Hackasat 2: Take out the trash writeup
Rahil Shah
I competed in hackasat 2 this weekend with DiceGang and we ended up getting 4th place. I solved this at 9 am after not sleeping so the thought process to solve this was a bit all over the place. Additionally, there was a lot of physics and less hacking than a usual ctf prompt. Still, it was a really fun experience and I'm excited for finals in September
Description
A cloud of space junk is in your constellation's orbital plane. Use the space lasers on your satellites to vaporize it! Destroy at least 51 pieces of space junk to get the flag.
The lasers have a range of 100 km and must be provided range and attitude to lock onto the space junk. Don't allow any space junk to approach closer than 10 km.
Command format:
[Time_UTC] [Sat_ID] FIRE [Qx] [Qy] [Qz] [Qw] [Range_km]
Command example:
2021177.014500 SAT1 FIRE -0.7993071278793108 0.2569145028089314 0.0 0.5432338847750264 47.85760531563315
This command fires the laser from Sat1 on June 26, 2021 (day 177 of the year) at 01:45:00 UTC and expects the target to be approximately 48 km away. The direction would be a [0,0,1] vector in the J2000 frame rotated by the provided quaternion [-0.7993071278793108 0.2569145028089314 0.0 0.5432338847750264] in the form [Qx Qy Qz Qw].
One successful laser command is provided for you (note: there are many possible combinations of time, attitude, range, and spacecraft to destroy the same piece of space junk):
2021177.002200 SAT1 FIRE -0.6254112512084177 -0.10281341941423379 0.0 0.773492189779751 84.9530354564239
We are given two TLE files to show the paths of the satellites and the asteroids they need to shoot. The skyfield API can process these very easily, so there’s no real point in looking into the format too much. I initially plotted all of the paths on a 3d graph, and saw all objects had relatively simiar paths. At this point, I started scripting min distance between all satellites for all asteroids to see when they will be too close and make us lose. This was all fairly simple to do, but then came calculating the quaternions. I didn’t particularly understand the concept, but in the end one of my teammates found a simple and easy way to leverage the scipy library with the TLEs to calculate the quaternion between a satellite and asteroid at a specific time.
After sucessfully calculating quaternions, we ran into a problem that wasn’t stated in the description, the lasers needed to cool down. We weren’t really sure how the laser cooldown system worked, but we eventually just attempted a minute between shots per satellite and it seemed to work fine. The full solve script is written below:
Final Solve Script
#calculate quaternion
def get_rot(dst):
src = np.array([0, 0, 1])
v = np.cross(src, dst)
c = np.dot(src, dst)
V = np.matrix([
[0, -v[2], v[1]],
[v[2], 0, -v[0]],
[-v[1], v[0], 0]])
R = np.identity(3) + V + 1/(1+c) * V @ V
return Rotation.from_matrix(R)
#calculate 3d distance
def calcDistance(x1,y1,z1,a1,b1,c1):
return np.sqrt((x-a)**2+(y-b)**2+(z-c)**2)
from scipy.spatial.transform import Rotation
from scipy.spatial import distance
from skyfield.api import load, EarthSatellite
from skyfield.functions import mxm
from skyfield.timelib import Time
from skyfield.framelib import ecliptic_J2000_frame
import numpy as np
import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D
#initialize satellites and junk
satellites = load.tle_file('sats.tle')
junk = load.tle_file('spacejunk.tle')
#initialize objects
ts = load.timescale()
used = dict()
minu = np.arange(0,2500)
time = ts.utc(2021, 6, 26, 0, minu)
success = dict()
for junks in junk:
success[junks] = 0
final = ""
for junks in junk:
goal = junks
Jpos = goal.at(time).position.km
for i in range(len(satellites)):
if success[junks] == 1:
break
Rpos = satellites[i].at(time).position.km
if True:
x, y, z = Rpos
a,b,c = Jpos
distance = calcDistance(x,y,z,a,b,c)
#check for possible deaths greedily
for d in distance:
if d <= 10:
print("fail" + str(distance))
print(satellites[i])
print(junks)
#check if within range with a 5 km buffer
for d in distance:
if success[junks] == 1:
break
if d < 95:
for k in range(len(distance)):
if d == distance[k]:
curr_t = ts.utc(2021, 6, 26, 0,k)
if satellites[i] not in used:
used[satellites[i]] = list()
used[satellites[i]].append(k)
else:
if k in used[satellites[i]]:
break
used[satellites[i]].append(k)
tim = k
hours = int(tim/60)
minutes = int(tim % 60)
thing = str(satellites[i]).split(" ")
frame = (junks - satellites[i]).at(curr_t).position.km
dst = frame / np.linalg.norm(frame)
q = get_rot(dst).as_quat()
#super obnoxious formatting
final += (("2021177.{:02d}{:02d}00 " + thing[0].upper() +" FIRE "+ str(q[0]) + " "+ str(q[1]) + " "+str(q[2]) + " " + str(q[3]) + " " + str(d) + "\n").format(hours, minutes))
success[junks] = 1
break
#sorting the commands by time
x = final.split("\n")
x.sort()
for i in x:
print(i)