-
Notifications
You must be signed in to change notification settings - Fork 0
/
Copy pathw3challs_striptease.html
356 lines (336 loc) · 17.8 KB
/
w3challs_striptease.html
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
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
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8" />
<meta http-equiv="X-UA-Compatible" content="IE=edge" />
<meta name="viewport" content="width=device-width, initial-scale=1" />
<meta name="author" content="F3real" />
<meta name="keywords" content="crackme,elf" />
<meta name="description" content="Solution to W3challs striptease challenge" />
<title>W3challs striptease - EnSec blog</title>
<link href="https://f3real.github.io/theme/css/combined.css" rel="stylesheet" />
<!-- Feeds -->
</head>
<body data-spy="scroll" data-target="#scrollspy">
<div id="wrapper">
<!-- Sidebar -->
<nav id="sidebar-wrapper-small" class="twitchy-background">
<ul id="accordion-small" class="sidebar-nav sidebar-nav-small">
<li>
<a href="https://f3real.github.io" title="EnSec blog" class="collapsed">
<span class="fas fa-home"></span>
</a>
</li>
<li class="nav-divider"></li>
<li>
<a href="https://f3real.github.io/archives.html" title="Recent Articles" class="collapsed">
<span class="fas fa-th-list"></span>
</a>
</li>
<li class="nav-divider"></li>
<li>
<a data-toggle="collapse" data-parent="#accordion-small" href="#collapse-social-small" title="Social" class="collapsed">
<i class="fas fa-users padding-small"></i>
</a>
</li>
<li class="panel anti-panel"><ul id="collapse-social-small" class="collapse ">
<li>
<a href="https://github.com/F3real" title="Github"><i class="fab fa-github-square padding-small"></i></a>
</li>
<li>
<a href="https://www.linkedin.com/in/stefan-ili%C4%87-61a004111" title="Linkedin"><i class="fab fa-linkedin padding-small"></i></a>
</li>
</ul></li>
<li class="nav-divider"></li>
<li>
<a href="#" title="Back to top" class="collapsed">
<span class="fas fa-arrow-up"></span>
</a>
</li>
</ul>
</nav>
<nav id="sidebar-wrapper" class="twitchy-background">
<ul id="accordion" class="sidebar-nav">
<li class="sidebar-brand">
<a href="https://f3real.github.io/">
<span class="fas fa-home padding-small"></span>
EnSec blog
</a>
</li>
<li>
<a href="https://f3real.github.io/archives.html">
<span class="fas fa-th-list padding-small"></span>
Archives
</a>
</li>
<li class="nav-divider"></li>
<li>
<a data-toggle="collapse" data-parent="#accordion" href="#collapse-social">
<i class="fas fa-users padding-small"></i>
Contact
</a>
</li>
<li class="panel anti-panel"><ul id="collapse-social" class="sidebar_submenu collapse ">
<li>
<a href="https://github.com/F3real" title="Github">
<i class="fab fa-github-square padding-small"></i>
Github
</a>
</li>
<li>
<a href="https://www.linkedin.com/in/stefan-ili%C4%87-61a004111" title="Linkedin">
<i class="fab fa-linkedin padding-small"></i>
Linkedin
</a>
</li>
</ul></li>
<li class="nav-divider"></li>
<li class="panel anti-panel"><ul id="collapse-pages" class="sidebar_submenu collapse ">
</ul></li>
<li class="nav-divider"></li>
<li>
<a data-toggle="collapse" data-parent="#accordion" href="#collapse-categories">
<i class="fas fa-folder-open padding-small"></i>
Categories
</a>
</li>
<li class="panel anti-panel"><ul id="collapse-categories" class="sidebar_submenu collapse ">
<li >
<a href="https://f3real.github.io/category/ctf.html">
<i class="fas fa-folder-open padding-small"></i>
ctf
<span class="badge badge-secondary float-right categorybadge">28</span>
</a>
</li>
<li >
<a href="https://f3real.github.io/category/misc.html">
<i class="fas fa-folder-open padding-small"></i>
misc
<span class="badge badge-secondary float-right categorybadge">15</span>
</a>
</li>
<li class="active">
<a href="https://f3real.github.io/category/reversing.html">
<i class="fas fa-folder-open padding-small"></i>
reversing
<span class="badge badge-secondary float-right categorybadge">6</span>
</a>
</li>
<li >
<a href="https://f3real.github.io/category/tutorial.html">
<i class="fas fa-folder-open padding-small"></i>
tutorial
<span class="badge badge-secondary float-right categorybadge">5</span>
</a>
</li>
</ul></li>
</ul>
</nav>
<!-- /#sidebar-wrapper -->
<!-- open/close sidebar -->
<button onclick="toggleMenu();return false;" class="btn btn-primary" id="menu-toggle">
<span id="right-arrow" class="fas fa-chevron-right" title="expand sidebar"></span>
<span id="left-arrow" class="fas fa-chevron-left" title="minimize sidebar"></span>
</button>
<!-- /open/close sidebar -->
<!-- Page Content -->
