HACKvent 2019: Day 1

CTF: Hackvent 2019
Link to challenge: https://academy.hacking-lab.com

Date Completed: 2 December 2019

Challenge

HV19.01 censored

I got this little image, but it looks like the best part got censored on the way. Even the tiny preview icon looks clearer than this! Maybe they missed something that would let you restore the original content?

1	I got this little image, but it looks like the best part got censored on the way. Even the tiny preview icon looks clearer than this! Maybe they missed something that would let you restore the original content?

Solution

The hint indicates that we should have a clearer smaller image available to us.
At first we attempt to shrink the image, resharpen and enlarge but there is too much degradation for that to work. We also open the image in GIMP and mess with levels/contrast with no luck.

Finally, we use binwalk on the image to find hidden binaries and find:

$ python .\binwalk .\f182d5f0-1d10-4f0f-a0c1-7cba0981b6da.jpg
DECIMAL       HEXADECIMAL     DESCRIPTION
--------------------------------------------------------------------------------
0             0x0             JPEG image data, EXIF standard
12            0xC             TIFF image data, little-endian offset of first image directory: 8
332           0x14C           JPEG image data, JFIF standard 1.01

$ python .\binwalk .\f182d5f0-1d10-4f0f-a0c1-7cba0981b6da.jpg

DECIMAL HEXADECIMAL DESCRIPTION

--------------------------------------------------------------------------------

0 0x0 JPEG image data, EXIF standard

12 0xC TIFF image data, little-endian offset of first image directory: 8

332 0x14C JPEG image data, JFIF standard 1.01

We extract the second JPEG image in this file which gives us a smaller preview thumbnail but with a crisp QR code!

This image is good enough to scan and gives us our flag!

Flag: HV19{just-4-PREview!}

CTF: Advent Of Code
Link to challenge: http://adventofcode.com

Date Completed: 01/12/2015

The Challenge

--- Day 1: Not Quite Lisp ---
Santa was hoping for a white Christmas, but his weather machine's "snow" function is powered by stars, and he's fresh out! To save Christmas, he needs you to collect fifty stars by December 25th.
Collect stars by helping Santa solve puzzles. Two puzzles will be made available on each day in the advent calendar; the second puzzle is unlocked when you complete the first. Each puzzle grants one star. Good luck!
Here's an easy puzzle to warm you up.
Santa is trying to deliver presents in a large apartment building, but he can't find the right floor - the directions he got are a little confusing. He starts on the ground floor (floor 0) and then follows the instructions one character at a time.
An opening parenthesis, (, means he should go up one floor, and a closing parenthesis, ), means he should go down one floor.
The apartment building is very tall, and the basement is very deep; he will never find the top or bottom floors.
For example:
(()) and ()() both result in floor 0.
((( and (()(()( both result in floor 3.
))((((( also results in floor 3.
()) and ))( both result in floor -1 (the first basement level).
))) and )())()) both result in floor -3.
To what floor do the instructions take Santa?
--- Part Two ---
Now, given the same instructions, find the position of the first character that causes him to enter the basement (floor -1). The first character in the instructions has position 1, the second character has position 2, and so on.
For example:
) causes him to enter the basement at character position 1.
()()) causes him to enter the basement at character position 5.
What is the position of the character that causes Santa to first enter the basement?

--- Day 1: Not Quite Lisp ---

Santa was hoping for a white Christmas, but his weather machine's "snow" function is powered by stars, and he's fresh out! To save Christmas, he needs you to collect fifty stars by December 25th.

Collect stars by helping Santa solve puzzles. Two puzzles will be made available on each day in the advent calendar; the second puzzle is unlocked when you complete the first. Each puzzle grants one star. Good luck!

Here's an easy puzzle to warm you up.

Santa is trying to deliver presents in a large apartment building, but he can't find the right floor - the directions he got are a little confusing. He starts on the ground floor (floor 0) and then follows the instructions one character at a time.

An opening parenthesis, (, means he should go up one floor, and a closing parenthesis, ), means he should go down one floor.

The apartment building is very tall, and the basement is very deep; he will never find the top or bottom floors.

For example:

(()) and ()() both result in floor 0.

((( and (()(()( both result in floor 3.

))((((( also results in floor 3.

()) and ))( both result in floor -1 (the first basement level).

))) and )())()) both result in floor -3.

To what floor do the instructions take Santa?

--- Part Two ---

Now, given the same instructions, find the position of the first character that causes him to enter the basement (floor -1). The first character in the instructions has position 1, the second character has position 2, and so on.

For example:

) causes him to enter the basement at character position 1.

()()) causes him to enter the basement at character position 5.

What is the position of the character that causes Santa to first enter the basement?

Solution

Before part 2 of the problem was revealed I decided to tackle the problem using the command line. I cam up with the following command:

# Day 1 - Part 1
echo $((`cat input.txt | tr -d '\)' | wc -c` - `cat input.txt | tr -d '\(' | wc -c`))

1 2	# Day 1 - Part 1 echo $((`cat input.txt \| tr -d '\)' \| wc -c` - `cat input.txt \| tr -d '\(' \| wc -c`))

We echo the input and remove the ) symbol which leaves only ( and we use wc to count the number of ( symbols which tells us how many times we went to a higher floor. We then subtract his number from the number of times we went to a lower floor (using bash arithmetic) which gives us our answer.

Part 2 of the problem would be annoying to do on the command line so I decided to write a quick python script to solve it. The solution also solves the first part of the problem (lol).

counter = 0
level = 0
basement_pos = 1
basement_found = False
with open("input.txt", "r") as file:
for line in file:
for c in line:
if c == ')':
level -= 1
elif c == '(':
level += 1
counter += 1
# Check if in basement
if (level < 0 and not basement_found):
basement_pos = counter
basement_found = True
# Conclusion (we did part 1 too lol)
print "Final level:", level
print "First basement level position:", basement_pos

counter = 0

level = 0

basement_pos = 1

basement_found = False

with open("input.txt", "r") as file:

for line in file:

for c in line:

if c == ')':

level -= 1

elif c == '(':

level += 1

counter += 1

# Check if in basement

if (level < 0 and not basement_found):

basement_pos = counter

basement_found = True

# Conclusion (we did part 1 too lol)

print "Final level:", level

print "First basement level position:", basement_pos

Mo Beigi