Description: What do the flags mean?
Difficulty: Medium
Author: Danny
Summary
This challenge focuses on decoding a message hidden using Maritime Signal Flags, a visual communication method used by ships.
The challenge provides an image file named flag.png, which displays a sequence of maritime flags.
Decoding can be done manually using the reference chart or automatically through online tools like Maritime Signals decoder
Analysis
We are given a PNG file, and upon opening it, we can see a series of flags resembling country or signal flags:
However, I noticed that the flag format PICOCTF{…} is preserved, the curly braces remain intact.
This suggests that the encoding technique only affects letters and numbers, leaving symbols like { and } unchanged.
A quick search suggests that this is likely using the Navy Signals Cipher, a system that maps letters and digits to specific maritime flags.
What Are Nautical Signal Flags?
The International Code of Maritime Signals (ICS) is a visual communication system used in maritime navigation to transmit messages between ships or between a ship and the shore.
It uses a set of distinct flags (also called pennants or semaphores), and each flag represents a specific letter or number.
How Does the Navy Signals Cipher Work?
The International Maritime Signal Code can also be used as a monoalphabetic substitution cipher,
each nautical flag stands for a letter (A–Z) or a number (0–9).
Example:
Number Flags
Digits have distinct flag designs and correspond to lowercase ASCII characters:
How to Decrypt Maritime Signal Flags
Decryption is straightforward, substitute each flag with its corresponding letter or digit.
Example:
Recognizing Maritime Flag Ciphertext
Ciphertext made with this method typically contains:
- Square flags in blue, white, yellow, red, or black (no green or mixed colors).
- Patterns or themes related to ships, navy, or seafaring clues (like lighthouses or distress signals).
These visual hints indicate that the challenge likely involves Nautical Signal Flags.
Why Use Maritime Signal Flags?
The International Maritime Signal Flags system allows ships to communicate when radio or electronic communication is unavailable.
It’s used for transmitting navigation instructions, warnings, and emergency messages, forming a critical part of naval operations.
Solution
To decode the message, I substituted each maritime flag in the PNG file with its corresponding letter or digit.
I used the Maritime Signals decoder.
The decoded flag is PICOCTFF1AG5AND5TUFF.
Since the curly braces {} were not part of the encryption, I added them back to match the flag format:
⚡ Raikiri🎉 Flag pwned! The Navy Signals Cipher has been successfully decoded.
PICOCTF{F1AG5AND5TUFF}💡 TL;DR / Lesson LearnedThe International Maritime Signal Flags system was primarily a visual communication tool for ships, not a cryptographic system. Its main purpose was safe, reliable signaling over distances when radio or other means weren’t available.
This system was originally used by ships to communicate visually over distances. However, this type of communication is no longer used for security purposes, as it is easily breakable.