<div id="page-content-wrapper">
<div class="container-fluid">
<section id="content">
<article>
<div class="row">
<div class="col-lg-10">
<header class="page-header">
<h1>
<a href="https://f3real.github.io/w3challs_striptease.html"
rel="bookmark"
title="Permalink to W3challs striptease">
W3challs striptease
</a>
<small>
<div class="post-info">
<div class="publish-info-block">
<small>
<span class="published">
<i class="fa fa-calendar padding-small"></i><time datetime="2020-02-16T10:02:00+01:00"> Sun 16 February 2020</time>
</span>
<span class="category">
<i class="fa fa-folder-open padding-small"></i><a href="https://f3real.github.io/category/reversing.html">reversing</a>
</span>
<span class="tags">
<i class="fa fa-tags padding-small"></i>
<a href="https://f3real.github.io/tag/crackme.html">crackme</a> / <a href="https://f3real.github.io/tag/elf.html">elf</a> </span>
</small>
</div>
</div><!-- /.post-info --> </small>
</h1>
</header>
</div>
</div>
<div class="row">
<div class="col-lg-10">
<div class="entry-content">
<p>In this post, we will take a quick look at one of the simpler reversing challenges from W3challs, striptease.</p>
<p>If we open it in <code>radare2</code> and look at the entry function we see:</p>
<div class="highlight"><pre><span></span><code> 0x08048075 be00000000 mov esi, 0
│ ; CODE XREF from fcn.08048075 @ +0x46
│ ┌─> 0x0804807a 8b1dd0900408 mov ebx, dword [segment.LOAD1] ; [0x80490d0:4]=188
│ ╎ 0x08048080 83c303 add ebx, 3
│ ╎ 0x08048083 391dd4900408 cmp dword [0x80490d4], ebx ; [0x80490d4:4]=255
│ ┌──< 0x08048089 7d06 jge 0x8048091
│ │╎ 0x0804808b 2b1dd4900408 sub ebx, dword [0x80490d4] ; [0x80490d4:4]=255
│ │╎ ; CODE XREF from fcn.08048075 @ 0x8048089
│ └──> 0x08048091 b899800408 mov eax, 0x8048099
└ ╎ 0x08048096 ffe0 jmp eax
╎ 0x08048098 83891dd09004. or dword [ecx + 0x490d01d], 8
╎ 0x0804809f bf5a910408 mov edi, 0x804915a
╎ 0x080480a4 8b0435d89004. mov eax, dword [esi + 0x80490d8]
╎ 0x080480ab 31d8 xor eax, ebx
╎ 0x080480ad 890435d89004. mov dword [esi + 0x80490d8], eax
╎ 0x080480b4 46 inc esi
╎ 0x080480b5 81fe81000000 cmp esi, 0x81 ; 129
└─< 0x080480bb 75bd jne 0x804807a ; fcn.08048075+0x5
┌─< 0x080480bd e916100000 jmp 0x80490d8
│ 0x080480c2 b801000000 mov eax, 1
│ 0x080480c7 bb00000000 mov ebx, 0
│ 0x080480cc cd80 int 0x80
</code></pre></div>
<p>This is the only function in binary that we can immediately inspect, so we need to find what does it do. If we look closely, we see that it is a simple loop performing XOR decryption with incrementing key. After loop finishes, control will jump to decrypted code. We can dynamically debug binary and see the decrypted code or we can write a simple python script to do it for us. </p>
<p>The flag is a hardcoded string in part of code that gets decrypted.</p>
<p>Solution:</p>
<div class="highlight"><pre><span></span><code><span class="n">TARGET_BYTE_OFFSET</span> <span class="o">=</span> <span class="mh">0xd8</span>
<span class="n">REPETITIONS</span> <span class="o">=</span> <span class="mh">0x81</span> <span class="c1"># 129</span>
<span class="n">INCREMENT</span> <span class="o">=</span> <span class="mi">3</span>
<span class="n">TARGET_BIN</span> <span class="o">=</span> <span class="s1">'striptease-1be7d788d5e8de3ea92166b9d9fdbd5ce62d97e76136675c3ef12ef7b3db3602'</span>
<span class="n">TARGET_BIN_MODIFIED</span> <span class="o">=</span> <span class="s1">'striptease-modified'</span>
<span class="c1"># load encrypted part of binary:</span>
<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="n">TARGET_BIN</span><span class="p">,</span> <span class="s1">'rb'</span><span class="p">)</span> <span class="k">as</span> <span class="n">target</span><span class="p">:</span>
<span class="n">target</span><span class="o">.</span><span class="n">seek</span><span class="p">(</span><span class="n">TARGET_BYTE_OFFSET</span><span class="p">)</span>
<span class="n">encrypted_instructions</span> <span class="o">=</span> <span class="nb">bytearray</span><span class="p">(</span><span class="n">target</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="n">REPETITIONS</span><span class="p">))</span>
<span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s1">'Length of encrypted part of binary: </span><span class="si">{</span><span class="nb">len</span><span class="p">(</span><span class="n">encrypted_instructions</span><span class="p">)</span><span class="si">}</span><span class="s1">'</span><span class="p">)</span>
<span class="c1"># decode instructions</span>
<span class="n">esi</span> <span class="o">=</span> <span class="mi">0</span>
