46[_0x2d8f05];return _0x4d74cb;};function verify(){checkpass=document[_0x4b5b('0x0')]('pass')[_0x4b5b('0x1')];split=0x4;if(checkpass[_0x4b5b('0x2')](0x0,split*0x2)==_0x4b5b('0x3')){if(checkpass[_0x4b5b('0x2')](0x7,0x9)=='{n'){if(checkpass[_0x4b5b('0x2')](split*0x2,split*0x2*0x2)==_0x4b5b('0x4')){if(checkpass[_0x4b5b('0x2')](0x3,0x6)=='oCT'){if(checkpass[_0x4b5b('0x2')](split*0x3*0x2,split*0x4*0x2)==_0x4b5b('0x5')){if(checkpass['substring'](0x6,0xb)=='F{not'){if(checkpass[_0x4b5b('0x2')](split*0x2*0x2,split*0x3*0x2)==_0x4b5b('0x6')){if(checkpass[_0x4b5b('0x2')](0xc,0x10)==_0x4b5b('0x7')){alert(_0x4b5b('0x8'));}}}}}}}}else{alert(_0x4b5b('0x9'));}}
</script>
```

為了增加程式可讀性，我們丟到[jsnice](http://jsnice.org/)裡面讓他幫我們自動整理，整理後結果如下
```
'use strict';
/** @type {!Array} */
var _0x5a46 = ["f49bf}", "_again_e", "this", "Password Verified", "Incorrect password", "getElementById", "value", "substring", "picoCTF{", "not_this"];
(function(data, i) {
  /**
   * @param {number} isLE
   * @return {undefined}
   */
  var write = function(isLE) {
    for (; --isLE;) {
      data["push"](data["shift"]());
    }
  };
  write(++i);
})(_0x5a46, 435);
/**
 * @param {string} level
 * @param {?} ai_test
 * @return {?}
 */
var _0x4b5b = function(level, ai_test) {
  /** @type {number} */
  level = level - 0;
  var rowsOfColumns = _0x5a46[level];
  return rowsOfColumns;
};
/**
 * @return {undefined}
 */
function verify() {
  checkpass = document[_0x4b5b("0x0")]("pass")[_0x4b5b("0x1")];
  /** @type {number} */
  split = 4;
  if (checkpass[_0x4b5b("0x2")](0, split * 2) == _0x4b5b("0x3")) {
    if (checkpass[_0x4b5b("0x2")](7, 9) == "{n") {
      if (checkpass[_0x4b5b("0x2")](split * 2, split * 2 * 2) == _0x4b5b("0x4")) {
        if (checkpass[_0x4b5b("0x2")](3, 6) == "oCT") {
          if (checkpass[_0x4b5b("0x2")](split * 3 * 2, split * 4 * 2) == _0x4b5b("0x5")) {
            if (checkpass["substring"](6, 11) == "F{not") {
              if (checkpass[_0x4b5b("0x2")](split * 2 * 2, split * 3 * 2) == _0x4b5b("0x6")) {
                if (checkpass[_0x4b5b("0x2")](12, 16) == _0x4b5b("0x7")) {
                  alert(_0x4b5b("0x8"));
                }
              }
            }
          }
        }
      }
    }
  } else {
    alert(_0x4b5b("0x9"));
  }
}
;
```

但是網頁作者為了不讓我們看懂程式碼，將變數名稱經過編碼，我們要做的事情就是將這些變數名稱還原

我的作法如下:

打開[JS bin](https://jsbin.com/patalokoxe/edit?js,console,output)
(也可以用google f12內建的console)

將程式輸入console(註解記得拿掉)，按下enter，會輸出undefined結果

![](https://hackmd.io/_uploads/S1Lduzj63.png)

圖片文字內容如下:

```
>   var _0x5a46 = ["f49bf}", "_again_e", "this", "Password Verified", "Incorrect password",                 "getElementById", "value", "substring", "picoCTF{", "not_this"];
    (function(data, i) {
      var write = function(isLE) {
        for (; --isLE;) {
          data["push"](data["shift"]());
        }
      };
      write(++i);
    })(_0x5a46, 435);

    var _0x4b5b = function(level, ai_test) {
      level = level - 0;
      var rowsOfColumns = _0x5a46[level];
      return rowsOfColumns;
    };

    function verify() {
      checkpass = document[_0x4b5b("0x0")]("pass")[_0x4b5b("0x1")];
      split = 4;
      if (checkpass[_0x4b5b("0x2")](0, split * 2) == _0x4b5b("0x3")) {
        if (checkpass[_0x4b5b("0x2")](7, 9) == "{n") {
          if (checkpass[_0x4b5b("0x2")](split * 2, split * 2 * 2) == _0x4b5b("0x4")) {
            if (checkpass[_0x4b5b("0x2")](3, 6) == "oCT") {
              if (checkpass[_0x4b5b("0x2")](split * 3 * 2, split * 4 * 2) == _0x4b5b("0x5")) {
                if (checkpass["substring"](6, 11) == "F{not") {
                  if (checkpass[_0x4b5b("0x2")](split * 2 * 2, split * 3 * 2) == _0x4b5b("0x6")) {
                    if (checkpass[_0x4b5b("0x2")](12, 16) == _0x4b5b("0x7")) {
                      alert(_0x4b5b("0x8"));
                    }
                  }
                }
              }
            }
          }
        }
      } else {
        alert(_0x4b5b("0x9"));
      }
    }
    ;
=========
>    undefined
```

之後將一個個經過編碼的變數輸入至console，就可以看到他經過編碼前的樣子，我們在做的事情就是把程式碼還原成人類看的懂的模樣

![](https://hackmd.io/_uploads/SyBLczoa2.png)

我們要將我們反組譯後的變數寫回去程式，這裡我會推薦使用vscode，可以利用vscode的`變更所有發生次數`，`Change all occurrences`來一次改寫大量變數

*備註:由於是mkv轉gif，導致有一些網格紋路*

![](https://i.imgur.com/bGu6EFH.gif)


整理完後，程式碼如下
```
function verify() {
  checkpass = document["getElementById"]("pass")["value"];
  /** @type {number} */
  split = 4;
  if (checkpass["substring"](0, split * 2) == "picoCTF{") {
    if (checkpass["substring"](7, 9) == "{n") {
      if (checkpass["substring"](split * 2, split * 2 * 2) == "not_this") {
        if (checkpass["substring"](3, 6) == "oCT") {
          if (checkpass["substring"](split * 3 * 2, split * 4 * 2) == "f49bf}") {
            if (checkpass["substring"](6, 11) == "F{not") {
              if (checkpass["substring"](split * 2 * 2, split * 3 * 2) == "_again_e") {
                if (checkpass["substring"](12, 16) == "this") {
                  alert("Password Verified");
                }
              }
            }
          }
        }
      }
    }
  } else {
    alert("Incorrect password");
  }
}
;
```

[substring()](https://www.fooish.com/javascript/string/substring.html)的是一種查詢的function()
```
var str = "abcdefg"

console.log(str.substring(0,3)) ##輸出 abcd
console.log(substring(4,6)) ##輸出 ef
```

最後得到結果
```
picoCTF{not_this_again_ef49bf}
```
