Hi there, here is some nudges, hopes it help. If you are still stuck, PM me and I will answer as soon as I can Spoiler First, there is not a single key. So the dcode site won't help. You will have to decode this on your own manually with the help of the XOR calculator (unless you are good at programming ). Read carefully the first two paragraphs of the section 3.3 from the paper. Then, try to link that with the post from berzerk0 https://forums.aliceandsmith.com/threads/mission-25.1330/page-2#post-23715 (25-2, specially the part about "What is the XOR function and why is it used?"). Then use the XOR calculator. Spoiler: HUGE Nudge, basically the answer... Like the paper says, this is a "rolling XOR", which means that the "key" to decrypt a byte is the previous byte! The beginning is the end!!