The previous versions of WhatsApp Messenger used .crypt8 files, plenty of users tried to recover the chat history from them, which is exactly what crypt8 to html conversion represents, because the chat history is actually stored in HTML format. Nowadays crypt12 is used for the same purpose, but the modus operandi is pretty much the same.
You have to first get the original crypt version from your crypt8 file and then proceed to extract the chat history with a program known as WhatsApp Xtract.Open CRYPT8 file Open HTML file
An android forensic decoder
Online service for WhatsApp databases