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A reverse challenge that was available during SigSegV2 CTF (2019)
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noraj/SigSegV2.reverse_6
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| Name | Name | Last commit message | Last commit date | |
|---|---|---|---|---|
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| Date | Author | Contact | Version | Comment |
|---|---|---|---|---|
| 27/10/2019 | noraj (Alexandre ZANNI) | noraj#0833 on discord | 1.0 | Document creation |
Information displayed for CTF players:
- Name of the challenge /Nom du challenge:
Matz 2.3 - Category /Catégorie:
Reverse - Internet: not needed
- Difficulty /Difficulté: easy / facile
Haaaaa the script was removed by SIben, only this weird file is left :/Helppppppp me!Flag format: sigsegv{flag}author: [noraj](https://pwn.by/noraj/)- Hint1: Ruby
- Hint2: bytecode
Just share the.rbc file.
.rb(Ruby) -->.rbcsame as.py(Python) -->.pyc, the c stands for compiled.rbcis some ruby bytecode- (The title
Matz 2.3can help;Matz is the ruby creator's nickname and2.3is the ruby version where bytecode compilation was introduced) - OSINT skill:How to compile Ruby?:https://stackoverflow.com/questions/5902334/how-to-compile-ruby/44613584#44613584
- OSINT skill:RubyVM::InstructionSequence:https://ruby-doc.org/core-2.6.5/RubyVM/InstructionSequence.html
- disassemble the bytecode in human readble instructions (use
load.rb)
Obtains something like that:
$ ruby load.rb == disasm: #<ISeq:<main>@script.rb:1 (1,0)-(44,19)> (catch: FALSE)local table (size: 1, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])[ 1] a198563@00000 newarray 0 ( 1)[Li]0002 setlocal_WC_0 a198563@00004 getlocal_WC_0 a198563@0 ( 2)[Li]0006 putobject 290008 putobject 1160010 opt_aset <callinfo!mid:[]=, argc:2, ARGS_SIMPLE>, <callcache>0013 pop0014 getlocal_WC_0 a198563@0 ( 3)[Li]0016 putobject 160018 putobject 990020 opt_aset <callinfo!mid:[]=, argc:2, ARGS_SIMPLE>, <callcache>0023 pop0024 getlocal_WC_0 a198563@0 ( 4)[Li]0026 putobject 310028 putobject 950030 opt_aset <callinfo!mid:[]=, argc:2, ARGS_SIMPLE>, <callcache>...a198563 is the array name,29 is the array index,116 is the value assigned to it.
Retrieve all values. You will end with something like inscript.rb.
Then transform the array of int into ASCII text:
putsa198563.map{|i|i.chr}.join
sigsegv{you_c4n_c0mp1le_ruby_t0_bytec0de}
A reverse challenge that was available during SigSegV2 CTF (2019).
8 teams on 36 flaged this challenge.
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