DownUnderCTF 2025 — Down To Modulate Frequencies! (100 pts)

Overview

This challenge involved decoding a DTMF (Dual-tone multi-frequency) signal. We received a text file containing a long string of numbers to decode using telephone keypad frequencies and multi-tap text entry.

Solution

DTMF generates two specific audio frequencies per keypad button. Each button can be identified by the sum of its row and column frequencies.

Three steps to solve:

1. Frequency Mapping — Convert 4-digit frequency sums to keypad digits:

Sum	Digit
1979	4
2033	2
2418	#
…	…

2. Sequence Parsing — Split the digit sequence by # separators.

3. Multi-tap Conversion — Apply classic phone keypad text entry (pressing 2 once = A, twice = B, three times = C, etc.).

# Split 428-char input into 4-digit chunks
# Map each chunk to its keypad digit
# Split by '#'
# Convert each group using multi-tap lookup

Flag

DUCTF{ONLYNINETIESKIDSWILLREMEMBERTHIS}