Finding encryption key for Apple Messages chat

I'm trying to back up some message transcripts, including photos. I found I the chat transcripts from ~/Library/Messages/ can be opened on another Mac, but the images don't load. I searched my local drive for the image names, and found them in various places in ~/Library/Messages/Attachments. On a guess, I tried opening the transcript in MesaSQLite, and I was asked for the encryption key. Is it possible to find that encryption key, and if so, where is it? That's the main question. I'd also need to find the paths for those attachments. Hopefully they're in the transcript database file. If anyone answering that main question happens to know or have some insight into the attachment paths, that'd be great. I'm running macOS 10.13.6.