0CTF 2015 Quals -- (Baby)PolyQuine

Different people see different me. But I am always myself. <202.112.26.114:12321>

Make the output of your program exactly the same as your source code. All 5 correct required to get this flag (Only need 3 correct for BabyPolyQuine)

$python2 –version Python 2.7.6

$python3 –version Python 3.4.0

$gcc –version gcc (Ubuntu 4.8.2-19ubuntu1) 4.8.2

$ruby –version ruby 1.9.3p484 (2013-11-22 revision 43786) [x86_64-linux]

$perl –version This is perl 5, version 18, subversion 2 (v5.18.2) built for x86_64-linux-gnu-thread-multi

The challenge reminds me of HITCON CTF 2014 – Polyglot, except this time, it want us to write a program, which is not only a polyglot, but also a quine.

Well, by searching the answer with the help of the almighty google, we can easily solve the BabyPolyQuine challenge by submitting this program, which pass almost all the languages check – except for Python3.

It seems that Python3 has encounter some syntax error. After changing def printf(a,*b):p a%b to def printf(a,*b):p (a%b), Python3 can run it without any error message now, but it print an extra newline, which doesn’t match the challenge’s requirement. So now we’re focusing on how to make both python2 & python3 have the same printing behavior. After doing some trial & error, I found that for C, Ruby & Perl, they will ignore the line def printf(a,*b):p (a%b), except Python. So we can add some code at that line.

As for my opinion, I add the following code:

from sys import* ; p = lambda x:stdout.write(x)
def printf(a,*b):p (a%b)

For Python, it will import the sys module, and set a lambda p as a stdout.write() function. The stdout.write() function won’t print newline, so python2 & python3 will act like the same.

The whole program:

#include/*
s='''*/<stdio.h>
main(){char*_;/*==;sub _:lvalue{$_}<<s;#';<<s#'''
from sys import* ; p = lambda x:stdout.write(x)
def printf(a,*b):p (a%b)
s
#*/
_=" #include/*%cs='''*/<stdio.h>%cmain(){char*_;/*==;sub _:lvalue{\%c_}<<s;#';<<s#'''%cfrom sys import* ; p = lambda x:stdout.write(x)%cdef printf(a,*b):p (a%%b)%cs%c#*/%c_=%c%s%c;printf(_,10,10,36,10,10,10,10,10,34,_,34,10,10,10,10);%c#/*%cs='''*/%c}//'''#==%c";printf(_,10,10,36,10,10,10,10,10,34,_,34,10,10,10,10);
#/*
s='''*/
}//'''#==

After we submit the code, we get the flag as well: 0ctf{"Yields falsehood when preceded by its quotation" yields falsehood when preceded by its quotation}