MS-DOS task writeup from CTFZONE 2019 Quals
@ Rakovsky Stanislav | Sunday, Dec 1, 2019 | 2 minutes read | Update at Sunday, Dec 1, 2019

reverse, easy > Baby rev

EZ task for beginners in RE.
Don't forget to add ctfzone and curly braces before submitting flag (like ctfzone{FLAG_HERE}).

There is a MS-DOS binary file. The _main function is located at seg000:13DB

It has a pretty cute tricking option. It:

  1. says This program cannot be run in DOS mode.
  2. sleeps 5 seconds (0x1388)
  3. sets foreground color to green
  4. clears the console
  5. and then makes its work

This behaviour may confuse the beginner reverser :3

The next things are loop initialisation code (blue), reading from the file (purpre) and the chain of checks (yellow).

There are 8 similar functions (i named them check0 - check7) that look like a big if-else chain.

It checks each byte from our input file using si register as base address. Let’s write the idapython script to parse them all:

from idautils import *
from idaapi import *

si = {}
ea = BeginEA()
for funcea in Functions(SegStart(ea), SegEnd(ea)): # walk through all the functions
    functionName = GetFunctionName(funcea)
    if "check" in functionName:# if the function name contains "check"
        for (startea, endea) in Chunks(funcea):
            for head in Heads(startea, endea):
                if GetMnem(head) == "cmp" and "si" in GetOpnd(head, 0): # if it's an operation "cmp" with si register
                    val = int(GetOpnd(head, 1).replace("h", ""), 16)
                    if "[si]" in GetOpnd(head, 0):
                    offs = GetOpnd(head, 0)
                    offs = offs[offs.index("+")+1:offs.index("]")].replace("h", "")
                    si.update({int(offs, 16):val})
                    #print functionName, ":", "0x%08x"%(head), ":", GetDisasm(head), ":"

print si 
print "1 Ok"

t = list(si.keys())
m = "".join([chr(si[i]) for i in t])

with open("out", "w") as f:
print "2 Ok!"

Okay! How it looks:

Let’s type out flag using dos (I actually wrote the flag from the text, @silmaulviel said about type command)

Tip of the day: Functions _print and _put take the string from dseg section.

mov ax, 374h; start of “This program…” string in dseg
push ax
call _print

author, editor: Rakovsky Stanislav, Unicorn CTF


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