Just an FYI, this no longer works. The latest WhatsApp and Android 7+ versions don't allow you to run the legacy WA, so there's not going to be a backup of the "key" to download.
The only way to get it, is to have your device rooted.
I'm frankly stunned that after such long time WhatsApp still doesn't offer a simple toolset to merge and restore old databases and keys, so that people can store backups of their entire chat history in HTML format etc.
Yes, there are numerous third party tools, one even less reliable and insecure than the next, it's a disaster.
So strange how these WA developers, with an income in the millions $US per year, seem too lazy to provide export/merge options for old dated crypt* files. How hard can that be? It's *their* code, for crying out loud. You just build an app next to WA that allows you to open and export *.crypt12 files from the same device where the valid key is under /data/* and you make sure it's by the same dev team, so WhatsApp Inc. so everybody can trust it. But nooooo, the world has to jump through mountains of adb and java (of all) hoops to get there.