Boss training staff will occasionally post supplementary material here to help you solve the case. You should use it alongside any data or clues you find in the official texts published on the Challenge pages.
Agent Madness has drafted a guide to breaking the Vigenere cipher which you may find very helpful with Mission Briefing 6A. We strongly recommend that you take a look unless you already know how to tackle these. Don’t forget that Mission Briefing 5A also told you the key length for this cipher, which should greatly simplify things.