interencdec picoCTF 2024 Solution

Description

1. Step 1Read the initial Base64
   cat enc_flag prints a long Base64 string ending in ==. Decode it once to reveal a Python byte literal (b'...').
   bash

   base64 -d enc_flag

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   Base64 is an encoding scheme, not encryption. It converts arbitrary binary data into a safe printable-ASCII string using 64 characters (A-Z, a-z, 0-9, +, /). Every 3 bytes of input become 4 Base64 characters, which is why Base64-encoded data is always about 33% larger than the original.

   The == at the end is padding. Base64 works in 3-byte groups; if the input isn't a multiple of 3 bytes, one or two = characters are appended as placeholders so the length is always a multiple of 4.

   Base64 is everywhere: email attachments (MIME), embedding images in CSS (data:image/png;base64,...), JWTs (the header and payload are Base64URL-encoded), and passing binary data through systems that only handle text. Seeing a string that ends in == or is unusually long and uses only alphanumeric characters is a strong hint to try Base64 decoding it.

2. Step 2Strip quotes and decode again
   Remove the leading b' and trailing ' (cut -d "'" -f2 works on the typical bytecode print format), then Base64-decode the inner string to obtain a Caesar-shifted message.
   bash

   base64 -d enc_flag | cut -d "'" -f2 | base64 -d

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   The b'...' wrapper is Python's syntax for a bytes literal. When Python prints a bytes object it adds this prefix so you can tell it apart from a regular string. It's not part of the data, just how Python represents it in text form.

   cut -d "'" -f2 splits on the single-quote character and takes the second field, which works cleanly when the inner Base64 contains no quotes (it never will, since the Base64 alphabet is A-Za-z0-9+/=) and there is no whitespace before the opening b'. If you ever see surrounding whitespace or a different quoting style (b"..."), reach for sed -E "s/^b['\\"]//; s/['\\"]$//" instead.

   The key insight: encoding is not encryption. No secret key is involved; anyone who recognizes the encoding can reverse it. Real encryption (AES, RSA) requires a key you don't have. Encoding is purely a format transformation.

3. Step 3Apply ROT13 / Caesar
   The decoded text starts with something like 'cvpbPGS{...}' - a Caesar-shifted picoCTF{...}. Use CyberChef's ROT13 (the easiest path) or pipe through bsdgames caesar, then scan the 25 candidate lines for the one starting with 'pico'.
   bash

   base64 -d enc_flag | cut -d "'" -f2 | base64 -d | caesar

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   caesar from bsdgames spits out a single line for each shift 1-25 with no labels, so the readable line jumps out by eye. CyberChef's ROT13 recipe with "Brute force all rotations" checked is the friendliest option.

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   A Caesar cipher shifts each letter by a fixed number of positions in the alphabet. ROT13 is a Caesar cipher with a shift of 13; it's its own inverse (applying it twice gives you back the original), which made it popular for hiding spoilers in early internet forums.

   With only 25 possible shifts, the right one is whichever produces the recognizable picoCTF{ prefix. That single landmark, plus the known flag format, is the only signal you need; full English frequency analysis is overkill on a 40-character flag and is the standard tool for the Vigenère-style longer ciphertexts where you don't already know the plaintext skeleton.

   Historically, Julius Caesar reportedly used a shift of 3. The cipher was reasonably secure in an era when most people were illiterate, but provides zero real security today. It's the ancestor of the Vigenère cipher (a repeating-key Caesar), which itself was cracked in the 1800s using the Kasiski test.

No-terminal path (recommended)

Flag

Two Base64 layers plus a Caesar shift are all that stand between you and the flag.