Deja Vu

Platform: CUSTOM | Category: Reversing | Type: Challenge | Difficulty: Easy | OS: N/A | **Author: XESXOR Techniques: arithmetic_reverse, image_code_reading

Summary

Walkthrough of the CUSTOM Reversing Deja Vu. Language: Python. Source code provided as an image (screenshot). Flag checker with arithmetic verification.

Description

Language: Python. Source code provided as an image (screenshot). Flag checker with arithmetic verification.

Analysis

Code from the image:

 python read-only • copy allowed Copy
import sys
verify = [521, 339, 1028, 365, 1132, 352, 833]
if len(sys.argv[1]) != len(verify): sys.exit(-1)
for i in range(0, 7):
    char = ord(sys.argv[1][i])
    char = (char + i) * 13 - 337
    if char != verify[i]: sys.exit(-1)
print "[v] correct!"

Solution

Reverse formula: ord(char) = (verify[i] + 337) / 13 - i ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

 python read-only • copy allowed Copy
verify = [521, 339, 1028, 365, 1132, 352, 833]
flag = ''.join(chr((v + 337) // 13 - i) for i, v in enumerate(verify))
# REDACTED

‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Key Takeaways / Lessons

• Mastering arithmetic_reverse, image_code_reading was key to this challenge.
• Always check for common misconfigurations and verify assumptions.

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Crafted by XESXOR Research • 2026-03-09 • CUSTOM • Deja Vu • Difficulty: Easy