Послание от Француза (A Message from the Frenchman)

bug-makers

Task: a 143-byte ciphertext letter from a Frenchman plus a photo of 'Pixie' with a note that her name is the key but not in Russian. Solution: recognized a Vigenere cipher (preserved formatting, flag-shaped suffix), derived the key 'pixie' (English spelling per the hint) and decrypted to recover the flag.