morse-code picoCTF 2022 Solution

Description

1. Step 1

   Run the audio through a decoder

   Observation

   I noticed the challenge provided a WAV file named morse_chal.wav and the description explicitly mentioned Morse code, which suggested feeding the audio directly into an online Morse audio decoder to extract the encoded letters.

   The file is only ~30 seconds long; most online Morse decoders output the letters immediately.

   What didn't work first

   Tried: Trying to listen to the audio and manually transcribe dots and dashes by ear.

   Manually transcribing Morse audio is extremely error-prone, especially for digits mixed with letters. A beginner will mishear short tones as long ones and lose track of word boundaries. Online audio decoders like morsecode.world handle the entire conversion automatically in seconds.

   Tried: Using a text-based Morse decoder tool before decoding the audio first.

   Text-based Morse decoders expect dot-dash input like .-. --. not a WAV file. The first step is to feed the WAV to an audio-specific decoder to get the dot-dash (or letter) text, and only then use a text decoder if you have the raw symbols.

   Learn more

   Morse code represents letters and digits as sequences of short signals (dots) and long signals (dashes), separated by pauses. It was developed in the 1830s for telegraph communication and is still used in amateur radio. Each character has a unique dot-dash pattern: A is .-, B is -..., and so on.

   In audio Morse, the dots and dashes are tones of different durations. Automated decoders work by analyzing the waveform: they measure the duration of each tone and silence, classify them as dots, dashes, or separators based on timing ratios, and map the resulting patterns to characters. Online tools like morsecode.world or morse.withgoogle.com can decode audio files without any local software. Once you have the dot-dash text, you can also use our Morse Code Decoder to convert it to plain text instantly.

   For those who prefer command-line tools, fldigi is a popular open-source amateur radio application that includes a Morse decoder. More programmatically, Python libraries like pyaudio combined with signal processing can detect tone frequencies and timings from WAV files.

2. Step 2

   Apply the required formatting

   Observation

   I noticed the problem statement explicitly said to replace spaces with underscores before wrapping in picoCTF{...}, and the decoder output was uppercase with word boundaries, which meant two transformations (lowercase conversion and space-to-underscore substitution) were needed before the flag would pass the submission check.

   The decoded message reads WH47 H47H 90D W20U9H7 - that's letter-and-digit Morse, where 4, 7, 9, 0, 2 are real digits in the Morse alphabet (....-, --..., ----., -----, ..---), not garbled letters. Spaces become underscores per the prompt. Wrap with picoCTF{ and } to assemble the final flag, e.g. picoCTF{wh47_h47h_90d_w20u9h7} ending in a closing brace so the format check passes.

   What didn't work first

   Tried: Treating the digits in the decoded output as garbled letters and re-decoding them.

   Morse code has a full numeral set (0-9) so digits like 4, 7, 9 in the output are correct and intentional, not a decoding error. Re-interpreting them as letters produces nonsense. Trust the decoder output and keep the digits as-is.

   Tried: Leaving spaces between words instead of replacing them with underscores.

   The challenge prompt explicitly says to replace spaces with underscores before wrapping in picoCTF{}. Submitting with spaces (e.g. picoCTF{wh47 h47h ...}) will fail the flag check. Use tr or a simple find-and-replace to swap every space for an underscore.

   Learn more

   The pipeline demonstrates several useful shell text-transformation tools in sequence. tr ' ' '_' translates (replaces) every space with an underscore - the tr (translate) utility maps individual characters one-to-one, making it ideal for single-character substitutions. tr '[:upper:]' '[:lower:]' then lowercases the entire string using character classes, which handles the full alphabet without listing each letter.

   The final two sed substitutions prepend the flag prefix and append the closing brace. sed 's/^/picoCTF{/' replaces the start-of-line anchor with the prefix text, and sed 's/$/}/' appends to the end of the line. Chaining these transformations is idiomatic Unix shell scripting.

   In CTF flag formatting, word separators commonly use underscores rather than spaces because spaces can cause issues in URLs and command-line arguments. Recognizing this convention helps when you decode a phrase and need to decide how to format it for submission.

Flag

Simple signal-processing exercise, so there's no need for heavy tooling.