In order to recover the chat history out of WhatsApp backups (.crypt12), you have to use WhatCrypt Tools utility which has been designed for exactly for the purpose of recovery of various WhatsApp message backups. The export procedure saves the chat history in a ZIP-compressed HTML file, which upon decompression, can be viewed using any web browser.
You can also further export the HTML to plain text format, which essentially achieves crypt12 to text conversion for you. Just keep in mind that this process will also remove any and all the formatting and you will just get the text.Open CRYPT12 file Open TEXT file
Online service for WhatsApp databases