Vigenere picoCTF 2022 Solution

Description

1. Step 1

   Identify the cipher

   Observation

   I noticed the challenge is explicitly named 'Vigenere' and the problem statement provides a five-letter key CYLAB, which indicated that no key-recovery step was needed and the only task was feeding the ciphertext and key into a standard Vigenere decoder.

   The problem statement explicitly names Vigenère. Tools like CyberChef → From Vigenere make decoding trivial.

   What didn't work first

   Tried: Trying frequency analysis to decode the ciphertext before entering the key.

   Frequency analysis works on simple Caesar/monoalphabetic ciphers where each plaintext letter always maps to the same ciphertext letter. Vigenère is polyalphabetic - the same plaintext letter shifts differently at each position - so the frequency distribution is flattened and single-letter analysis fails. Since the key CYLAB is given, you just need to plug it into a decoder directly.

   Tried: Treating the key CYLAB as a numeric shift and applying a single Caesar shift of 5 (the length) or 2 (value of C).

   A Caesar cipher uses one fixed shift for every letter, but Vigenère applies each key letter's value as a separate shift at each position, cycling through C(2), Y(24), L(11), A(0), B(1). Applying just the first letter's shift (2) partially decodes only the letters at positions 1, 6, 11, etc. Use a Vigenère decoder that accepts the full keyword.

   Learn more

   The Vigenère cipher is a polyalphabetic substitution cipher that uses a repeating keyword to determine the shift at each position. For a key of length n, position i is shifted by the value of the (i mod n)-th key letter. This means the same plaintext letter produces different ciphertext letters depending on its position - defeating simple frequency analysis.

   Vigenère was considered unbreakable for nearly 300 years and was called le chiffre indéchiffrable (the indecipherable cipher). It was finally broken systematically by Charles Babbage (1854) and Friedrich Kasiski (1863). The key insight of Kasiski examination: repeated segments of plaintext (like "the") that happen to align with the same part of the repeating key produce repeated ciphertext, revealing the key length. Once you know the key length, each position becomes an independent Caesar cipher, breakable by frequency analysis.

   When the key is provided (as here), decoding is instant: subtract each key letter's value from the corresponding ciphertext letter (modulo 26). Our Vigenère Cipher tool, CyberChef, dCode.fr, and cryptii.com all implement this. The key CYLAB repeats: C(2), Y(24), L(11), A(0), B(1), C(2), Y(24), ...

2. Step 2

   Apply the key CYLAB

   Observation

   I noticed that the first seven characters of the ciphertext ('rgnoDVD') did not match any recognizable prefix on their own, and that subtracting the key CYLAB letter by letter from positions 1-7 produces 'picoCTF', which confirmed the key is correct and that a straight Vigenere decode would yield the flag.

   Enter CYLAB as the key and paste your cipher.txt contents as the input (example from one instance: rgnoDVD{O0NU_WQ3_G1G3O3T3_A1AH3S_...}). The trailing hash in the ciphertext and flag varies per instance. The decoder outputs the correct picoCTF flag.

   What didn't work first

   Tried: Entering the key as lowercase 'cylab' instead of uppercase 'CYLAB'.

   Most online Vigenère tools are case-insensitive for the key and treat both forms identically since only the alphabetical index (A=0, B=1, ...) matters. However, some tools distinguish case or only accept uppercase. If the output looks garbled, try switching the key to all-uppercase CYLAB and make sure the tool is set to Decrypt, not Encrypt.

   Tried: Looking at the decoded output and not recognizing the flag because non-alphabetic characters like digits and underscores look unchanged.

   Vigenère only shifts alphabetic characters; digits, underscores, and curly braces pass through unmodified. So in the ciphertext 'rgnoDVD{O0NU_WQ3_...}', the '{', '0', '_', '3' characters are already in plaintext form. The decoded alphabetic letters will spell out 'picoCTF{D0NT_US3_...' and the full string is the flag as-is.

   Learn more

   The decryption formula for Vigenère is: plaintext[i] = (ciphertext[i] - key[i mod keylen] + 26) mod 26. Non-alphabetic characters (digits, underscores, braces) are passed through unchanged since the cipher only affects letters. The flag wrapper picoCTF{...} confirms correctness - rgnoDVD must decrypt to picoCTF.

   You can verify this manually for the first letter: r (17) - C (2) = 15 = p. Second: g (6) - Y (24) + 26 = 8 = i. Third: n (13) - L (11) = 2 = c. And so on. This illustrates why knowing the key makes Vigenère trivial to break.

   Without the key, breaking Vigenère on short ciphertexts is harder but still feasible. The index of coincidence method estimates key length by testing how "English-like" the ciphertext looks when letters at spacing k are grouped. Once the key length is known, each Caesar sub-cipher is solved independently. Python's pycipher library and the Dcode.fr Vigenère analyzer automate this entire process.

   For a worked walk through Kasiski examination, IC scoring, and column-wise frequency attack against an unknown-key Vigenère ciphertext, see the CTF Encodings reference.

Flag

The trailing hash (e.g. 2951a89h) is generated per instance and will differ from the value shown for other instances. The human-readable prefix is always D0NT_US3_V1G3N3R3_C1PH3R. Vigenère is not secure - once the key is known, decoding is instantaneous.