46 - Простая геометрия (Simple Geometry)
46 - Простая геометрия (Simple Geometry)
Platform: CUSTOM | Category: Reversing | Type: Challenge | Difficulty: Medium | OS: N/A | **Author: XESXOR Techniques: angle_computation, dot_product_calculation, euclidean_distance, static_binary_analysis, vector_operations
Summary
Walkthrough of the CUSTOM Reversing 46 - Простая геометрия (Simple Geometry). Task: Reverse engineer a binary that validates geometric properties of 6 points in 2D space. Solution: Analyze the binary to identify perpendicularity check (dot product = 0) and distance constraints
Description
A reverse engineering challenge involving a binary called simple_geo that validates geometric properties of points in 2D space. The binary accepts 6 points (p1 through p6), each with x and y coordinates, and verifies specific geometric relationships between them.
Analysis
Binary Structure
The challenge requires inputting 6 points in 2D space (12 coordinates total). The binary constructs 3 line segments from pairs of points:
- Line 1: from p1 to p2
- Line 2: from p3 to p4
- Line 3: from p5 to p6
Key Validation Conditions
The binary implements geometric checks with the following requirements:
-
Perpendicularity Check: The angle between Line 1 and Line 2 must be exactly 90 degrees
- Calculated using the dot product of direction vectors
- Formula: (dot_product / (length1 * length2)) = 0 (for perpendicular vectors)
-
Distance Constraints:
- Length of Line 1 must be exactly 4.0
- Length of Line 2 must be exactly 5.0
- Length of Line 3 must be exactly 4.0
Key Functions in Binary
fcn.00001199 - Distance Calculator
Calculates Euclidean distance between two points:
distance = sqrt((x2 - x1)^2 + (y2 - y1)^2)
fcn.0000123a - Dot Product
Computes the dot product of two vectors:
dot_product = v1.x * v2.x + v1.y * v2.y
fcn.000012e8 - Angle Computation
Calculates the absolute cosine of the angle between two line segments:
cos_angle = abs(dot_product / (length1 * length2))
For perpendicular lines, this returns 0.
Solution
Mathematical Approach
To satisfy the constraints:
- Create two perpendicular lines of lengths 4.0 and 5.0
- Create a third line of length 4.0
A simple solution uses axis-aligned vectors:
-
Line 1 (horizontal): p1 = (0, 0) to p2 = (4, 0)
- Direction: (4, 0)
- Length: 4.0
-
Line 2 (vertical): p3 = (0, 0) to p4 = (0, 5)
- Direction: (0, 5)
- Length: 5.0
- Dot product with Line 1: 40 + 05 = 0 ✓ (perpendicular)
-
Line 3: p5 = (0, 0) to p6 = (4, 0)
- Length: 4.0
Python Solution
python read-only • copy allowed#!/usr/bin/env python3 import math # Define the points that satisfy all geometric constraints points = [ (0, 0), # p1 (4, 0), # p2 - Line 1 length = 4.0 (0, 0), # p3 (0, 5), # p4 - Line 2 length = 5.0, perpendicular to Line 1 (0, 0), # p5 (4, 0) # p6 - Line 3 length = 4.0 ] def distance(p1, p2): """Calculate Euclidean distance between two points.""" return math.sqrt((p2[0] - p1[0])**2 + (p2[1] - p1[1])**2) def dot_product(v1, v2): """Calculate dot product of two vectors.""" return v1[0] * v2[0] + v1[1] * v2[1] def verify_solution(points): """Verify that the solution meets all constraints.""" p1, p2, p3, p4, p5, p6 = points # Create direction vectors line1_dir = (p2[0] - p1[0], p2[1] - p1[1]) line2_dir = (p4[0] - p3[0], p4[1] - p3[1]) line3_dir = (p6[0] - p5[0], p6[1] - p5[1]) # Calculate lengths len1 = distance(p1, p2) len2 = distance(p3, p4) len3 = distance(p5, p6) # Check dot product (should be 0 for perpendicular) dot = dot_product(line1_dir, line2_dir) print(f"Line 1 length: {len1} (expected 4.0)") print(f"Line 2 length: {len2} (expected 5.0)") print(f"Line 3 length: {len3} (expected 4.0)") print(f"Dot product (Line 1 × Line 2): {dot} (expected 0 for perpendicular)") # Verify constraints assert abs(len1 - 4.0) < 0.0001, f"Line 1 length must be 4.0, got {len1}" assert abs(len2 - 5.0) < 0.0001, f"Line 2 length must be 5.0, got {len2}" assert abs(len3 - 4.0) < 0.0001, f"Line 3 length must be 4.0, got {len3}" assert abs(dot) < 0.0001, f"Lines must be perpendicular, dot product: {dot}" print("\nAll constraints satisfied!") return True # Verify the solution verify_solution(points) # Output for use with the binary print("\nInput coordinates for the binary:") for i, (x, y) in enumerate(points, 1): print(f"p{i}: {x} {y}")
Interactive Input
When running the binary, provide the following sequence of coordinates:
0 0 4 0 0 0 0 5 0 0 4 0
Or when prompted individually for each point:
- p1: 0 0
- p2: 4 0
- p3: 0 0
- p4: 0 5
- p5: 0 0
- p6: 4 0
Techniques and Concepts
Vector Mathematics
- Dot Product: For checking perpendicularity, if dot product = 0, vectors are perpendicular
- Euclidean Distance: Standard 2D distance formula
- Vector Normalization: Used implicitly in angle calculations
Geometric Constraints
- Axis-aligned vectors simplify calculations (fewer rounding errors)
- Perpendicular vectors can be generated by swapping coordinates and negating one
- Distance constraints are verified exactly using the Euclidean formula
Alternative Solutions
While the simple axis-aligned solution works, other valid inputs could include:
- Any two perpendicular vectors with magnitudes 4.0 and 5.0
- Example: (3, 0) to (7, 0) for a horizontal line of length 4.0
- Example: (2, 0) to (2, 5) for a vertical line of length 5.0
The key is maintaining exact distances and perpendicularity through floating-point calculations.
Key Takeaways / Lessons
- Mastering angle_computation, dot_product_calculation, euclidean_distance, static_binary_analysis, vector_operations was key to this challenge.
- Always check for common misconfigurations and verify assumptions.
Crafted by XESXOR Research • 2026-01-09 • CUSTOM • 46 - Простая геометрия (Simple Geometry) • Difficulty: Medium