Next one was about Reverse Engineering. Well, that was tough for a beginner.
Did you get this flag?
CBA{Reverse_engineering_is_awesome!}
Well, who did not :) That function, flag_generator(), first encodes the file with a form of compression. I do not care the details of it, but i know that the last step needs to reverse the compression. The next part of the function base64 encode; remember, everything is about base64 in the end. It does this inside a loop, which will occur a random number of times between 0 and 10. It’s clear that in the solution i need to base64 decode the text number of times before performing the decompression.
PYTHON DECODE
from base64 import b64decode as b64d
from base64 import b64encode as b64e
from random import randint
def flag_generator():
flag = “<redacted>”
flag = flag.encode(‘zlib’)
for i in range(randint(0, 10)):
flag = b64e(flag)
return b64e(flag)
if __name__ == ‘__main__’:
flag = “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”
#flag = flag_generator()
print (“Try to get the flag reversing the python script that generated it!”)
print (“Flag string: “ + flag)
from base64 import b64decode as b64d
from base64 import b64encode as b64e
from random import randint
def flag_generator():
flag = “CBA{Reverse_engineering_is_awesome!}”
flag = flag.encode(‘zlib’)
for i in range(randint(0, 10)):
flag = b64e(flag)
return b64e(flag)
**The last step of the generator is base64 encoding a random number of times between 0–10, 0 and 10 included.
**So create a loop that repeats 11 times.
**Use a try block to attempt to decode step. If it fails, keep looping, if not
# end the loop early with a break, then
# decode the file to reverse the zlib encoding on line 8
# print the flag
def flag_degenerator(flag):
for i in range(11):
try:
flag.decode(‘zlib’)
break
except:
print (“Count no: “ + str(i))
flag = b64d(flag)
currentFlag = flag.decode(‘zlib’)
print (“The flag is: “ + currentFlag)
# Alternative solution — Using recursion to find the solution
# Use a try block to test whether we can successfully decode the file.
# If it can’t, the function does a single base64 decode, then calls itself recusively until it can decode the file.
# When it can decode the file, it does so and then prints, knowing that there are no more base64 decodes required.
def flag_degenerator_recusion(flag):
try:
print (“The flag is: “ + flag.decode(‘zlib’))
except:
flag_degenerator_recusion(b64d(flag))
if __name__ == ‘__main__’:
flag = “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”
#flag = flag_generator()
print (“Try to get the flag reversing the python script that generated it!”)
print (“Flag string: “ + flag)
flag_degenerator(flag)
#flag_degenerator_recusion(flag)
FLAG: CBA{Reverse_engineering_is_awesome!}
Yeah, nobody said it was easy…
