Now AI5WIN's, father of AI6WIN and grandfather of Silky Engine, .arc archives had also fallen. Alas, I hath nor archives of ancient AI1-AI4 engines, nor the means (I could find only AI5WIN ports of the games) to obtain it. Therefore I hath no choice but to stop hacking archives of Elf-Silky archives line. But probably it is not the big deal, since games on earlier engines does not work on modern operating systems.
Here is the tool -- AI5WINArcTool. As usual, unpacking and packing with correct (de)compression at both operations.
AI5WIN's .arc probably is the most complicated from Elf-Silky archives line. Data compression and storing are like in the other archives in this series, and header not very far from AI6WIN's, but... creators of the engine decided to obfisificate header with 3 (!) different keys. Fortunately, the way they implemented it was not too problematic to deal with to the extend that I implemented system that authomaticaly hacks all three keys from any (well, there are some conditions, but I could not find any archives that does not match it) AI5WIN .arc archive (yes, even not from .exe) and therefore should work with all it's games.