friends

Friends

Description

I made a really complicated math function. Check it out.

nc chall.csivit.com 30425

Analysis

Let's take a quick look at namo.py:

import math
import sys

def fancy(x):
    a = (1/2) * x
    b = (1/2916) * ((27 * x - 155) ** 2)
    c = 4096 / 729
    d = (b - c) ** (1/2)
    e = (a - d - 155/54) ** (1/3)
    f = (a + d - 155/54) ** (1/3)
    g = e + f + 5/3
    return g

def notfancy(x):
    return x**3 - 5*x**2 + 3*x + 10

def mathStuff(x):
    if (x < 3 or x > 100):
        exit()

    y = fancy(notfancy(x))

    if isinstance(y, complex):
        y = float(y.real)

    y = round(y, 0)
    return y

print("Enter a number: ")
sys.stdout.flush()
x = round(float(input()), 0)
if x == mathStuff(x):
    print('Fail')
    sys.stdout.flush()
else:
    print(open('namo.txt').read())
    sys.stdout.flush()

Ignore the mathy stuff and look at how the input is parsed:

print("Enter a number: ")
sys.stdout.flush()
x = round(float(input()), 0)
if x == mathStuff(x):
    print('Fail')   
    sys.stdout.flush()
else:
    print(open('namo.txt').read())
    sys.stdout.flush()

If we enter a regular number, we hit the 'Fail' path:

kali@kali:~/Downloads$ nc chall.csivit.com 30425
Enter a number: 
4
Fail
kali@kali:~/Downloads$ nc chall.csivit.com 30425
Enter a number: 
5
Fail

But this is a float, so what about nan? This stands for "not a number" which causes all math comparisons to return false. Passing nan as input triggers the else branch and prints namo.txt:

kali@kali:~/Downloads$ echo nan | nc chall.csivit.com 30425                                         
Enter a number:                                                                                     
Mitrooon                                                                                            
bhaiyo aur behno "Enter a number"
mann ki baat nambar

agar nambar barabar 1 hai {
        bhaiyo aur behno "s"
}

nahi toh agar nambar barabar 13 hai {
        bhaiyo aur behno "_"
}


nahi toh agar nambar barabar 15 hai {
        bhaiyo aur behno "5"
}


nahi toh agar nambar barabar 22 hai {
        bhaiyo aur behno "4"
}


nahi toh agar nambar barabar 28 hai {
        bhaiyo aur behno "k"
}


nahi toh agar nambar barabar 8 hai {
        bhaiyo aur behno "y"
}


nahi toh agar nambar barabar 17 hai {
        bhaiyo aur behno "4"
}


nahi toh agar nambar barabar 9 hai {
        bhaiyo aur behno "_"
}


nahi toh agar nambar barabar 4 hai {
        bhaiyo aur behno "t"
}


nahi toh agar nambar barabar 3 hai {
        bhaiyo aur behno "c"
}


nahi toh agar nambar barabar 20 hai {
        bhaiyo aur behno "r"
}


nahi toh agar nambar barabar 12 hai {
        bhaiyo aur behno "n"
}


nahi toh agar nambar barabar 0 hai {
        bhaiyo aur behno "c"
}


nahi toh agar nambar barabar 23 hai {
        bhaiyo aur behno "t"
}


nahi toh agar nambar barabar 27 hai {
        bhaiyo aur behno "0"
}


nahi toh agar nambar barabar 10 hai {
        bhaiyo aur behno "n"
}

nahi toh agar nambar barabar 11 hai {
        bhaiyo aur behno "4"
}


nahi toh agar nambar barabar 7 hai {
        bhaiyo aur behno "m"
}


nahi toh agar nambar barabar 25 hai {
        bhaiyo aur behno "c"
}


nahi toh agar nambar barabar 24 hai {
        bhaiyo aur behno "_"
}


nahi toh agar nambar barabar 6 hai {
        bhaiyo aur behno "{"
}


nahi toh agar nambar barabar 16 hai {
        bhaiyo aur behno "_"
}


nahi toh agar nambar barabar 18 hai {
        bhaiyo aur behno "_"
}


nahi toh agar nambar barabar 2 hai {
        bhaiyo aur behno "i"
}


nahi toh agar nambar barabar 5 hai {
        bhaiyo aur behno "f"
}


nahi toh agar nambar barabar 19 hai {
        bhaiyo aur behno "g"
}


nahi toh agar nambar barabar 14 hai {
        bhaiyo aur behno "1"
}


nahi toh agar nambar barabar 21 hai {
        bhaiyo aur behno "3"
}


nahi toh agar nambar barabar 26 hai {
        bhaiyo aur behno "0"
}


nahi toh agar nambar barabar 29 hai {
        bhaiyo aur behno "}"
}

nahi toh {
        bhaiyo aur behno ""
}

achhe din aa gaye

Copy those phrases into google translate and it's just a set of rules defined in Hindi. 0 = c, 1 = s, 2 = i, and so on.

Solution

I just started with the basic nan trick and then piled on parsing commands on top of that:

kali@kali:~/Downloads$ echo nan | nc chall.csivit.com 30425 | grep -A1 'hai {' | sed 's/agar nambar barabar //' | sed 's/nahi toh //' | sed 's/ hai {$/ =/' | sed 's/^\tbhaiyo aur behno \"//' | sed 's/\"$//' | sed 's/--//' | sed ':a;N;$!ba;s/=\n/ /g' | sort -n | uniq | awk '{print $2}' | tr -d '\n'; echo ''
csictf{my_n4n_15_4_gr34t_c00k}

This strips away all the text except for the key:value pairs defined by the rules, then sorts each line by the key, removes all the duplicate empty lines, prints the value of each key, and finally strips out all the newline characters to give us the flag.