repetitions picoCTF 2023 Solution

Description

1. Step 1Detect how many layers you have
   Loop base64 decoding and stop when the output contains picoCTF{. That confirms the depth without guessing.
   python

   python3 - <<'PY'
   import base64, pathlib
   data = pathlib.Path('enc_flag').read_bytes().strip()
   for i in range(1, 20):
       try:
           data = base64.b64decode(data)
       except Exception as e:
           print(f'layer {i}: decode failed ({e})')
           break
       if b'picoCTF{' in data:
           print(f'layer {i}: found flag -> {data.decode(errors="replace")}')
           break
       print(f'layer {i}: still encoded ({data[:32]!r}...)')
   PY

   Copied!
   Learn more

   Nested Base64 encoding means the data has been Base64-encoded several times in a row. Each layer wraps the previous output, so to recover the original you decode exactly as many times as it was encoded. For this challenge that turns out to be six.

   The loop above is more robust than counting layers by eye: it stops as soon as picoCTF{ appears and tells you how many decodes that took. If the output ever stops looking like Base64 (mixed case + digits + = padding) before the flag shows, the next layer is probably hex, gzip, or a different encoding instead.

2. Step 2Apply six base64 decodes (the fixed-depth path)
   Once you know the depth is six, a flat shell pipeline reads enc_flag once and pipes through five more decodes.
   bash

   base64 --decode enc_flag | base64 --decode | base64 --decode | base64 --decode | base64 --decode | base64 --decode

   Copied!
   Learn more

   The first base64 --decode enc_flag reads the file directly (no UUOC) and emits decoded bytes. Each subsequent | base64 --decode peels another layer. After six total decodes you land on plain ASCII containing the flag.

   Want a sanity-check sentinel inside the pipeline? Append | grep -o "picoCTF{[^}]*}". If the flag has not surfaced, grep prints nothing and you know the depth was off by at least one.

3. Step 3Alternative: CyberChef Magic
   CyberChef's Magic operation auto-detects multi-layer Base64 and decodes until the output stops looking encoded.
   Learn more

   CyberChef ships a Magic operation that fingerprints the input and tries cascades of decoders until the output stops looking encoded. For pure multi-layer Base64 it nails this challenge in one click.

   Magic's limits matter though: it does not handle hex-or-compressed layers cleanly, so if a layer mid-stack is gzipped, XOR'd with a key, or reversed, Magic gives up. In those cases drop into the recipe panel and chain operations manually.

Flag

Each Base64 decode peels a layer; six iterations reveal the final picoCTF flag.