AF-1DFB-VG · Kriptanaliz / Klasik

Polyalphabetic Break

DOWNLOADABLE ciphertext. The main message is Vigenere-encrypted; recover the key via frequency analysis (Kasiski / index of coincidence / chi-squared) and find the hidden passphrase in the plaintext. The simple Caesar line in the file is a decoy.

Scenario

An intercepted message. The main content is polyalphabetic (Vigenere); it covers English plaintext, so frequency/Kasiski analysis recovers the key. The file also has a simple substitution (Caesar) line; it cracks easily but leads to a wrong (fake) key = decoy. Skill: not jumping to the weak one, but breaking the real Vigenere with genuine cryptanalysis and extracting the hidden passphrase (PASSPHRASE...END).

Anti-forensics techniques

Polyalphabetic cipher (Vigenere)
Decoy: easy Caesar line (fake key)

Provided artifacts

Ciphertext file (Vigenere + Caesar decoy)

Sample questions

q1: The recovered Vigenere key?
q2: The hidden passphrase in the decrypted plaintext?
trap1: Do NOT report the fake key (PASSWORD) from the Caesar decoy line.

Soundness trap

The Caesar line easily yields PASSWORD but it is fake; presenting it as the key is heavily penalized. The real key only comes from Vigenere cryptanalysis.

Scoring

Cryptanalysis accuracy (key+passphrase) + Caesar-decoy resistance (soundness).

DOWNLOADABLE

Download and solve

Download artifact

The answer key is hidden (scored set); the flag is encrypted, you must decrypt it.