<span class="n">ebx</span> <span class="o">=</span> <span class="mh">0xbc</span>
<span class="k">while</span> <span class="n">esi</span> <span class="o">!=</span> <span class="n">REPETITIONS</span><span class="p">:</span>
<span class="n">ebx</span> <span class="o">=</span> <span class="n">ebx</span> <span class="o">+</span> <span class="n">INCREMENT</span>
<span class="k">if</span> <span class="n">ebx</span> <span class="o">>=</span> <span class="mi">255</span><span class="p">:</span>
<span class="n">ebx</span> <span class="o">-=</span> <span class="mi">255</span>
<span class="n">encrypted_instructions</span><span class="p">[</span><span class="n">esi</span><span class="p">]</span> <span class="o">^=</span> <span class="n">ebx</span>
<span class="n">esi</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="n">TARGET_BIN_MODIFIED</span><span class="p">,</span> <span class="s1">'wb'</span><span class="p">)</span> <span class="k">as</span> <span class="n">res</span><span class="p">:</span>
<span class="n">res</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="n">encrypted_instructions</span><span class="p">)</span>
</code></pre></div>
</div>
<footer class="text-right">
<p>- F3real</p>
</footer>
<div id="show-comments" class="span7 text-center">
<a href="https://f3real.github.io/w3challs_striptease.html#disqus_thread"
data-disqus-identifier="w3challs_striptease"
class="btn btn-primary twitchy-background">Show Comments</a>
</div>
<section id="comments" class="comments hidden">
<hr/>
<h2>Comments</h2>
<div id="disqus_thread"></div>
<noscript>Please enable JavaScript to view the <a href="http://disqus.com/?ref_noscript">comments powered by
Disqus.</a></noscript>
<a href="http://disqus.com" class="dsq-brlink">comments powered by <span class="logo-disqus">Disqus</span></a>
</section>
</div>
</div>
</article>
</section>
<footer>
<hr>
<div class="row">
<div class="col-lg-10 text-center">
<p><small>
Built by <a href="http://docs.getpelican.com/en/latest">Pelican</a> / <a href="https://github.com/F3real/pelican-twitchy">pelican-twitchy</a>
· © 2024 F3real
</small></p>
</div>
</div>
</footer> </div>
</div>
<!-- /#page-content-wrapper -->
</div>
<!-- /#wrapper -->
<!-- disqus -->
<script type="text/javascript">
/* * * CONFIGURATION VARIABLES: EDIT BEFORE PASTING INTO YOUR WEBPAGE * * */
var disqus_shortname = 'https-f3real-github-io'; // required: replace example with your forum shortname
var disqus_identifier = 'w3challs_striptease';
var disqus_url = 'https://f3real.github.io/w3challs_striptease.html';
var disqus_config = function () {
this.language = "en";
};
var commentsDiv = document.getElementById('show-comments');
commentsDiv.onclick = function() {
/* * * DON'T EDIT BELOW THIS LINE * * */
(function () {
var dsq = document.createElement('script');
dsq.type = 'text/javascript';
dsq.async = true;
dsq.src = '//' + disqus_shortname + '.disqus.com/embed.js';
(document.getElementsByTagName('head')[0] || document.getElementsByTagName('body')[0]).appendChild(dsq);
})();
this.style.display = 'none';
};
</script>
<!-- /disqus -->
<script>
const wrapper = document.getElementById('wrapper');
const sidebarBig = document.getElementById('sidebar-wrapper');
const sidebarSmall = document.getElementById('sidebar-wrapper-small');
const triggers = Array.from(document.querySelectorAll('[data-toggle="collapse"]'));
for (var i = 0; i < triggers.length; i++) {
triggers[i].addEventListener('click', (ev) => {
const elm = ev.currentTarget;
ev.preventDefault();
const selector = elm.getAttribute('href').replace('#','');
elm.classList.toggle('collapsed');
document.getElementById(selector).classList.toggle('show');
}, false);
}
function showBigNav() {;
sidebarBig.style.display = 'block';
sidebarSmall.style.display = 'none';
}
function showSmallNav() {
sidebarBig.style.display = 'none';
sidebarSmall.style.display = 'block';
}
const mediaQuery = window.matchMedia('(min-width:768px)');
mediaQuery.onchange = e => {
if (wrapper.classList.contains('toggled')) {
wrapper.classList.remove('toggled');
} else {
if (e.matches) {
showBigNav();
} else {
showSmallNav();
}
}
}
function setNavbar() {
var condition = wrapper.classList.contains('toggled');
if (!mediaQuery.matches) {
condition = !condition;
}
if (condition) {
showSmallNav();
} else {
showBigNav();
}
}
function toggleMenu(){
wrapper.classList.toggle('toggled');
setNavbar();
}
window.onload = setNavbar;
</script>
<script data-goatcounter="https://f3real.goatcounter.com/count"
async src="//gc.zgo.at/count.js"></script>
</body>
</html>